Fundamentals Of Nervous System & Nervous Tissue

FUNDAMENTALS OF NERVOUS SYSTEM & NERVOUS TISSUE Study of nervous system and receptors is called neurology. In humans, physiological functions are coordinated by both the nervous and endocrine systems. The nervous system has highly specialised cells called neurons. Neurons detect and receive several information(stimuli) from different sensory organs (receptors) and process all such information to decide the type of response of the living body. The sensory information is coded in the form of electrical impulses by the nervous system and transmitted to other cells for their response. Nervous system of higher organism performs three basic function. (a) receiving sensory input from internal and external environment by nerves to the brain. (b) processing the input information in the brain (central brain) (c) responding to stimuli transmitting motor commands from the brain to determine the response of the body parts or cells. NERVOUS SYSTEM OF HUMAN Human nervous system has two contrasting functional part viz. central nervous system (CNS) and peripheral nervous system (PNS). The central nervous system (CNS) consists of the brain and spinal cord, which lies within the bony cases of the skull and spine. CNS is the site of processing information within the nervous system. The parts of the nervous system outside the skull and spine make up the peripheral nervous system. The PNS consists largely of nerve fibres of axons which (a) carry nerve impulses from the sensory receptors of the body inward to the CNS (sensory or afferent pathway) (b) carry nerve impulse to the muscles and glands for their excitation from the central nervous system (motor or efferent pathway) The PNS has two divisions : the somatic nervous system and the autonomic nervous system. In the somatic nervous system motor fibers activate the striped muscles of the body, such as those that move the arms and legs, while the sensory fibers of this system come from the major receptor organs of the body - the eyes, the touch receptors and so on. In the Autonomic Nervous System (ANS) motor fibers activate the smooth muscles of such bodily organs as the stomach, causes secretion from certain glands such as the salivary glands, and regulate activity in the special types of muscles found in the heart. Sensory fibers in the autonomic nervous system carry information from the internal body organs that is perceived as pain, warmth, cold or pressure. The ANS has two sub divisions : the sympathetic nervous system and the parasympathetic nervous system. In general, the sympathetic nervous system is activate in stressful situations. The parasympathetic nervous system is active in rest or quiet state. Apart from ANS there is the neuroendocrine system, which is a network of endocrine glands whose hormone production is controlled by commands from the CNS. Neurons (Nerve cells) are functional units of nervous system. They are classified structurally or functionally.

Neurons are grouped structurally into three types according to the number of processes extending from their cell body. (a) Unipolar neurons : (Pseudounipolar neurons) have a single process further dividing into two processes. One of these processes represents the axon and the other is functionally a dendrite but its structure is like that of an in the dorsal root of gangilia cell of olfactory bulbs (part of the olfactory lobes of the brain). True unipolar neurons with an axon and no dendrite are found in invertebrates and vertebrate embryos. (b) Bipolar neurons : It have only two processes, an axon at one end and a dendrite at another end. Found in the retina of eyes. Olfactory epithelium and cochlear and vertibular ganglia (cochlea and vertibule are the parts of membranous labyrinth of internal ear). (c) Multipolar neurons : These are most common and have several dendrites and an axon e.g. motor neurons and interneurons. Found in pyramidal cells in cerebral coetex and in spinal cord. The functional (classification of neurons is according to the direction in which the nerve impulse travel relative to the CNS. They are of three types viz. (a)

sensory or afferent neurons

(b)

motor or efferent neurons

(c)

association neurons or inter neurons (lie between sensory and motor neurons where intergration occurs most confined within the CNS.

Masses of neurons (nerve cells) inside the CNS are called nuclei but those which lie in PNS are called ganglia. The bundles of nerve fibres within the CNS are called tracts and those within PNS are called nerves. A nerve consists of several bundles of nerve fibres called fasciculi (singular : fasciculum). Each fasciculum is surrounded by a layer of connective tissue called the perineurium. Each nerve fibre is surrounded by a layer of connective tissue called the endoneurium. A dense layer of connective tissue that surrounds the entire nerve is called the epineurium. Each nerve has its own supply of small blood vessels.

