BASICS OF NERVOUS SYSTEM
Updated: May 11, 2020
NERVOUS SYSTEM is the chief CONTROLLING and COORDINATING system of the body.
NERVOUS SYSTEM IS BASICALLY CLASSIFIED INTO:
1.CENTRAL NERVOUS SYSTEM
2.PERIPHERAL NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM
System whose basic function is to collect information from surroundings and our body through SENSORY INPUT, and then compare and contrast it with our past memories and finally generate a decision through MOTOR OUTPUT.
TWO COMPONENTS OF CNS
Average weight of adult brain is 15,00 grams, since brain floats in CEREBROSPINAL FLUID, it only weighs 50 grams which is comfortable.
PARTS OF BRAIN
IT HAS 3 MAIN PARTS
1. FOREBRAIN OR PROCENCEPHALON
it has two subdivisions telencephalon and diencephalon.
TELENCEPHALON consists of both cerebral hemispheres and it lies on the top and outer surface.
DIENCEPHALON consists of thalamus and hypothalamus and it lies in the center of forebrain.
2. MIDBRAIN OR MESENCEPHALON
it is the middle part of brain containing various structures and it connects forebrain to hindbrain.
3. HINDBRAIN OR RHOMBENCEPHALON
it also further has two subdivisions metencephalon and myelencephalon.
METENCEPHALON consist of PONS and CEREBELLUM.
MYELENCEPHALON consists of MEDULLA OBLANGATA.
(it will be discussed in a separate article link will be available soon.)
ANATOMICALLY CNS HAS GRAY MATTER AND WHITE MATTER
GRAY MATTER : collection of cell bodies within CNS.
it is present inside CNS in two forms.
gray matter which is applied on the surface of brain.
EXAMPLE: CEREBRAL CORTEX AND CEREBELLAR CORTEX.
some gray matter is surrounded by white matter in CNS.
EXAMPLE: 3rd NERVE NUCLEUS, NUCLEUS FOR TASTE, SALIVATORY NUCLEUS etc.
WHITE MATTER : collection of axons within CNS.
it is present inside CNS in three forms on the basis of direction of axons.
These are of two types.
Those bundles of axons taking information from lower part of CNS to upper part of CNS are ascending tracts.
Those bundles of axons taking information from upper part of CNS to lower part of CNS are descending tracts.
2. COMMISURAL FIBRES : fibres which connect right and left half of brain.
EXAMPLE : CORPUS CALLOSUM (largest commisure connect both cerebral hemisphere.)
3. ASSOCIATION FIBRES : fibres which connect brain on anteroposterior axis (connects frontal and occipital lobes)
PERIPHERAL NERVOUS SYSTEM
System whose basic function is to send SENSORY INPUT to CNS via SENSORY or AFFERENT FIBRES and to transfer MOTOR OUTPUT from CNS to BODY via MOTOR or EFFERENT FIBRES.
TWO COMPONENTS OF PNS
It transfers SENSORY INPUT from BODY to CNS.
It receives two types of sensations.
SPECIAL SENSES: senses which can only be felt from specific/special parts of the body.
EXAMPLES 1) OLFACTION (from nose)
2) VISION ( from eyes)
3) TASTE ( from tongue )
tongue has both sensations special (taste) and general ( touch, pain and temperature)
4) HEARING ( from ear)
5) SENSE OF BALANCE ( from internal ear )
GENERAL SENSES: senses which are generalized and can be felt from more than one part of body.
EXAMPLES 1) TOUCH
4) PROPRIOCEPTION ( all the sensations coming from locomotor system in a group known as proprioception )
It transfers MOTOR OUTPUT from CNS to BODY.
It has two categories
SOMATIC MOTOR: system which transfers motor output to voluntary parts and mainly control VOLUNTARY functions of body.
EXAMPLE- skeletal muscles
AUTONOMIC MOTOR: system which transfers motor output to involuntary parts and mainly controls INVOLUNTARY functions of body.
It works in two pathways.
SYMPATHETIC: In normal conditions it works with parasympathetic system equally to maintain a state of balance but when situations become stressful it DOMINATES.
