Nerve Lecture 3 - GALALA UNIV. - PDF

MEDICAL PHYSIOLOGY FACULTY OF Physical Therapy FALL 2023  The Nerve (Part II) By Prof. Dr. Magdi Ali El-Damarawi Professor of Medical Physiology Faculty of Medicine 2 ...

MEDICAL PHYSIOLOGY FACULTY OF Physical Therapy FALL 2023  The Nerve (Part II) By Prof. Dr. Magdi Ali El-Damarawi Professor of Medical Physiology Faculty of Medicine 2  Intended Learning Outcomes - By the end of this lecture, you should be able to: - List the different types of nerve degeneration. - Recognize the time of nerve regeneration. - Mention the types and the characteristics of electric stimuli. - List the factors that determine the effectiveness of the stimulus. - Understand the strength-duration curve. - Define chronaxie, rheobase, and minimal time. - List the types of cell membrane ionic channels. - Understand the effect of subthreshold stimulus on the nerve cell membrane. 3  Degeneration Degeneration reaction (RD): (due to cutting of the nerve) Wallerian degeneration 3 types Retrograde degeneration Transneural degeneration (Rare)  Wallerian degeneration 1. It occurs in the distal fragments. 2. Starts 3-4 days after the cut. 3. There is degeneration of the axon and myelin sheath (their debris are removed by macrophages). 5 Retrograde degeneration 1. It occurs in the proximal part of the nerve. 2. It extends to the first node of Ranvier proximal to the injury. 3. It occurs also in the cell body. Regeneration - It starts after 20 days and is completed after 80 days from the cut. 6 Remarks on nerve stimulation 7  Types of electric stimuli 1. From Galvanic current: a) low intensity. b) longer duration. c) obtained from battery. 2. From Faradic current: a) high intensity b) shorter duration c) Alternative current. 8  Factors that determine the effectiveness of the stimulus 1. Intensity of the stimulus:- # Subthreshold stimulus- No response. # Threshold stimulus- Action potential (Response). # Suprathreshold stimulus:- i. In a single nerve fiber- the same response as threshold. ii. In a mixed nerve – greater response. 9 2. Duration of the stimulus:- - Any stimulus to give a response, it must be applied for a certain time called theutilization time. - As the stimulus intensity increases, this time will be shorter (within limits). - Strength-duration curve:- It represents the relation between the intensity of the stimulus and its duration of application. 10  Stimuli of too short duration could not excite whatever its intensity. Very weak stimuli can not produce stimulation whatever its duration. N.B.:- A. Rheobase: is the minimal intensity of galvanic current that can produce excitation. B. Chronaxie: is the time needed by a current intensity double the rheobase to produce excitation. It is a measure for the excitability. - Prolonged chronaxie means decreased the excitability and vice versa. C. Minimal time (T): It is the time below which no stimulation occurs whatever the intensity of the stimulus. 11 12  3. Rate of application of the stimulus: # if the rate of application rises rapidly, it will give a response. # if the rate of rise is slow, there will be no response due to accommodation of the nerve. - Mechanism:- due to activation of the sodium channels inactivation gates at the same time with the activation gates. # Stimulation of the nerve by constant (G) current gives response only on the make and break of the circuit. - Mechanism:- Accommodation of the nerve to the constant current. 13  Excitable vs non-excitable tissues Excitable non-excitable Example Nerve and muscles (Skeletal, Liver, pancreas, red blood cells cardiac, and smooth muscles.) (RBCs)….etc… Resting membrane potential - Present - Present (RMP) - About -70 to -90 mv - About -10 to – 30 mv On stimulation It develops action potential (AP) No action potential (AP) 14  Types of ionic channels in the cell membrane There are 2 types: 1. Leak channels (passive). 2. Gated channels (active). The leak channels: - They have no gates. - Always are opened. - They are watery pathways through protein molecules. - Most of them are highly selective due to its diameter and shape and the charges of ions that pass through it. - Examples: Sodium leak channels and potassium leak channels. 15  Gated channels: - They have gates (from outside or inside). - They are either: i. Voltage gated channels: - respond to changes in the voltage of the membrane potential. - Examples: Rapid voltage gated sodium channels and slow potassium voltage gated channels. ii. Ligand gated channels (Chemically stimulated): - it opens when its specific neurotransmitter or hormones bind to its specific receptors. 16  Effect of subthreshold stimulus Application of a subthreshold stimulus does not produce propagated action potential. It produces non-propagated potentials: i. Electrotonic potentials (Electrotonus). ii. Local response. 17  Electrotonic potentials (Electrotonus) Results from the application of subthreshold galvanic current (less than 7 mv). It includes 2 different changes: i. Catelectrotonus. ii. Anelectrotonus. 18  Catelectrotonus It is a state of partial depolarization in the region of the cathode. It is a passive process. The membrane potential at this site is reduced passively by the addition of negative charges by the cathode on the outer surface of the cell membrane. The excitability is increased and the chronaxie is decreased. 19  Anelectrotonus It is a state of hyperpolarization in the region of the anode. Also, it is a passive process. The membrane potential at this site is increased passively by the addition of positive charges by the anode on the outer surface of the cell membrane. The excitability is reduced and the chronaxie is increased. With strong currents, the excitability is lost completely at the anode (Anodal block) 20 Note that, There are three types of depolarization: - Catelectrotonus. - local response. - firing level. 21 Propagation of the action potential in unmyelinated nerve 22 23  Assignment 4 - Write a summary about the mechanism of action potential conduction in myelinated nerves. 24 25