Download Nervous System Organization and Components

Nervous System - Organization and Components
I. Systems of Internal Communications
The survival of a multicellular organism depends on regulation and integration of activities of its cells. There
are two systems that serve as means of internal communication within the organism. The nervous
system acts rapidly, for a short duration and endocrine system acts slowly, for a long duration.
Both systems integrate and coordinate activities to assure proper body function. These systems allow for
communication and control of body functions to maintain homeostasis of the body.
Now we will consider the nervous system, next semester you will study the endocrine system.
II. General Plan of the Nervous System
Neurons (nerve cells) are the fundamental structural and functional units of the nervous system, but in the
intact nervous system the reflex is the fundamental structural and functional unit of the nervous system.
The neurons carry messages in the form of electrica1 signals, called nerve impulses or action potentials.
The nervous system has 3 basic functions:
A. Sensory - the sense organs or receptors perceive or sense a change in the internal or external environment,
called a stimulus. When a stimulus of sufficient intensity activates the receptor, the energy of the stimulus
is transduced into the electrical energy of the nerve impulse. The nerve impulses once initiated in the
receptors are carried in the sensory neurons to the central nervous system, spinal cord and brain (CNS).
B. Integrative - in the spinal cord and brain, interconnecting neurons carry the impulses to other areas of the
CNS. This information is analyzed, stored and integrated with other information so an appropriate
response is made to the stimulus.
C. Motor - Ultimately the nerve impulses are carried by motor neurons away from the CNS to the effectors
(muscle cells or gland cells) to cause a coordinated, meaningful response to the stimulus.
III. Organization (Divisions ) of the Nervous System
There are various divisions of the nervous system based on structure, location and function.
The nervous system is divided into central and peripheral nervous systems.
A. Central Nervous System (CNS)
Structure
The CNS consists of the brain and spinal cord.
Location
The CNS is located in the dorsal body cavity. The brain is in the cranial cavity of the skull and the
spinal cord is in the vertebral canal of the vertebral column.
Function
The CNS is the integrative and control center of the nervous system. It receives sensory input from the
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receptors, integrates and correlates incoming information and it formulate responses to the sensory
input. In addition, it stores information to provide for emotion, memory, reasoning and
conceptualization.
B. Peripheral Nervous System (PNS)
Structure
The receptors and effectors are connected to the CNS by nerves (groups of neurons) of the PNS.
These nerves carry impulses to and from the CNS.
Location
There are 12 pairs of cranial nerves, which arise from the ventral surface of the brain. There are 31
pairs of spinal nerves, which arise from the dorsal and ventral regions of the spinal cord.
Function
The PNS is functionally divided into:
1. Afferent (Sensory) division - the afferent division has 2 subdivisions.
a. Somatic sensory division - it is composed of sensory neurons to the CNS. The receptors are
located in the skin, muscles, fascia and joints. From the receptors these neurons carry nerve
impulses to the CNS.
b. Visceral sensory division - it is composed of sensory neurons that carry nerve impulses from
receptors (proprioceptors) located the viscera of the body to the CNS. This division is the
sensory part the autonomic nervous system.
2. Efferent (Motor) division - it has 2 subdivisions.
a. Somatic nervous system (SNS) or Voluntary nervous system refers to nerves associated with
structures of the body wall. Somatic nervous system’s functions are consciously
controlled. The system consists of motor (somatic) neurons from the CNS to the skeletal
muscle. These neurons carry nerve impulses to produce contractions of the skeletal muscle.
b. Autonomic nervous system (ANS), Involuntary nervous system or Visceral nervous system
refers to nerves associated with internal body (visceral) organs. Autonomic nervous system’s
functions cannot be consciously controlled. The system is composed of visceral motor neurons.
The nerve impulses go to the smooth muscle, cardiac muscle, and glands and to the viscera of
the body. The motor part of the ANS is divided functionally into sympathetic and parasympathetic
divisions. Most viscera organs receive dual innervations as the 2 divisions have opposing
actions. The sympathetic involves the use of energy. The parasympathetic restores and conserves
the body's energy.
IV. Components of the Nervous System
Nerve tissue is composed of 2 types of cells that differ structurally and functionally.
A. Neurons or Nerve Cells - these cells transmit the nerve impulses.
B. Neuroglial Cells (Nonnervous nerve cells)
1. Neurilemmal or Schwann cells which form the segmented myelin covering around the processes of
the neurons of the PNS.
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2. Other neuroglial cells that serve as support, nutrition and protective tissue for the neurons of the
CNS. Some of these neuroglial cells also form the myelin for the neurons in the CNS.
Nerves of the PNS are formed by bundles of sensory neuron’s dendrites and motor neuron’s axons,
neurilemmal cells and connective tissue. The peripheral nerves connect the spinal cord and brain (CNS) to the
receptors and effectors. Bundles of interneuron’s axons in the CNS are called nerve tracts. They are composed
of neurons and neuroglial cells, but no connective tissue. The nerve tracts connect spinal cord and brain
together in the white matter of the spinal cord.
A. Neurons
Neurons function to conduct nerve impulses. The shape of the neuron makes it well suited for
transmitting information, as the processes are long and thin resembling a wire.
Structure of a Neuron
1. Perikaryon, Cell Body or Cyton - each neuron is composed of a perikaryon and cytoplasmic processes
that extend from the perikaryon. In the perikaryon is a large spherical nucleus with a nucleolus. The
cytoplasm contains cellular organelles, such as the Golgi complex, rough endoplasmic reticulum and
neurofibrils.
