Nervous System

Definition

The nervous system is an organ system that contains a network of specialized cells called neurons. This is the master controlling and communicating system of the body. It coordinates the action of an animal and transmits signals between the different parts of the body. Every thought, movement and emotions reflect the activity of the nervous system.

Functions of the NERVOUS SYSTEM

1. To monitor changes that takes place inside and outside the body. The nervous system utilizes the million sensory receptors to carry out this function. Any changes or stimuli occurring are noted by the nervous system and the gathered data is now called a sensory input.
2. Another important function of the nervous system is to process and interpret the sensory input or gathered data. It is the working of this system to make decision about what should be done at each moment. This is the process known as INTEGRATION.
3. As the nervous system has reached a decision of what response and appropriate action to be done in response to the stimuli, it theneffects a response by activating muscles or glands through motor output.

Structural Classification of the Nervous system

Structurally, the nervous system is classified into the central nervous system and the peripheral nervous system.

• Central nervous system. The CNS consists of the brain and the spinal cord. These organs occupy the dorsal body cavity and act as the INTEGRATING and COMMAND CENTERS of the nervous system. It is the CNS that interprets an incoming sensory information and sends and instruction basing on the past experience and current condition.
• Peripheral Nervous System. The PNS is consisting of the nerves that extend from the brain and the spinal cord. It is the part of the nervous system outside the CNS. There are varieties of nerves. The spinal nerves carry impulses to and from the spinal cord. The cranial nerves, on the other hand, carry impulses to and from the brain. These nerves serve as the communication lines of the body.

Functional Classification of the Nervous System

The functional classification of the nervous system is only concerned about the structures of the peripheral nervous system (PNS). The PNS in this classification is divided into two principal subdivisions:

• Sensory or afferent division. This subdivision is composed of the nerve fibers that convey impulses to the central nervous system (CNS) from the sensory receptors. These sensory receptors are located in the different parts of the body. With the presence of these sensory fibers the CNs is constantly informed of the events going on both inside and outside the body.

1. The fibers responsible for delivering impulses from the skin, skeletal muscles and joints are called the somatic sensory fibers.
2. Fibers that transmit impulses from the visceral organs are called the visceral sensory fibers.

• Motor or efferent division. This division is responsible for carrying impulses from the CNS to the effector organs, muscles and glands. In response these impulses, activate muscles and glands and they effect a motor response. The two classification of motor or efferent division are:

1. Somatic nervous system. This subdivision is also referred as the voluntary nervous system. The somatic NS allows a person to consciously or voluntarily control aperson’s skeletal muscles.
2. Autonomic nervous system (ANS). The ANS regulates the events that are automatic or INVOLUNTARY such as the activity of the smooth and cardiac muscles and glands. The two parts of the ANS are the sympathetic and the parasympathetic systems.

Function and  Structure of the Nervous System

If you think of the brain as a central computer that controls all bodily functions, then the nervous system is like a network that relays messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.

The nervous system derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body. Even though it is complex, nervous tissue is made up of two principal types of cells namely, the supporting cells and the neurons.

SUPPORTING CELLS

The supporting cells in the CNS are “lumped together” as NEUROGLIA or GLIAL CELLS. Glial Cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin and participate in signal transmission in the nervous system. In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.

The functions of glial cells are:

1. to support neurons and hold them in place
2. to supply nutrients to neurons
3. to insulate neurons electrically
4. to destroy pathogens and remove dead neurons
5. to provide guidance cues directing the axons of neurons to their targets

Characteristics of Glial Cells:

1. Lumped together.
2. Not able to transmit impulses.
3. Never lose their ability to divide.

The CNS glia include:

• Astrocytes. These are star-shaped cells that account nearly half of the neural tissue. Astrocytes form a living barrier between capillaries and neurons and play a role in making exchanges between the two. This is to prevent harmful substances in the blood from entering the neurons. Aside from that, astricytes are also important in controlling the chemical environment in the brain. This is done by picking up excess ions and recapturing released neurotransmitters.
• Microglia. These are spiderlike phagocytes that dispose debris including dead brain cells and bacteria.
• Ependymal cells. These cells line the cavities of the brain and the spinal cord. Aside from lining the cavities of certain organs, these cells are very important in helping the CSF through their cilia to circulate and fill those cavities and form a protective cushion around the CNS.
• Oligodendrocytes. These are glial cells that wrap their flat extensions tightly around the nerve fibers, producing fatty insulating coverings called myelin sheaths.

NEURONS

Anatomy of the Neuron

The nervous system is defined by the presence of a special type of cell—the neuron (sometimes called “neurone” or “nerve cell”). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via SYNAPSES,  which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an AXON,  a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts. Axons frequently travel through the body in bundles called nerves.

