Nervous System




Major Functions of the Nervous System


The nervous system is the main control center for the entire body. It sends and receives messages using electrical impulses that are passed between nerve cells called neurons. The nervous system is made up of two parts:
Central Nervous System - includes the brain and spinal cord
Peripheral Nervous System - includes the nerves throughout the body

Your nervous system has two main functions:
1. Orders are sent from the brain to body organs and tissues along motor neurons
2. Information about the body's condition or the environment is sent to the brain along sensory neurons. This information is obtained using the 5 senses.

At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are two basic ways that a cell can send signals to other cells. The simplest is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.

At a more integrative level, the primary function of the nervous system is to control the body. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking this is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications.

The nervous system has three basic functions:
1. Sensory neurons receive information from the sensory receptors
2. Interneurons transfer and interpret impulses, and
3. Motor neurons send appropriate impulses/instructions to the muscles and glands.


Diagrams that Include the Major Parts - (Brain, parts of the brain), spinal cord, nerves, neurons)


With all the billions of neurons that are present in the body, things would be a complete mess if these neurons were not organized in some way. So to help tidy things up a bit, the body arranges these neurons together as part of the nervous system.
The nervous system therefore acts very much like the body's communication network, as it contains the highways of information which our brain uses to send and receive information to and from different parts of the body. There are two main divisions of the nervous system. These are: the central nervous system and the peripheral nervous system with each serving a specific role.

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The Central Nervous System

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The peripheral nervous system consists of all the neural tissue that lies outside the central nervous system. It can be further subdivided into the autonomic nervous system and the somatic nervous system.

The Autonomic Nervous System

http://www.youtube.com/watch?feature=player_embedded&v=JvuugFKe1PE#

The autonomic nervous system mainly deals with involuntary (or automatic) responses. This means that it can sometimes act independently from the brain without receiving any information from it.
As a result, many of the process that occur in the body which you don't have to consciously think about (digestion, breathing, regulation of body temperature), are under control of the autonomic nervous system. These are known as involuntary responses.

In addition to this, the autonomic nervous system can itself be divided into two types (or divisions); the sympathetic division and the parasympathetic division,

The Sympathetic Division (Fight or Flight)
The sympathetic division tends to be activated when there is an increase in autonomic activity. For example, when you exercise, the sympathetic division of the autonomic nervous system will increase your pulse and rate of respiration.

The Parasympathetic Division (Time to Rest)
The parasympathetic division of the autonomic nervous system works in reverse to the sympathetic division. It is activated when there is a decrease in autonomic activity. For example, when you are very relaxed (such as during meditation) the parasympathetic division causes your pulse and rate of respiration to decrease.

We can summarize both divisions of autonomic nervous system by saying:

The sympathetic division speeds things up and prepares your body for activity.

The parasympathetic division progressively slows things down so that you can relax.


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The Somatic Nervous System

The somatic nervous system is the other major division of the peripheral nervous system, and its role is to control the actions of the body (soma). For example, walking, talking or any other form of voluntary bodily movementnsdiv4365536ide.jpg


Spinal Nerves

Thirty-one pairs of spinal nerves originate from the spinal cord. They are all mixed nerves, and they provide a two-way communication system between the spinal cord and parts of the arms, legs, neck and trunk of the body. Although spinal nerves do not have individual names, they are grouped according to the level from which they stem, and each nerve is numbered in sequence. Hence, there are eight pairs of "cervical nerves" (numbered C1 - C8), twelve pairs of "thoracic nerves" (T1 - T12), five pairs of "lumbar nerves" (L1 - L5), five pairs of "sacral nerves" (S1 - S5), and one pair of "coccygeal nerves". The nerves coming from the upper part of the spinal cord pass outward nearly horizontally, while those from the lower regions descend at sharp angles. This is derived from the consequence of growth. In early life, the spinal cord extends the entire length of the vertebral column, but with age, the column grows faster than the cord. As a result, the adult spinal cord ends at the level between the first and second lumbar vertebrae, so the lumbar, sacral, and coccygeal nerves descend to their exits beyond the end of the cord

Brain

The brain is a jelly-like substance, which in adults weighs about three pounds. It is divided into three parts: the brain stem, which is an extension of the spinal cord, the forebrain (which consists mainly of the cerebruim) and the cerebellum. The forebrain and cerebellum are divided into two hemispheres which are linked by a thick band of nerve fibers and these hemispheres have areas, called "lobes," which perform specific functions. The brain's surface lies in rather ugly, wrinkled folds. Traditionally referred to as one's "gray matter," it does, indeed, contain gray nerve cell bodies which surround a smaller mass of white nerve fibers. The brain, like the heart, is protected by a buffer zone. This, in the form of fluid, may be the source of "water on the brain," but it is very necessary to our survival. Only these pools of fluid and the skull protect the brain from the bumps and grinds of daily living which would damage this fragile organ. With them, we are able to think, reason, love, forgive, create and remember, as well as to survive through automatic processes such as breathing and digesting, and we have reflexes which signal in case of "fight or flight" emergencies. Just think of it!

