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Excitable tissue. "Nerve and Muscle". Nerves and muscle are excitable structures, meaning that they have the ability to respond to stimuli. A stimulus is defined as a sudden change in the environment such a change can be: a. Mechanical e.g. tapping on the nerve fiber.
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Excitable tissue "Nerve and Muscle" Nerves and muscle are excitable structures, meaning that they have the ability to respond to stimuli.
A stimulusis defined as a sudden change in the environment such a change can be: a. Mechanical e.g. tapping on the nerve fiber. b. Chemical e.g. adding weak acids or alkalis. c. Thermal e.g. heating or cooling the nerve fiber. d. Electrical by using electrical current.
PHYSIOLOGY OF THE NERVE Neuron is made up of a cell body and cell processes I. The cell body: contains a large central nucleus, no centrosome indicating that nerve cells are not able for cell division
The cytoplasm of the neuron contains the following: a. Neurofibrils. b. Mitochondria. c. Nissl's granules. d. Golgi complex.
II. The cell processes: a. The dendrites: short branches conduct impulses towards the cell body. b. The axon or nerve fiber: a long branch conducts nerve impulses away from the cell body.
Types of nerve fibers:are of 2 types. • Unmyelinatednerve fiber: has a neurilemma, which is essential for nerve regeneration. • Myelinatednerve fiber: the axon is surrounded with a sheath of protein-lipid complex called themyelin sheath, in addition to the outer neurilemma.
The myelin sheath is interrupted by constrictions called the nodes of Ranvier. • The myelin sheath is insulator for electrical changes.
Resting membrane potential (RMP) • Is the potentialdifference between the outer and inner surfaces of the cellmembrane during the resting state. • a polarized state: This denotes that the outside of the cell is positively charged relative to the inside.
The RMP is about -70 mv in nerve and -85 mv in skeletal muscle Resting membrane potential (RMP)
Causes of the resting membrane potential is unequal distribution of electrically charged ions : cations at the outer surface and anions on the inner surface of the cell membrane due to: • Selective permeability of the membrane. • Sodium-potassium pump:transports Na+ (3 ions) out of the cell and K+ (2 ions) into the cell.
Changes occurring in the nerve on stimulation I- Electrical changes: (Action potential, spike and after potentials) The action potential is the change in the membrane potential of the nerve fiber after its excitation by an effective stimulus.
The action potential (A.P.) includes the following 2 stages: (a) Stage of depolarization: Is due to Na+ influx, decreases the membrane potential and this is known asdepolarization of the membrane. At -50mv firing level The peak is +35 mv the magnitude of the A.P. is 70 mv + 35 mv = 105 mv.
(b) Stage of repolarization More K+ outflux with no Na+ entry, so the membrane potential rapidly falls towards the resting level. Action potential and after potential
(2) Conduction in myelinated nerve fiber (salutatory conduction): • the myelin sheath is highly insulator to ions exchanges. • atnodes of Ranvier the myelin sheath is absent. • depolarization occurs at node of Ranvier and conduction occurs by local current flow between the nodes and the next one. • salutatory conduction means jumping of the A.P. from one node of Ranvier to the next.
