Neuro Physiology - Nerves, Action Potentials, and the NMJ Flashcards
What is the resting membrane potential?
The steady state voltage across the cell membrane at rest, normally -70mV.
Determined by:
Ions present and their concentrations
Membrane permeability to these ions (100x more permeable to K than Na, causing it to leak down its gradient)
Ion pumps that alter these gradients (Na-K ATPase 3Na out for every 2K in)
Negatively charged phosplate and protein ions also contribute to the potential (Donan effect)
Draw and explain an action potential in a peripheral nerve
IMAGE
The spontaneous depolarisation of any excitable cell as a result of a stimulus, such as ligand binding or voltage gated channel opening, split into four phases.
Phase 1: Resting potential of -70mV rises towards threshold potential of -55mV
Phase 2: All-or-nothing action potential, voltage gated Na channels open, membrane potential of +30mV
Phase 3: Na channels close, K channels open, causing rapid repolarisation
Phase 4: Temporary hyperpolarisation before the NaKATPase pump restores resting potential
What are the types of refractory period in nerve cells?
Absolute: The duration of time after an action potential, where another action potential cannot be conducted, no matter the stimulus applied.
During this time, the membrane ptoential is more positvie than the threshold
Relative: Where a second action potential could be transmitted but it would need to be supramaximal. This occurs during the period of hyperpolariation (Phase 4)
What is the effect of hypokalaemia on the action potential?
Potassium is primarily intracellular, and hypokalaemia can be categorised into acute or chronic.
Acute: Extracellular potassium decreases, causing an increased concentration gradient - cell becomes hyperpolarised and less excitable, with reduced conductivity, explaining symptom of muscle weakness.
Chronic: Intracellular K has time to redistrible and restore the correct gradient, so muscle weakness isn’t seen
What are the different types of nerve fibres?
IMAGE
Aα 12-20 / 80-120 / Motor & Sensory (Propriocemption)
Aβ 5 / 30-70-12 / Sensory (Touch & pressure)
Aγ 3-6 / 15-30 / Motor to muscle spindles
Aδ 2-5 / 12-30 / Sensory (Pain, touch & temperature)
B < 3 / 3-15 / Autonomic (Preganglionic)
C 0.4-1.3 / 0.5-25 / Autonomic (Post-ganglionic) & Sensory (Pain, temperature)
Diameter (um) / Velocity (m/s) / Function
Myelinated fibres demonstrate saltatory conduction where the action potential jumps from one node of Ranvier to the next
What would an action potential look like in a compound nerve made up of all different nerve fibres?
IMAGE
Differing conduction velocities mean that if a supramaximal stimulus is applied, then all fibres will depolarse, but then conduct at different velocities.
