Lecture 5-Overview A&P function-Nerve Tissue(Choy 5.8.2024).pdf
- 1. Lecture 5: Overview of A&P Functions of Nerve Tissue: Neurophysiology By Ms Choy Kee Leong PPLK (5.8.2024) 1
- 2. LEARNING OBJECTIVES: 1. State the functions of neuron. 2. State the structures of a neuron, and explain their functions. 3. Describe the composition of grey mater and white mater. 4. Classify neurons according to structure and function. 5. State 6 types of supporting nerve cells and their functions. 6. Explain the action potential. 7. Explain the synaptic transmission. 2
- 3. NEURON 3
- 4. Cells Of The Brain The brain is made up of two types of cells: 1. Neurons – nerves impulse 2. Neuroglia – support and protect neurons 4
- 5. 1. NEURON (Actual Nerve cell) 5
- 6. 1. NEURON (Actual Nerve cell) • Neurons are amitotic. • If a neuron is destroyed, it cannot be replaced because neurons DO NOT undergo mitosis. • Function: Conducting nerve impulses. 6
- 7. • Neurons are electrical excitability = ability to respond to a stimulus and convert it into an action potential. • Stimulus = any change in the environment that strong enough to initiate an action potential. • Action potential (or impulse) = an electrical signal that propagates (travels) along the surface of the neuron’s membrane or a muscle fiber. NEURON cont’d……. 7
- 8. STRUCTURES OF NEURON Each neuron has 3 basic parts: 1. Cell body (soma) 2. Dendrites 3. Axon 8
- 9. Neuron Cell body Node of Ranvier Dendrite Axon 9
- 11. CELL BODY • Cell body known as soma. • Contains a nucleus surrounded by cytoplasm. • No centriole in the neuron, NO MITOSIS. • The organelles are endoplasmic reticulum, lysosomes, mitochondria, and Golgi apparatus. 11
- 12. • Cell body form the grey matter of the nervous system. • Found at the periphery of the brain and the centre of the spinal cord. • Groups of cell bodies are called nuclei in the CNS and ganglia in PNS. • Function: Metabolic centre of the neuron cells. Cont’d… 12
- 13. Brain 13
- 14. DENDRITE • Consists of short processes and extension of cells bodies. • Short, tapering, highly branched, forming a tree- shaped) processes emerge from the cell body. • Receiving or input parts of neuron. 14
- 15. Cont’d……. Dendrite • Dendrite in motor neurons (efferent) → form part of synapses. • Dendrite in sensory neurons (afferent) → form the sensory receptors that respond to specific stimuli. • Examples: – Vision from eye receptor. – Hearing from inner ear receptor. • Function: – Receive & carry incoming impulses towards cell bodies. 15
- 16. DENDRITE Cell body Node of Ranvier Dendrite 16
- 17. AXON • Axons are extensions of cell bodies. Longer than dendrites, as long as 100 cm. • Long, cylindrical projection that joins the cell body at a cone-shaped elevation called axon hillock. • The axon terminals part called synaptic bulb. • These terminals contain hundreds of tiny vesicles called neurotransmitters → transmit nerve impulse away from the cell body. 17
- 18. CONT’D... • The membrane of the axon is called axolemma. • Cytoplasm in the axon is called axoplasm. • Function of axon: Carry nerve impulses away from the cell body to the other part of the body. 18
- 19. AXON Cell body Node of Ranvier Axons hillock Axon Synaptic bulb 19
- 21. AXON AND DENDRITE • Are cytoplasmic extensions, or processes, that project from the cell body. • They are sometimes referred to as nerve fibers. • Bundles of nerve fibers running through the CNS are call tracts, in PNS is called nerves. 21
- 22. MYELIN SHEATH • Most neurons are surrounded by a myelin sheath. • Composed of whitish, lipid & protein (lipoprotein). • Axon that surrounded by myelin sheath called myelinated neurone. • Axon not surrounded by myelin sheath called non- myelinated neurone. • Myelinated fibers make up white matter in the CNS. 22
- 23. Myelin Sheath and Neurilemma • Whitish, fatty (protein-lipid), segmented sheath around most long axons • Formed by Schwann cells in the PNS • It functions in: • Protection of the axon • Electrically insulating fibers from one another • Increasing the speed of nerve impulse transmission • Neurilemma – remaining nucleus and cytoplasm of a Schwann cell 23
