Acetylcholine (ACh) was the first neurotransmitter to be isolated and obtained by Otto Loewi in 1920 from the endings of parasympathetic neurons of the vagus nerve in frog heart. The two main neurotransmitters in vertebrate nervous system are acetylcholine (ACh) and noradrenaline although other neurotransmitters also exist. The post synaptic membrane contains large protein molecules which act as receptor sites for neurotransmitter and numerous channels and pores. This theory states that nerve impulse is an electro-chemical even governed by differential permeability of neurilemma to Na and \ across. Most accepted mechanism of nerve impulse conduction is ionic theory proposed by Hodgkin and Huxley. The transmission of excitation in a particular direction is called conductivity.ĭefinition of nerve impulse : A wave of reversed polarity or depolarization (action potential) moving down an axon is called a nerve impulse. (2) Conductivity : The excitation does not remain at the site of its origin. Excitation arises at the receptors on account of various stimuli such as light, temperature, chemical, electrical or pressure which constantly act on the organisms. (1) Excitability : It is the ability of the nerve cells and fibres to enter into an active state called the state of excitation in response to a stimulus. Main properties of nervous tissue : The nervous tissue has two outstanding properties excitability and conductivity. The latter are 0.1 mm to one or more (upto 10) metres long and about 0.025 m thick on an average. That is why, the term ‘ nerve fibres’ is usually applied to the axons. Thus, the reaction or response impulses are always carried to the effectors by axons. The axons are specialized as fibres conducting impulses away from the receiving parts. The dendrites and cytons usually constitute the impulse receiving parts which receive impulses directly from receptors, or from other adjacent neurons. Nerve fibres : Although, all parts of a neuron transmit excitations (= impulses), but the transmission is always unidirectional. Bottons of one neuron lie upon dendrites or cytons of adjacent neurons (figure), or upon muscle fibres or glands. Each telodendron bears a terminal knob or botton. The latter is terminally branched into short telodendria. Of these, muscle cells are further specialized for contraction while nerve cells are further specialized for receiving stimuli (as sensory or receptor cells) and transferring excitations from one to the other.Ī typical neuron consists of a nucleated cell body (= cyton, soma or perikaryon), five to seven short, slender and branched (= arborized) dendrites, and a single, relatively thicker and longer fibrous axon. However, the nerve cells and muscle fibres are specialized excitable cells of body, capable of transmitting or conducting excitations along their membranes. Consequently, every living cell becomes excited when stimulated. Nerve cells (= neurons) : Irritability is a basic characteristic of the “living substance”, i.e., the protoplasm.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |