Physiology - Nerve & Muscle Cells Flashcards
Define resting membrane potential
- describes the difference in charge across a cell membrane at rest as a result of separation of positive and negative charges
- requires a concentration gradient with different membrane permeabilities and unequal distribution of ions (mainly Na+/K+)
How is resting membrane potential created?
- Na+K+ATPase maintains the gradient by pumping 3 Na+ out for 2 K+ in
- Na+ and K+ then move down their concentration gradient (Na+ moves in, K+ moves out)
- at rest there are more open K+ channels than Na+ channels, thus permeability of K+ is greater and net movement of K+ out
- K+ concentration gradient is the main determinant of the resting membrane potential
- inside of cell is negative relative to outside
- RMP neuron = -70, RMP muscle cell = -90
Why is a cell more excitable in hyperkalaemia
RMP moves closer to threshold potential for eliciting an action potential
Describe the sequence of events in skeletal muscle contraction/relaxation
1) resting membrane potential in skeletal muscle is -90mV
2) build up of EPSP, not a full action potential yet
- ligand-gated Na+ channels are open and Na+ moving into the cell, moving towards the threshold potential of -55mV
3) threshold potential is reached and voltage-gated Na+ channels open with rapid influx of Na+ into the cell (depolarisation)
- action potential spreads along T tubules, activating voltage-gated DHPR
- DHPR interacts with RYR on SR, causing release of calcium from the SR
- calcium binds to troponin C, causing a shift in tropomyosin to uncover myosin binding site on actin
- myosin now able to interact with actin, forming cross bridge and the power stroke
4) membrane potential moves towards equilibrium potential for Na+, causing closure of Na+ channels at around +20
5) voltage-gated K+ channels open, causing K+ to move out of cell and fall in membrane potential (repolarisation)
- calcium is pumped back into SR and released from tropomyosin, thus ending interaction between actin and myosin
6) slow return of K+ channels to resting state (after hyper-polarisation)
7) return to resting potential
Draw a skeletal muscle action potential
- initial action potential travels down axon and causes opening of voltage-gated calcium channels
- this causes release of NT acetylcholine from pre-synaptic vesicles
- ACh binds to nicotinic post-synaptic membrane receptors, increases Na+ conductance, causing an EPSP
- once threshold level is reach, there is an action potential
Describe summation of contraction
- summation of contraction occurs when there is repeated stimulus before relaxation occurs
- the contractile mechanism itself does not have a refractory period, so there is repeated contractile response
- with rapidly repeated stimulation, individual responses fuse into one continuous contraction = tetanic contraction
In the synapse, where can inhibition occur
- pre-synaptic: occurs on axon to axon synapses, causes reduction in Ca+2 conductance and thus NT release
- post-synaptic: occurs by GABA causing increased Cl- conductance, altering RMP so less likely to fire
What are the major different types of skeletal muscle
- type 1 = slow, oxidative, red colour, small diameter SO
- type 2 = fast, glycolytic, white colour, large diameter FG
Describe the contraction of visceral smooth muscle
- starts with binding of acetylcholine to muscarinic receptors
- causes increased Ca+2 influx into cell from extracellular fluid and not SR
- calcium binds to calmodulin, activating the calmodulin-dependent myosin light chain kinase
- phosphorylation of myosin -> binding to actin and contraction
- dephosphorylation of myosin light chain by phosphatase -> relaxation of muscle
What influences intestinal smooth muscle contraction
- increased by: cold, stretch, ACh, phospholipase C
- decreased by: sympathetic stimulation, cAMP
Where are ion channels distributed in myelinated neurons
voltage-gated Na+ channels are concentrated at Nodes of Ranvier
What affects conduction
myelinated vs demyelinated
saltatory vs non-saltatory
diameter
direction of conduction
What are the classes of nerve fibers
- A alpha: somatic motor and proprioception
- B beta: touch and pressure
- A gamma: motor to muscle spindle
- A delta: pain and temperature
- B: preganglionic autonomic
- C dorsal roots: pain and temperature
- C sympathetic: postganglionic
What is the relevance of fibre types to emergency medicine
small diameter pain fibres are more susceptible to local anaesthetic than motor
Draw and describe the action potential of a neuron
1) resting membrane potential of -70mV, action potential on pre-synaptic membrane causes release of ACh
ACh binds to post-synaptic nicotinic receptors, opening ligand-gated Na+ channels
2) ligand-gated Na+ channels are open and Na+ moving into the cell, moving towards the threshold potential of -55mV
3) threshold potential is reached and voltage-gated Na+ channels open with rapid influx of Na+ into the cell (depolarisation)
4) membrane potential moves towards equilibrium potential for Na+ → Na+ channels close
5) voltage-gated K+ channels open → K+ moves out of cell → repolarisation
6) slow return of K+ channels to closed state = after-hyper-polarisation
7) return to resting potential
