Nerve and Muscle By Dr Abdel Aziz M

Nerve and Muscle By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology

Nerve and muscle , Abdelaziz Hussein 3

Nerve and muscle , Abdelaziz Hussein 4

Organization of Human Body • The human body is made up of different systems e. g. digestive system, whose functions are coordinated and integrated together. • Each system consists of many organs that made up of many tissues of complementary functions. • Each tissue consists of millions of similar cells. • The cell is the basic unit of structure and function in the body Nerve and muscle , Abdelaziz Hussein 5

Nerve and muscle , Abdelaziz Hussein 6

Nerve and muscle , Abdelaziz Hussein 7

Nervous System • Nervous system is divided into; • i) Central NS→consists of the brain and the spinal cord. • ii) Peripheral NS→ consists of; • 12 pairs of cranial nerves arising from the brain. • 31 pairs of spinal nerves arising from the spinal cord. Nerve and muscle , Abdelaziz Hussein 8

Nerve and muscle , Abdelaziz Hussein 9

Functions of NS Nerve and muscle , Abdelaziz Hussein 10

Functions of NS • 1 -Sensory functions: • It receives the sensory stimuli coming from skin, viscera and special sense organs (eye, nose, and ear). • 2 -Motor functions: • It controls the activity of skeletal ms, smooth ms and cardiac ms. • 3 -Higher mental functions: • As learning and memory. Nerve and muscle , Abdelaziz Hussein 11

Types of cells in NS • NS is composed of 2 types of cells: Neurons (Nerve cells) Neuroglial cells Def. They are the structural They support and functional units of protect the neurons the NS No. one trillion neurons 10 to 50 times No. of neurons Nerve and muscle , Abdelaziz Hussein 12

Types of cells in NS Nerve and muscle , Abdelaziz Hussein 13

Neuron (Nerve Cell) • 1) Def. , • It is the structural and functional unit of the nervous system. • 2) No. • One trillion neurons • 3) Classification of neurons: • a) Histological classification: • Neurons are either multipolar, bipolar and unipolar neurons Nerve and muscle , Abdelaziz Hussein 14

Neuron (Nerve Cell) • b) Functional (physiological) classification: • i) Sensory (afferent) neurons: • They conduct impulses from receptors to the CNS. • ii) Motor (efferent) neurons: • They conduct impulses from the CNS to the effector organs. • iii) Interneurons: • They are small neurons. • They serve the integrative functions of the NS. • They are about 99% of all neurons. Nerve and muscle , Abdelaziz Hussein 15

Structure of Neuron • 4) Structure of neuron: • It is formed of cell body and cell processes Nerve and muscle , Abdelaziz Hussein 16

Cell Body or Soma • The cell body is the enlarged part of the neuron, which contains the nucleus. • It controls the metabolic processes and provides nutrition for the whole neuron. • It contains; mtiochondria, microtubules and microfilaments etc Nerve and muscle , Abdelaziz Hussein 17

Cell Processes • There are 2 cell processes; Axon (Axis Definition Functions Conduction direction cylinder) Single longer process Conduction of impulses. Dendrites Multiple short processes -↑ surface area of the cell body - Collection of impulses and transmit them to the cell body. away from the soma toward the cell body Nerve and muscle , Abdelaziz Hussein 18

Cell Processes • There are 2 cell processes; Nerve and muscle , Abdelaziz Hussein 19

Axonal Sheaths • There are two sheaths that cover the axons: Nerve and muscle , Abdelaziz Hussein 20

Axonal Transport • Transport of materials in the axons can be done by 2 mechansims; Slow axonal transport Fast axonal transport Speed 1 mm/day. 400 mm/day. Mechanism done by protoplasmic streaming done through the microtubules Transports new cytoplasm and proteins Materials (nutrients and metabolites) Direction from the cell body toward the be either away from the cell axon terminals i. e. one direction body or towards it i. e. 2 directions Importance Regeneration of axons and Nutrition and removal of renewing the cytoplasm of the metabolites axon. Nerve and muscle , Abdelaziz Hussein 21

Nerve Fibers • 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. Nerve and muscle , Abdelaziz Hussein 22

