Neural Control and Coordination

Vagus nerve arises from the side of medulla oblongata. It innervates the larynx, trachea, oesophagus, stomach, lungs, heart and intestines. It is a mixed nerve. It controls the visceral sensations and visceral movements, i.e., heart beat, respiratory movements, peristalsis, sound production, etc. Movement of the tongue is controlled by hypoglossal nerve as it innervates the muscles of the tongue.

In a resting state of the neural membrane, Na⁺ concentration is higher on the outer side and K⁺ concentration is more within the cell. This concentration gradient is maintained by voltage gated channels. Hence if diffusion is allowed Na⁺ would enter the cell and K⁺ would leave.

Cell body of a nerve cell contains basophilic granules called Nissl’s granules. These granules appear to be cisternae of rough endoplasmic reticulum with numerous attached and free ribosomes. They probably synthesize proteins for the cell.

Neurons are the chief functional units of the nervous system. An ordinary neuron has a soma or cyton and a long thread called axon which is enclosed in a multilayered myelin sheath, made by Schwann cells. The myelin sheath is interrupted at the spaces between schwann cells to form gaps. These gaps are called Nodes of Ranvier. These nodes and the myelin sheath create condition that speed up the nerve impulses.

It takes some time for rhodopsin to split into scotopsin and retinal (bleading) and release of transmitter passing nerve inpulse via bipolar and ganglion cells to the optic nerves.

This is a case of adaptation. It differs from accomodations which is a reflex mechanism by which the focus of the eye change to make the images of distant and near objects sharp on the retina.

Cornea forms the anterior one-sixth of the fibrous coat. It is transparent, circular and fully visible from infront. It is composed of a peculiar variety of connective tissue covered externally by stratified nonkeratinized squamous epithelium and internally by simple squamous epithelium. It lacks blood vessels. It is nourished by lymph from adjacent area.

Depolarization of a nerve cell membrane occurs during the passage of an action potential along the axon where the nerve is transmitting an impulse. During depolarization, the activation gates of Na⁺ channels open and the K⁺ channels remain closed. Na⁺ rush into the axon. Entry of sodium ions leads to depolarization (reversal of polarity) of the nerve membrane, so that the nerve fibre contents become electropositive with respect to the extracellular fluid.