Q 1. Define the following terms: (a) Neuron, (b) axon, (c) dendrite, (d) myelin sheath, (e) afferent neuron, (f) efferent neuron, (g) interneuron or associative neuron, (h) neurolemocyte, (i) neurofibril node, (j) resting membrane potential, (k) action potential, (I) generator potential, (m) synapse, (n) neurotransmitter, (o) reflex, (p) stimulus, (q) neurolemma, (r) neuroglia, (s) nucleus
Ans. (a) Neuron: Neuron or nerve cell is the basic functional unit of nervous system. It consists of a cell body with a nucleus, and its process.
(b) Axon: Axon is relatively long cylinerical process of a neuron that conducts impulses away from the cell body towards the synaptic terminals.
(c) Dendrite: Dendrites are thread-like extensions of the cell body of a neuron that conduct impulses towards the cell body.
(d) Myelin sheath: In the vertebrates and some complex invertebrates, the axon is often covered with a lipoprotein sheath called myelin sheath.
(e) Afferent neuron: Afferent neurons commonly called as sensory or receptor neurons are neurons which either act as receptors of stimuli themselves or are activated by receptors.
(f) Efferent neurons: Also called motor or effector neurons are those neurons that carry processed information via a signal (impulse) from central or peripheral nervous system to effectors such as gonads which in response secrete or muscles which in response contract.
(g) Interneuron or Associative neurons: These are neurons between a sensory neuron and a motor neuron. These function as integrating centres. These receive signals from sensory neurons and transmit them to motor neurons.
(h) Neurolemmocyte: Formerly called Schwann cells are cells that surround a fibre of a peripheral nerve and form the neurolemmal sheath and myelin.
(i) Neurofibril node: Formerly called as nodes of Ranvier are constrictions of myelenated nerve fibres at regular intervals at which myelin sheath is absent and the axon is enclosed only by cell process.
(j) Resting membrane potential: The difference in electrical charge between the inside and outside of the membrane at any given point is due to the relative numbers of the positive and negative ions in the fluid on either side of the membrane, and to the permeability of plasma membrane to these ions. The resting potential is about – 70 mV in a neuron, due to unequal distribution of Na+ ions on outside (high) and e and negative protein ions inside (highly concentrated).
(k) Action potential: It is the sequence of electrical changes when a nerve cell is exposed to a stimulus that exceeds its threshold. Along the neuron plasma membrane, it is a wave of depolarization and repolarizafion.
Generator potential: It is the gradual potential that travels only a short distance along the plasma membrane of a sensory cell.
Synapse: Synapse is the junction between the axon end of one neuron and the dendrite or cell body of another neuron or effector cell. Synapses can be electrical or chemical. In an electrical synapse, nerve impulses transmit directly from neuron to neuron when positively charged ions move from one neuron to the next. In chemical synapse, two cells communicate via neurotransmitter, which the presynaptic neuron releases.
(n) Neurotransmitter: Any chemical substance that the terminal end of an axon secretes that either stimulates or inhibits a muscle fibre contraction, or an impulse in another neuron, such as acetylcholine or epinephrine. More than 50 neurotransmitters are known.
(o) Reflex: A reflex is a predictable, involuntary response to a stimulus.
(p) Stimulus: A stimulus is any change or form of energy an animal can detect with its receptors.
Neurolemma: Neurolemma is the delicate nucleated outer sheath of a nerve cell; also called as sheath of Schwann.
(r) Neuroglia: Neuroglia are the supporting structures of nervous system (Virchow, 1854). It consists of a fine web of tissue made up of modified
ectodermal elements in which are enclosed peculiar branched cells known as neuroglial cells or glial cells. Some .types of glial cells play a role in myelin formation, transport of material to neurons, and maintenance of ionic environment of neurons.
(s) Nucleus: In neurophysiology usage, a nucleus is a small aggregation of nerve cell bodies within the central nervous system.
Q. 2. What is the difference between a nerve and a tract (nerve tract)?
Ant,. Nerve processes (usually axons) are bundled together in a well-formed wrapping of connective tissue called; a nerve in peripheral nervous system, and a tract or nerve tract in central nervous system.
Q. 3. Give two properties of neurons?
Ans. Neurons are specialized to produce signals (nerve impulses) that can be communicated over short to long distances, from one part of an animal’s body to another. Neurons have two important properties i.e. (1) excitability, and (2) conductivity, the ability to conduct a signal.
Q. 4. What are neurotoxins?
Ans. Neurotoxins are chemicals that are poisonous to or destroy nerve tissue, such as venom of cobras, sea snakes, and coral snakes.
Q. 5. What are neuroglial or glial cells? Why glial cells are much numerous in vertebrate brain? Offer examples of functions glial cells perform in the peripheral nervous system and in the central nervous system.
Ans. Neuroglial or glial cells are non-nervous cells that surround the neurons. These have a special relationship to neurons.Ghat cells are extremely numerous in the vertebrate brain, where they outnumber neurons (10: 1) and may form almost half the volume of brain. Some important functions of glial cells are:
(1) Some glial cells form intimate insulating sheaths of lipid-containing myelin around nerve .fibres. Vertebrate nerves are often enclosed by myelin, an insulating sheath laid down in concentric rings by special glial cells called Schwann cells in the peripheral nervous system, and oligodendrocytes in the central nervous system.
(2) Certain glial cells called astrocytes, because of their radiating, star like shape, serve as nutrient and ion reservoirs for neurons.
(3) Astrocytes serve as a scafolding during brain development, enabling migrating neurons to find their destinations from points of origin.
(4) Astrocytes, and smaller microglial cells, are essential for the regenerative process that follows a brain injury.
(5) Unfortunately, astrocytes also participate in several diseases of the nervous system, including Parkinsonism and multiple sclerosis.
Other functions are being determined