Explain the process of conduction of a nerve impulse along a nerve fibre.

1. Neural System:

(i) The neuroendocrine system regulates, integrates, and coordinates all body functions.

(ii) Nervous System is found only in animals and absent in plants.

(iii) Nervous tissue is a specialized tissue with the property of excitability and conductivity. 

(iv) The nervous tissue is of ectodermal origin.

(v) Nervous System comprises two different cell types: neurons and glial cells.

(vi) Neurons are the functional units of the nervous system.

(v) Glial cells are the supporting cells.

2. Neuron as Structural and Functional Unit of Neural System:

(i) Nerve impulses are electrical signals produced by the plasma membrane of a neuron.

(ii) Conduction of nerve impulses is an electrochemical process.

(iii) Membrane of a non-conducting neuron is positive on the outside and negative inside.

(iv) The difference in charge is called resting potential.

(v) The nerve impulse is conducted along the axon membrane in the form of a wave of depolarisation and repolarisation.

(vi) In white fibres or myelinated fibres, the nerve impulse jumps from one node of Ranvier to next and is called saltatory propagation, which increases the speed of the impulse.

(vii) The junction between two neurons is called a synapse.

(viii) A synapse is formed by the membranes of a presynaptic neuron and a postsynaptic neuron which may or may not be separated by a gap called synaptic cleft.

(ix) Chemicals involved in transmitting impulses at chemical synapses are neurotransmitters.

(x) Acetylcholine is the most common and the best-studied neurotransmitter.

3. Human Neural System:

(i) Human nervous system consists of two main subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

(ii) The central nervous system is composed of the brain and the spinal cord.

(iii) The brain and spinal cord are covered by three connective tissue membranes called meninges: dura mater, arachnoid mater, and pia mater.

(iv) Fluid in and around the brain and spinal cord is known as cerebrospinal fluid.

(v) The main function of cerebrospinal fluid is to act as a cushion and shock absorber to the brain and spinal cord.

4. Central Neural System:

(i) The brain may be divided into forebrain, midbrain and hindbrain.

(ii) The forebrain consists of cerebrum, thalamus and hypothalamus.

(iii) The cerebrum is the largest part of the human brain.

(iv) The cerebrum is longitudinally divided into two cerebral hemispheres that are connected by the corpus callosum.

(v) The midbrain receives and integrates visual, tactile and auditory inputs.

(vi) The hindbrain comprises pons, cerebellum and medulla.

(vii) The cerebellum or the “little cerebrum”, the second-largest part of the brain.

(viii) The cerebellum controls involuntary muscular coordination, maintains posture, orientation and equilibrium of the body.

(ix) The medulla contains centres, which control respiration, cardiovascular reflexes and gastric secretions.

(x) The cavity of the spinal cord is known as the central canal or neurocoel.

(xi) A nerve carries many nerve fibres and based on the nature of fibres, there are three types of nerves: sensory or afferent, motor or efferent and mixed nerve.

(xii) In human beings, there are $12$ pairs of cranial nerves.

(xiii) The number of spinal nerves in man is $31$ pairs.

5. Reflex Action and Reflex Arc:

(i) Reflex action is an immediate involuntary response to a stimulus without the intervention of the brain, controlled by the spinal cord.

(ii) Reflex arc is the arrangement of neurons in the pathway that always pass through the central nervous system and is formed by receptor-spinal cord-effector (muscle or gland).

(iii) Autonomic nervous system consists of two divisions: sympathetic and parasympathetic.

(iv) Sympathetic and parasympathetic exert opposite influences on the same organs or set of organs.

6. Sensory Reception and Processing:

(i) Meissner’s corpuscles are found in the dermis of the skin of the fingertip, lips, and nipples; these senses of touch and gentle pressure.

(ii) Organs for special senses are eyes, ears, nose, and tongue.

(iii) The retina is the innermost layer of the wall of the eye.

(iv) The primary function of the retina is image formation.

(v) Photoreceptor neurons consist of rods and cones; rods are longer, slender, and filamentous; cones are shorter and thicker.

(vi) The blindspot (optic disc) is the point from where the optic nerves emerge.

(vii) Blindspot in the eye is where neither rods nor cones are present.

(viii) The ear can be divided into the outer ear, the middle ear, and the inner ear.

(ix) The middle ear contains three ossicles called the malleus (hammer), incus (anvil), and stapes (stirrup).

(x) The passage connecting the middle ear with the pharynx is the eustachian tube.

(xi) The internal ear contains the organs of hearing and balance and consists of two main divisions: bony labyrinth and membranous labyrinth.

(xii) Corti's organ contains hair cells that act as auditory receptors and are located on the basilar membrane.

(xiii) Mechanism of hearing involves receiving the sound wave by the external ear and directing them to the eardrum. The vibration of the eardrum leads to the vibration of the ear ossicles. The vibration reaches the cochlea which generates waves in the lymph. The waves generate ripples in basilar membrane and hair cells in them. As a result, nerve impulses are generated in an afferent neuron that passes to the brain via auditory nerves.

(xiv) The nose performs two functions: respiration and olfaction.

(xv) Olfactory receptor cells function as chemoreceptors.

(xvi) Tongue is a gustatory receptor, and receptors for gustatory sensations are located in the taste buds.