Nervous System
The human neural system is divided into two parts : 1. Central nervous system(CNS)and 2. Peripheral nervous system(PNS).
The central nervous system the brain and the spinal cord and is the site of information processing and control. The Peripheral nervous system comprises all the nerves of the body associated with CNS(central nervous system).
The nerve fibre of PNS (peripheral nervous system) are of two types :
1. Afferent fibres and 2. Efferent fibres
The peripheral nervous system is divided into two divisions called the somatic neural system and the autonomic neural system.
The somatic neural system relays impulse from the central nervous system to the skeletal muscles while the autonomic neural system transmits the lmpulses from the central nervous system to the involuntary organs and to the smooth muscle of the body.
The autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system.
Neuron:
Neuron is the structural and functional unit of the nervous system.
Based on the number of the axon and dendrites, the neurons are divided into three types :
1.Multipolar neuron : Neuron with one axon and two or more dendrites, found in cerebral cortex.
2.Bipolar neuron : Neuron with one axon and one dendrite, found in rating of eye.
3. Unipolar neuron : Neuron in which cell body with one axon only,found usually in embryonic stage.
There are two types of axons, namely myelinated axon and non myelinated axon.
The myelinated nerve fibres are enveloped with Schwan's cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called Node of Ranvier. Myelinated nerve fibres are found in spinal and cranial nerves.
Unmyelinated nerve fibres is enclosed by Schwan's cell that dose not form a myelin sheath around the axon and is commonly found in autonomic neural system and in somatic neural system.
Transmission of impulse :
There are two types of synapses namely the chemical synapses and the electrical synapses.
ELECTRICAL SYNAPS : At electrical synapses, the membranes of pre - and post - synaptic neurons are in very close proximity. In electrical synaps electric current can flow directly from one neuron to other neuron across the synapses. Transmission of an impulse across electrical synaps is very similar to impulse conduction through a single axon. In the case of electrical synaps impulse transmission is always faster than that across a chemical synapses. Electrical synapses are rare in our system.
CHEMICAL SYNAPSEA : At a chemical synapses, the membrenes of pre - and post - synaptic neuron are separated by a fluid - filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of lmpulse at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an lmpulse(action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vasicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft and the released neurotransmitters binds to their specific receptors,present on the post - synaptic membrane. This binding opens the ion channels allowing the entry of ions which can generates a new potential in the post - synaptic neuron. The new potential developed may be either excitation or inhibitory.
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