Lecture 4 RESTING MEMB POTENTIAL - Functions of Body Functions and Biophysics PDF

Foundations of Body Functions and Biophysics Neuronal Membrane and Resting Membrane in Neurons Prof Dr. Abdelaziz M. Hussein Prof and Chairman of Medical Physiology Department Lecture Objectives At the end of this lecture you will be able to...

Foundations of Body Functions and Biophysics Neuronal Membrane and Resting Membrane in Neurons Prof Dr. Abdelaziz M. Hussein Prof and Chairman of Medical Physiology Department Lecture Objectives At the end of this lecture you will be able to: 1. Identify types and functions of neuroglial cells 2. Recognize the structure and types of neurons 3. Define the resting membrane potentials 4. Explain how to calculate and measure the resting membrane potential 5. Explain the mechanisms of development of RMP in neurons Contents 01 Types of neuroglial cells and their functions 02 Definition, structure and types of neurons 03 Types of membrane potentials in neurons 04 Definition, calculation and measurement of RMP 05 Causes of RMP in neurons  Central Nervous System (CNS) 12 pairs Peripheral Nervous System (PNS) 31 pairs 5  Neurons (Nerve cells) Neuroglial Cells Structural units of the NS Support and protect the neurons 100 billion neurons 10 to 50 times No. of neurons Types of neurons: Sensory , motor ,and interneuron 6 Def - Neurons are the structural units of the nervous system. No. - Is about 100 billion (109) in the nervous system Structure of Neuron: - It is formed of 4 parts; i) Cell body ii) Axon iii) Dendrites iv) Presynaptic terminals 7  The nerve (or nerve trunk) is composed of a larger number of nerve fibers  Each nerve fiber is an axon covered by a myelin sheath and a Schwann sheath  The only function of nerve fibers is the conduction of nerve impulses 8  Excitability Conductivity It is the ability of the It is the ability of the nerve nerve fibers to respond fibers to conduct nerve to stimuli impulses from one site to another 9 Def, Excitability is a bioelectric phenomenon in which the nerve fibers respond to stimuli by rapid changes in their resting membrane potential and conduct action potentials along the nerve fibers to their terminals 10 1) Resting membrane potential (RMP): -Is the potential difference between outside and inside of the nerve fiber during rest -Normally, it is about – 70 mV 2) Graded potential: -Is a local change in the resting membrane potential of nerve fibers e.g. a)Receptor potential in receptor region b)Postsynaptic potentials (PSPs) in dendrites or cell body 3) Action potential: -Is the electrical changes that occur in resting membrane potential as a result of stimulation by an effective stimulus. 11 Def., - It is the potential difference between outside and inside of the nerve fiber during rest. Value: - Is negative because the inner surface of the cell membrane is negatively charged relative to the interstitial fluid - Its value varies according to the excitable tissue: o Nerves: -70 mV. o Skeletal muscles: -90 mV. o Cardiac muscle: -90 mV. o Smooth muscles: -60 mV. 12 Measurement of RMP - By two microelectrodes with very fine tips less than 1 µm connected with a special voltmeter. - One electrode is placed on the surface and the 2nd is inserted in the nerve fibers 13  Causes of RMP 1. Distribution of Ions 2. Selective permeability 3. Na-K pump Charge separation 14 15 1) Distribution of ions inside and outside the cell: ECF: ICF ratio for Na+ ions is 10: 1 ECF: ICF ratio for K+ ions is 1: 35 ECF: ICF ratio for Cl- ions is 25: 1 16 2) Selective permeability of the cell membrane - Cell membrane contains ion channels allow only specific ions to pass while blocking others because of their size, charge, or state of hydration. - There are 3 types of channels (leak or passive, voltage gated and ligand gated channels that allow diffusion of ions - Diffusion of ions thorough leak channels is the cause of RMP 17 Diffusion of ions through the cell membrane: Cell membrane offers absolute barrier to proteins. In the resting neuron, a) K+ diffusion: (the main cause of RMP) K+ ions diffuse from inside the cell to outside due to: 1) Concentration gradient: (inside 30-40 times > outside). 2) High permeability of the membrane to K ions (the cell membrane is 100 times more permeable to K ions than for Na ions. K+ ions diffusion continues until positive charges outside the membrane repel the outflow of more K+. 18 b) Na+ and Cl diffusion Na+ ions tend to diffuse from outside to inside due to: 1. Concentration gradient (outside > inside 10 times) 2. Electric gradient (inside is negative) Na+ ions pass through the passive Na+ channels with difficulty Cl- ions tend to diffuse from outside to inside due to its concentration gradient (outside > inside 25 times). This diffusion is prevented by repulsion force by negative charges inside the cell 19  K+ K+ K+ K+ ECF + + + + ++ + + + + + + + + + + + + + + + + + + + + ------------------------------------ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Pr- Pr- Pr- Pr- Pr- ICF 20 3) Na+ - K+ Pump: - Na+-K+ ATPase or pump is electrogenic pump that pumps 3 Na+ ions outside the cells and 2 K+ ions inside the cell. - To maintain a high concentration of Na+ ion outside, and a high concentration of K+ ion inside the nerve cells. - This occurs in spite of the continuous diffusion of these ions across the membrane. - Any condition decreases the metabolic activity of the cell e.g. by cooling will inhibits Na-K pump, so, Na+ ions will accumulate inside the cell and neutralize the -ve charges of protein ions. - K ions that held on the outer surface escape away, and RMP becomes progressively lost. 21  90-95 % of RMP is a passive process due to diffusion of ions thorough leak ion channels 5-10% of RMP is active process due to the electrogenic Na-K pump i.e. about 4-5 mV  Neuroglial cells support and protect neurons  Neurons are about 100 billions in NS  Nerve fibers have 2 properties; excitability and Summary & Wrap up conductivity  Membrane potentials in nerve fibers include RMP, graded potentials and action potentials  RMP in nerve fibers is about – 70 mV  RMP is caused by distribution of ions, selective permeability and Na-K pump  Questions 1. Sensory (afferent) nerve fibers a) Are about 99% of neurons b) Carry impulses from receptors to central nervous system c) Carry impulses from central nervous system to 3. The resting membrane potential organs in nerve fibers is d) Have integrative functions a) – 70 mV e) Located mainly inside the CNS b) – 90 mV c) – 10 mV 2. Main cause of resting membrane potential is : d) – 120 mV a) Na/k pump e) – 30 mV b) high Na concentration outside c) Unequal distribution of ions d) K diffusion through leak channel e) High intracellular k ion  References 1. Costanzo, Linda S. "BRS Physiology (Board Review Series)." (2018). 2. Ganong, William F. "Review of medical physiology." (2020). Discussion and Feedback THANK YOU