Sciatic Nerves

The sciatic nerves are branches of the lumbar nerves and are the largest and longest nerves in the body. They descend into the buttock and into the thighs, where they divide into the "tibial" and "common peroneal" nerves. The many branches of these nerves supply nerve impulses to and from the muscles and skin in the hip joints and thighs, the lower legs, feet and most of the skin below the knee.

Common Plantar Digital Nerve

The three common digital nerves, stemming from the medial plantar nerve, pass between the divisions of the plantar aponeurosis; each splits into the two "proper digital nerves." Those of the first common digital supply the adjoining side of the great and second toes; those of the second, the adjoining side of the second and third toes; those of the third, the sides of the third and fourth toes.

Facial Nerves

The facial nerve is the seventh cranial nerve. It begins in parts of the brain stem and branches into the face, neck, salivary glands and outer ear. The facial nerve performs both motor and sensory functions. Branching up from the brain stem, it divides into smaller nerves that reach into the face, neck, salivary glands and the outer ear. These branches control the muscles of the neck, the facial expressions, and the muscles of the forehead. They also stimulate secretions of the lower jaw and those salivary glands which are in the front of the mouth. Along with this, they report taste sensations from the front two-thirds of the tongue and carry sensations from the outer ear. Although a spontaneous grin and a deliberate smile both use lip and cheek muscles, each involves a different neural pathway from the brain.

Femoral Nerves

Femoral nerves are branches which stem from the lumbar nerves and divide into many smaller branches to supply motor impulses to the muscles of the thighs and legs; they receive sensory impulses from the skin of the thighs and the lower legs.

Lateral Plantar Nerve
The lateral plantar nerve supplies the skin of the fifth and lateral half of the fourth toes, as well as most of the deep muscles of the foot; its distribution can be compared to the ulnar nerve in the hand. It passes along the outside of the foot, dividing into a superficial and a deep branch.

Medial Nerves
Medial nerves are branches which stem from the cervical nerves, which supply impulses to the muscles of the forearms, and to the muscles and skin of the hands.

Medial Plantar Nerve

The medial plantar nerve, the larger of the two terminal branches of the tibial nerve (which stem from the sciatic nerve), accompanies the medial plantar artery. From its origin under the "flexor retinaculum" it passes deep into the muscles of the toes, and becomes the "proper plantar digital nerve" to the great toe. It finally divides opposite the bases of the toes into the three "common digital nerves."

Musculocutaneous Nerve

The musculocutaneous nerve in the arm is formed by splitting the brachial plexus at the inside border of the pectoral muscles into two branches. These branches continue to split as they run down through the upper arm, forearm, and into the wrist and hand. In the leg, the "superficial peroneal nerve" (also called musculocutaneous nerve) passes between the peronei and the extensor digitorum longus, pierces the deep tissues of the lower third of the leg, and divides into a "medial" and "intermediate dorsal cutaneous nerve."

Palmar Nerve

The palmar nerve is a division of the ulnar nerve that supplies the sides of the digits

Posterior Femoral Cutaneous Nerve

The posterior femoral cutaneous nerve (small sciatic nerve) is distributed to the skin of the perineum and the back surface of the thigh and leg. It stems from the sacrum and leaves the pelvis through the greater sciatic foramen (opening in the bone). It accompanies the inferior gluteal artery to the gluteus maximus (large muscle in the buttocks) and runs down the outer thigh and deep into the tissue at the back of the knee. It accompanies the small saphenous vein to the middle of the back of the leg.

Pudenal Nerve

The pudendal nerve arises from the sacrum and leaves the pelvis through the greater sciatic foramen (opening through the bone). It then crosses the spine of the ischium and reenters the pelvis through the lesser sciatic foramen. It splits into two terminal branches as it approaches the urogenital diaphragm (which fills the space within the pubic arch).

Saphenous Nerve
The saphenous nerve is the largest and longest branch of the femoral nerve; it supplies the skin on the inside of the leg.

Sciatic Nerves

The sciatic nerves are branches of the lumbar nerves and are the largest and longest nerves in the body. They descend into the buttock and into the thighs, where they divide into the "tibial" and "common peroneal" nerves. The many branches of these nerves supply nerve impulses to and from the muscles and skin in the hip joints and thighs, the lower legs, feet and most of the skin below the knee.

Sural Nerve
The sural nerve (short saphenous nerve) lies with the small saphenous vein. It supplies the branches to the skin on the back of the leg and then continues as the "lateral dorsal cutaneous nerve" along the outside of the foot and little toe.

Ulnar Nerves

The ulnar nerves are branches stemming from the cervical nerves, which supply impulses to and from the muscles of the forearms and hands and from the skin of the hands.