• Cell body – the metabolic center of the neuron. This part of neuron contains the usual organelles except for the centrioles. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch out into still finer extensions. This is well protected and is located in the bony skull or vertebral column and is essential to well-being of the nervous system. The cell body carries out most of the metabolic functions of a neuron.
• Nissl substance and Neurofibrils – found in the cell body that is essential in maintaining cell shape.
• Dendrites – neuron processes that covey incoming messages TOWARD the cell body.
• Axons – neuron processes that generate nerve impulses AWAY from the cell body.
• Axon hillock – a cone-like region of the cell body where the axon arises.
• Axon terminals – – located at the terminal end of the axons that contains tiny vesicles or membranous sacs that contains chemicals called neurotransmitters. When impulses reach the axon terminals, they stimulate the release of neurotransmitters into the extracellular spaces.
• Synaptic cleft – a tiny gap that separates axon terminal from the next neuron.
• Myelin – a whitish, fatty material that covers long nerve fibers. It has a waxy appearance that protects and insulates the fibers and increases the rate of nerve impulses.
• Schwann cells – myelinates the axon outside the nervous system. Schwann cells are specialized supporting cells that enclose themselves tightly around the axon jelly-roll fashion.
• Myelin sheath – a tight coil of wrapped membranes created after the Schwann cells enclose the axon.
• Neurilemma – part of the Schwann cell external to the myelin sheath.
• Nodes of Ranvier – gaps or indentations between the myelin sheaths.

Classification of Neurons

Functional Classification of Neurons

Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include SENSORY NEURONS that transmute physical stimuli such as light and sound into neural signals, and MOTOR NEURONS that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons receive all of their input from other neurons and send their output to other neurons. An ITERNEURON is always found completely within the CNS and conveys messages between parts of the system

In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with sheath. These cells maintain the tissue by supporting and protecing the neurons. They also provide nutrients to neurons and help to keep the tissue free of debris. The neurons require a great deal of energy for the maintenance of the ionic imbalance between themselves and their surrounding fluids, which is constantly in flux as a result of the opening and closing of channels through the neuronal membranes.

Structural Classification of Neurons

• Multipolar neurons – These are several processes extending from the cell body. All motor and association neurons are multipolar and this is the most common structural type.
• Bipolar neurons – These are neurons with two processes – an axon and a dendrite. Bipolar neurons are rare in adults and are only found in some special sense organs such as the eye or nose where they act in sensory processing as receptor cells.
• Unipolar neurons – These neurons have single process emerging from the cell body. it is very short and divides almost immediately into proximal (central) and distal (peripheral) processes.

Neurons are dynamically polarized, so that information flows from the fine dendrites into the main dendrites and then to the cell body, where it is converted into all-or-none signals, the action potentials, which are relayed to other neurons by the axon, a long wire-like structure. The neuron is actually a very poor conductor; the signal drops to 37% of its original strength in only about 0.15 mm. Thus it needs amplification all along its length in the form of sodium-potassium pumps and gates.

Sodium ions rush into the neurons from the extracellular fluid, resulting in a transient change in the voltage difference between the neuron and the surrounding environment. The action potential travels like a wave from the cell body down the neuron via the repeating amplifications. Thus, the action potential enables the neuron to communicate rapidly with other neurons over sizable distances, sometime more than a meter away with a speed from 20 -200 m/sec. When the action potential reaches an axon terminal, it causes the terminals to secrete a chemical messenger (neurotransmitter), generally an amino acid or its derivative, which binds to receptors in the post-synaptic neurons on the far side of the synaptic cleft. When the postsynaptic potential has reached a specific value an action potential is triggered and the signal is passed to the next neuron.

THE CENTRAL NERVOUS SYSTEM

The Central Nervous System (CNS) is composed of the brain and spinal cord. The CNS is surrounded by bone-skull and vertebrae. Fluid and tissue also insulate the brain and spinal cord. During embryonic development, the brain first forms as a tube, the anterior end of which enlarges into three hollow swellings that form the brain, and the posterior of which develops into the spinal cord.

Anatomy of the CNS

Brain

When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example, if you accidentally touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away. Luckily, this neurological relay race takes a lot less time than it just took to read about it. Considering everything it does, the human brain is incredibly compact, weighing just 3 pounds. Its many folds and grooves, though, provide it with the additional surface area necessary for storing all of the body’s important information.

The four main regions of the brain are:

• Cerebral hemispheres
• Diencephalon
• Brain stem
• Cerebellum

Cerebral Hemispheres

The paired cerebral hemispheres are the most superior part of the brain and are collectively called the cerebrum.

1. Gyri or gyrus (singular) – elevated ridges of tissue found on the entire surface of the cerebral hemisphere.
2. Sulci or sulcus (singular) – shallow grooves that separates the gyri.
3. Fissures – deeper groves which separates the larger regions of the brain. The cerebral hemispheres are separated by a single deep fissure called the LONGITUDINAL FISSURE.

The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, frontal, parietal and temporal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.

The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobeconducts three functions:

1. motor activity and integration of muscle activity
2. speech
3. thought processes

Language comprehension is found in Wernicke’s area. Speaking ability is in Broca’s area. Damage to Broca’s area causes speech impairment but not impairment of language comprehension. Lesions in Wernicke’s area impair ability to comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with higher thought processes, planning, memory, personality and other human activities.

