Neural System and Neuron Structure
நரம்பு மண்டலமும் நியூரான் அமைப்பும்
Nissl's granules are just free ribosomes and RER packed into the cell body — they make proteins. They are not DNA, not lipids. One NCERT line, one guaranteed mark.
The idea
The neural system of all animals is composed of highly specialised cells called neurons, which can detect, receive and transmit different kinds of stimuli. In vertebrates the system is more developed. The human neural system is divided into the central neural system (CNS), which includes the brain and spinal cord and is the site of information processing and control, and the peripheral neural system (PNS), which comprises all the nerves of the body associated with the CNS. The nerve fibres of the PNS are of two types: afferent fibres transmit impulses from tissues or organs to the CNS, and efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues or organs. The PNS is divided into the somatic neural system, which relays impulses from the CNS to skeletal muscles, and the autonomic neural system, which transmits impulses from the CNS to the involuntary organs and smooth muscles of the body; the autonomic system is further classified into sympathetic and parasympathetic neural systems. The visceral nervous system is the part of the PNS comprising the whole complex of nerves, fibres, ganglia and plexuses by which impulses travel from the CNS to the viscera and from the viscera to the CNS. A neuron is composed of three major parts — cell body, dendrites and axon. The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl's granules. Short fibres that branch repeatedly and project out of the cell body, also containing Nissl's granules, are dendrites; they transmit impulses towards the cell body. The axon is a long fibre whose distal end is branched; each branch terminates as a bulb-like structure called a synaptic knob, which possesses synaptic vesicles containing chemicals called neurotransmitters. Based on the number of axons and dendrites, neurons are multipolar (one axon and two or more dendrites; found in the cerebral cortex), bipolar (one axon and one dendrite; found in the retina of the eye) and unipolar (cell body with one axon only; found usually in the embryonic stage). There are two types of axons — myelinated and non-myelinated. Myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon; the gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated fibres are found in spinal and cranial nerves. Non-myelinated nerve fibres are enclosed by a Schwann cell that does not form a myelin sheath; they are commonly found in autonomous and somatic neural systems.
Why it matters
A body with trillions of cells needs a fast wiring system that can sense, decide and command within milliseconds — that is the job of neurons organised into CNS and PNS.
NEET lens
Know: CNS = brain + spinal cord; PNS → somatic (to skeletal muscle) and autonomic (sympathetic, parasympathetic); afferent = to CNS, efferent = from CNS. Neuron parts; Nissl's granules in cell body and dendrites (free ribosomes and RER, protein synthesis); synaptic knob with vesicles of neurotransmitter. Multipolar — cerebral cortex; bipolar — retina; unipolar — embryonic stage. Myelin sheath formed by Schwann cells; nodes of Ranvier; myelinated fibres in spinal and cranial nerves.
Clinical significance
In multiple sclerosis the immune system damages myelin, slowing or blocking nerve impulses and causing weakness and vision problems. Local anaesthetics block impulse conduction in peripheral nerves so that a dentist can work without pain.
Introduction
The neural system is made of neurons and is organised into the CNS (brain and spinal cord) and the PNS (all other nerves). The neuron is its structural and functional unit.
Core concept
Divisions: CNS (brain, spinal cord) — information processing and control. PNS — afferent fibres (tissues → CNS), efferent fibres (CNS → tissues); somatic (CNS → skeletal muscles), autonomic (CNS → involuntary organs, smooth muscles) → sympathetic and parasympathetic; visceral nervous system = nerves, fibres, ganglia, plexuses between CNS and viscera. Neuron: cell body (organelles + Nissl's granules), dendrites (short, branched, contain Nissl's granules, carry impulses towards cell body), axon (long; branched distal end; synaptic knobs with synaptic vesicles containing neurotransmitters; carries impulses away from cell body). Types: multipolar — cerebral cortex; bipolar — retina; unipolar — embryonic stage. Myelinated axon: Schwann cells form myelin sheath; nodes of Ranvier between sheaths; in spinal and cranial nerves. Non-myelinated: Schwann cell without myelin; in autonomous and somatic neural systems.
Go deeper
Hydra has a network of neurons (a simple neural system); insects have a brain and several ganglia and neural tissues. Vertebrates show a much more developed system.
From Class 10
Class 10 showed a neuron with dendrites, cell body and axon. NEET adds Nissl's granules, myelin from Schwann cells and the divisions of the PNS.