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The Brain
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Map of Cortexes

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Internal Structure of the Brain
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Sagittal View
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Spinal Cord

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The spinal cord is the largest nerve in the body. Nerves are stranded, cord-like structures made up of many nerve fibers; the spinal cord is made up of 31 pairs of nerves. The spinal cord acts as a telephone cable connecting the brain with other parts of the body. If you think of the spinal cord as a telephone cable, it connects the main office (the brain) to many other offices (different parts of the body). Messages are sent through the spinal cord in two directions.
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Nerves

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Nerves can run from the brain to the finger tip, or to the toe, and be seven feet long. (Actually this thing we call a nerve is really called anaxon). They can send their electrical charge that distance in a millisecond. Each nerve has insulation (a myelin sheath) and a ´power station´ called a Neuron. At one end the filaments, dendrites, connect to cells and other stimuli.
Nerves operate in channels up and down the body via the spinal column and when a ´current´ runs there is a magnetic field generated outside it. They co-ordinate our muscles andour actions and monitor our organs.

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Explain How Neurons Transmit Impulses Within and Between Themselves


The brain of an adult human weighs about 3 pounds and contains billions of cells. The two distinct classes of cells in the nervous system are neurons (nerve cells) andglia (glial cells).
The basic signaling unit of the nervous system is the neuron. The brain contains billions of neurons; the best estimates are that the adult human brain contains 1011neurons. The interactions between neurons enable people to think, move, maintain homeostasis, and feel emotions. A neuron is a specialized cell that can produce different actions because of its precise connections with other neurons, sensory receptors, and muscle cells. A typical neuron has four morphologically defined regions: the cell body, dendrites, axons, and presynaptic, or axon, terminals
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The neuron, or nerve cell, is the functional unit of the nervous system. The neuron has processes called dendrites that receive signals and an axon that transmits signals to another neuron.

The cell body, also called the soma, is the metabolic center of the neuron. The nucleus is located in the cell body, and most of the cell’s protein synthesis occurs in the cell body.
A neuron usually has multiple processes, or fibers, called dendrites that extend from the cell body. These processes usually branch out somewhat like tree branches and serve as the main apparatus for receiving input into the neuron from other nerve cells.
The cell body also gives rise to the axon. Axons can be very long processes; in some cases, they may be up to 1 meter long. The axon is the part of the neuron that is specialized to carry messages away from the cell body and to relay messages to other cells. Some large axons are surrounded by a fatty insulating material called myelin, which enables the electrical signals to travel down the axon at higher speeds.
Near its end, the axon divides into many fine branches that have specialized swellings called axon, or presynaptic, terminals. These presynaptic terminals end in close proximity to the dendrites of another neuron. The dendrite of one neuron receives the message sent from the presynaptic terminal of another neuron.
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Neurons transmit information to other neurons. Information passes from the axon of the presynaptic neuron to the dendrites of the postsynaptic neuron.


Summary
The point at which two neurones meet is called the synapse. At this point there are 2 situations:
1: there is no gap, the neurones are physically connected by gap-junctions. This is a rarer situation but does occur. In this case the nerve impulse continues down the second neurone (known as the post-synaptic nerve)

2: there is a small gap (a few micrometers) between the cells. In this case the first nerve, carrying the impulse, (pre-synaptic) causes a release of a chemical known as a neurotransmitter. There are various types of neurotransmitters and each are involved in different situations. When the neurotransmitter reaches the 2nd nerve it binds to the membrane (in most situations) which either causes the cell to depolarize and continue the impulse, or causes hypo-polarization which prevents any other nerve stimulating it.


http://science.education.nih.gov/supplements/nih2/addiction/guide/lesson2-1.htm




Describe the Path A Nerve Impulses Travels Throughout the Body from Stimulus to Response


It can depend on what the stimulus was:

1. You step on a rock and it is uncomfortable, so you move your foot : this isn't a reflexive action, so sensory nerves on the part of your skin in discomfort send signals to your spinal cord, which takes it all to the brain. The brain processes what's going on and "decides," so to speak, that moving the foot would eliminate discomfort. It sends signals to the spinal cord, which sends signals to the leg muscles, which are instructed to "move the foot."

2. You touched a hot stove : the impulse will be carried via reflex arc.
Basically, a reflex is an automatic movement in response to a certain stimulus. In the simplest ones, you sense something (like the heat of the stove burning your hand) and those nerves just completely bypass the rest of the system and tell certain muscles "MOVE. NOW."

In more complex ones (like how if you're holding a bowl and something is dropped into it, you keep the bowl level instead of dropping it - this is the stretch reflex) can travel to the spinal cord THEN muscles.

