Neurotransmission & Neuromuscular Junctions 2024 PDF (RCSI)

RCSI Royal College of Surgeons in Ireland Medical University of Bahrain Neurotransmission & Neuromuscular Junctions Class Year 1 Course The Body: Movement & Function Code MED 1 - 102 Title...

RCSI Royal College of Surgeons in Ireland Medical University of Bahrain Neurotransmission & Neuromuscular Junctions Class Year 1 Course The Body: Movement & Function Code MED 1 - 102 Title Neurotransmission & Neuromuscular Junctions Lecturer Dr. Patrick Walsh Date November 2024 Learning Objectives:  Differentiate between autonomic and sensory-somatic nervous system  Differentiate between sensory and motor neurons  Define a neurotransmitter  Describe the sequence of events occurring during neurotransmission at the neuromuscular junction  Describe the role of acetylcholinesterase in neurotransmission at the neuromuscular junction  Characterise the nicotinic receptors at the neuromuscular junction ORGANIZATION OF THE Brain NERVOUS SYSTEM Central nervous system (CNS)  Brain  Spinal cord Spinal Cord Peripheral nervous system (PNS)  Sensory nerves (afferent nerves)  Motor nerves (efferent nerves) Peripheral Nerves  THE NERVOUS SYSTEM Brain and spinal cord Central nervous system Peripheral Afferent division Efferent division nervous system Central nervous system Sensory Visceral Somatic Autonomic Local visceral stimuli stimuli nervous system nervous system stimuli Motor Sympathetic Parasympathetic Enteric neurons nervous system nervous system nervous system Skeletal muscle Smooth muscle Cardiac muscle Digestive organ Exocrine glands only Peripheral Some endocrine glands nervous system STRUCTURAL UNITS OF THE NERVOUS SYSTEM: NEURONES PRESYNAPTIC CELL POSTSYNAPTIC CELL Nissl bodies (RER and free ribosomes) Body or soma Dendrites Dendritic spines Axon Dendrite Axon hillock Initial segment of Axon axon Telodendron Golgi Neurofilament Mitochondria Synaptic terminal Nucleus Nucleolus SIGNAL TRANSDUCTION: Dendrites or the cell body (soma) receive in-put signals, leading to a – depolarisation or – hyperpolarisation of the plasma membrane Axons propagate out-put signals: – Action potentials WAYS TO CLASSIFY NEURONES (I) Morphology (shape) –Bi-polar neurons vs multi-polar neurones number of projections from cell body Afferent and Efferent nerves –Neurones that transmit information towards the CNS are afferent (arriving) –Neurones that transmit information from the CNS are efferent (exiting, effector organ) Neurotransmitter – Substance / chemical they release (e.g. dopamine, acetylcholine) FUNCTIONAL WAYS TO CLASSIFY NEURONS / NERVES (II) Sensory nerves send information to the central nervous system about the internal and external environment. Motor nerves control the activity of the body by controlling muscle and gland functions (contraction, relaxation, secretion). A TYPICAL SPINAL NERVE SHOWING AFFERENT & EFFERENT AXONS FUNCTIONAL WAYS TO CLASSIFY NEURONS / NERVES (II) Sensory nerves send information to the central nervous system about the internal and external environment. Motor nerves control the activity of the body by controlling muscle and gland functions (contraction, relaxation, secretion). Motor response to sensory input depends on integration of information - Interconnections between nerves – e.g. reflex arcs  The Axon Synapse is the junction Synapse between one neurone & the Synaptic next cell terminal Specialised structure at which electrical impulse is converted to a chemical signal for © RCSI (Koenig); The journal of cell science 2012 communication between cells (electro-chemical coupling) Type of communication: Nerve-Nerve Nerve-Organ / Organ-Nerve Nerve-Muscle Nerve-Gland Synapses between nerve and muscle cells are also called neuromuscular junctions or motor end plates NEUROTRANSMITTERS Typically small, rapid-acting molecules – e.g. Acetylcholine, Dopamin, glutamate, noradrenaline, GABA Generally, neurones release one type of major NT THE CHEMICAL SYNAPSE The presynaptic neurone releases a chemical, a neurotransmitter The neurotransmitter diffuses across the synaptic cleft The neurotransmitter binds to specific receptor proteins on the plasma membrane of the postsynaptic cell to alter its membrane potential EXCITATORY SYNAPSES Synapses are either excitatory or inhibitory The neurotransmitter