Action potential 2024-09-18 07_50_16.pdf

XY2141 Anatomy II Action potential and neuromuscular junctions SGM104 Gillian Lewis [email protected] Organisms exist in a changing environment. They must be able to detect changes and coordinate an appropriate response. This may be achieved using the nervous...

XY2141 Anatomy II Action potential and neuromuscular junctions SGM104 Gillian Lewis [email protected] Organisms exist in a changing environment. They must be able to detect changes and coordinate an appropriate response. This may be achieved using the nervous system Sensory Receptors Specialised cells that detect changes in our environment Each type responds to a different stimulus Energy transducers – convert one form of energy to another and generate nerve impulses The more receptors in an area the more sensitive it is  Stimulus detectable change Receptor cells which can sense changes Coordinator Central nervous system (brain/spinal cord) Effector Muscle or gland Response Action taken  Transmission of Nerve Impulses Neurones rapidly transmit impulses as electrical signals These signals are brief changes in the distribution of electrical charges across the plasma membranes of neurones The change in electrical charge is caused by the movement of sodium ions and potassium ions across the plasma membrane  Resting Potential Before an impulse can be generated in a neurone there must be a negative charge across the neurone membrane This is called the resting potential The membrane is said to be polarised  The Action Potential To Do: Use information from the following slides to complete your templates Na+ Key: Na+ Na+ Sodium-potassium pump K+ K+  The Action Potential To Do: Use information from the following slides to complete your templates Key: K+ K+ K+ Leakage channel K+  The Action Potential To Do: Use information from the following slides to complete your templates Key: Na+ Na+ Na+ Na+ Na+ Sodium ion channel Na+  The Action Potential To Do: Use information from the following slides to complete your templates Key: K+ Potassium ion channel K+ K+  Ion Channels Ions (Na+, K+, Cl-, Ca2+ ) diffuse much faster across the membrane than predicted Rates vary in different cells Highly selective 2 conformational states – Open – Closed  Types of ion channels The two basic types of ion channels are gated and leakage (nongated). – Gated channels open and close in response to some sort of stimulus: voltage changes, ligands (chemicals), and mechanical pressure – Leakage (nongated) channels are always open. Gated Ion Channels Voltage-gated channels respond to a direct change in the membrane potential (voltage) Gated Ion Channels Ligand-gated channels respond to a specific chemical stimulus  Gated Ion Channels Mechanically gated ion channels respond to mechanical vibration (hearing) and sense of touch (pressure)  Action Potential Action potentials are a brief reversal of the resting potential across the cell membrane of a neurone They cause a potential difference across the membrane of +40mV  Action Potential Stages of the Action Potential are: 1. Resting potential  Action Potential Stages of the Action Potential are: 2. Depolarisation http://www.psych.ualberta.ca/~ITL/ap/ap.swf  Action Potential Stages of the Action Potential are: 1. Resting potential 2. Depolarisation 3. Repolarisation 4. Hyperpolarisation 5. Resting potential http://www.psych.ualberta.ca/~ITL/ap/ap.swf  Direction of Transmission An action potential is usually only transmitted in one direction – ahead of the previous action potential Because the region behind an action potential takes time to recover from its own action potential and is temporarily incapable of generating a new one – refractory period  Membrane potential Membrane potential= difference in voltage (or electrical potential) between the inside and outside of a cell Non resting cells vs resting cells Measuring membrane potentials – Conventions: outside of a cell always zero and the inside is compared to the outside  Diffusion and Equilibrium potential Diffusion potential – potential difference generated across a membrane when a an ion diffuses down its concentration gradient – only if the membrane is permeable to that ion Equilibrium potential – diffusion potential that exactly balances or opposes the tendency for diffusion down the concentration difference – chemical and electrical driving forces are equal and opposite  Nernst equation If the membrane is permeable to only a single ion we can predict the membrane voltage (potential)→ Nernst equation Converts a concentration difference for an ion into a voltage E – equilibrium potential in millivolts (mV) Co– extracellular concentration of ion Ci – intracellular concentration of ion z – valency of the ion e.g. - Na+ & K+ valence is +1 - Cl- valence is -1 - Ca2+ valence is +2 R, F – constants z – valency of the ion. Na+ & K+ valence is +1 T – temperature in 0K Cl- valence is -1 ln – natural logs Ca2+ valence is +2  At 200C this can be simplified to : E (mV) = 58.log10 Co z Ci At 370C this can be simplified to : E (mV) = 61.log10 Co z Ci Calculating the equilibrium potentials E (mV) = 58.log10 Co z Ci For sodium (Na+): Co/Ci 150/15=10 log10() log(10)=1 