Nerve Physiology Lecture Notes PDF

Lecture(4) Nerve Physiology Prof. Dr. Nanees El-Malkey Assistant professor, Sinai University sinaiuniversity.net  Item no.1: general organization of nervous system Item no.2: types of neurons Item no.3: action potential INDEX...

Lecture(4) Nerve Physiology Prof. Dr. Nanees El-Malkey Assistant professor, Sinai University sinaiuniversity.net  Item no.1: general organization of nervous system Item no.2: types of neurons Item no.3: action potential INDEX @Sinaiunieg [email protected] www.su.edu.eg  It is the structural (anatomical) unit of the nervous system @Sinaiunieg [email protected] www.su.edu.eg  Classification of neurons 1. According to myelination: A. Myelinated nerve fiber. Myelinated B. Non-myelinated Non-myelinated @Sinaiunieg [email protected] www.su.edu.eg 2. According to the function @Sinaiunieg [email protected] www.su.edu.eg  Define Afferent: Carries sensations from tissues towards the central nervous system (CNS). Efferent: Carries orders from CNS to the tissues. Interneuron: a neuron which transmits impulses between other neurons @Sinaiunieg [email protected] www.su.edu.eg Anatomical unit of Nervous system: Neurons Functional unit of nervous system: Reflex action @Sinaiunieg [email protected] www.su.edu.eg Reflex action: ✓ It is the involuntary response to adequate stimulus. Reflex arc: 1. Receptor: detects stimuli. 2. Afferent (sensory neuron): 1. receptor 3. Center of the reflex. 2. Afferent neuron 4. Efferent (motor neuron): 3. Inter neuron. 5. Effector organ: a muscle or a gland 5. Effector organ 4.efferent 1. Receptor: detects stimuli. 2. Afferent (sensory neuron): 3. Center of the reflex. 4. Efferent (motor neuron):. 5. Effector organ: a muscle or a gland  Excitability It is the ability of the living tissue to respond to an adequate stimulus. The stimulus: It is the change in the environment of the living tissue. @Sinaiunieg [email protected] www.su.edu.eg  Types of stimuli According to its nature: a. Electrical b. Chemical c. Mechanical d. Thermal According to its intensity: a.Threshold stimulus. b. Subthreshold c.Suprathreshold stimulus @Sinaiunieg [email protected] www.su.edu.eg  Threshold stimulus is the minimal stimulus intensity which produces a response in the excitable tissues (i.e. the intensity is adequate). @Sinaiunieg [email protected] www.su.edu.eg  Suprathreshold stimulus ub threshold  Nerve fiber Nerve trunk Subthreshold No response No response Threshold Full response Not full response Suprathreshold Full response Full response  Subthreshold stimulus → produces no response (i.e. the intensity is not adequate). Suprathreshold stimulus → produces full repone in the single nerve fiber and nerve trunk Threshold stimulus produce full response in single neuron only threshold stimulus cannot produce full response in nerve trunk @Sinaiuni [email protected] www.su.edu. All or none law: When a single nerve fiber is stimulated, it either gives a full response or no response at all. The Resting Membrane Potential (RMP) Define resting membrane potential  Define resting membrane potential It is the potential difference between inside and outside the cell membrane with inside relatively negative to outside. @Sinaiunieg [email protected] www.su.edu.eg  Ion distribution across cell membrane: outside inside ❖Na+ (142 mM), + K (140 mM) ❖ Ca+2 (110 mM), +2 Ca (4 mM) - Cl (30 mM) ❖Cl- (103 mM), ❖HCO3 - (28 mM). protein- (40 mM).  It is not equal in all type of cells : –9 to –100 mV. ✓ In RBCs: -30 mV. ✓ In nerve cells (neurons-70 to –90 mV ✓ In the muscle: –90 mV.  Causes of RMP unequal distribution of ions across the cell membrane Selective permeability of the cell membrane Na+ - K+ pump The membrane is impermeable to the intracellular protein anions  Causes of unequal ion distribution across the cell membrane [causes of RMP] [I] Selective permeability of the cell membrane: two types of channels: A-Leakage channels (Passive): B-Gated channels (Active):  Leakage gated passive active Not gated Has gates Opened all the time. Open during activation of cells K+ permeability is 50-100 2 types: times greater than Na+ --Voltage-gated channels. permeability --Chemical (ligand)-gated channels  Causes of RMP unequal distribution of ions across the cell membrane Selective permeability of the cell membrane Na+ - K+ pump The membrane is impermeable to the intracellular protein anions  [II] Na+ - K+ pump: pump Na+ outside : against concentration and electrical gradient. pump K+ inside the cell against the concentration gradient only.  [III] The membrane is impermeable to the intracellular protein anions with large molecular weight → more negative charges inside the cell. Selective permeability of the cell membrane Na+ - K+ pump The membrane is impermeable to the intracellular protein anions  Action potential and its ionic basis It is a transient change in the resting membrane potential as a result of application of a threshold stimulus. @Sinaiunieg [email protected] www.su.edu.eg  Phases of action potential 1. Latent period 2. Spike potential Depolarization Repolarization @Sinaiunieg [email protected] www.su.edu.eg  Latent period: It represents the time that the nerve impulse (response) takes to travel from the stimulating to recording electrode. @Sinaiunieg [email protected] www.su.edu.eg  Spike potential The membrane potential rises from RMP to a high level, then returns back to RMP in two main steps: a.Depolarization(ascending limb): ✓Excessive Na+ influx by activation of Na+ voltage gates. @Sinaiunieg [email protected] www.su.edu.eg  ✓ steps of repolarization: return of the membrane potential to the resting state i. Rapid repolarization ii.Negative after potential (after depolarization) iii. Positive after potential (after hyperpolarization):. @Sinaiunieg [email protected] www.su.edu.eg  i. Rapid repolarization: During which the membrane restores 70% of its resting condition. Causes: A) Inactivation of Na+ gates ( permeability to inside). B) Activation of K+ gates ( permeability to outside).  ii. Negative after potential (after depolarization): the rate of repolarization becomes slow. ????????? Decrease in K+ gradient → slow K+ efflux → delayed repolarization. @Sinaiunieg [email protected] www.su.edu.eg  ii. Positive after potential (hyperpolarization): Overshooting of membrane potential about 1- 2 mV beyond RMP, then the membranes return to normal RMP.???? Delayed K+ channels closure → more K+ efflux → more hyperpolarization @Sinaiunieg [email protected] www.su.edu.eg hyperpolarization  Na+ - K + pump restores the normal ionic distribution of the RMP Maintenance of Na+ (extra cellular) and K+ (intracellular). @Sinaiunieg [email protected] www.su.edu.eg  Ionic basis of Action potential 1.During depolarization: Changes in voltage-gated fast Na+ channels: ❖ Outer gate (activation gate): opens at the start of depolarization causing Na+ influx. Na+ ❖ Inner gate (inactivation gate): closes at the end of depolarization. Na+ During rest: the activation gate is closed & the inactivation gate is opened → no Na+ permeability.  1.During repolarization: ❖Changes in voltage-gated K+ channels: K+ channel has a single inner gate ▪ Na+ ❖During rest: the gate is closed. ❖During activation: slow opening of K+ channels → repolarization. @Sinaiunieg [email protected] www.su.edu.eg  Excitability changes during action potential 1. Absolute refractory period: [depolarization]: No response to any stimulus (loss excitability) 2. Relative refractory period: [upper part of repolarization]. ▪ Stronger stimulus → response (low excitability). 3. Supernormal phase: [lower part of repolarization [negative after potential] ▪ Weak stimulus → response (high excitability). 3. Subnormal phase: [positive after potential]. ▪ Stronger stimulus → response (low excitability). @Sinaiunieg [email protected] www.su.edu.eg  Wrap up 9. Neurons either Myelinated nerve fiber or Non myelinated. 10. Neurons either afferent, efferent or interneurons. 11. Reflex arc: receptor→→ afferent →→ center →→ efferent →→ effector organ. @Sinaiunieg [email protected] www.su.edu.eg Wrap up 12. Excitability: It is the ability of the living tissue to respond to an adequate stimulus. 13. Adequate stimulus= give response = threshold stimulus. 14. Excitable tissue has RMP. 15. RMP: It is the potential difference between inside and outside the cell membrane with inside relatively negative to outside Wrap up 16. Action potential: It is a transient change in the resting membrane potential as a result of application of a threshold stimulus. 17. Phases of action potential: latent period, depolarization, repolarization @Sinaiunieg [email protected] www.su.edu.eg Wrap up 1.Nervous system is composed of: CNS & PNS. 2.CNS is composed of: brain and spinal cord. 3.PNS is composed of spinal and cranial nerves. @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg THANK YOU Dr. Nanees El-Malkey Assistant professor, Faculty of Physical Therapy Sinai University