at excitatory synapses depolarises the postsynaptic membrane. Example: acetylcholine (ACh) Binding of acetylcholine to its receptors on the postsynaptic cell opens up ligand-gated sodium channels. These allow an influx of Na+ ions, reducing the membrane potential. If depolarisation of the postsynaptic membrane reaches a threshold, an action potential is generated in the postsynaptic EXCITATORY SYNAPSES NT receptor synaptic cleft + + + + + + + + + Postsynaptic - - - - - - - - - membrane EXCITATORY SYNAPSES NT receptor synaptic cleft + + + + + + + + + Postsynaptic - - - - - - - - - membrane NT (e.g. Ach) binding opens Na+ channel synaptic cleft - Depolarization Postsynaptic -70 -60 -50- - -50 -60 -70 membrane Na+ Na+ Na+  +60 +50 +40 Membrane potential +30 +20 +10 0 -10 -20 -30 -40 -50 -60 -70 -80 Resting Membrane Potential -90  +60 +50 +40 Membrane potential +30 +20 +10 0 -10 -20 -30 -40 -50 Depolarization -60 -70 -80 Resting Membrane Potential -90 POTENTIALS (I): GRADED POTENTIALS Two basic forms of Graded potentials die electrical signals: out over short distances Graded potentials Action potentials Graded potentials: local Graded potentials can changes in membrane be integrated to potential not reaching generate an action threshold potential -> serve as long- distance signals POTENTIALS (I): GRADED POTENTIALS: CURRENT FLOW DURING GRADED POTENTIALS POTENTIALS (I): GRADED POTENTIALS: DIE OUT QUICKLY Occurs in small, specialized region of excitable cell membranes Magnitude of graded potential varies directly with the magnitude of the triggering event  ACTION POTENTIALS However, If Graded Potentials summate sufficiently to reach the threshold voltage (-50 mV), voltage-gated channels in the adjacent region of the membrane open and sodium ions flood into the cell, leading to an Action Potential. This constitutes a NERVE IMPULSE which moves down the axon.  POTENTIALS (II): ACTION POTENTIALS Brief, rapid, large (≅100mV) changes in membrane potential during which potential actually reverses Involves only a small portion of the total excitable cell membrane at a certain time Do not decrease in strength as they travel from their site of initiation throughout remainder of cell  +60 +50 +40 Membrane potential +30 During an AP voltage-gated +20 Na+ channels open in the PM +10 that allows Na+ movement into 0 the cell -10 PM depolarizes -20 Resting Membrane Potential -30 -40 -50 -60 -70 +- -80 -90 -+  +60 +50 +40 Membrane potential +30 +20 Voltage-gated K+ channels +10 subsequently open which 0 allows K+ to leave the cell down -10 its electrochemical gradient -20 bringing the MP back toward -30 resting (-70mV) -40 -50 -60 +- - -70 -80 + -90 +60 +50 +40 +30 +20 +10 0 -10 1. Opening of -20 VGSCs -30 -40 -50 threshold potential -60 -70 -80 -90 0.1msec +60 +50 +40 +30 Caused by Na+ influx +20 +10 0 -10 1. Opening of -20 VGSCs -30 -40 -50 threshold potential -60 -70 -80 -90 0.1msec +60 2. Closing of VGSCs +50 3. VGPCs open +40 +30 Caused by Na+ influx +20 +10 0 -10 1. Opening of -20 VGSCs -30 -40 -50 threshold potential -60 -70 -80 -90 0.1msec +60 2. Closing of VGSCs +50 3. VGPCs open +40 +30 Caused by Na+ influx +20 caused by K+ efflux Re-polarization +10 0 -10 1. Opening of -20 VGSCs -30 -40 -50 threshold potential -60 -70 -80 -90 0.1msec +60 2. Closing of VGSCs +50 3. VGPCs open +40 +30 Caused by Na+ influx +20 caused by K+ efflux Re-polarization +10 0 -10 1. Opening of -20 VGSCs -30 -40 -50 threshold potential -60 -70 -80 -90 0.1msec 4. VGPCs close POTENTIALS (II): ACTION POTENTIALS When membrane reaches threshold potential – Voltage-gated Na+-channels in the membrane undergo conformational changes Flow of sodium ions into the ICF reverses the membrane potential from -70 mV to +30 mV – Flow of potassium ions into the ECF restores the membrane potential to the resting state  What is the threshold potential at which an action potential is generated? A. 50 mV B. 0 mV C. -50 mV B. –70 mV  What is the threshold potential at which an action potential is generated? A. 50 mV B. 0 mV C. -50 mV B. –70 mV  THE REFRACTORY PERIOD – ONE WAY TRAFFIC Refractory period is the period when a further stimulus applied to the neurone (or muscle fiber) will not trigger another action potential. Due to inactivation of sodium channels (absolute refractory period) and repolarisation brought about by opening of potassium channels and potassium ions (K+) movement out of cell (relative refractory period). In some neurons, the refractory period lasts only 0.001 - 0.002 seconds. In other words, some neurons can transmit up to 500- 1000 impulses per second.  