The Nissl bodies are revealed by the E/M to be enlarged rough ER. The microtubules and fibrils
compose the neurofibrils. The fibrils may provide support to the cell. Transport of material occurs in
the microtubules.
Location of the perikarya - nuclei and ganglia
Perikarya of most neurons are located in the CNS. The perikarya in the CNS are in clusters called
nuclei form the gray matter of the spinal cord and brain. Perikarya outside, in the PNS, are grouped
together in structures called ganglia.
2. Processes of the Neurons
These are thin extensions of the cell.
a. Dendrites - these are processes of the neuron that conduct the nerve impulse toward the perikaryon.
The number, length and extent of branches of the dendrites varies in different types of neurons.
Dendrites possess similar cytoplasmic structures found in the perikaryon. Dendrites usually are not
myelinated.
b. Axons - there is usually one axon per cell. The axon conducts the nerve impulse away from the
perikaryon. The axon is usually myelinated. The length of the axon is long. From the axon
hillock, an elevation on the perikaryon, the axon arises. A branch of the axon is its collateral.
The axon contains in its axoplasm (cytoplasm) mitochondria, microtubules, neurofibrils and at its
terminal branches neurosecretory vesicles but no Nissl bodies.
B. Neuroglial Cells
Most axons are covered with a fatty insulating substance, myelin. Such axons are called myelinated axons
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or fibers. Unmyelinated axons or fiber are not covered by visible amounts of myelin. Due to the lipid
content of myelin, myelinated axons appear white. The white matter of the nervous system is due to the
myelinated axons.
1. Neurilemmal cells
Outside the myelin, surrounding the axon is a sheath of cells, the neurilemma or Schwann sheath.
The neurilemma and myelin are not continuous but are interrupted at intervals along the length of the
axon. The point of interruption is the neurofibril node (node of Ranvier).
Axons of the PNS
Surrounding both myelinated and unmyelinated axons of the PNS are neurilemmal (Schwann) cells
which are arranged end to end along the length of the axon.
a. Myelinated axons - the myelin sheath is formed around a short segment of axon as the neurilemmal cell
wraps itself around the axon. The myelin sheath is composed alternating layers of the lipid and protein,
which are remains of the cell membrane of the neurilemmal cell. The neurofibril nodes are bare axon
between successive neurilemmal cells and segments of myelin. The nodes are important for rapid
(saltatory) conduction or transmission of the nerve impulse.
b. Unmyelinated axons - axons of several neurons are surrounded by a single neurilemmal cell and no
visible myelin is formed. The neurilemmal cells cover the unmyelinated axons.
Axons of the CNS
2. Other Neuroglial Cells - Neurons in the CNS are myelinated. In the CNS one of the neuroglial cells, the
oligodendrocyte, spirals around the axons to form the myelin sheath. There are no neurilemmal cells in
the CNS. The myelinated axons form the white matter of the CNS.
C. Types of Neurons
1. Classification according to structure.
This classification is based on the number of processes that extend from the perikaryon.
a. Bipolar neuron - this cell has one dendrite and one axon that extend from each end of the perikaryon.
Location -The bipolar neurons are in the organs of special senses, such as the retina of the eye,
cochlea of the inner ear and the olfactory area of the nose.
b. Unipolar or Pseudounipolar neuron - this cell has a single process arising from the perikaryon. The
single process immediately divides into two processes. One of the processes functions as the dendrite.
The other process functions as the axon. Functionally these are the sensory neurons of the PNS.
Location – The unipolar neurons are in the spinal nerves and cranial nerves.
c. Multipolar neuron - this cell has one long process from the perikaryon, which functions as the axon,
and shorter more numerous other processes from the perikaryon which function as the dendrites.
Functionally these neurons are the interneurons in the brain and spinal cord of the CNS. In the
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PNS they are the motor neurons.
Location – The multipolar neurons are in the spinal nerves and cranial nerves, white matter of CNS.
2. Classification according to function or direction the neurons transmit the nerve impulses.
a. Sensory (Afferent) neurons - these cells transmit nerve impulses from receptors to the CNS.
The bipolar and unipolar neurons are sensory neurons.
b. Interneurons or Association neurons – These cells are within the CNS. They connect one neuron to
another. Some multipolar neurons are interneurons.
c. Motor (Efferent) neurons - these cells transmit nerve impulses away from the CNS to the effector.
Some multipolar neurons are motor neurons.
D. Nerves and Nerve Tracts
A nerve is a bundle of nerve processes (axons and dendrites) held together by connective tissue.
Outside the CNS, each nerve fiber is wrapped in a connective tissue covering, the endoneurium. The
fascicles (bundles) of nerve fibers are wrapped in connective tissue, the perineurium. The connective tissue
around the entire nerve is the epineurium.
A nerve can vary in size and composition. Most peripheral nerves contain processes of both sensory and
motor neurons. Nerve impulses in a mixed peripheral nerve travel to and from the CNS at the same time.
There are a few purely sensory nerves that contain only processes of sensory neurons that carry nerve
impulses to the CNS.
Inside the CNS, fascicles of neurons are the nerve tracts. This is the white matter of the CNS. The nerve
tracts are composed of axons of the interneurons and neuroglial cells, but no connective tissue coverings,
these neurons connect the spinal cord and brain together. Some nerve tracts are sensory as they carry nerve
impulses from receptors (sense organs) through the spinal cord to the brain. Other nerve tracts are motor as
they carry nerve impulses from the brain through the spinal cord to the effectors (muscles and glands).
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Proprioceptors – Sense organs which gives information concerning movement and position of the body.
These organs occur in muscles, tendons and inner ear.
Interoceptors – Sense organs located in the viscera of the body.
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