1.Introduction to Nervous System
3.Classification of Nerve Fibers
Primarily, nervous system is divide into two parts:
1. Central nervous system
2. Peripheral nervous system.
Fig.1:- Parts of central nervous system
It is formed by neurons and supporting cells called neuroglia. Central nervous system (CNS) includes brain and spinal cord.Structures of brain and spinal cord are arranged in two layers, namely gray matter and white matter.
Brain is situated in the skull. It is continued as spinal cord in the vertebral canal through the foramen magnum of the skull bone. Brain and spinal cord are surrounded by three layers of meninges called the outer dura mater, middle arachnoid mater and inner pia mater.
Parts of Brain
Fig.2:- Parts of brain
Brain consists of three major divisions:
Prosencephalon is otherwise known as forebrain. It is further divided into two parts:
1.Telencephalon, which includes cerebral hemispheres, basal ganglia, hippocampus and amygdaloid nucleus
2.Diencephalon, consisting of thalamus, hypothalamus, metathalamus and subthalamus
Mesencephalon is also known as midbrain. Rhombencephalon or hindbrain is subdivided into two portions:
1.Metencephalon, formed by pons and cerebellum
2. Myelencephalon or medulla oblongata
It is again divided into two subdivisions:
1. Somatic nervous system
2. Autonomic nervous system.
1. Somatic Nervous System
Somatic nervous system is responsible for muscular activities and movements of the body
2. Autonomic Nervous System
Autonomic nervous system is concerned with regulation of visceral or vegetative functions. Autonomic nervous system consists of two divisions, sympathetic division and parasympathetic division.
Fig.3:- Organization of nervous system
Neuron or nerve cell is defined as the structural and functional unit of nervous system.However, it is different from other cells by two ways
1. Neuron has branches or processes called axon and dendrites
2. Neuron does not have centrosome. So, it cannot undergo division.
Neurons are classified by three different methods.
A. Depending upon the number of poles
B. Depending upon the function
C. Depending upon the length of axon.
1. Unipolar neurons
2. Bipolar neurons
3. Multipolar neurons.
1. Unipolar Neurons– Unipolar neurons are the neurons that have only one pole.
2. Bipolar Neurons– Neurons with two poles are known as bipolar neurons.
3. Multipolar Neurons– Multipolar neurons are the neurons which have many poles.
On the basis of function, nerve cells are classified into two types:
1. Motor or efferent neurons
2. Sensory or afferent neurons.
1. Motor or Efferent Neurons
Motor or efferent neurons are the neurons which carry the motor impulses from central nervous system to peripheral effector organs like muscles, glands, blood vessels, etc.
2. Sensory or Afferent Neurons
Sensory or afferent neurons are the neurons which carry the sensory impulses from periphery to central nervous system.
Depending upon the length of axon, neurons are divided into two types:
1. Golgi type I neurons
2. Golgi type II neurons.
Neuron is made up of three parts:
1. Nerve cell body
Like any other cell, it is constituted by a mass of cytoplasm called neuroplasm, which is covered by a cell membrane. Nissl bodies and neurofibrils are found only in nerve cell and not in other cells.
Dendrite transmits impulses towards the nerve cell body. Usually, the dendrite is shorter than axon.
Axon is the longer process of nerve cell. Axon transmits impulses away from the nerve cell body.
Each nerve is formed by many bundles or groups of nerve fibers. Each bundle of nerve fibers is called a fasciculus.
The whole nerve is covered by tubular sheath, which is formed by a areolar membrane. This sheath is called epineurium. Each fasciculus is covered by perineurium and each nerve fiber (axon) is covered by endoneurium.
Internal Structure of Axon – Axis CylinderAxon has a long central core of cytoplasm called axoplasm.
Axoplasm is covered by the tubular sheathlike membrane called axolemma. Axoplasm contains mitochondria, neurofibrils and axoplasmic vesicles. Axis cylinder of the nerve fiber is covered by a membrane called neurilemma.
Nerve fiber described above is the non-myelinated nerve fiber, which is not covered by myelin sheath.
