Action and membrane potential + Synaptic transmission

Resting membrane potential of cell

-70 mV

How does a cell maintain its ionic charge

Using active transport pumps (e.g. Na+ K+ ATPase) to counteract the concentration potential trying to pump K+ out. Electrostatic attraction will also draw some K+ into the cell, as it is naturally negatively charged. The biggest factor, however is K+ efflux through K+ leak channels which increase permeability of K+ to the membrane

Nernst equation at 37C and its use

61mV/valence * log ( [ion outside]/[ion inside] )

Used to calculate equilibrium potential of one membrane permeable ion

ICF and ECF concentrations and _eion values for K+

ICF - 100 mM

ECF - 5mM

eIon - -80mV

ICF and ECF concentrations and _eion values for Na+

ICF - 15 mM

ECF - 150mM

eIon - +62mV

ICF and ECF concentrations and _eion values for Cl-

ICF - 13 mM

ECF - 150 mM

eIon - -65mV

ICF and ECF concentrations and _eion values for Ca2+

ICF - 0.0002

ECF - 2mM

eIon - +123mV

Goldmann equation at 37C and its use

Used to calculate overall membrane resting potential, accounting for all ions and their permeability

What effect would hypokalaemia (low [K+] in ECF) have on resting membrane potential?

Would make it more negative, as it would shift the concentration gradient out the cell, causing a greater efflux of positively charged potassium out of the cell, making it more negative.

What voltage-gated channels open when a stimulus is applied

VGNaC open relative to stimulus applied (stronger stimulus, more Na+, which depolarises RMP)

What voltage-gated channels are activated to open when a the action threshold is reached?

VGNaC and VGKC, but VGKC open slower

What causes the action potential to spike so much once an action threshold is reached

VGNaC are fully open, causing a massive Na+ influx into cell, causing the potential to become very Positive (+30 mV)

What causes the the spike of action potential to go down

VGNaC deactivate and close, and the VGKC channels finish opening, causing a massive K efflux, dropping the potential and repolarising the cellW

Why does the membrane potential hyperpolarise after an action potential

VGKC stay open a little after the membrane is repolarised, causing more K+ to efflux and drop the membrane potential

Difference between absolute and relative refractory period

Absolute - No matter the strength of the stimulus, it will not trigger another action potential

Relative - A strong enough stimulus will cause another action potential

Why are absolute refractory period absolute

VGNaCs already opening (during depolarisation) or inactive (during repolarisation), and so cannot open again to trigger another aciton potential

Why are refractory periods important

1. Sets the maximum frequency action potentials are generated

2. Limits the rate that signals can be transmitted

   1. Ensures one-way travel of action potential

3 factors that affect conduction velocity

Myelination - Insulated parts means it doesn’t need ion channels along those parts, and so less leakage occurs. Only need ion channels at Nodes of Ranvier

Axon Fibre Radius - action potential scales with diameter

Functional Ion Channel Number - less channels = less leakage = faster conduction (almost like maintain water pressure in a pipe)

Main example of a synchronised synapse use. Also how it achieves synchronisation

Cardiac muscle needs to be synchronised.

Achieves synchronisation by gap junctions

Define a reflex

A stereotyped, involuntary action caused by a specific stimulus

3 general functions of reflexes

1. Protective

   1. (limb withdrawal, coughing etc)

2. Postural

   1. (ensuring you don’t fall whilst walking, balance etc.)

3. Homeostasis

   1. Maintaining blood pressure etc.

Reflex arc 5 (or 6) components

Func

Function of muscle spindle

Causes muscle reflex to tense when under tension and relaxed to prevent damage

What makes the muscle spindle reflex arc different to a normal reflex arc

No interneurone, goes directly from sensor in spindle, through spinal cord to effector in muscle (only one synapse between effector and sensor (monosynaptic))

Why can’t you tense two antagonistic muscle simultaneously (e.g. quad and hamstring)

When tensing one, (voluntarily or no) interneurones sent an inhibitory NT to the other, preventing contraction

Purpose of Golgi tendon organs

To release inhibitory NT from the interneurone to the muscle when too much strain is placed on the tendon

Also control fine muscle tension (e.g. in fingers)