Lecture 8 + Assignment 7 Flashcards
Vestibular labyrinth contains 5 things
Two otolith organs
- utricle and saccule
= detect head tilt and linear acceleration
Three semicircular canals
- superior, posterior, horizontal
= detect head rotation
Otolith organ orientations
utricle
= horizontal hair cells
- face towards striola
saccule
= vertical hair cells
- face away from striola
Otolith organ components
otoconia chalk pieces (cal. carbo. CaCO3)
= 1-5 micron diameter
otolithic jelly membrane
hair cells
supporting cells
Otolith organ tilt / lin. acceleration detection
otoconia add mass+inertia to the otolithic membrane
makes jelly lag behind
Adaptation of hair cells in otolith organs
Transduction:
Head tilt
- more tip link tension
- opens K/Ca channels
- depolarization
adaptation:
- more Ca inside stereocilium
- tip link motor slips
- closes K/Ca channels
3 planes of rotation
Yaw
= rotation around z-axis
Pitch
= rotation around y-axis
Roll
= rotation around x-axis
forward and back
Semicircular canal organization
are orthogonal to one another (90 degrees)
roughly in the three planes
each detect rotation in their own plane
Semicircular canal components
Ampulla: bulge in the bony canal
Cupula: gelatinous substance in which stereocilia are embedded
Crista: epithelial cell layer in which the hair cell bodies are embedded
Semicircular canal detecting acceleration
- cupula displaced in the opposite direction of movement
- due to endolymph inertia
Rotation stimulus graph
transduction (up)
quickly adapts back to the baseline (no more acceleration)
off response (down)
- bumps into cupula from other side when you stop
hair cells hyperpolarize
= firing rate below baseline
Vestibulo-ocular reflex circuitry
- scarpa’s ganglion / cranial nerve 8 comes into the medulla
- synapses onto neurons in vestibular nucleus
- decussates in rostral medulla
- synapses onto abducens nucleus
- connects to 2 neurons
- one goes out through 6th cranial nerve
= releases acetyl choline into muscle (lateral rectus of opposite eye) - causes contraction
- other decussates and ascends into the midbrain
- release glutamate onto oculomotor nucleus
- onto 3rd cranial nerve
- signals to release acetyl choline onto medial rectus
- causes turn to opposite side
- both eyes rotate in the opposite direction of head movement
- inhibitory relaxes the antagonist
Do peripheral nerves decussate
no
anything that leaves central does not decussate
3 nerves involved in VOR
CN 3 - oculomotor nerve
- motor
- eye movements
- pupil constriction
- upper eyelid muscle
CN 6 - abducens nerve
- abducens or lateral eye movements
CN 8 - vestibulocochlear (auditory) nerve
- hearing, balance
VOR during left rotation
activates:
1. the hair cells on the left horizontal semicircular canal
2. the left vestibular nucleus
3. the right abducens nucleus -> right lateral rectus muscle
4. the left abducens nucleus -> left lateral rectus muscle
= rightward eye rotation
Oscillopsia
- bilateral loss of VOR
- causes thinks to look like they’re shaking + vision blurring
- can be due to some antibiotics
ex. hair cells destroyed by ototoxic medications such as streptomycin
Benign paroxysmal positional vertigo (BPPV)
- dislodged otoconia enter posterior semicircular canal
- causes dizziness in certain head positions
- can be helped by Epley Maneuver which tries to return them to correct spot
Positional alcohol nystagmus
- alcohol enters cupula
- makes it lighter than endolymph = buoyant
- causes it to deflect in certain head positions
Primary motor and premotor cortex
premotor
- in front
- brod. area 6
- lesion here causes apraxia = inability to PLAN and execute complex voluntary motor tasks
- much larger in humans than other animals
primary motor
- further back
- brod. area 4
- correlates to animal size
Motor homunculus
looks like somatosensory homunculus
- corticospinal tract
- top
- upper and lower extremities
- axons go all the way down to the spinal cord
- cortical efferent
- commands muscle movements - corticobulbar tract
- face
- goes to brain stem motor neurons
cortex layer 5
Corticospinal and corticobulbar tracts
do this
LCST and VCST
LCST
- 90% of the CST axons
- decussates in the medulla
- terminates contralaterally in the lateral part of the ventral horn
DISTAL muscles
VCST
- 10% of CST axons
- does not decussate in the medulla
- terminates bilaterally (mainly contralaterally) in the medial ventral horn
AXIAL/PROXIMAL muscles
Lateral corticospinal tract topography
on the sides of the butterfly
SLTC CTLS
Directional tuning of a M1 neuron
primary motor complex
- respond preferentially to a certain angle of movement
- can do a population vector to see resulting vector
- ones that fire more have more influence
Dr. Apostolos Georgeopolous
- Came up with idea of population vector for the motor cortex
- M1 neurons basically vote on what to movement to cause
Motor neuron pool
- group of alpha motor neurons in the ventral horn of the spinal cord that all go to one muscle
- can span multiple spinal segments
Motor unit
- alpha (lower) motor neuron connects to multiple muscle fibers and causes contraction
- larger muscles = more fibers innervated by a single alpha motor neuron
- gives more control and dexterity, more graded movement
ex.