- 24. MYELIN SHEATH 24
- 25. 25 Myelin Sheath and Neurilemma Myelinated Axon Unmyelinated Axon 25
- 26. CONTD… • Unmyelinated regions between the myelin segments are called the nodes of Ranvier. • Nodes of Ranvier located in between the myelin segments in the axon. 26
- 27. CONTD… • Functions: 1. Act as an insulation (penebat) for the nerve fibers. 2. Protect axon from injury. 3. Increases the transmission rate of nerve impulses. 27
- 28. • Myelin sheath consists of a series of Schwann cells arranged along the length of the axon. • In the CNS oligodendrocytes produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate. • Neurilemma plays an important role in the regeneration of nerve fibers. CONT’D... 28
- 29. Common Disorders • Multiple sclerosis: Disease that caused progressive destruction of myelin sheaths of neurons in the CNS. Symptoms: Heaviness or weakness in the muscles Abnormal sensations Double vision Unable to breath 29
- 30. CLASSIFICATION OF NEURONS Both structural and functional features are used to classify the various neurons in the body. 1. Structural classification: (a) Multipolar neurons (b) Bipolar neurons (c) Unipolar neurons 2. Functional classification: (a) Sensory (afferent neurons) (b) Motor (efferent neurons) (c) Interneurons (association neurons) 30
- 31. (a) Multipolar neurons • Have several dendrites + 1 axon. • Location: Most in the brain and spinal cord. 31
- 32. (b) Bipolar Neuron • Have one main dendrite + one axon. • Location: Retina of the eye. Inner ear Olfactory area (smell). 32
- 33. (C) Unipolar Neurons • Have dendrites and one axon. • Fused together to form a continuous process. • Begin in the embryo as bipolar neurons. • Function: as sensory receptors (touch, pressure, pain, or thermal stimuli). 33
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- 35. CONT’D... 2. Functional classification: (a) Sensory (afferent neurons) - contains sensory receptors. - once get stimulated = forms action potential in axons and conveyed into CNS. (b) Motor (efferent neurons) - convey action potential away from CNS to effectors (muscle or glands) in PNS through cranial & spinal nerves. 35
- 36. CONT’D... (c) Interneurons (association neurons) - between sensory & motor neurons. - integrate incoming sensory information from sensory to motor response. 36
- 38. TYPE STRUCTURE FUNCTION AFFERENT (SENSORY) • Dendrite - long • Axon - short • Cell body located in the ganglia in PNS. • Dendrite in PNS • Axon extends into CNS. • Transmit impulses from peripheral sense receptors to CNS. EFFERENT (MOTOR) • Dendrite - short • Axon – long • Dendrite & cell body located within CNS. • Axons extend to PNS. • Transmits impulses from CNS to effectors such as muscles & glands in periphery. ASSOCIATION (INTERNEURON) • Dendrite – short. • Axon may be short or long. • Located entirely within CNS. • Transmits impulses from afferent neurons to efferent neurons. 38
- 39. Three types of neuron 39
- 40. 2. Neuroglia 40
- 41. 41 2. Neuroglia • Cells of the brain that provide neurons with nourishment, protection, and structural support. • There are about 10 to 50 times more neuroglia than nerve cells • Glia = glue nervous tissue together. • Make up about ½ the volume of CNS. • Smaller than neurons, 5 – 50 times more numerous. 41
- 42. 2. Neuroglia cont’d… • Does NOT generate & conduct nerve impulses. • Give mechanical support and metabolism to the neurons: Support the neurons. Nourish the neurons. Protect the neurons. Capable of mitosis. 42
- 43. 6 TYPES OF NEUROGLIA 1. Astrocytes 2. Oligodendrocytes 3. Microglia 4. Ependymal cells 5. Schwann cells 6. Satellite cells 43
- 44. 2. Neuroglia 1. CNS • Astrocytes • Oligodendrocytes • Microglia cells • Ependymel cells 2. Peripheral neuroglia •Schwan cells •Satellite cells 44
- 45. 2. Neuroglia cont’d… • Astroglia or astrocytes transport nutrients to neurons, hold neurons in place, digest parts of dead neurons, and regulate the blood brain barrier. • Oligodendroglia cells provide insulation (myelin) to neurons. • Ependymal cells line the ventricles and secrete cerebrospinal fluid (CSF). • Microglia digest dead neurons and pathogens. 45