Fig.4:- Cross section of a nerve
Fig.5:- A. Myelinated nerve fiber; B. Non-myelinated nerve fiber.
Nerve fiber which is insulated by myelin sheath is called myelinated nerve fibers. The area where myelin sheath is absent is called node of Ranvier. Segment of the nerve fiber between two nodes is called internode. Myelin sheath is responsible for white color of nerve fibers.
Myelin sheath is formed by concentric layers of proteins, alternating with lipids. Formation of Myelin Sheath – MyelinogenesisFormation of myelin sheath around the axon is called the myelinogenesis. It is formed by Schwann cells in neurilemma. In the peripheral nerve, the myelinogenesis starts at 4th month of intrauterine life. It is completed only in the second year after birth. Schwann cells wrap up and rotate around the axis cylinder in many concentric layers. The concentric layers fuse to produce myelin sheath but cytoplasm of the cells is not deposited.
1.Faster conduction– In myelinated nerve fibers, the impulses jump from one node to another node. This type of transmission of impulses is called saltatory conduction2.
2.Insulating capacity– Myelin sheath has a high insulating capacity. Because of this quality, myelin sheath restricts the nerve impulse within single nerve fiber and prevents the stimulation of neighboring nerve fibers.
Neurilemma is a thin membrane, which surrounds the axis cylinder. It is also called neurilemmal sheath or sheath of Schwann. In non-myelinated nerve fiber, the neurilemma surrounds axolemma continuously. In myelinated nerve fiber, it covers the myelin sheath. At the node of Ranvier (where myelin sheath is absent), neurilemma invaginates and runs up to axolemma in the form of a finger-like process.
In non-myelinated nerve fiber, the neurilemma serves as a covering membrane. In myelinated nerve fiber, it is necessary for the formation of myelin sheath (myelinogenesis).
1. Facilitate initial growth and development of nerve cells in central and peripheral nervous system
2. Promote survival and repair of the nerve cells
3. Play an important role in the maintenance of nervous tissue and neural transmission.
Nerve growth factor (NGF) is a neurotrophin found in many peripheral tissues.
1. NGF promotes early growth and development of neurons. NGF also promotes the growth of cholinergic neurons in cerebral hemispheres.
2. Commercial preparation of NGF extracted from snake venom and submaxillary glands of male mouse is used to treat sympathetic neuron diseases.
3. NGF plays an important role in treating many nervous disorders such as Alzheimer disease, neuron degeneration in aging and neuron regeneration in spinal cord injury.
Fig.6:- Different methods to classify nerve fibers
i. Myelinated Nerve Fibers
Myelinated nerve fibers are the nerve fibers that are covered by myelin sheath.
ii. Non-myelinated Nerve Fibers
Nonmyelinated nerve fibers are the nerve fibers which are not covered by myelin sheath.
i. Somatic Nerve FibersSomatic nerve fibers supply the skeletal muscles of the body.
ii. Visceral or Autonomic Nerve Fibers .Autonomic nerve fibers supply the various internal organs of the body.
1.Cranial Nerve FibersNerve fibers arising from brain are called cranialnerve fibers.
2. Spinal Nerve FibersNerve fibers arising from spinal cord are called spinal nerve fibers.
i. Sensory Nerve Fibers
Sensory nerve fibers carry sensory impulses from different parts of the body to the central nervous system. These nerve fibers are also known as afferent nerve fibers.
ii. Motor Nerve Fibers
Motor nerve fibers carry motor impulses from central nervous system to different parts of the body. These nerve fibers are also called efferent nerve fibers.
1.Adrenergic Nerve FibersAdrenergic nerve fibers secrete noradrenaline.
2. Cholinergic Nerve Fibers .
i. Type A nerve fibers
ii. Type B nerve fibers
iii. Type C nerve fibers.
Type A nerve fibers are divided into four types:
a. Type A alpha or Type I nerve fibers
b. Type A beta or Type II nerve fibers
c. Type A gamma nerve fibers
d. Type A delta or Type III nerve fibers.
Velocity of impulse through a nerve fiber is directly proportional to the thickness of the fiber.