midterm 1 Flashcards
who was the first person to coin the term motor unit + claim the contraction MU as a *Fundamental functional unit of
contraction
Charles Sherrington
Motor unit definition
Alpha motor neuron and all skeletal
muscle fibres innervated by its axon
*
What are the two ways force is controlled
- altering the # of active MUs (RECRUITMENT)
- changing the frequency of activation (RATE CODING)
Where do excitatory post synaptic potentials summate to generate an action potential
axon hillock
Motor Neuron labeled
who is Lugi Galvani
Discovered that frog leg muscles twitch when electricity added
saltatory conduction
propagation of action potential down an axon
myelination
insulates axon, prevents movements of ions (Na+,K+) across the membrane.
Nodes of ranvier
gaps in the myelin sheath
Ion movement at the nodes of ranvier causes…
an AP at one node to bring the next node to threshold to initiate another AP
Saltatory conduction + Myelination PROS
- Increase conduction VELOCITY (without a change in axon diameter)
- Reduced METABOLIC cost (only small segments of the axon require NA/K+ pump to restore resting membrane potential)
Neuromuscular junction synapse: MN AP + Muscle Fibre AP relationship
1:1 relationship
What neurotransmitter is released in the neuromuscular junction
ACH (acetylcholine)
Safety factor of neurotransmitter in NMJ
3-5x ACH is released, needed for muscle fibre AP
Curare function
prevents BINDING ACH to ACH receptor
what happens when curare prevents ACH binding to receptor?
muscle fibers cannot generate an action potential
how long does curare last
8-30 hours
how was curare used -historically-?
used as a medical paralyzing agent + treatment of tetanus
Botox function
prevents RELEASE of ACH, causes botulism
what happens when botox prevents release of ACH?
Inability for excitation along the sarcolemma
How long does botox last
6-8 weeks
what are the two treatments botox is used for?
- overactive muslces
- cosmetic improvements
3 motor unit classification types
type 1
type 2a
type 2x
Type 1
INNERVATE Slow oxidative muscle fibers, FORM Slow MU
type 2a
INNERVATE FOG: fast oxidative-glycolytic, FORM fatigue resistant MU
type 2x
INNERVATE: G: glycolytic,
FORM: fast fatiguable MU
twitch
physiological response of MU to summation
twitch can cause
summation; increased firing rate causing summation to achieve tetanus
how is contraction speed of motor units determined
behaviour of 3 types of motor units determined by the neuron and muscle fibre they’re connected with
So, what would be the benefit of:
* Low threshold (type I) motor neurons or small motor
neurons innervate slow oxidative muscle fibres?
* Large motor neurons, high threshold (Type II) motor
neurons innervate fast contracting, fast fatiguable or fast
glycolytic muscle fibers?
- Size principle allows for fatigue resistance. Since smaller motor neuron’s have lowest activation principle they are recruited first. This allows us to save our energy until activity demands
what are the two neurotoxins we learnt about
curare and botox
what is a refractory period
period after an action potential where the neuron or muscle fibre is temporarily unavailable to generate another action potential
what is a neural strategy to grade force
- firing rate
- motor unit recruitment
Input resistance definition
how easy it is to excite the motor neuron: ex) diameter of hose
what type of input resistance do small MNs (type 1) have
High input resistance-greater response, easier to excite
Rheobase
direct measure of the current that has to be put into a neuron to get it to fire
ex) flow of water
small MN are __ to excite
easier
Rheobase is ___ in small MNs
low
Conduction velocity is __ in small MNs
slow (because small MN are easy to excite)
Conduction velocity in large MN
shorter after-hyperpolarization
what law is the action potential propagation along the axon determined by
Ohm’s law (current x resistance= voltage)
conduction velocity ex
ex) pressure of water
what is size principle- how are MN recruited
from smallest to largest
axon terminal
where synapses occur
Neuromuscular junction
how the axon connects to muscle
SAG
Fast MU have the property of SAG where force decreases over time
what is the MU size principle good for
simplifies the task of force modulation:
- ensures smooth increase in force production
- minmizes fatigue
*like a dimmer switch
negative effects of size principle
cannot selectively choose which MU to recruit
recruitment threshold
amount of force needed to turn a motor unit on
force frequency curve
sigmoidal relationship
firing rate matches
contractile speed
In MUs, who’s the boss
Motor neuron always the boss; the muscle will change properties
how are we able to produce a steady/smooth contraction at low firing MU rates
each MU are able to produce partially fused tetanus: because units fire asynchronously with each other, the net force is smooth
surface EMG
measures superficial muscle activity non invasive (electrodes placed on skin)
Indwelling EMG
somewhat invasive but can observe a single motor unit (needle) unlike regular EMG (can only see summation)
MVC
maximal voluntary contraction
how does force modulation/recruitment vary depending on the muscle type
-varies between muscles because of their function, fibre composition and control demands. more useful muscles with increase its force to maximal right away whereas less used muscles like the adductor pollicis use more rate coding and low and slow recruitment to get them all recruited
How does EMG work
picks up action potentials going across muscle fibre, picks up summation of AP but can’t see individual AP because of summation
how can we hold a steady hand with MU active at the same time
they are active asynchronously allowing use to hold a steady hand because force from MU combined
Eric is exercising at Dwight Schrute’s Gym for Muscles and progressively increases the weight that he is
lifting. What neural strategies, in context of a motor unit, will allow him to increase his force?