- 46. 46 46 46
- 47. 47 Types Of Neuroglia Cells Cell type Decription Function 1. Astrocytes (CNS) Star shape Bind by to nerves. (Blood brain Barrier) 2. Ependymal cells (CNS) Columnar cells with cilia Important in formation & circulation of CSF 3. Microglia (CNS) Small cells with long processes Protection; phagocytic 4. Oligodendrocytes (CNS) Cells with long process around axon Form myelin sheath in CNS 5. Schwann cells (PNS) Flat cells, wraps around an axon Form myelin sheath in PNS 6. Satellite cells (PNS) Similar to Schwann cells Support nerve cell bodies within ganglia 47
- 48. 1. ASTROCYTES 48
- 49. Cont’d... DESCRIPTION LOCATION • CNS STRUCTURE • Star shaped • Numerous radiating processes with bulbous ends for attachment. FUNCTION • Bind blood vessels to nerves. • Form ‘blood-brain barrier’. (Allowing entrance of essential nutrients, restriction of harmful substances) • Regulate composition of fluid around neurons. 49
- 51. Cont’d... DESCRIPTION LOCATION • CNS STRUCTURE • Small cells with few, but long processes that wrap around axons. FUNCTION • Form myelin sheaths around axons in CNS. 51
- 52. Microglia 52
- 53. Cont’d... DESCRIPTION LOCATION • CNS STRUCTURE • Small cells with long processes. • Modified macrophages. FUNCTION • Protection • Become mobile & phagocytic in response to inflammation. 53
- 55. Cont’d... DESCRIPTION LOCATION • CNS (line ventricles of brain & central canal of spinal cord. STRUCTURE • Columnar cells with cilia. FUNCTION • Active role in formation and circulation of cerebrospinal fluid (CSF). 55
- 57. Cont’d... DESCRIPTION LOCATION • PNS STRUCTURE • Flat cells with long, flat process that wraps around axon in PNS. FUNCTION • Form myelin sheaths around axons in PNS. • Active role in nerve fiber regeneration. 57
- 59. Cont’d… DESCRIPTION LOCATION • PNS STRUCTURE • Flat cells, similar to Schwann cells. FUNCTION • Support nerve cell bodies within ganglia. • Eg. Regulate concentration of Sodium, Potassium & CO2. 59
- 63. Action Potentials (AP) • Only cells with excitable membranes (neurons and muscle cells) can generate AP. • Membrane potential = -70mV. • AP do not decrease in strength over distance. • Threshold stimulus is needed to generate the AP • An axon is capable of generating an AP. 63
- 64. Transmission of Nerve Impulses 1. Polarization of the neuron membrane 2. Resting membrane potential 3. Depolarization / Action potential 4. Repolarization 5. Hyperpolarization 6. Refractory period 64
- 65. 1. Polarization of the neuron’s membrane: Na is on the outside & K is on the inside • When neuron is not stimulated, its membrane is polarized. • Being polarized, the electrical charge outside is positive and inside is negative. • Na+ is on the outside & K+ is on the inside. • Na+ / K+ pumps on the membrane pump N+ back outside and K+ back inside. • The charge of an ion inhibits the membrane permeability. 65
- 66. 2. Resting potential • When the neuron is inactive and polarized, it is said to be at its resting potential. • It remains polarized until a stimulus come along • A resting potential of -70mV 66
- 67. 3. Action potential: Na+ ions move inside the membrane • When neuron is stimulated, the Na+ ion gated open and allow Na+ rush into the cell. • As this happen, the neuron goes from being polarized to depolarization. • More positive ion is now inside the cell until the threshold is reached. • Each neuron has a threshold level • Once stimulus is above the threshold, more gated Na+ ion channel open and allow more Na+ inside the cell 67
- 68. 3. Action potential: Na+ ions move inside the membrane (cont’d) • This causes complete depolarization of the neuron and an action potential is created. • Na channel continues to open • “All or none phenomenon” – if a stimulus does not exceed the threshold level and cause the gates to open, no action potential results. 68
- 69. 4. Repolarization: K+ ion moves outside & Na ion stays inside the membrane • After inside cell is flooded with Na, the K gated ion channel open and K moves outside the membrane. • Both ions restores electrical charge balance • Just after the K gates open, the Na gates close 69