The path a nerve impulse travels in order is the following:
1. Stimulus
2. Sensory receptor
3. Sensory neuron
4. Central Nervous System (CNS) interneurones
5. Motor neuron
6. Muscle/Effector (whatever part of the body the stimulus is going to)
7. Response
This process involves many neurons and neurotransmitters mainly Acetycholine

Any thing that makes an organism react is called a stimulus. When an organism reacts to a stimulus it is called a response. There are two kinds of responses; a voluntary response and an involuntary response. Voluntary responces are when you control the response such as swatting a fly. Your heart beat however is involuntary.

Each nerve impulse begins in the dendrites of a neuron. It moves toward the cell body, and on to the axon tip. the nerve impulse travels along with the neuron in the form of electrical and chemical signals. When the impulse reaches the end of the neuron, it can pass to another neuron or it can travel to a muscle or organ. This point is called a synapse. When a nerve impulse passes at a synapse the dendrite and axon tip do not touch. The axon tips release chemicals that carry the impulse across.



Describe the Structure of a Neuron (Labelled diagram)
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Neurons are the basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information. Each part of the neuron plays a role in the communication of information throughout the body. Follow the links below to learn more about the functions of each part of a neuron.


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Dendrites are treelike extensions at the beginning of a neuron that help increase the surface area of the cell body and are covered with synapses. These tiny protrusions receive information from other neurons and transmit electrical stimulation to the soma.
Dendrite Characteristics


  • Most neurons have many dendrites
  • Short and highly branched
  • Transmits information to the cell body


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The soma is where the signals from the dendrites are joined and passed on. The soma and the nucleus do not play an active role in the transmission of the neural signal. Instead, these two structures serve to maintain the cell and keep the neuron functional.
The support structures of the cell include mitochondria, which provide energy for the cell, and the Golgi apparatus, which packages products created by the cell and secretes them outside the cell wall.

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The axon hillock is located at the end of the soma and controls the firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire a signal (known as an action potential -

part of the process that occurs during the firing of a neuron)

down the axon.


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The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons.
Axon Characteristics


  • Most neurons have only one axon
  • Transmit information away from the cell body
  • May or may not have a myelin covering

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The terminal buttons are located at the end of the neuron and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse. Neurotransmitters (

a chemical messenger that carries, boosts and modulates signals between neurons and other cells in the body)

are used to carry the signal across the synapse to other neurons.






Explain What Occurs During the Reflex Arc (Diagram)

If you stand on something sharp then you move your foot very quickly. You don't have to think about it - it feels like it happens even before you feel the pain. This is because the process is a reflex and doesn't involve the brain.
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Receptors and Effectors

The diagram shows someone stepping on a drawing pin. Receptors in the skin of the foot will register the fact that this has happened and will send a signal along a sensory neuron to the spinal cord. Inside the spinal cord an interneuron will transfer the nerve signal to another nerve cell. This cell is a motor neuron and it carries the signal to themuscles in the leg. The leg muscles will contract and, hopefully, the foot will be moved away from the source of pain.
Each of the nerve cells are separated by synapses.

A reflex is a response to a perturbing stimulus that acts to return the body to homeostasis. This may be subconscious as in the regulation of blood sugar by the pancreatic hormones, may be somewhat noticeable as in shivering in response to a drop in body temperature; or may be quite obvious as in stepping on a nail and immediately withdrawing your foot.
A reflex arc refers to the neural pathway that a nerve impulse follows. The reflex arc typically consists of five components (3):
1. The receptor at the end of a sensory neuron reacts to a stimulus.
2. The sensory (afferent) neuron conducts nerve impulses along an afferent pathway towards the central nervous system (CNS).
3. The integration center consists of one or more synapses in the CNS.
4. A motor (efferent) neuron conducts a nerve impulse along an efferent pathway from the integration center to an effector.
5. An effector responds to the efferent impulses by contracting (if the effector is a muscle fiber) or secreting a product (if the effector is a gland).

Reflexes require a minimum of two neurons, a sensory neuron (input) and a motor neuron (output). The sensory neuron (such as a pain receptor in the skin) detects the stimuli and sends a signal towards the Central Nervous System. This sensory neuron synapses with a motor neuron which innervates the effector tissue (such as skeletal muscle to pull away from the painful stimuli). This type of reflex is the "withdrawal" reflex and is monosynaptic, meaning only one synapse has to be crossed between the sensory neuron and the motor neuron. It is the simplest reflex arc and the integration center is the synapse itself. Polysynaptic reflexes are more complex and more common. They involve interneurons which are found in the CNS. More complex reflexes may have their integration center in the spinal cord, in the brainstem, or in the cerebrum where conscious thoughts are initiated.
Many people conider only the simplest types of responses as "reflexes", those that are always identical and do not allow conscious actions. We must not confuse these with "reactions", which are different from reflexes in that they are voluntary responses to a stimulus from the environment. For example,while the body has various subconscious physiological responses to mitigate cold, as humans we can simply choose to put on more clothes. This is a conscious order made by the cerebrum, not an involuntary response to a stimulus. This is a very complex response involving millions of neurons and some time to process the voluntary response. In contrast, spinal reflexes occur much faster, not only because they involve fewer neurons, but also becuase the electrical signal does not have to travel to the brain and back. Spinal reflexes only travel to the spinal cord and back which is a much shorter distance. Because of this and the complexity of conscious reactions, they take more time to complete than a reflex. On average, humans have a reaction time of 0.25 seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus.