- motor unit recruitment + size principle
- rate coding
- motor unit synchronization
- increased neuromuscular efficiency
Communication between neuron’s is…
1:1
Divergence
a single neuron synapses on multiple neuron’s
Convergence
multiple neurons converge on fewer neurons
peripheral structures
can be extrafusal fibres or intramural fibres
extrafusal fibers
skeletal muscle fibers
intrafursal fibers
muscle spindles
direction of neural information can be
afferent or efferent
afferent
to the brain
efferent
away from the brain
role of afferent inputs
projects CENTRALLY to the spinal cord and more superior REFLEXS
where is the cell body in afferent fibers
in the dorsal root ganglion
what type of receptors do afferent fibers have
sensory receptors
how are afferents labeled
based on cross sectional diameter (1=largest 4=smallest)
conduction velocity depends on….
diameter
larger diameter=
faster conduction
what type of neurons are the fastest
1A
group 1a senses
length and velocity
group 2 senses
static length
what shape are muscle spindle receptors in
fusiform (football) shape: lie in parallel with force-producing muscle
what type of muscles have highest density
hang muscles (more distal muscles)
What type of muscles have fewer muscle spindles
more proximal muscle
what are the two types of receptors
- Bag (bag 1=dynamic, bag 2=static)
- Chain= static
what are the two types of afferents
- type 1a (primary)=length & velocity
- type 2 (secondary)= length
what do type 1a (primary) afferents innervate
bag 1, bag 2, chain
what do type 2 (secondary) afferents innervate
bag 2, chain `
how can we record muscle spindle action
microneurography
what does a microneurography look at
single unit action potentials
type 1as record what?
velocity and length
type 2s record what?
length
what does a tendon tap do
stretches intrafusal muscle spindles
what responds to tendon taps
Primaries (1a) are very sensitive to taps and vibrations, can stop firing on release (unloading)
efferent system consists of
fusimotor or gamma system
Describe the efferent systesm
- only receptor to have it’s own efferent motor system
- consists of dynamic and static MN
- skeletalmotor vs fusimotor
alpha motor neurons sends info to
skeletal muscle fibers
Gamma motor neurons sends info to
muscle spindles
dynamic gamma sends info to
bag 1
static gamma sends info to
bag 2 and chain
Importance of gamma system
Muscles stretched and contracts muscles with NO AP (uses its own motor system to shorten)
Turning on the gamma system does what?
prevents the spindle from becoming unloaded during shortening contractions: keeping it sensitive to stretch
gamma dynamic makes the spindle?
more velocity sensitive
gamma static makes the spindle more?
length sensitive
activating the gamma (mn) STATIC efferent causes
decreased dynamic response
activating the gamma (mn) DYNAMIC efferent causes
increased dynamic response
co-activation of the alpha-gamma systems causes
the muscle spindle to maintain it’s sensitivity, be more easily excited
why does the soleus have lower motor unit firing rates (less rate coding) then a bigger muscles
- they rely on recruitment rather then rate coding
smaller muscles rely on
recruitment
Rate coding
used when all motor units are already recruited for force production
recruitment
used for gradual force increase
what can we do to maintain a steady/smooth contraction
Summation of twitches, motor units firing ASYNCHRONOUSLY
how does the size of the action potential threshold change depending on fast or slow contractions
action potential threshold remains the same
what is the function of a muscle spindle
- detect stretch, velocity, and length
what is alpha gamma co-activation
when both alpha and gamma MN activate together, the muscle spindle can remain it’s sensitivity during shortening and not have to be reactivated again
why does the tendon tap cause a reflexive kick
when you hit the tendon, you get a stretch of the muscle, 1a spindles are sensitive to stretch and activate MN due to Monosynaptic relfex
monosynaptic reflex
muscle stretch reflex that facilitates communication between sensory neurone and innervating the muscle
why are gamma motor neurone so important
allow spindles to maintain their sensitivity, tightness, and prevents muscle spindles from becoming unloaded
GTOs
Golgi tendon organs
where are GTOs located
tendon junction
where do GTO spindles lie
In bundles within a capsule including nerve endings and collagen fibres in parallel with the muscle fibres, they are spread throughout muscle belly
GTO main goal
to sense force: and the amount of force being produced,
Nerve endings interdigitate among?