Diencephalon

The diencephalon or interbrain sits atop the brainstem and is enclosed by the cerebral hemispheres. The major structures of the diencephalon are:

1. Thalamus – The thalamus is a relay station for sensory impulses passing upward the sensory cortex.
2. Hypothalamus – Plays a role in body temperature regulation, water balance and metabolism. It is also the center for many drives and emotion such as thirst, appetite, sex, pain and pleasure. Aside from that, the hypothalamus regulates the pituitary gland and produces two hormones of its own.
3. Epithalamus – The epithalamus contains the pineal body and the choroid plexuses. The choroid plexuses form the cerebrospinal fluid.

Brain Stem

The brain stem is about the size of a thumb in diameter and is approximately 3 inches long. It provides a pathway for ascending and descending tracts. The structures of the brain stem are:

1. Midbrain – The midbrain, located underneath the middle of the forebrain, acts as a master coordinator for all the messages going in and out of the brain to the spinal cord. It is composed primarily of two bulging fiber tracts called the cerebral peduncles, which convey ascending and descending impulses.
2. Pons – the pons have an important nuclei in the control of breathing.
3. Medulla oblongata – most inferior part of the brain stem. It contains many nuclei that regulate vital visceral activities. The medulla oblongata contains centers that control heart rate, BO, breathing, swallowing, vomiting and others.
4. Reticular Formation – the neurons of the reticular formation are involved in the motor control of the visceral organs. A special group of reticular formation neurons, the reticular activating system (RAS) plays a role in consciousness and the awake/sleep cycles.

Cerebellum

The cerebellum is the third part of the hindbrain, but it is not considered part of the brain stem. Functions of the cerebellum include fine motor coordination and body movement, posture, and balance. This region of the brain is enlarged in birds and controls muscle action needed for flight.

Spinal Cord

The spinal cord runs along the dorsal side of the body and links the brain to the rest of the body. Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column.

The gray matter of the spinal cord consists mostly of cell bodies and dendrites. The surrounding white matter is made up of bundles of interneuronal axons (tracts). Some tracts are ascending (carrying messages to the brain), others are descending (carrying messages from the brain). The spinal cord is also involved in reflexes that do not immediately involve the brain.

Nerves divide many times as they leave the spinal cord so that they may reach all parts of the body. The thickest nerve is 1 inch thick and the thinnest is thinner than a human hair. Each nerve is a bundle of hundreds or thousands of neurons (nerve cells). The spinal cord runs down a tunnel of holes in your backbone or spine. The bones protect it from damage. The cord is a thick bundle of nerves, connecting your brain to the rest of your body.

Peripheral Nervous System

The Peripheral Nervous System contains only nerves and connects the brain and spinal cord (CNS) to the rest of the body. The axons and dendrites are surrounded by a white myelin sheath. Cell bodies are in the central nervous system (CNS) or ganglia.  Ganglia are collections of nerve cell bodies. Cranial nerves in the PNS take impulses to and from the brain (CNS). Spinal nerves take impulses to and away from the spinal cord. There are two major subdivisions of the PNS motor pathways: the somatic and the autonomic.

Two main components of the PNS:

1. sensory (afferent) pathways that provide input from the body into the CNS.
2. motor (efferent) pathways that carry signals to muscles and glands (effectors).

Most sensory input carried in the PNS remains below the level of conscious awareness. Input that does reach the conscious level contributes to perception of our external environment.

Autonomic Nervous System

The Autonomic Nervous System is that part of PNS consisting of motor neurons that control internal organs. It has two subsystems. The autonomic system controls muscles in the heart, the smooth muscle in internal organs such as the intestine, bladder, and uterus. TheSympathetic Nervous System is involved in the fight or flight response. TheParasympathetic Nervous System is involved in relaxation. Each of these subsystems operates in the reverse of the other (antagonism). Both systems innervate the same organs and act in opposition to maintain homeostasis. For example: when you are scared the sympathetic system causes your heart to beat faster; the parasympathetic system reverses this effect.

Motor neurons in this system do not reach their targets directly (as do those in the somatic system) but rather connect to a secondary motor neuron which in turn innervates the target organ.

Somatic Nervous System

The Somatic Nervous System (SNS) includes all nerves the muscular system and external sensory receptors. External sense organs (including skin) are receptors. Muscle fibers and gland cells are effectors. The reflex arc is an automatic, involuntary reaction to a stimulus. When the doctor taps your knee with the rubber hammer, she/he is testing your reflex (or knee-jerk). The reaction to the stimulus is involuntary, with the CNS being informed but not consciously controlling the response. Examples of reflex arcs include balance, the blinking reflex, and the stretch reflex.

Sensory input from the PNS is processed by the CNS and responses are sent by the PNS from the CNS to the organs of the body.

Motor neurons of the somatic system are distinct from those of the autonomic system. Inhibitory signals, cannot be sent through the motor neurons of the somatic system.

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