Excitable tissue

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Excitable tissue

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Abstract

*CNS contains about 1011 neurons.
* The neuron is the basic building blocks of the N.S.
* Muscle cells→ contraction

Neurons → integration and transmission of nerve impulses
* C.N.S neurons have different shapes
a- 5 -7 processes → dendrites, extended outward from the cell body.
Dendrites have small knobby projections called dendritic spines.

b- Long fibrous axon; originate from some what thickened area of the cell body - the axon hillock.
-The first part of the axon → initial segment.
-The axon divides →terminal branches → each end in a number of synaptic knobs also called terminal buttons.
- They contain granules or vesicles contain synaptic transmitters (stored).
The axons of many neurons are myelinated i.e. acquire a sheath of myelin (protein -lipid complex)
* Outside the CNS, the myelin is produced by Schwann cells.
* the myelin sheath envelopes the axon except at its ending and at the nodes of Ranvier.

Nodes of Ranvier = periodic 1μm constrictions that are about 1mm apart.
-The loss of myelin is associated with delayed or blocked conduction in the demyelinated axons.

Introduction
Zones of neurons
Protein synthesis and axoplasmic transport
Excitation and conduction
Resting membrane potential
Refractory period
Distribution of ion channels in myelinated neurons
Nerve fiber types and function
Sequence of events during transmission
Nerve endings in smooth and cardiac muscle

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[...] Propagated disturbances action potentials or nerve impulses (the only electrical responses of neurons and other excitable tissues). * Due to changes in the conduction of ions across the cell membrane that are produced by alternations in ion channels. * The impulse normally transmitted (conducted) along the axon to its termination. * Conduction is an active, self propagation process and the impulse move along the nerve at a constant amplitude and velocity. -The electrical events are small measured in (ms). - The potential changes also are small measured in (mv). [...]

[...] * The Na+ conductance is short lived. The Na+ channels rapidly enter a closed state called the inactivated state for milliseconds. * The direction of the electrical gradient for Na+ is reversed during the overshoot because the membrane potential is reversed→ limits Na+ influx. * The direction of the electrical gradient for Na+ is reversed during the overshoot because the membrane potential is reversed→ limits Na+ influx. *Opening of voltage-gated channels efflux movement of +ve charge out of the cell→ completes the process of repolarization. [...]

[...] * The stimulus artifact is followed by an isopotential interval (latent period) that ends with the start of the A.P. =correspond to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrodes. Its duration is proportionate to the distance between the stimulating and recording electrodes and inversely proportionate to the speed of conduction. Beginning depolarization of the membrane:- * After an initial 15mv of depolarization, the rate of depolarization increases. [...]

[...] Will excite the nerve. During after depolarization the threshold is again decreased. During after-hyperpolarization, it is increased. Electrogenesis of the A.P:- * The nerve cell membrane is polarized at rest with +ve charge outside of cell membrane and -ve charges along the inside. * During the A.P. this polarity is reversed. * Myelin is an effective insulator and current flow through it negligible. * Depolarization in myelinated axons jumps from one node of Ranveir to the next. This jumping of depolarization from node to node is called salutatory conduction. [...]

[...] There also is a numerical system ( Ia, Ib, II, III,IV) to classify sensory fibers. Fibers differ Speed of conduction. Fiber diameter. Sensitivity to hypoxia and anesthetics: local anesthetics depress transmission in the group C fibers before they affect the touch fibers in As the axon supplying a skeletal muscle fiber approaches its termination, it loses its myelin sheath and divides into a number of terminal buttons or end-feet. The end-feet contain many small, clear vesicles that contain Acetylcholine (the transmitter at these junctions) the endings fit into depression, in the motor end plate = the thickened portion of the muscle membrane of the junction. [...]

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