Reaction times vary from individual to individual. Because of the higher degree of neural processing, reaction times can be influenced by a variety of factors. Reaction times can decrease with practice, as often times athletes have faster reaction times than non-athletes. Sleepiness, emotional distress, or consumption of alcohol can also impact reaction time.

Did you know?

The Patellar Reflex (Knee-Jerk)
You may have seen doctors on the television (or maybe it's happened to you) where they test someone's reflexes by tapping just under the kneecap (patella) with a small rubber hammer. If this spot is hit just right then it will cause some special sensory cells (called amuscle spindle) to send signals off to the spinal cord. The signal passes through the interneuron and then via a motor neuron to the quadriceps muscle. This causes the leg to kick forward.
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No matter how much you concentrate you cannot stop this from happening as it's a reflex action and doesn't involve the brain.

http://www.youtube.com/watch?v=Y5nj3ZfeYDQ

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Central Nervous System

The central nervous system is one of the most important organs of the nervous system. It is made up of two of the most vital organs in the human body - the brain and the spinal cord. The brain weighs approximately 3 lbs, while the spinal cord is about 42 to 45 cm in length. Both of them make up a large part of the entire nervous system. Central nervous system functions include coordinating the activities between the various parts of the human body. Working in collaboration with the peripheral nervous system, central nervous system plays a fundamental role in controlling the behavior in various multicellular organisms.

The central nervous system is made up of the
The spinal cord
The brain
  • receives sensory input from the spinal cord as well as from its own nerves (e.g., olfactory and optic nerves)
  • devotes most of its volume (and computational power) to processing its various sensory inputs and initiating appropriate — and coordinated — motor outputs.


Central Nervous System Functions

As we said earlier, the central nervous system comprises the brain and spinal cord, both of which play an important role on physical as well as mental aspects of our life. The brain plays a major role in controlling the various body functions, which include movement, sensation, thinking, memory, speech, etc. On the other hand, the spinal cord is connected to the brain at a particular section of the brain referred to as the brainstem. The brain is divided into two halves, the right brain and the left brain. Right brain functions include visual and spatial skills, memory storage in auditory and visual modalities, whereas left brain functions include sequential analysis, memory storage in particular order, logical interpretations etc. The brain is further divided into several regions, each of which is assigned a specific function. For instance, the frontal lobe deals in cognition and memory, while the parietal lobe looks after processing of sensory input. The spinal cord and central nervous system neurons located within it are primarily assigned the responsibility of transmitting messages back and forth between the brain and the peripheral nerves. Although the brain and spinal cord work together to control various functions of the body, reflex movements can occur through spinal cord pathways, without any of the structure of brain getting involved in the process.

Any damage caused to the head or spine can lead to some adverse effects on the individual's body and hamper various nervous system functions. Our brain is protected by the skull, whereas the spinal cord is protected by the vertebrae or the spinal column, but this doesn't mean both these organs are safe from injuries. In fact, in several causes the spinal cord is damaged or punctured by the vertebrae itself. These injuries may range from a shock which may last for a few hours, to more severe conditions, such as complete paralysis.

This was brief information about the central nervous system function and its structure. These central nervous system functions play an important role in various activities, which means any damage caused to the central nervous system can affect our day-to-day life. One has to take some precautionary methods, especially when indulging in various physical activities to ensure that the brain and nervous system are not harmed by any injury.