(situations like FFF fright, fight and flight are a trigger for this system)
It stimulates our CARDIAC ACTIVITY and inhibits our GIT ACTIVITY.
PARASYMPATHETIC: In normal conditions it works with sympathetic system equally to maintain a state of balance but when situations become extremely stress free or when our mind is in complete relaxation it becomes DOMINANT.
It stimulates our GIT ACTIVITY and inhibits CARDIAC ACTIVITY.
CELLULAR ARCHITECTURE OF NERVOUS SYSTEM
NERVOUS TISSUE has mainly two types of cells NERVE CELLS/NEURON and SUPPORTING CELLS/NEUROGLIAL CELLS.
This is the functional and structural unit of nervous system. (RANGE FROM 180-200 BILLIONS)
1.CELL BODY : It forms gray matter in CNS and ganglia in PNS.
2. CELL PROCESSES : They are of two types.
DENDRITES- They are short and richly branched and are various in numbers.
AXON : It is a single elongated process of neuron collectively forms white matter in CNS and nerves in PNS.
(branches of axon arise at right angle to each other and are known as COLLATERALS.)
3. MYELIN SHEATH : Inside CNS it is formed my OLIGODENDROCYTES and inside PNS it is formed by SCHWANN CELLS.
CLASSIFICATION OF NEURONS
ON THE BASIS OF NUMBER OF PROCESSES : 5 TYPES
1. APOLAR : They only have dendrites and no axons.
examples- retina of eyeball as amacrine cells.
2. UNIPOLAR : They have a single process.
examples- mesencephalic nucleus of trigeminal nerve, occur during featal life, and most common in vertebrates.
3.BIPOLAR : They have two processes.
example- confined to 1st neuron of retina, ganglia of eight cranial nerve, and olfactory mucosa.
4. PSEUDOUNIPOLAR : They are unipolar to begin with but become bipolar functionally.
examples- dorsal nerve root ganglia and sensory ganglia of cranial nerves.
5. MULTIPOLAR : They have many dendrites and a single elongated axon.
examples- most common in man, all motor and internuncial neurons.
ON THE BASIS OF LENGTH OF AXON : 2 TYPES
GOLGI TYPE 1 : These have long axons and short dendrites.
examples- pyramidal cells of cerebral cortex, purkinje cells of cerebellum, anterior horn cells of spinal cord.
GOLGI TYPE 2 : These have small axons , establish synapse with neighbouring neurons.
examples- cerebral cortex and cerebellar cortex.
ON THE BASIS OF SIZE : 2 TYPES
MACRONEURONS: Neurons having size more than 7 micrometre.
MICRONEURONS: Neurons having size of less than 7 micrometre.
FUNCTIONAL CLASSIFICATION : 4 TYPES
SENSORY NEURONS: further 3 types
1. PRIMARY/1st ORDER NEURONS- present as spinal or sensory neurons in the dorsal root ganglion of spinal nerves.
2. SECONDARY/2nd ORDER NEURONS- present in the gray matter of spinal cord and inside the brainstem.
3. TERTIARY/3rd ORDER NEURONS- present in thalamus.
MOTOR NEURONS: further 2 types
1.UPPER MOTOR NEURONS- situated in motor area of brain, synapse with cranial nerve nuclei and anterior horn of spinal cord.
2. LOWER MOTOR NEURONS- located in cranial nerve nuclei and anterior horn of spinal cord, nerves emerging from these nuclei supply the various part of skeletal muscle.
These are connective tissue cells inside CNS which performs supporting functions.
1. ASTROCYTES: STAR shaped cell concerned with the nutrition of nervous tissue and their main function is to form BLOOD BRAIN BARRIER.
(they are absent in pineal gland and posterior pituitary)
2. OLIGODENDROCYTES: These are counterparts of schwann cells and their main function is to form myelin sheath around axons inside CNS.
3. MICROGLIA: They behave like macrophages of CNS.
4. EPENDYMAL CELLS: These are columnar cells lining the cavity of CNS and are responsible for the formation of CSF.
(Proliferation of glial cells is called GLIOSIS since the glial cells are capable of division they can form tumors inside CNS.)