- 70. 5. Hyperpolarization : more K+ ions are on the outside than Na ions on the inside • When K gated is close, the neuron has more K on the outside than Na on the inside • This causes the membrane potential to drop slightly lower than the resting potential • The membrane is said to be hyperpolarized because it has greater potential • After the impulse has traveled through the neuron, the action potential is over, the cell membrane returns to normal ( known as resting potential) 70
- 71. 6. Refractory period: everything back to normal (K+ return inside & Na+ returns outside) • While neuron is busy returning to normal, it does not respond to any incoming stimuli. • The neuron is back to polarized state and stays in the resting potential until another impulse comes along 71
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- 78. Factors Affecting the Speed of Conductance: Myelin, Axon Diameter, Temperature • Impulses travel faster in myelinated neurones → SALTATORY CONDUCTION - Schwann cells prevent diffusion of ions - Flow of current between adjacent nodes of Ranvier - Depolarisation only at nodes of Ranvier - Action potential jumps from node to node 78
- 79. Conduction Velocities of Axons • Conduction velocities vary widely among neurons • Rate of impulse propagation is determined by: 1. Axon diameter – the larger the diameter, the faster the impulse 2. Degree of a myelin sheath – myelination dramatically increases impulse speed 3. Speed of conduction 80
- 80. Action Potential: Role of the Sodium- Potassium Pump • Repolarization • Restores the resting electrical conditions of the neuron • Does not restore the resting ionic conditions • Ionic redistribution back to resting conditions is restored by the sodium- potassium pump 81
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- 82. Saltatory Conduction in Myelinated Axons •Myelin sheathing has bare patches of axon called nodes of Ranvier •Action potentials jump from node to node Fig. 48.11 83
- 84. Factors Affecting the Speed of Conductance: Myelin, Axon Diameter, Temperature • Temp affects speed of conduction of impulses Higher temp increases rate of diffusion of ions • Impulses faster in an axon with larger diameter Small cells / large surface area : volume ratio / ion leakage weakens membrane Myelin stops ion leakage / diameter only important for unmyelinated neurones 85
- 85. SYNAPES: The space where 2 neurons communicate in one direction. 86
- 86. Electrical Synapses • Are less common than chemical synapses • Correspond to gap junctions found in other cell types • Contain intercellular protein channels • Permit ion flow from one neuron to the next • Are found in the brain and are abundant in embryonic tissue 87
- 87. Chemical Synapses Specialized for the release and reception of neurotransmitters • Typically composed of two parts: • Axonal terminal of the presynaptic neuron, which contains synaptic vesicles • Receptor region on the dendrite(s) or soma of the postsynaptic neuron 88
- 88. Synapse • Nerve impulse from one neuron is transmitted to the next neuron • Junction between the 2 neurons is called synapes • Synapse has 3 parts: 1. Synaptic bulb/knob/button on the presynaptic neuron 2. Synaptic cleft 3. Postsynaptic membrane of postsynaptic neuron 89
- 89. 90 Synapses • A junction that transfer information from one neuron: • To another neuron • To an effector cell • Presynaptic neuron – conducts impulses toward the synapse • Postsynaptic neuron – transmits impulses away from the synapse 90 90
- 91. Synaptic Transmission • Synapses is he point at which the nerve impulse passes from one neuron to another neuron. • In synapse, neurons communicate with other neurons or effectors called synaptic transmission. • Each axon terminal is separated from the next neuron by a tiny gap called the synaptic cleft (about 20nm (nanometer) width. 92
- 92. Synaptic Cleft • Fluid-filled space separating the presynaptic and postsynaptic neurons • Prevent nerve impulses from directly passing from one neuron to the next • Transmission across the synaptic cleft: • Is a chemical event (as opposed to an electrical one) • Ensures unidirectional communication between neurons 93