Sodium channels open during depolarization by positive feedback As the action potential develops at one point in the plasma membrane, it regenerates an identical action potential at the next point in the membrane. Therefore, it travels along the plasma membrane undiminished. Na+ ECF + - + - + - ICF - + - + - + K+ Cell body Axon terminals © RCSI, 2010, Dept. of Physiology (Koenig) THE ACTION POTENTIAL IS ALL-OR- NONE The strength of the action potential is an intrinsic property of the cell. So long as they can reach the threshold of the cell, strong stimuli produce no stronger action potentials than weak ones. However, the strength of the stimulus is encoded in the frequency of the action potentials that it generates.  INHIBITORY SYNAPSE (I) The neurotransmitter at inhibitory synapses hyperpolarises the postsynaptic membrane. Example: gamma aminobutyric acid (GABA) Binding of GABA to its receptor on the postsynaptic neurone opens up ligand-gated chloride (Cl−) channel. The entry of negatively charged chloride ions increases the membrane potential (e.g. -70 to -90 mV), meaning even more negative charge inside the cell This increased membrane potential (or more negative charges inside the cell) counteracts any INHIBITORY SYNAPSE (I) GABA receptor synaptic cleft + + + + + + + + + Postsynaptic - - - - - - - - - membrane NT binding opens Cl- channel synaptic cleft - Hyperpolarization Postsynaptic -70 -70 -80- - -80 -70 -70 membrane Cl- Cl- Cl-  +60 +50 +40 Membrane potential +30 +20 +10 0 -10 -20 -30 -40 Resting Membrane Potential -50 threshold potential -60 -70 -80 -90 Hyperpolarization  Myelination of neurones The axons of most neurones are encased in a fatty sheath called the myelin sheath. - Myelin functions as an “electrical insulator” - restricts current flow It is the expanded plasma membrane of a neighboring cell called the Schwann cell (or oligodendrocyte in the CNS). Where the sheath of one Schwann cell meets the next, the axon is unprotected. The voltage-gated sodium channels of myelinated neurones are confined to these spots (called nodes of Ranvier). The influx of sodium ions at one node creates enough depolarisation to reach the threshold of the next. Thereby the action potential http://highered.mheducation.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html jumps from one node to the next. This results in a faster propagation  CATEGORIZATION OF MUSCLE TYPES - the cytoplasm: mostly actin and myosin filaments - nuclei and organelles are pushed to the edge - the endoplasmic (sarcoplasmic) reticulum is arranged as a system of tubes around groups of myofibrils. Tubes of the sarcoplasmic reticulum drain into large tubes called T-tubes. From: Human Physiology by Lauralee Sherwood, Cengage Learning SKELETAL MUSCLE  Peripheral Nervous System & nerve- muscle junctions The “output” of PNS consists of motor neurones running from the CNS to the muscles and glands - called effectors - that take action. Nerve (somatic motor nerves) – skeletal muscle – neuromuscular junction (NMJ) Nerve (Autonomic NS) – cardiac or smooth muscle MOTOR NEURONS INNERVATE SKELETAL MUSCLE FIBRES Cell bodies of motor neurons located in ventral horn Thick, myelinated axons (somatic efferent fiber) Motor neurons lose myelin sheath at motor end plate o Several fine branches with many varicosities (swellings), called synaptic boutons o Synaptic cleft o Boutons lie over postsynaptic junctional folds on muscle o Each axon terminal forms a neuromuscular junction with a single  THE NEUROMUSCULAR JUNCTION (I) Motor neurones and skeletal muscle fibres are chemically linked at the Neuromuscular Junction (NMJ) Action potentials traveling down (large, myelinated) motor neurons of the sensory-somatic efferent branch of the nervous systemMEPP cause the contraction of skeletal muscle fibers at which they terminate – always excitatory – create miniature end plate potentials (mEPP) from single vesicle release The junction between the terminal of a motor neuron Neuromuscular Junction What is a Neuromuscular Junction? 