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Describe the structure of Neuron.
The structural and functional unit of the nervous system is the nerve cell or neuron. Approximately 100 billion neuron are present in the nervous system.They are highly differentiated cells and do not contain centriole ( essential for cell division). Therefore, they cannot divide or undergo mitotic division. Neuron consists of the following parts:
Cell body/Perikaryon
- It is that part of the neuron which contains nucleus & most of the cytoplasm with cell organelles.
- The nucleus is vesicular with prominent nucleolus.
- The cytoplasm contains basophilic Nissl substance (Nissl granules – made up of aggregations of rough endoplasmic reticulum).
- Nissl substance is absent in the axon hillock (beginning of axon).
- Cell bodies of neurons in Central nervous system (CNS) form the Gray matter or Nuclei.
- Cell bodies of neurons in Peripheral nervous system form the sensory or autonomic ganglion.
Dendrites
- These are short, branched, tapering processes spreading out from the cell body.
- They receive impulses from other neurons and sensory receptors and carry them towards the cell body or axon.
Axon or Nerve Fiber
- It is the long, thin process of a neuron. It carries nerve impulses away from the cell body.
- The initial segment is called Axon hillock which is devoid of Nissle substance.
- It may have one or more side branches, called axon collaterals along its course.
- Each axon ends in a number of short, fine branches called telodendria/axon terminals.
- The enlarged tips of the axon terminals are the terminal boutons, which form synapses with other neurons, muscles or glands and contains vesicles filled with neurotransmitters.
- Bundles of axons of neurons in the Central nervous system are located in the white matter and are called tracts, fasciculus or lemniscus.
- Bundles of axons of neurons in Peripheral nervous system are called nerves.
Classify Neurons (Types of Neurons).
Neurons are classified into various types :
1. According to the shape and number of processes:
- Pseudounipolar neuron: single process arises from the cell body which divides to form dendrite and axon ( central and peripheral process). e.g. dorsal root ganglion cells of the spinal cord.
- Bipolar neuron: two processes arise from the cell body, one from each pole. One of them is dendrite, the other axon. e. g. bipolar neurons in the retina, olfactory epithelium and vestibulo-chochlear ganglia.
- Multipolar neuron : several dendrites and one axon arise from the cell body. e.g. pyramidal cells of cerebral cortex, motor neurons of anterior horn of spinal cord.
2. According to the Function
- Sensory or afferent neuron: neurons that transmit impulses from a sensory receptor to the CNS. e.g. dorsal root ganglion cells of spinal cord.
- Motor or efferent neurons : neurons that transmit impulses from the CNS to an effector organ (muscle or gland). e.g. pyramidal cells of cerebral cortex.
- Interneurons: These are located entirely within the CNS and synapse with other neurons.
3.According to the length of the axon
- Golgi Type I neuron: These neuorns have a long axon (e.g. pyramidal cells of cerebral cortex, Purkinjee cells of cerebellar cortex).
- Golgi type II neurons: They have short axons or no axon at all.( e.g. interneurons).