1A03 - CRANIAL NERVES + MEMBRANE POTENTIAL

Cranial Nerves

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