collagen ( this allows them to fire AP )
the afferent finer that innervates the Golgi tendon organ is
the 1B afferent
Golgi tendon organs are bundles within a capsule including
nerve endings and collage fibers
Whats the role of the muscle spindle receptors
sense length and velocity of muscle
what is the function of the Golgi tendon organ receptors
- provide feedback to the CNS about muscle force/tension
- primarily active force/tension
what technique allows you to record action potentials from sensory afferents in awake humans
Electromyography (from muscle activity) and Microneurography (from sensory afferents)
How are GTOs excited
Muscle tension (by level of force)
how much force can excite GTOs and activate AP
30-90 millinewtons
what type of forces are GTOs sensitive to
actively generate forces rather than passive stretch
A _____________ is the force output of a muscle
in response to one stimulus
twitch
how do GTOs respond to contract
respond to increased muscle tension during a twitch
What is the muscle function of the GTO
INHIBITS the AGONIST muscle
how do GTOS provide motor feedback
through the 1B afferent
How does the 1B inhibitory interneuron connect
it has a disynaptic connection to the motor neurons
Disynaptic connection to motor is called
autogenic inhibition reflex
autogenic inhibition
inhibits the muscle that’s doing the contraction inhibiting the agonist muscle
Golgi tendon organs Inhibitory agonist reflex is for 2 aspects
protective mechanisms and force modulation
where are joint receptors located
within joint capsule: joint ligaments + loose tissue
Where are there no receptors
in the cartilage of joint and in synovial membrane
role of joint receptors
- respond at limits of joint movements
- respond to joint pressure
- code ambiguously for joint movement
- Protective role
what does coding ambiguously mean
it responds the same way for all joint movements because it can’t tell the difference between movements
Stretch reflex is called the monosynaptic reflex because the 1a afferent synapses onto the 1a interneuron prior to synapsing onto the alpha motor neuron (t/f)
False two neurons so we need two synapses
two types of sensory receptors within the inner ear
semicircular canals
otolith organs
3 types of semicircular canals
- anterior
- posterior
- horizontal
2 types of otolith organs
- utricle
- saccule
what are mechanoreceptors
hair cells (stereocillia and kinocilium)
what do hair cells do
transform mechanical energy into neural energy
Kinocilium
apex of hair cell (tallest point)
Stereocilium
linked stair like structure (shorter)
what happens when sterocillia are pushed towards the kinocilium
the hair cell depolarizes (increased firing rates)
what happens when the sterocillia are pushed away from the kinocilium
the hair cell hyperpolarizes (decreased firing rates)
mechanoreceptors respond to
acceleration or gravity in line with hair cells
at rest, what happens with hair cells
they have a baseline firing rate (due to leaky channels)
what is the name of fluid within canals
endolymph
whats a cupula
house the hair cells in the crista
semi circular canal detects
angular acceleration
acceleration of the canals lead to
increased firing rates
during periods of constant velocity, hair cells return to
normal leakiness, baseline firing rates
Horizontal canal
yaw, Z: spins around top of head
Anterior canal
pitch, Y: spins around ear
Posterior canal
roll, x: spins around nose
Balance of left and right excitation and inhibition leads to
sensation of head rotation
sensation of head rotation is caused by
balance of left and right excitation and inhibition
How does head rotation effect endolymph
causes opposite endolyphm fluid
sterocillia pushed towards kinocillum (to left) in semicircular canal=
excitation
steriocillia pushed to left =
pushed towards kinocillium
sterocillia pushed away from kinocillum (to right) in semicircular canal=
inhibition
whats the difference between semicircular canals and otolith organs?
