1A03 - CRANIAL NERVES + MEMBRANE POTENTIAL

Cranial Nerves

12 pairs of peripheral nerves that carry messages to and from the brain

Cranial Nerves - Sensory-Only

general senses + special senses

i,ii, viii,

Cranial Nerves - Motor-Only

for skeletal muscles or for the parasympathetic nervous system (muscles + glands)

iii, iv, vi, xi, xii

Olfactory (I)

- smell (sensory only)
- cerebrum

Optic (II)

- vision (sensory only)
- cerebrum

Occulomotor (III)

- eye + upper eyelid movement (motor - somatic)
- pupil constriction + lens focus (motor - parasympathetic)
- brain stem

Trochlear (IV)

- eyeball movement (motor)
- brain stem

Trigeminal (V)

- facial structures (sensory): upper (opthalamic), middle (maxillary), lower (mandibular)
- mastification (motor - mandibular)
- brain stem

Abducens (VI)

- lateral eye movement (motor)
- brain stem

Facial (VII)

- taste (sensory)
- facial expressions (motor)
- salivary + lacrimal glands (motor - parasympathetic)
- brain stem

Vestibulocochlear (VIII)

- balance + hearing (sensory)
- brain stem

Glossopharyngeal (IX)

- taste + swallowing muscles + blood pressure/gases (sensory)
- swallowing (motor)
- parotid gland + saliva (motor - parasympathetic)
- brain stem

Vagus (X)

- blood pressure/gases + taste (sensory)
- voice production + swallowing (motor)
- GI control + respiration + lowers HR (motor - parasympathetic)
- brain stem

Accessory (XI)

- head movements (trapezius + sternocleidomastoid) + swallowing (motor)
- spinal cord

Hypoglossal (XII)

- speech + swallowing (motor)
- brain stem

Electrical Nature of Neurons

polarized / electrically excitable

Concentration Gradient

separates molecules / move things from high concentration to low concentration

Electrical Gradient

separates charges / positive from negative

Resting State

- 70 mV / outside (+) / inside (-) /

Extracellular Fluid at Resting State

high concentration of chloride, sodium, calcium

Intracellular Fluid at Resting State

high concentration of potassium and protein

Causes of Ion Concentration Differences

sodium potassium pump + membrane permeability

Causes of Ion Concentration Differences - Sodium Potassium Pump

creates gradient of sodium + potassium ions / requires ATP / 2 K+ in and 3 Na+ out

Sodium-Postassium Pump

1. ATP binds; becomes ADP and inorganic phosphate; causes shape change
2. Na+ moves towards outside of cell
3. K+ binds, inorganic phosphate moves away; return to initial shape
4. K+ moves in, ADP and inorganic phosphate dissociate and pump stops

Causes of Ion Concentration Differences - Membrane Permeability

number of open channels / size of ions / number of gated channels

Leak Channels

always open / ions move with gradient / ion specific / more K+ channels than Na+ channels

Establishing Resting Membrane Potential

1. Na+ out, K+ in } pumped
2. K+ out, Na+ in } passive transport
3. K+ move positive charge outside cell
4. negative proteins drawn towards membrane
5. K+ drawn towards inside due to charge differences
6. K+ moves due to gradients } in (electrical); out (concentration)

Gated Ion Channels

have gates / open and close on demand / ligand, mechanical, voltage

Ligand-Gated Ion Channels

respond to chemical stimuli / have specific receptors / mostly in soma + dendrites

Mechanically-Gated Ion Channels

respond to mechanical vibration or pressure / physically open gates / mostly in specialized dendrite regions

Voltage-Gated Ion Channels

respond to a direct change in the membrane potential / found in axons and pre-synaptic terminals

Depolarization

voltage moves closer to 0 / sodium and calcium

Hyperpolarization

voltage moves further from 0 / potassium and chloride

Graded (local) Potentials

short distances / ligand + mechanically / in cell body and dendrite regions / can summate

Action Potentials

long + short distances / voltage-gated / all-or-none / magnitude stays constant (-70 mV to 35 mV)

All-or-None Principle

the law that the neuron either fires at 100% or not at all

Action Potential: At Rest

K+ gate = closed
Na+ activation gate = closed
Na+ inactivation gate = opened

Action Potential: Depolarizing

- GP reaches threshold, opens activation gate
- Na+ rushes in
- reaches 35 mV
- K+ gate opens slowly

Action Potential: Repolarizing

- inactivation gate closes at depolarization peak
- Na+ can no longer move
- K+ moves out until resting state is reached

Action Potential: Post-Repolarization

- hit threshold, closes activation gate
- K+ outflow returns membrane potential to -70 mV
- inactivation gate opens, activation gate closes

Action Potential: Afterpotential

- K+ channels stay open longer than they should
- K+ continues to leave
- membrane potential drops to roughly -90 mV

Action Potential: Returning to Rest

- K+ channels close
- sodium/potassium pump + leak channels bring membrane potential back to -70 mV

Action Potential: Refractory Period

when neutron resists production of new signal after AP is produced; less sensitive to 'next' stimulus

Absolute Refractory

even maximum stimulus will not begin AP / must return to resting state

Relative Refractory

supra threshold will start AP / K+ gate open, Na+ gate closed

Suprathreshold

a stimulus that exceeds the threshold to generate an action potential

Stimulus Strength

- determined by frequency of action potentials, until the max rate is reached
- increase in stimulus = increase in GP size → trigger more action potentials
- reach max. stimulus (make AP as fast as possible)
- stimuli greater than max cannot be detected