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) (b) (c) sensory or afferent neurons motor or efferent neurons 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