1. A chemical synapse between motor neuron and muscle fiber. 2. An electrical synapse between a motor unit and a muscle. What is a Neuromuscular Junction? 1. A chemical synapse between motor neuron and muscle fiber. 2. An electrical synapse between a motor unit and a muscle. THE NEUROMUSCULAR JUNCTION (II) Axon terminal of motor neuron forms neuromuscular junction with a single muscle cell Signals are passed between nerve terminal and muscle fiber by means of neurotransmitter ACh Released ACh binds to receptor sites on motor end plate of muscle cell membrane Binding triggers opening of specific channels in motor end plate Ion movements depolarize motor end plate, producing end-plate potential (EPP or End plate spike) Local current flow between depolarized end plate and adjacent muscle cell membrane brings adjacent areas to threshold Action potential is initiated and propagated throughout muscle fiber At approach to muscle, axon branches and loses myelin sheath Action Potential Each terminal branch innervates a single muscle cell ACETYLCHOLINE IS THE NEUROTRANSMITTER AT THE SKELETAL NEUROMUSCULAR JUNCTION Chemical messenger (neurotransmitter) carries signal from nerve to muscle. Acetylcholine was one of the first neurotransmitters discovered. Acetylcholine is produced in the pre-synaptic terminal of the motor neuron by the enzyme choline acetyltransferase which uses acetyl coenzyme A and choline as substrates. Acetylcholine in the central nervous system is released within the cholinergic system (excitatory)  Presynaptic Terminal Vesicles filled with neurotransmitter Postsynaptic Membrane RELEASE OF ACETYLCHOLINE FROM SYNAPTIC VESICLES The terminals of motor axons contain thousands of vesicles filled with the neurotransmitter acetylcholine (ACh). When an action potential reaches the axon terminal, hundreds of these vesicles release their ACh onto a specialized area of the postsynaptic membrane on the muscle fiber. This area contains a cluster of ligand-gated ion channels that are opened by ACh and let sodium ions (Na+) diffuse in.  1 AP propagated in motor neurone 2 VGCCs open 3 ACh released into cleft 4 ACh binds receptor 1 5 Receptor’s ion channel opens Presynaptic nerve terminal Terminal 6 Ach destroyed by button acetylcholinesterase Synaptic vesicle Ca2+ 2 4 Synaptic cleft 5 3 4 6 ACh receptor 5 Muscle fibre END PLATE POTENTIAL Muscle fiber has resting membrane potential of −80 mV. Influx of sodium ions reduces this potential, creating an end plate potential by depolarization – magnitude depends on amount/duration of ACh. Depolarizing effect of EPP opens voltage-gated Na+ channels eliciting an action potential in the fiber. The action potential sweeps down the length of the fiber and leads to the contraction of the muscle.  Closed ACh receptor ECF + + + + + + + + + + ICF - - - - - - - - - - Muscle fibre ACh binds - opens channel (Na+ moves in) -80 -60 -50- -50 -60 -80 + +  Closed ACh receptor ECF + + + + + + + + + + ICF - - - - - - - - - - Muscle fibre ACh binds - opens channel (Na+ moves in) VGSCs open VGSCs open Direction of AP Direction of AP -80 -60 -50- -50 -60 -80 + + Muscle contraction following Ca2+- release in excitation- contraction coupling NICOTINIC RECEPTORS Receptor for ACh at postsynaptic membrane of skeletal NMJ is the muscle-type nicotinic receptor (NM or N1) The drug nicotine also activates this receptor Nicotinic receptors are ionotropic  ion channel is intrinsic part of the receptor Nicotinic ACh receptors mediate very rapid responses  NICOTINIC RECEPTORS found at the neuromuscular junction of skeletal muscles (NM or N1) - also found in the autonomic nervous system (ganglion) and the central nervous system* *ganglia-type nicotinic receptors (ganglionic/autonomic nACh receptor, NN ACh must be cleared from the synaptic cleft quickly after it is released, to prevent constant muscle stimulation. How is ACh cleared quickly from the synaptic cleft? 1. Broken down by an enzyme and recycled back into synaptic vesicle 2. Taken up by muscle when attached to receptor 3. Slowly diffuses away ACh must be cleared from the synaptic cleft quickly after it is released, to prevent constant muscle stimulation. How is ACh cleared quickly from the synaptic cleft? 