Properties of nerve fibers • The nerve fibers have 2 properties, 1. Excitability is the ability of the nerve fibers to respond to stimuli, and convert these stimuli into nerve impulses 2. Conductivity is the ability of the nerve fibers to conduct nerve impulses from one site to another Nerve and muscle , Abdelaziz Hussein 23

Stimulus • 1) Def. • It is a change in the surrounding environment (either external or internal environment) • 2) Types of stimuli: • a) Electrical stimuli: • b) Chemical stimuli • c) Physical stimuli Nerve and muscle , Abdelaziz Hussein 24

Factors that determine the effectiveness of the stimulus • a) Strength of the stimulus Nerve and muscle , Abdelaziz Hussein 25

Factors that determine the effectiveness of the stimulus • b) Duration • -The stimulus to be effective must act for a certain length of time known as the excitation time. Within limit, the stronger the stimulus, the shorter is excitation time Nerve and muscle , Abdelaziz Hussein 26

Strength and Duration curve (Excitability curve) Nerve and muscle , Abdelaziz Hussein 27

Factors that determine the effectiveness of the stimulus • c) Rate of rise of intensity Nerve and muscle , Abdelaziz Hussein 28

Nature of Excitability • 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 Nerve and muscle , Abdelaziz Hussein 29

Resting membrane potential (RMP) • 1 -Def. • It is the potential difference between the outside and inside of nerve fiber during rest. • 2 -Value: • It is about 70 m. V (in nerve fiber) and 90 (in skeletal ms) • It is -70 m. V because the inner surface of the cell membrane is -vely charged relative to the interstitial fluid. Nerve and muscle , Abdelaziz Hussein 30

Resting membrane potential (RMP) • 3 -Measurement of the resting membrane potential • By two microelectrodes with very fine tips less than 1 um connected with a special voltmeter. • One electrode is introduced inside the nerve fiber and the other electrode is placed on its outer surface. Nerve and muscle , Abdelaziz Hussein 31

Resting membrane potential (RMP) • 3 -Measurement of the resting membrane potential Nerve and muscle , Abdelaziz Hussein 32

4 -Causes of the resting membrane potential Nerve and muscle , Abdelaziz Hussein 33

Causes of RMP • A) Distribution of ions inside and outside the nerve fiber Nerve and muscle , Abdelaziz Hussein 34

Causes of RMP • B) Selective permeability of the cell membrane • The cell membrane is made up of double layers of lipids, with specialized proteins penetrating the double layers. • These proteins form channels which regulate the movements of ions across the membrane. • Certain ions (Na, K, Cl and Ca) can cross the membrane only through these protein pores channels Nerve and muscle , Abdelaziz Hussein 35

AS Biology, Cell membranes and Transport 36

Causes of RMP • B) Selective permeability of the cell membrane • These channels are 3 types 1. Passive ion channels 2. Chemically activated ion channels 3. Voltage activated ion channels Nerve and muscle , Abdelaziz Hussein 37

Causes of RMP • • • B) Selective permeability of the cell membrane Diffusion of ions through the cell membrane a) K diffusion: (the main cause of RMP) b) Na diffusion c) Cl diffusion Nerve and muscle , Abdelaziz Hussein 38

Causes of RMP • Diffusion of ions through cell membrane Nerve and muscle , Abdelaziz Hussein 39

Causes of RMP C) Sodium and potassium pump: (Na- K Pump): i) Structure: The pump is formed of a large protein molecule (act as a carrier) in the cell membrane, which has: a. ATPase property (ability to split ATP). b. Na binding site→ at the inner surface c. K binding site→ at the outer surface of the cell membrane. • • • Nerve and muscle , Abdelaziz Hussein 40

Causes of RMP • C) Sodium and potassium pump: (Na- K Pump): • ii) Operation: Nerve and muscle , Abdelaziz Hussein 41