58x1=58 Valency z for Na is +1 58 58/1=+58 z So E (mV) =+58mV for the sodium ion  A real membrane potential Real membranes are permeable to more than one ion The exact value of the resting membrane potential depends on: – Polarity of the electrical charge of the ion – Concentration gradient for each ion – The permeability of the membrane to each ion Measure by the Goldman equation:  Resting membrane potentials Two cell types: nerve and muscle cells use these characteristics to produce a resting membrane potential. Period between action potential These cells communicate with each other by changing the resting membrane potential. The membrane of a resting neurone is positive outside and negative inside (between - 70mV and -80mV) due to: the distribution of different ions across the membrane the relative permeability of the membrane toward permeability at rest → K + and Cl − resting membrane potential close to equilibrium potential permeability at rest→ Na + and Ca 2+ resting membrane potential far from the equilibrium potential They produce→ action potentials or graded potentials  Action potential (phases) 1. Resting membrane potential - Membrane potential around -70mV - K + conductance or permeability is high - Na + conductance is low 2. Action potential - Inward current depolarise the membrane - Opening Na+ channels (permeability increases) 3. Repolarization of the action potential - Na + channels respond to depolarization by closing but slowly - Opening K + channels faster - K + conductance much higher than the Na + conductance 4. Hyperpolarizing afterpotential (undershoot) - K + conductance is higher than at rest  Action Potential Temporary depolarisation of the membrane where the action potential is causes a “local circuit” to be set up between the depolarised region and the resting regions either side Action Potential This depolarises regions next to the action potential and causes voltage gated sodium channels to open Action Potential Sodium ions flood in A few milliseconds later potassium ions flood out Causes an action potential Action Potential  Conduction velocity Speed at which the action potential is propagated along the nerve fibre Physiologically, inform of the of the speed at which information can be transmitted Cable properties 1. Time constant (τ) → how quickly a cell membrane depolarizes o hyperpolarizes in response to an inward/ outward current 2. Length constant (λ) → how far a depolarizing current will spread along a nerve  Increasing conduction velocity 1. Increasing nerve diameter 2. Myelination Electrical insulator → decreases ion flow through the membrane Myelin sheath around the axon Schwann cell → sphingomyelin Action potential faster in myelinated fibres as they “jump” Saltatory Conduction in Myelinated Fibres from Node to Node – Node of Ranvier→ small uninsulated area action potentials can occur. Neuromuscular junction Motor neurones are the nerves that innervate muscle fibers anterior horns of the spinal cord Motor unit : single motor neurone and the muscle fibre innervate Neuromuscular junction is the junction between Motor neurone and Skeletal muscle fibre. They are linked chemically. Neurotransmitter at neuromuscular junction is Acetylcholine. As axon approaches muscle , it divides into many terminal branches and loses its myelin sheath Axon terminal branch ends in an enlarged knob-like structure called Terminal button. Vesicles which contain chemical transmitter are present in terminal button. Motor end-plate A motor end plate is a chemical synapse between nerve terminals that invaginate into the surface of the muscle fibre 1. Axon Terminal - Contains around 300,000 vesicles which contain acetylcholine (Ach) 2. Synaptic Cleft : - 20 – 30 nm space between the axon terminal & the muscle cell membrane. 3. Synaptic Gutter ( Synaptic Trough) - It is the muscle cell membrane which is in contact with the nerve terminal. It has many folds called Subneural Clefts. Ach receptors are located here Events at the neuromuscular junction 1. An action potential to the axon terminal (terminal button) 2. AP at the axon terminal causes the opening of Ca2+ channels 3. Ca2+ triggers the release of acetylcholine (ACh) 4. ACh diffuses across the synaptic cleft and binds with RECEPTORS on motor end plate 5. This binding causes opening of Na+ channels and Na+ entry into muscle cell 6. Na+ entry causes depolarization of Motor end plate called END PLATE POTENTIAL ( EPP) 7. The resultant Na+ entry initiates action- potential in the muscle fiber 8. Propagation of the action potential 9. Acetylcholine is destroyed by enzyme acetylcholinesterase Acetylcholine Formation and Release Acetylcholine binding to the receptor  End plate potential Provoked by the influx of Na+ inside of the muscle fiber Local potential→ end plate It is the initiator of the AP Threshold Action potential  Release of Ca+ from the T tubule–sarcoplasmic reticulum system 1. Action potential 2. Dihydropyridine receptors→ feel voltage changes 3. Pull the ryanodine receptor channels out in the sarcoplasmic reticular cisternae 4. Ca+ release 5. Releasing causes contraction 6. Re-uptake of the calcium ions by a calcium pump by ATP