Brain Structures and their Functions

The nervous system is your body's decision and communication center. external image nerve.gifThe central nervous system (CNS) is made of the brain and the spinal cord and the peripheral nervous system (PNS) is made of nerves. Together they control every part of your daily life, from breathing and blinking to helping you memorize facts for a test. Nerves reach from your brain to your face, ears, eyes, nose, and spinal cord... and from the spinal cord to the rest of your body. Sensory nerves gather information from the environment, send that info to the spinal cord, which then speed the message to the brain. The brain then makes sense of that message and fires off a response. Motor neurons deliver the instructions from the brain to the rest of your body. The spinal cord, made of a bundle of nerves running up and down the spine, is similar to a superhighway, speeding messages to and from the brain at every second.
The brain is made of three main parts: the forebrain, midbrain, and hindbrain. The forebrain consists of the cerebrum, thalamus, and hypothalamus (part of the limbic system). The midbrain consists of the tectum and tegmentum. The hindbrain is made of the cerebellum, pons and medulla. Often the midbrain, pons, and medulla are referred to together as the brainstem.
The Cerebrum: The cerebrum or cortex is the largest part of the human brain, associated with higher brain function such as thought and action. The cerebral cortex is divided into four sections, called "lobes": the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. Here is a visual representation of the cortex:
Image of Cerebral Cortex
Image of Cerebral Cortex
What do each of these lobes do?
  • Frontal Lobe- associated with reasoning, planning, parts of speech, movement, emotions, and problem solving
  • Parietal Lobe- associated with movement, orientation, recognition, perception of stimuli
  • Occipital Lobe- associated with visual processing
  • Temporal Lobe- associated with perception and recognition of auditory stimuli, memory, and speech
Note that the cerebral cortex is highly wrinkled. Essentially this makes the brain more efficient, because it can increase the surface area of the brain and the amount of neurons within it.
A deep furrow divides the cerebrum into two halves, known as the left and right hemispheres. The two hemispheres look mostly symmetrical yet it has been shown that each side functions slightly different than the other. Sometimes the right hemisphere is associated with creativity and the left hemispheres is associated with logic abilities. The [[javascript:Start('definitions/def-corpuscallosum.html')|corpus callosum]]is a bundle of axons which connects these two hemispheres.
Nerve cells make up the gray surface of the cerebrum which is a little thicker than your thumb. White nerve fibers underneath carry signals between the nerve cells and other parts of the brain and body.
The neocortex occupies the bulk of the cerebrum. This is a six-layered structure of the cerebral cortex which is only found in mammals. It is thought that the neocortex is a recently evolved structure, and is associated with "higher" information processing by more fully evolved animals (such as humans, primates, dolphins, etc).

The Cerebellum:The cerebellum, or "little brain", is similar to the cerebrum in that it has two hemispheres and has a highly folded surface or cortex. This structure is associated with regulation and coordination of movement, posture, and balance.
The cerebellum is assumed to be much older than the cerebrum, evolutionarily. What do I mean by this? In other words, animals which scientists assume to have evolved prior to humans, for example reptiles, do have developed cerebellums. However, reptiles do not have neocortex.
Limbic System:The limbic system, often referred to as the "emotional brain", is found buried within the cerebrum. Like the cerebellum, evolutionarily the structure is rather old.
This system contains the thalamus, hypothalamus, amygdala, and hippocampus. Here is a visual representation of this system, from a midsagittal view of the human brain:
Image of the Limbic System
Image of the Limbic System

Thalamus

Thalamus- a large mass of gray matter deeply situated in the forebrain at the topmost portion of the diencephalon. The structure has sensory and motor functions. Almost all sensory information enters this structure where neurons send that information to the overlying cortex. Axons from every sensory system (except olfaction) synapse here as the last relay site before the information reaches the cerebral cortex.

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Hypothalamus

Hypothalamus- part of the diencephalon, ventral to the thalamus. The structure is involved in functions including homeostasis, emotion, thirst, hunger, circadian rhythms, and control of the autonomic nervous system. In addition, it controls the pituitary.

a coronal view
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Amygdala

Amygdala- part of the telencephalon, located in the temporal lobe; involved in memory, emotion, and fear. The amygdala is both large and just beneath the surface of the front, medial part of the temporal lobe where it causes the bulge on the surface called the uncus. This is a component of the limbic system.

a coronal view of the amgydala
amygdala
amygdala


Hippocampus

Hippocampus- the portion of the cerebral hemisphers in basal medial part of the temporal lobe. This part of the brain is important for learning and memory . . . for converting short term memory to more permanent memory, and for recalling spatial relationships in the world about us

a coronal view of the hippocampus
hippocampus
hippocampus

Brain Stem: Underneath the limbic system is the brain stem. This structure is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. Scientists say that this is the "simplest" part of human brains because animals' entire brains, such as reptiles (who appear early on the evolutionary scale) resemble our brain stem.
The brain stem is made of the midbrain, pons, and medulla.

Midbrain/Mesencephalon

Midbrain/ Mesencephalon- the rostral part of the brain stem, which includes the tectum and tegmentum. It is involved in functions such as vision, hearing, eyemovement, and body movement. The anterior part has the cerebral peduncle, which is a huge bundle of axons traveling from the cerebral cortex through the brain stem and these fibers (along with other structures) are important for voluntary motor function.

external image midbrain.jpg
Pons

Pons- part of the metencephalon in the hindbrain. It is involved in motor control and sensory analysis... for example, information from the ear first enters the brain in the pons. It has parts that are important for the level of consciousness and for sleep. Some structures within the pons are linked to the cerebellum, thus are involved in movement and posture.

external image pons.jpg
Medulla

Medulla Oblongata- this structure is the caudal-most part of the brain stem, between the pons and spinal cord. It is responsible for maintaining vital body functions, such as breathing and heartrate

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The nervous system is divided into the somatic nervous system which controls organs under voluntary control (mainly muscles) and the Autonomic Nervous System (ANS) which regulates individual organ function and homeostasis, and for the most part is not subject to voluntary control. It is also known as the visceral or automatic system.