Causes of RMP C) Sodium and potassium pump: (Na- K Pump): iii) Function of Na-K pump: It helps to maintain a high concentration of Na ion outside and a high concentration of K ion inside the nerve cells • Any condition decreases the metabolic activity of the cell e. g. by cooling →inhibits Na-K pump →Na+ ions will accumulate inside the cell and neutralize the -ve charges of protein ions and K ions that held on the outer surface escape away, and RMP becomes progressively lost. • • • Nerve and muscle , Abdelaziz Hussein 42

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Action Potential • A) Def. • It is the electrical changes, which occur in the RMP as a result of stimulation of N. F. by an effective stimulus • B) Initiation or phases of action potential • i) Depolarization • ii) Repolarization • iii) Redistribution of ions inside and outside Nerve and muscle , Abdelaziz Hussein 44

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i) Depolarization • The stimulus ↑es the permeability of the cell membrane (several hundred folds) to Na ions, which diffuse inside causing gradual change in the membrane potential from the resting potential (-70 m. v) to the threshold potential or the firing level (-55 m. v). • At -55 m. v, m gates of the voltage activated sodium channels open and Na ions flow into the cell (Na influx). • As a result of sudden Na influx, the membrane potential quickly reaches zero potential and then overshoots to about +35 mv, so there is a momentary reversal in polarity Nerve and muscle , Abdelaziz Hussein 46

i) Depolarization Nerve and muscle , Abdelaziz Hussein 47

ii) Repolarization • At the threshold potential (-55 mv), the voltage sensitive K channels also open, but begin to act after a slight delay time. • Repolarization results from closure of h gates of voltage gated Na channels and opening of the n gates of voltage gated K channels, allowing K ions to diffuse out of the cell (K efflux or outflux). • Diffusion of K ions outside returns the inside of the membrane to its original -ve potential l(-70 m. v) i. e. restore the RMP i. e. repolarization Nerve and muscle , Abdelaziz Hussein 48

ii) Repolarization Nerve and muscle , Abdelaziz Hussein 49

iii) Redistribution of ions inside and outside • After depolarization and repolarization, the ionic composition inside and outside the cell membrane is slightly disturbed. • Redistribution of Na and K ions to the normal resting condition is established by the Na-K pump which actively transports Na out and K into the cell Nerve and muscle , Abdelaziz Hussein 50

Propagation or Conduction of Action Potential • The action potential must be propagated in order to transfer information from one place in the nervous system to another site • There 2 types of propagation of action potential; a. Saltatory conduction in myelinated nerve fibers b. Continuous conduction in unmyelinated nerve fibers • conduction velocity is ↑ by fiber size & myelination. Nerve and muscle , Abdelaziz Hussein 51

Continuous Conduction • occurs in unmyelinated nerve fibers • relatively slow→ 0. 5 -2. 0 m/sec • Mechanism Nerve and muscle , Abdelaziz Hussein 52

Saltatory Conduction • occurs in myelinated nerve fibers • fast (may reach up to 120 m/sec) • Mechanism Nerve and muscle , Abdelaziz Hussein 53

Saltatory Conduction • Significance • a- It ↑es the velocity of conduction • b- It ↓es the energy needed for the Na-K pump → restricted to the nodes of Ranvier. • Myelinated fibers use about 1% of the energy used by unmyelinated fibers. Nerve and muscle , Abdelaziz Hussein 54

Recording of the action potential • i) Aparatus • The cathode ray oscilloscope (CRO) consists cathode ray tube which fires a beam of electrons directed to a special screen Nerve and muscle , Abdelaziz Hussein 55

Recording of the action potential • ii) Methods • 2 methods for recording of action potential • A) Biphasic action potential: • a) Method: • By two micro electrodes placed on the outer surface of the nerve fibers→ connected with the CRO. • b) Record: Nerve and muscle , Abdelaziz Hussein 56

Recording of the action potential • • ii) Methods B) Monophasic action potential a) Method: recorded by one of the following methods: i) If the nerve is damaged in between the two electrodes used in recording the biphasic action potential. Nerve and muscle , Abdelaziz Hussein 57

Recording of the action potential • • ii) Methods B) Monophasic action potential a) Method: ii) If one micro electrode (recording electrode) is introduced inside the nerve fiber and the other electrode (reference electrode) is placed in the extracellular fluid far away from the excited region. Nerve and muscle , Abdelaziz Hussein 58

