Contents
Parts of nervous system
=> (1) Nervous system is divided into three parts:
=> (i) Central nervous system (CNS):
(a) In all the vertebrates including man, CNS is dorsal, hollow and non-ganglionated while in invertebrates when present, it is ventral, solid and ganglionated.
(b) CNS is formed of two parts:
Brain – Upper and broader part lying in the head; and
Spinal cord – Lower, long and narrow part running from beginning of neck to trunk.
=> (ii) Peripheral nervous system (PNS):
(a) It is formed of long, thin, whitish threads called nerves which extend between CNS and body parts (muscles, glands and sense organs).
(b) It controls the voluntary functions of the body.
(c) It has cranial and spinal nerves.
=> (iii) Autonomic nervous system (ANS):
(a) It is formed of nerve fibres extending upto visceral organs and controls the involuntary functions of visceral organs of body like heart beat, peristalsis etc.
(b) It is again formed of two systems: sympathetic and para-sympathetic nervous system which has opposing functions.
Central nervous system:
(1) Central nervous system is made up of brain and spinal cord. CNS is covered by 3 meninges and its wall has two type of matter.
(2) Types of matter: CNS of vertebrates is formed of two types of matter –
(a) Grey matter: It is formed of cell-bodies and non-medullated nerve fibres.
(b) White matter: It is formed of only medullated nerve fibres which appear white due to presence of medullary sheath.
Brain (Encephalon):
It is soft, whitish, large sized and slightly flattened structure present inside cranial cavity of cranium of the skull. In man, it is about 1200-1400 gm in weight and has about 10,000 million neurons. Brain is made up of 3 parts
(1) Fore brain (Prosencephalon)
(i) Olfactory lobe – Rhinencephalon
(ii) Cerebrum – Telencephalon
(iii) Diencephalon – Diencephalon
(2) Mid brain (Mesencephalon)
(i) Optic lobes – Mesencephalon
(3) Hind brain (Rhambencephalon)
(i) Cerebellum – Metencephalon
(ii) Medulla oblongata – Myelencephalon
Reflex action
(1) The reflex actions are involuntary actions because these are not under the conscious control of the brain.
(2) The spinal cord and brain stem are responsible for most of the reflex movements.
(3) A few examples of the reflex actions are withdrawal of hand or leg if pricked by a pin, secretion of saliva as soon as one thinks of delicious food or mere its sight causes salivation, if the body part is touched with acid or hot object it is automatically, without thinking and planning is withdrawn, cycling, motor driving etc.
=> (4) Component of reflex action: The whole of the reflex are includes six parts –
(a) Receptor organs: Receptors are windows of the body or guards of the body. These are situated on all, important organs, for example – eyes, nose, ear, tongue, integument etc. These perceive the stimuli from outside the body.
(b) Sensory neurons: These are also termed afferent neurons. These carry the stimuli from receptors to spinal cord. These neurons are situated in the ganglion on the dorsal side of spinal cord.
(c) Nerve centre: Spinal cord is termed as nerve centre. Synaptic connections are formed in it.
(d) Association neurons: These are also called intermediate neurons or interstitial neurons. These are found in spinal cord. They transfer the impulses from sensory neurons to motor neurons.
(e) Motor neurons: These are situated in the ventral horn of spinal cord. These carry the impulses to effector organs.
(f) Effector organs: These are the organs, which react and behave in response to various stimuli, for example – muscles and glands.
=> (5) Type of reflexes: The reflexes are of following types –
(a) Monosynaptic reflex
(b) Polysynaptic Spinal Reflex
(c) Polysynaptic Spinal/Brain Reflexes
(d) Unconditioned or Simple reflex
(e) Conditioned or Acquired reflex