- semicircular canals have a cupula surrounded by endolyphm fluid (detect angular acceleration)
- otolith organs have an otolith membrane (with gel like substance) and crystals (detect linear acceleration)
Otolith organs detect
liner acceleration
semi circular canals detect
angular acceleration
otoliths or otoconia contain
- small calcium carbonate crystals ebbed into gelatinous material
- hair cells with cilia projecting up into overlying membrane
utricle
otolith organ, detects linear acceleration
saccule
otolith organ detects linear acceleration
what causes cilia to move in the otolithic membrane
shearing of the membrane
what happens to the otolith organs when the head tilts or accelerates
gravity causes the otoliths to slide, pulling on steriocilia (cause depolarization and in turn acceleration)
what type of acceleration are otoliths sensitive to
linear acceleration
explain how the spins occur
alcohol lowers the density of the blood , the cupula has a supply of blood flow, alcohol seeps into the cupula allowing it to be less dense then the endolyphm (the balance is disrupted), lesser density in the cupula causes it to float and the hair cells to move artificially (hair cells deflected falsely), causing a spinning sensation
describe the process of the spins leaving and coming back
when alcohol seeps into the endolymph the spins go away but come back because the alcohol is removed from cupula and now there’s another imbalance. This time the endolyphm is less dense, hair cells are deflected artificially again
why does drinking more alcohol help the spins
it allows alcohol to seep back into the cupula and rebalance the density of fluid (doesn’t work in the long term,)
what is BPPV
Benign Paroxysmal Position Vertigo
- non life threatening
- sudden, brief
- symptoms triggered by a certain position
- dizziness: false sense of rotation
BPPV occurs in
Older adults
BPPV is
idiopathic (not one source of cause)
what happens when BPPV occurs
- otolith crystals dislodged into semi-circular canals
when is BPPV most likely to occur
when lying down (canals become more sensitive)
what is the treatment for BPPV
Epley maneuver
how does the epley maneuver work
it consists of moving the head and mimicking positions of the canals to move the hair cells , directing the crystal out of canal
saccule movement
VERTICAL linear acceleration
Utricle movement
HORIZONTAL linear acceleration
Ménière’s disease
*presented unilaterally
- excess fluid in the labyrinth (canals) which causes
- Increase in edolymphatic pressure
- results in decreased firing in affected side and increased firing on the intact side
- sensation of spinning
Menieres disease cause
idiopathic
Menieres diseases causes __ to happen
- swelling causes distortion of information
- decreased and increased firing rate
types of sensory receptors
- chemoreceptors
- thermoreceptors
- nociceptors
- mechanoreceptors
types of nociceptor fibers
- A fibers: sense sharp localized pain
- C fibers: sense dull, burning, delayed pain
types of mechanoreceptors
- cutaneous
- baroreceptors
- proprioceptors
types of cutaneous receptors
merkel, meissner, pacininan, ruffini
how do cutaneous receptors work
- receptor potential
- integration at trigger zone
- action potential
- neurotransmitter released
tonic receptors
slowly adapt to continual stimulation
phasic receptors
rapidly to continual stimulation, reactivated when stimulus ends
Cutaneous receptors details
- small myelinated and unmyelinated
- sense pain and itch
- pain and temperature
what is a hot spot
the most sensitive spot in a sensory neuron, encompasses the spatial extent of the receptor surface
type 1 receptors
superficial receptors
- smaller receptive fields
- multiple points of maximal sensitivity
type 2 receptors
- deep receptors
- only one point of sensitivity
polysynaptic pathway
mediate flexion and cross extension reflexes
- excites muscle on contralateral limb : aid in withdrawal from the nociceptor afferent from the side in pain
slowly adapting sensory afferents
detect constant stimuli
- fire continuously when stimulus is present, never stops firing completely
- provides constant awareness
rapidly adapting afferents
detects changes in stimuli
- stops firing if stimulus stops or changes
- detects quick changes
Merkel cells
- superficial SA1
- highly sensitive to curvature and edges
- cells wedged along the border of dermis
Merkel cells receptive fields and sensitivity
- small receptive fields with multiple hotspots
(25% of hand receptors) - moderately low threshold
- most sensitive to 5Hz
- irregular discharge when stimulated
Meissner corpuscle
- superficial FA1
- stroking, velocity, or motion across skin
where are the meissners located
stacked of flattened disks in the dermis just below the epidermis
meissners receptive and sensitivity
- small receptive fields with multiple hotspots
- 40% of innervation of the hand
meissner threshold to indentation
6um
- sensitive to low frequency and vibration
merkel threshold to indentation
30um
Ruffini
- Deep (SA2)
- sense skin stretch
- branched fibres in cylindrical capsule
Ruffini sensitivity
- large receptive fields, only one hot spot
- 20% of hand innervation
- high threshold to indentation
Pacinian corpuscle
- deep
- FA2
- onion like capsule located deep in the skin
- vibration through an object
Pacinian corpuscle receptive fields
- large receptive fields only one hotspot
- 15% of innervation in the hand
- extremely low threshold (0.08)
What do the pacinian corpuscles sense
vibration: these receptors are very sensitive to alternating inputs such as those seen during mechanical vibration
- high frequency complex vibrations are elicited in the skin when scanning textures
- allow us to distinguish microscopic difference in texture
How are pacinian corpuscles helpful to blind people
allow them to distinguish microscopic differences in texture: ie. reading brail in between two fabrics