1. Broken down by an enzyme and recycled back into synaptic vesicle 2. Taken up by muscle when attached to receptor 3. Slowly diffuses away ACETYLCHOLINESTERASE (AChE) concentrated on the external surface of the postsynaptic membrane and in the synaptic cleft. enzyme breaks down the neurotransmitter ACh in the neuromuscular junction ( 25,000 molecules per second). the sodium channels close, and the field is cleared for the arrival of another nerve impulse.  NEUROMUSCULAR BLOCKING AGENTS Nicotinic cholinergic receptors at the NMJ can be blocked by several drugs Induces paralysis and/or cessation of breathing – respiratory muscles are skeletal muscles, too  NEUROMUSCULAR BLOCKING AGENTS Nicotinic cholinergic receptors at the NMJ can be blocked by several drugs Curare contains substances that inhibit binding of ACh to nAChR Curare paralyses skeletal muscles Treatment? Amazon: South American Indians using poison dart. (extract from plants- not the frogs!)  NEUROMUSCULAR BLOCKING AGENTS Nicotinic cholinergic receptors at the NMJ can be blocked by several drugs Curare contains substances that inhibit binding of ACh to nAChR Curare paralyses skeletal muscles Treatment? Amazon: South American Indians using Acetylcholinesterase inhibitors poison dart. (extract from plants- not the frogs!) D-Tubocurarine and related agents used as neuromuscular  NEUROMUSCULAR BLOCKING AGENTS Botulinum toxin (“Botox”) blocks release of ACh - prevents muscles responding to nerve impulses works by cleaving synaptic proteins required for vesicle release Ingesting 0.0001 mg can kill an adult Caused by improperly canned food Treats dystonias – disorders including spasms, involuntary twitches Cosmetic use: reduce wrinkles/frown lines – permanently contracted muscles  Myasthenia gravis Myasthenia gravis, disease in which immune system attacks motor end-plate ACh receptor. Commonest primary disorder of neuromuscular transmission. Mainly in adulthood: 20 / 100,000 Too little ACh effect – extreme muscle contraction weakness Treated with AChE inhibitor (neostigmine) – prolongs effect of ACh, or immunosuppressants How would neuromuscular transmission be affected in the presence of an acetylcholinesterase (AChE) inhibitor? 1. Muscle contraction would be delayed. 2. Muscle contraction would be prolonged. How would neuromuscular transmission be affected in the presence of an acetylcholinesterase (AChE) inhibitor? 1. Muscle contraction would be delayed. 2. Muscle contraction would be prolonged. Feature Somatic nervous system Site of origin Ventral horn of spinal cord for most, (muscles in head cranial nerves) Neurons from One Origin in CNS to effector organs Target organs Skeletal muscles Type of innervation Effector organs innervated by motor neurons Neurotransmitter at Only ACETYLCHOLINE Effector Organ Effects on Effector Stimulation only (inhibition possible only centrally organs through IPSPs on dendrites and cell body of motor neurones) MEPP Types of control Subject to voluntary control; much coordination subconciously Higher Centres Spinal Cord, Motor Cortex, Basal Nuclei, involved in control Cerebellum, Brain Stem RECOMMENDED READING MATERIALS FUN1: PHYSIOLOGY (for links: click through library proxy required) Human Physiology by Lauralee Sherwood, Brooks/Cole-Cengage Learning Ch. 4 – https://www-dawsonera-com.proxy.library.rcsi.ie/readonline/9781408088838/startPage/133 Ganong, Ganong’s review of medical physiology, Chapter 4, and Chapter 7, in-depth description of neurotransmitters – http://accessmedicine.mhmedical.com.proxy.library.rcsi.ie/content.aspx?bookid=1587&sectio nid=97162455 – http://accessmedicine.mhmedical.com.proxy.library.rcsi.ie/content.aspx?sectionid=97162770 &bookid=1587&jumpsectionID=97162774&Resultclick=2 Guyton & Hall, Medical Physiology, Chapter 4 & 5, more in-depth reading – https://www-clinicalkey-com.proxy.library.rcsi.ie/#!/content/book/3-s2.0-B9781455743773000070 Online resources: Nerve impulse: http://highered.mheducation.com/sites/0072943696/student_view0/chapter8/animation__ voltage-gated_channels_and_the_action_potential__quiz_1_.html & http://highered.mheducation.com/sites/0072943696/student_view0/chapter8/animation__ voltage-gated_channels_and_the_action_potential__quiz_2_.html Transmission at the synapse: http://highered.mheducation.com/sites/0072495855/student_view0/chapter14/animation_ _transmission_across_a_synapse.html