leg muscles = 1000 innervated by 1
fingers = 10
extraocular = 3
Neuromuscular junction
- synapse between an alpha motor neuron and a muscle fiber
- axon releases acetyl choline into a cleft, binds to receptors
- muscle AP causes contraction
= movement
instead of reuptake, acetylcholine degraded by acetylcholinesterase enzyme
each vesicle = 10 000 ACh molecules
AChE = degrades 5000 molecules/second
Amyotropic lateral sclerosis
- Lou Gehrig’s disease
- death of lower and upper motor neurons
- flaccid paralysis (loss of muscle tone)
- muscles atrophy
Duchenne muscular dystrophy
- death of skeletal muscles
- x-linked recessive genetic mutation in gene for dystrophin protein
dystrophin
- helps hold muscle cells together, part of the cytoskeleton
Myasthenia Gravis
- autoimmune destruction of skeletal muscle ACh receptors
= lack of communication - alpha motor neurons and muscle NOT damaged
- normal life expectancy with treatment
ex. inject acetylcholinesterase inhibitor
= keeps ACh in cleft
Multi-electrode extracellular recording + Dr. Andrew Schwartz
Record from multiple electrodes at the same time
- Very close together on an array
Done by Dr. Andrew Schwartz
- Recordings in monkeys, looked at M1 and resulting arm movement
- M1 population vector just precedes arm movement bc command must travel down
- Could use M1 (brain) to move a robotic arm
Used for human paralysis
Inertia
an objects resistance to changing speed
due to otoconia or endolymph
Shear force direction
- opposite direction of acceleration
Muscle spindle
- set of small thin muscle fibers
- inside main extrafusal fibers, parallel to them
- 1a afferent axon weaves between them
when the muscle is stretched
- 1a mech gated ion channels open due to deformation
- signals lengthening
Golgi tendon organ
- not in parallel to the muscle
instead in series with it - full of collagen fibers and 1b afferent axon weaving between them
when the muscle is stretched
- collagen fibers stretch
- 1b mech gated ion channels open
- collagen hard to stretch = signals tension
Gamma motor neuron
- innervates the intrafusal muscle fibers (alpha innervates extrafusal)
- when muscle contracted and short, slack muscle spindle
- gamma tightens it up again
- resets it to detect changes again
makes 1a fire again
Knee-jerk / myotatic stretch reflex
- Tap tendon pulls knee down, stretches quadriceps muscle, makes leg kick out
- Stretch causes contraction
- Muscles have sensors
- Upon stretch, mech gated ion channels in muscle spindle cause 1a axons to fire APs
- Enters dorsal horn, release glutamate onto alpha lower motor neuron = returns to muscle + releases ACh = muscle contraction
Spinal inhibitory interneuron releases GABA onto flexor alpha motor that innervates antagonist muscle
ex.
Head tilting, stretch causes contraction to keep posture
Spring in step be stretch causes contraction again = efficient
Autogenic inhibition reflex
- so you don’t hit yourself in the face when lifting a drink
Golgi tendon organ: defects tension in the muscle=APs in 1B axon (btwn tendon/ muscles
Dorsal spinal cord -> ventral horn -> synapse onto inhibitory interneuron -> back to same muscle
REDUCES CONTRACTION
ALSO
- 1B excites excitatory purple interneuron
= activates triceps muscle to oppose movement
Flexion-crossed extension reflex
Step on something painful
= Adelta or C axon signal
- Goes to STT AND dorsal horn excitatory interneurons to either excite or inhibit
- same interneurons decussate
Realises pain
- flexor muscle makes painful foot pull up and relaxes the antagonist muscle
Other side
- opposite
* hamstring relaxed, extensor muscles activated so quad straightens
* you don’t fall over when you put all your weight on one foot
Polysynaptic reflex (multiple synapses)
Polysynaptic excitation of right flexor and left extensor
Polysynaptic inhibition of right extensor and left flexor
