Short Notes and Formulas of class 8 science chapter-Nervous System
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Short Notes and Formulas of class 8 science chapter-Nervous System
May 13, 2022, 16:45 IST
Class- 8 science chapter-Nervous System
Except sponges, all multicellular animals possess simple or complex nervous system. In all these animals, nervous system is comprised of nervous tissue having specialized cells called neurons or nerve cells to respond to stimuli and coordinate animals' activities. Nerve cells or neurons are, in fact, the structural and functional units of nervous system. In higher multicellular animals, the nervous tissue consists of nerve cells or neurons, nerve fibres, bundle of nerve fibres forming nerves, packing cells (neuroglia), ependymal cells and neurosecretory cells.
Structure of Neuron
The units which make up the nervous system are called nerve cells or neurons. Neuron is the largest cell in the body. It carries messages in the form of electrical signals called nerve impulses. Neuron is an elongated branched cell having three components -cell body, dendrites and axon.
- Cell body or Cyton: Cell body is like a typical cell containing a central nucleus and surrounding cytoplasm. Around the nucleus there are granules called Nissl's granules. The cytoplasm has mitochondria, Golgi apparatus, neurofibrils, neurotubules. Cell body is concerned with metabolic maintenance and growth. It also receives nerve impulses from dendrites and transmits them to axon
- Dendrites: Dendrites are several short, tapering, much branched protoplasmic processes stretching out from the cell body of a neuron. They receive sensation or stimulus, which may be physical, chemical, mechanical or electrical. The stimulus is passed onto cyton.
- Axon: Axon is the longest part of the neuron. It is a single, elongated fibre arising from one side of cyton. It conducts impulses away from the cell body
Types of Neurons :
The neurons are of three types:
- Sensory (receptor) neurons,
- Motor (effector) neurons, and
- Relaying (connector) neurons.
Sensory (receptor) neurons: These often occur in sense organs, and receive stimuli by their dendrites.
The sensory neurons transmit impulses towards the central nervous system (brain and spinal cord) through their axons.
Motor (effector) neurons : The dendrites of these neurons synapse with axons of sensory neurons in central nervous system. They tranmit impluse from central nervous system towords effectors. Latter respond to stimuli.
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