Surrounding neurons are special companion cells known as glia (glue) or Neuroglia. About 50% of all brain cells are neuroglial cells. Neuroglia are astrocytes, oligodendrocytes and microglia. (a) The astrocytes separate and provide mechanical support to the neurons; insulate adjacent neurons so that impulses pass from one neuron to the next only over the synapse where packing cells are missing; communicate role in maintenance of the blood brain barrier; are responsible for repair of damaged areas of nervous tissue. (b) The oligodendrocytes form myelin sheaths around the axon that lie within the CNS. (c) The microglia engulf microbes and cellular debris; are phagocytic as well as scavengers. In the PNS Schwann cell, a type of glial cell, wrap around the axons of neutrons, thereby covering the axon with concentric layers of insulating plasma membrane. Nervous tissue is Ecodermal in Origin. Minimum power of regeneration is found in Nervous tissues (some time also consider power of regeneration is absent in Nervous tissue. ) Receive stimulation and conduction of impulses represented by it. Show irritability Cell division is absent after maturity. Unit of Nervous tissue is Neuron (Nerve Cell).

I. Neuron or Sensory cell. Neuron is unit of nervous tissue or nervous system made up of 2 main parts (a) main body – cyton or perikaryon or soma (b) Long process – Axon or nerve fibre. Cyton Forms central Nervous system or Ganglia It consists of Granular cytoplasm (= Neuroplasm), large Nucleus , mitochondria, Ribosomes, fat globules, Nissil’s granules (Nissil bodies) and Neurofibrils. Small processes arises from cyton. These are known as dendrons and small branching of Dendron known as Dendrites. To serve high energy needs for impulse conduction Neurons have many mitochondria.

Axon Also known as Axis or Axis Cylinder or Neuraxis May form Nerve fibre The part of cyton where axon arises is called Axon hillock. Axon ends with small branches on muscle fibres or other neurons. Terminal part of axon is known as Telodendria which terminates in the form of end bulb called Buttons or Button terminates or knobs. Axon terminals inter connect with dendrite terminals and forms synapses.

II Nerve fibres Nerve fibre is an axon together with its enveloping sheaths. In peripheral Nervous system (Nerves) Nerve fibre is surrounded by (a) Outer most sheath - Myelin sheath (b) Middle sheath – Sheath of Schwann cell or Neurolemma (c) Inner most sheath – Endoneural sheath Endoneural sheath is made up of connective tissue having Neuroglial cells, also known as Sheath of Henle or key Retzious.

The Myelin sheath appears as a tube around the axon At regular intervals (of about 1 mm in Humans ), the neurilemma is constricted and the myelin sheath is interrupted, forming the so called Nodes of Ranvier The segments between successive Nodes are called Internodes. The myelin sheath consists merely of the plasma membrane of over lying Schwann cells or (Oligodendrocytes) wrapped around the axon several times in a spiral fashion On the basis of structure Nerve fibres are of two types. 1. Myelinated Nerve fibres [medullated Nerve fibre] 2. Non – Myelinated Nerve fibres [ Non – medullated Nerve fibre ] Eg: - Certain axons of sympathetic Nervous system do not have a myelin sheath Each Internodal segment of axon and its myelin sheath is the product of a single schwann cell (oligodendrocyte) whose flattened nucleus is visible on external surface of the sheath Myelin sheath is lipoprotein but mainly lipoid in nature Nerve A nerve is a bundle of nerve fibres A single nerve fibre is surrounded by endoneurium Group of Nerve fibres are surrounded by perineurium Whole Nerve is covered by a sheath of connective tissue known as Epineurium. Most nerves (Cranial and Spinal) possesses only myelinated fibres, but some (Sympathetic) possess both types of fibres. Sensory Nerve fibres – It bring the impulse towards the central Nervous system. Motor Nerve fibres – It carries the impulses away from central Nervous System. The numerous fibres of a nerve are aggregated into several small bundles Called fasciculi.

MORPHOLOGICAL TYPES OF NEURON (i) (ii) (iii) (iv)

Unipolar neurons : In this type of neuron only one axon, often with dendrites or collateral processes, takes its origin from the cell body. Pseudounipolar neurons : In this type of neuron, there may be single process arising from the cell body and then dividing into an axon and a dendrite. Bipolar neurons : The neurons in which two processes take their origin from the cell body, one on either side of the cell body, are known as bipolar neurons. Multipolar neurons : Such type of neurons have a large number of cell processes, at least one of which is an axon.

III.

Neuroglia (Glial cell ) These are supporting cells which form a packing around the Neurons. This tissue divided into three kinds on the basis of their size, shape and functions. (a) Astrocytes – are found around the Blood vessels and appear to serve as regulator of the ionic environment of Neurons. (b) Oligodendroglial cells – form insulatory covering on the nerve fibres (c) Microglial cells – form reticulo – endothelial system.