Peripheral Nervous System


Peripheral Nervous System Divisions

The peripheral nervous system is divided into the following sections:

Peripheral Nervous System
  • Sensory Nervous System - sends information to the CNS from internal organs or from external stimuli.
  • Motor Nervous System - carries information from the CNS to organs, muscles, and glands.
    • Somatic Nervous System - controls skeletal muscle as well as external sensory organs.
    • Autonomic Nervous System - controls involuntary muscles, such as smooth and cardiac muscle.

  • Sympathetic - controls activities that increase energy expenditures.
  • Parasympathetic - controls activities that conserve energy expenditures.

Peripheral Nervous System Connections

Peripheral nervous system connections with various organs and structures of the body are established through cranial nerves and spinal nerves. There are 12 pairs of cranial nerves in the brain that establish connections in the head and upper body, while 31 pairs of spinal nerves do the same for the rest of the body. While some cranial nerves contain only sensory neurons, most cranial nerves and all spinal nerves contain both motor and sensory neurons.
There are two types of cells in the peripheral nervous system. These cells carry information to (sensory nervous cells) and from (motor nervous cells) the central nervous system (CNS). Cells of the sensory nervous system send information to the CNS from internal organs or from external stimuli.

Motor nervous system cells carry information from the CNS to organs, muscles, and glands. The motor nervous system is divided into the somatic nervous system and the autonomic nervous system.

The somatic nervous system controls skeletal muscle as well as external sensory organs such as the skin. This system is said to be voluntary because the responses can be controlled consciously. Reflex reactions of skeletal muscle however are an exception. These are involuntary reactions to external stimuli.
The somatic nervous system, or voluntary nervous system, is that part of the peripheral nervous system that regulates body movement through control of skeletal (voluntary) muscles and also relates the organism with the environment through the reception of external stimuli, such as through the senses of vision, hearing, taste, and smell. The somatic nervous system controls such voluntary actions as walking and smiling through the use of efferent motor nerves, in contrast with the function of the autonomic nervous system, which largely acts independent of conscious control in innervating cardiac muscle and exocrine and endocrine glands.

The nervous system comprises the brain and various types of nerves, including afferent nerves (from the Latin, ad = towards; ferro = I carry), which carry sensory impulses from all parts of the body to the brain and efferent nerves (ex = from; ferro = I carry) through which "messages" are conducted from the brain to the muscles and all of the organs of the body. The somatic part of the nervous system has sensory components which convey sensations from the eyes, the nose and other sensory organs to the brain (mainly the cerebral cortex) where most of the impulses reach our awareness, and motor components transmitting impulses to the skeletal muscles in the limbs and trunk permitting voluntary control of movements. The autonomic nervous system conveys sensory impulses from the blood vessels, the heart and all of the organs in the chest, abdomen and pelvis through nerves to other parts of the brain (mainly the medulla, pons and hypothalamus). These impulses often do not reach our consciousness, but elicit largely automatic or reflex responses through the efferent autonomic nerves, thereby eliciting appropriate reactions of the heart, the vascular system, and all the organs of the body to variations in environmental temperature, posture, food intake, stressful experiences and other changes to which all individuals are exposed.

Overview of human somatic nervous system

In humans, there are 31 pairs of spinal nerves and 12 pairs of cranial nerves.
The 31 pairs of spinal nerves emanate from different areas of the spinal cord and each spinal nerve has a ventral root and a dorsal root. The ventral root has motor (efferent) fibers that transmit messages from the central nervous system to the effectors, with the cell bodies of the efferent fibers found in the spinal cord gray matter. The dorsal root has sensory (afferent) fibers that carry information from the sensory receptors to the spinal cord (Adam 2001).
The 12 pairs of cranial nerves transmit information on the senses of sight, smell, balance, taste, and hearing from special sensory receptors. They also transmit information from general sensory receptors in the body, largely from the head. This information is received and processed by the central nervous system and then the response travels via the cranial nerves to the skeletal muscles to control movements in the face and throat, such as swallowing and smiling (Adam 2001).

Nerve signal transmission

The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons (upper motor neurons) within the precentral gyrus (which approximates the primary motor cortex). Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the corticospinal tract, via axons to control skeletal (voluntary) muscles. These stimuli are conveyed from upper motor neurons through the ventral horn of the spinal cord, and across synapses to be received by the sensory receptors of alpha motor neuron (large lower motor neurons) of the brainstem and spinal cord.
Upper motor neurons release a neurotransmitter, acetylcholine, from their axon terminal knobs, which are received by nicotinic receptors of the alpha motor neurons. In turn, alpha motor neurons relay the stimuli received down their axons via the ventral root of the spinal cord. These signals then proceed to the neuromuscular junctions of skeletal muscles.
From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (Nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.
In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. For vertebrates, however, the response of a muscle fiber to a neurotransmitter (always acetylcholine (ACh)) can only be excitatory or, in other words, contractile.