Monophasic Record of Action Potential • Monophasic action potential shows the following: • ◊ Latent period: • A) Spike potential • B) After potentials Nerve and muscle , Abdelaziz Hussein 59

Monophasic Record of Action Potential Nerve and muscle , Abdelaziz Hussein 60

Effects of stimulation of the nerve fibers • A) Stimulation by ineffective (subminimal) stimuli • Ineffective stimuli produce a localized area of depolarization which does not reach the firing level (-55 mv) known as "the local excitatory state". • B) Stimulation by effective stimuli produce a propagated action potential or a nerve impulse. 61

Local excitatory state • Local excitatory state is characterized by; • 1) It is localized to the site of stimulation and nearby area. • 2) During conduction, it decreases gradually with distance till it disappears (conducted with decrement) • 3) Its duration is very short (less than 1 m. sec). • 4) Can be graded: its magnitude is proportional with the strength of the subminimal stimulus i. e. it dose not obey the all or non law. Also, it has no threshold. 62

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Local excitatory state • Local excitatory state is characterized by; • 5) It can be summated to the local excitatory state of other subminimal stimuli. • Two types of summation can occur: • i) Spatial summation: By applying several subminimal stimuli at the same time at different sites near each other. • ii) Temporal summation: By applying several subminimal stimuli at the same site within a very short time (less than the duration of the local excitatory state) 64

Local excitatory state • Local excitatory state is characterized by; • 6) It is associated with increased excitability because the nerve fibers can respond to subminimal stimuli applied at the same time. • 7) It has no refractory period. 65

Properties of the nerve impulse • I. The nerve impulse is a wave: • It is characterized by Type A fibers Type B fibers Type C fibers Meylination thick myelinated thin unmyelinated Diameters 2 -20 um 1 -3 um 0. 5 -1 um Speed 10 -120 met/sec 3 -15 met/sec 0. 5 -2 meter/sec Examples i) somatic sensory -preganglionic i)postganglionic fibers autonomic fibers carrying sympath and cutaneous and deep parasympathetic sensation fibers. ii) Somatic sensory fibers carrying pain ii) Somatic motor sensation fibers to skeletal ms. pain). (delayed 66

Properties of the nerve impulse • I. The nerve impulse is a wave: • It is characterized by Type A fibers Type B fibers Type C fibers Nerve - high magnitude -moderate -low magnitude impulse –short duration magnitude -prolonged duration -0. 5 -1 m. sec -moderate -2. 0 m. sec duration -1. 2 m. sec Sensitive to prolonged deep O 2 lack local anaesthetics pressure Subtypes alpha, beta, gamma --------and delta fibers ---------67

Action potential in mixed nerve fibers • The AP in the mixed nerve is called the compound action potential • The mixed nerve is formed of bundles of nerve fibers of different types • The action potentials produced in these fibers are formed of multiple peaks 68

Properties of the nerve impulse • II. The nerve impulse obeys all or non law • The nerve impulse either occurs maximally or it does not occur at all, provided that all other conditions remain constant. So; • -All subminimal stimuli do not produce response • -Minimal (threshold) stimulus produces a maximal response. • -Further increase in the intensity of the stimuli (superminimal, maximal, supermaximal) do not produce any further increase in the response • All or non law is applied in the single nerve fiber and not applied in the nerve trunk. 69

Amplitude Local excitatory state Nerve impulse (Graded potential) (Action potential) Proportional with the intensity of the Not proportional with the intensity stimulus i. e. not obey all or non law. of the stimulus i. e. obey all or non law. Summation Can be summated→ temporal and Can't be summated spatial Threshold Has no threshold. Has a threshold. Refractory period Not followed by a refractory period. It is followed by a refractory period. Conduction Duration Localized to site of stimulation or Conducted by saltatory or nearby area. continuous conduction. Passive conduction. Active conduction. Conducted with decrement. Conducted without decrement. Very short 0. 5 -1 m. sec in thick myelinated fiber Nature May be a depolarization hyperpolarization. or Is a depolarization withovershoot. 70

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