- 93. Synaptic Cleft: Information Transfer • Nerve impulse reaches axonal terminal of the presynaptic neuron • Neurotransmitter is released into the synaptic cleft • Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron • Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect 94
- 94. Cont’d... • Presynaptic neuron = neuron that sending the signal. • Postsynaptic neuron = neuron that receiving the message. • There is no physical contact between these neurons. 95
- 95. Termination of Neurotransmitter Effects • Neurotransmitter bound to a postsynaptic neuron: • Produces a continuous postsynaptic effect • Blocks reception of additional “messages” • Must be removed from its receptor • Removal of neurotransmitters occurs when they: • Are degraded by enzymes • Are reabsorbed by astrocytes or the presynaptic terminals • Diffuse from the synaptic cleft 96
- 96. Chemical Synapse Synaptic vesicles within synaptic bulbs contain chemical called neurotransmitters (released when impulse reaches synaptic bulbs) • Neurotransmitters diffuse across synaptic cleft & bind to receptors on postsynaptic membrane • Eg: acetylcholine (neurotransmitter) inactivated by cholineterase to prevent prolonged reaction 97
- 97. Neurotransmitters • Are chemicals that cross the synaptic gap and either excite or inhibit post-synaptic neurons • Allows communication between neurons they are like a lock and key - they must match up 98
- 98. Examples Of Neurotransmitters • ACETYLCHOLINE - muscle activity, memory and learning, PNS, affecting the heart, stomach, liver, sweat glands, blood vessels • GABA (GAMMA AMINO BUTYRIC ACID) - Central Nervous System (CNS) main amino acid inhibitory neurotransmitter - induces sleep 99
- 99. Examples Of Neurotransmitters • CATECHOLAMINES - BIOGENIC AMINES 1. EPINEPHRINE - functions as both hormone and neurotransmitter - increases alertness and energy 2. NOREPINEPHRINE - aids in memory, learning, and emotion - affects mood and attention 3. DOPAMINE - deals with fine motor control (Parkinson disease sufferers have a deficit of this chemical 4. SERATONIN - linked with mood, emotions, sleep, consciousness, and digestion 100
- 100. HOW’S THE SIGNAL AT SYNAPSE? HOW’S THE CONDUCTIVITY? ▼ 101
- 101. Synaptic Transmission • Synaptic cleft (gap) of 20μm separates two neurones at a synapse (junction of 2 neurones) Presynaptic membrane is at the end of a neurone Postsynaptic membrane is at the next neurone in the chain • Synaptic knob of a presynaptic neurone contains Neurotransmitters in small vesicles Mitochondria to produce ATP needed for neurotransmitter synthesis 102
- 102. Aspects Of Synaptic Transmission Summation • Several presynaptic neurones release neurotransmitter • Cumulative effect reaches a threshold to depolarise postsynaptic membrane • E.g. rod cells when they synapse with relay neurones in the retina Spatial summation • Several impulses arrive at one neurone via several synapses • Cause sufficient depolarisation / open sufficient sodium ion channels • For threshold to be reached Temporal summation • Several impulses arrive at same neurone via same synapse • Threshold → action potential 103
- 103. Aspects Of Synaptic Transmission Inhibition • More inhibitory postsynaptic potentials IPSPs than excitatory postsynaptic potentials EPSPs • Reduces membrane potential / makes more negative • Hyperpolarisation of postsynaptic membrane • Cancels effect of action potential when several synapses 104
- 104. The Mechanisms Of Transmission At An Excitatory Synapse 1. Nerve impulse reaches synaptic knob/presynaptic membrane/neurone 2. Depolarisation opens Ca2+ gates / calcium ions enter 3. Ca2+ causes vesicles containing neurotransmitter to fuse with membrane 4. Release of neurotransmitter / into synaptic cleft / by exocytosis 5. Diffuse across synaptic cleft 6. Neurotransmitter binds to specific receptors in postsynaptic membrane 105