Reflex arcs

ReflexArc1.jpgexternal image magnify-clip.png
The mechanism of the reflex arc
A reflex arc is an automatic reaction that allows an organism to protect itself reflexively when an imminent danger is perceived. In response to certain stimuli, such as touching a hot surface, these reflexes are "hard wired" through the spinal cord. A reflexive impulse travels up afferent nerves, through a spinal interneuron, and back down appropriate efferent nerves.

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The autonomic nervous system controls involuntary muscles, such as smooth and cardiac muscle. This system is also called the involuntary nervous system. The autonomic nervous system can further be divided into the parasympathetic and sympathetic divisions.

There are two major components of the autonomic nervous system, the sympathetic and the parasympathetic systems. The afferent nerves subserving both systems convey impulses from sensory organs, muscles, the circulatory system and all the organs of the body to the controlling centers in the medulla, pons and hypothalamus. From these centers efferent impulses are conveyed to all parts of the body by the parasympathetic and sympathetic nerves. The impulses of the parasympathetic system reach the organs of the body through the cranial nerves # 3, 7, 9, & 10, and some sacral nerves to the eyes, the gastrointestinal system, and other organs. The sympathetic nerves reach their end-organs through more devious pathways down the spinal cord to clusters of sympathetic nerve bodies (ganglia) alongside the spine where the messages are relayed to other nerve bodies (or neurons) that travel to a large extent with the blood vessels to all parts of the body. Through these nervous pathways, the autonomic nerves convey stimuli resulting in largely unconscious, reflex, bodily adjustments such as in the size of the pupil, the digestive functions of the stomach and intestines, the rate and depth of respiration and dilatation or constriction of the blood vessels.

Transmission of Autonomic Stimuli
Like other nerves, those of the autonomic nervous system convey their messages to the appropriate end organs (blood vessels, viscera, etc.) by releasing transmitter substances to which the receptors of the target cells are responsive. The most important of these transmitters in the autonomic nervous system are acetylcholine and norepinephrine. In the parasympathetic system, acetylcholine is responsible for most of these transmissions between the afferent and efferent nerves of the system and between the efferent nerve endings and the cells or organs that they subserve. Acetylcholine also serves to transmit nerve-to-nerve messages in the afferent nerves and the brain centers of the sympathetic nervous system. However, the final transmission of messages from the sympathetic nerves to the end-organs or cells that they innervate is conveyed by the release of norepinephrine (noradrenaline) with at least one important exception, namely the sympathetically conveyed stimulus to the sweat glands which is transmitted by acetylcholine. A stimulus to contraction of the blood vessels is required in order to maintain the blood pressure when we arise from bed in the morning, so as to prevent fainting from excessive pooling of blood in the lower body. This stimulus is conveyed by norepinephrine release within the walls of the blood vessels from the nerve endings of the sympathetic nerves that innervate each blood vessel.
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The parasympathetic division controls various functions which include inhibiting heart rate, constricting pupils, and contracting the bladder. The nerves of the sympathetic division often have an opposite effect when they are located within the same organs as parasympathetic nerves. Nerves of the sympathetic division speed up heart rate, dilate pupils, and relax the bladder. The sympathetic system is also involved in the flight or fight response. This is a response to potential danger that results in accelerated heart rate and an increase in metabolic rate.
More information about the Sympathetic branch and the Parasympathetic branch.

VOCABULARY

Central Nervous System- the part of the nervous system comprising the brain and spinal cord.

Peripheral Nervous System- the portion of the nervous system lying outside the brain and spinal cord.

Autonomic Nervous System- the system of nerves and ganglia that innervates the blood vessels,heart,smoothmuscles,viscera,and glands and controls their involuntary functions,consisting of sympathetic and parasympathetic portions.

Sympathetic Nervous System- the part of the autonomic nervous system originating in the thoracic and lumbar regions of the spinal cord that in general inhibits or opposes thephysiological effects of the parasympathetic nervous system, as in tending to reduce digestive secretions or speed up the heart.




Parasympathetic Nervous System- the part of the autonomic nervous system originating in the brain stem and the lower part of the spinal cord that, in general, inhibits or opposes the physiological effects of the sympathetic nervous system, as intending to stimulate digestive secretions or slow the heart.

Ganglia- a mass of nerve tissue existing out side the central nervous system.

Neuron-a specialized,impulse-conducting cell that is the functional unit of the nervous system consisting of the cell body and its processes,the axon and dendrites.

Dendrite-



Resources
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