- 105. The Mechanisms Of Transmission At An Excitatory Synapse 7. Sodium channels open / sodium ions enter • Depolarisation of postsynaptic membrane • Threshold causes an action potential along postsynaptic neurone 8. Neurotransmitter are quickly removed from the postsynaptic membrane • Diffuse out of the synaptic cleft • Taken up by presynaptic membrane by endocytosis • Enzymes break down neurotransmitters into inactive substances 106
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- 108. SYNAPSE IMPULSE 109
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- 110. Action potential reaches axon terminal ▼ Neurotransmitter released from vesicles into synaptic cleft ▼ Vesicles fuses with plasma membrane ▼ Neurotransmitter released into synaptic cleft ▼ Neurotransmitter bind to receptors on the next post-synaptic membrane 111
- 111. EVENTS AT A SYNAPSE Nerve impulse (Action Potential) arrives at a synaptic bulb of presynaptic axon (axon terminal) ▼ Ca2 + channels open ▼ Ca2 + flows into the synaptic end bulb ▼ Ca2 + concentration increase ▼ Trigger synaptic vesicles to release neurotransmitter molecules into synaptic bulb 112
- 112. Cont…. Neurotransmitter molecules diffuse across the synaptic cleft ▼ Bind to neurotransmitter receptors in the postsynaptic neuron plasma membrane ▼ Excitability of Neuron Effectors (muscle, glands) 113
- 113. Neurotransmitter is then broken down and released by enzyme activity. Neurotransmitter: Acetylcholine Enzyme: Cholinesterase ▼ EXCITABILITY OF NEURON 114
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- 117. Thank You 118
- 118. Synaptic Delay • Neurotransmitter must be released, diffuse across the synapse, and bind to receptor • Synaptic delay – time needed to do this (0.3- 5.0 ms) • Synaptic delay is the rate-limiting step of neural transmission 119
- 119. Postsynaptic Potentials • Neurotransmitter receptors mediate changes in membrane potential according to: • The amount of neurotransmitter released • The amount of time the neurotransmitter is bound to receptor • The two types of postsynaptic potentials are: • EPSP – excitatory postsynaptic potentials • IPSP – inhibitory postsynaptic potentials 120
- 120. Excitatory Postsynaptic Potentials • EPSPs are graded potentials that can initiate an action potential in an axon • Use only chemically gated channels • Na+ and K+ flow in opposite directions at the same time • Postsynaptic membranes do not generate action potentials 121
- 121. Inhibitory Synapses and IPSPs • Neurotransmitter binding to a receptor at inhibitory synapses: • Causes the membrane to become more permeable to potassium and chloride ions • Leaves the charge on the inner surface negative • Reduces the postsynaptic neuron’s ability to produce an action potential 122
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- 123. Summation • A single EPSP cannot induce an action potential • EPSPs must summate temporally or spatially to induce an action potential • Temporal summation – presynaptic neurons transmit impulses in rapid-fire order • Spatial summation – postsynaptic neuron is stimulated by a large number of terminals at the same time 124
- 124. Neurotransmitters: Acetylcholine • Released at the neuromuscular junction • Synthesized and enclosed in synaptic vesicles • Degraded by the enzyme acetylcholinesterase (AChE) • Released by: • All neurons that stimulate skeletal muscle • Some neurons in the autonomic nervous system 125
- 125. Functional Classification of Neurotransmitters • Two classifications: excitatory and inhibitory • Excitatory neurotransmitters cause depolarizations (e.g., glutamate) • Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine) • Some neurotransmitters have both excitatory and inhibitory effects • Determined by the receptor type of the postsynaptic neuron • Example: aceytylcholine • Excitatory at neuromuscular junctions • Inhibitory with cardiac muscle 126
- 126. Neurotransmitter Receptor Mechanisms • Direct: neurotransmitters that open ion channels • Promote rapid responses • Examples: ACh and amino acids • Indirect: neurotransmitters that act through second messengers • Promote long-lasting effects • Examples: biogenic amines and peptides 127
- 127. ANY QUESTION? 128