313 midterm 1

where are interneurons

spinal cord

sensory/afferent neurons are located in ___

They carry sensory info to the spinal cord via ___

motor/efferent neuron exit the spinal cord from ___ via the ___

dorsal root ganglion

dorsal root of the spinal cord

anterior horn

ventral root

what is the motor neuron pool

its a tree full of motor neuron somas and dendrites that innervate a particular muscle

(all the motor neurons that innervate a particular muscle)

where are the motor neuron pools located in the spinal cord, how bout in different segments of spinal cord

depending on the muscle, MNP are located somewhere in the ventral root of the spinal cord

depending on the segment of spinal cord, MNP could locate differently around the ventral roots as well

what is cervical enlargement?

what lumbar enlargement?

its a enlarged gray area in cervical vertebrae bc it contains most of the motor neuron pools for the arm muscle

larger spinal cords with most motor neuron pools for the legs

both on the arm, where would the motor neuron pool for bicep brachii and fingers sit relative to each other in the ventral root? why?

MNP for bicep will sit more medially on the ventral root than the MNP of fingers bc bicep is more proximal than fingers

what is a motor unit

the cell body and dendrites of a motor neuron, the multiple branches of its axon, and all the muscle fibers that it innervates

what is the innervation ratio of a motor neuron

the number of muscle fiber innervated by a single motor neuron

what does a higher/lower innervation ratio indicate

higher innervation ratio means that this motor neuron innervates more muscle fiber, suggesting its pbl responsible for gross and powerful movements

lower innervation ratio means that this motor neuron innervates less muscle fibers, suggesting its pbl responsible for finner and more precise movements

how does innervation ratio changes with aging? why?

older we get, higher the innervation ratio

bc motor neurons will die with aging, leaving more muscle fibers for the remaining motor neurons to innervate

what is ALS? what effect does ALS have on muscle?

ALS = amyotrophic lateral sclerosis

higher innervation ratio (can’t do very fine movements)

more synchrony (contraction less smooth)

describe the axon size, conduction velocity, type of motor units they form of the 3 types of motor neurons + the muscle type they innervates

S motor neuron: smallest axon, slowest conductivity, innervate SO muscle fiber > form type S motor unit

FR motor neuron: in between size, conductivity, innervate fast-oxidative-glycolytic muscle fibers > form FR type motor units

FF motor neuron: largest axon, fastest conductivity, innervate FF muscle fibers > form FF type motor units

what does Rheobase measure

it measure the current it takes to activate a neuron

what Rheobase does a large motor neuron have

high rheobase for large MN

describe input resistant of a small vs large motor neuron

smaller MN have higher input resistant, meaning it is easier to excite/activate than a large MN

what is a twitch

a motor unit’s physiological response to stimulation

what does contraction time tell you

time it takes from start of contraction to peak force

what does half relaxation time tell you (HRT)

HRT tells you the time it takes from peak force to half force

how to achieve a unfused tetanus

by temporal summation, where you increase the discharge frequency of a motor neuron, leading to a increase in force output

How is fused tetanus achieved

doesn’t usually happen, only happen during forced stimulation (aka electrocution)

group the 3 types of motor units based on their behaviour (contraction time and half relaxation time)

S type= long contraction time, long HRT

FR type = short contraction time, long HRT

FF type= short CT, short HRT

how is motor unit activity recorded

1. metal microelectrode

   1. thin tungsten electrode (highly conductive and flexible)

   2. coated for insulation, but tip exposed

2. fine wires

   1. a pairs of twisted fine wire inserted using a gauge needle

   2. view wire location using ultrasound

why do we measure motor unit activity

because we cannot measure motor neuron (in spinal cord), so we measure muscle fiber activity, allowing us to infer motor neuron activity in spinal cord

pro and con of the 2 MU activity measurement

tungsten electrode con : could move around as muscle contract

fine wires: hook onto muscle, more stable and more accurate

how to differentiate different MU from the EMG

each shape represent a group of muscle fibers that are tied to the same motor neuron

2 ways to control muscle force

1. recruit more MU

2. fire at higher rate

describe Henneman’s size principle

All motor units in the motor neuron pool are recruited in the order of the size of motor neuron. Smaller motor units with smaller motor neurons gets recruited first.

Henneman argues that motor units are recruited based on motor neuron size due to what morphological issue:

rheobase (aka the amount of current it takes to activate a motor neuron)

what is the sequence of derecruitment according to Henneman

from large motor units to small motor units

describe muscle contraction force and frequenct relationship

non-linear, its sigmoidal (S shaped)

faster contraction doesn’t mean higher force output

force increase as frequency increase, but eventually force plateaus

what factor affects the relationship btw force and frequency

muscle length

at the same firing frequency, longer muscle can produce more force

contrast fast and flow twitch muscle’s force vs frequency relationship

slow twitch plateaus at lower frequency, meaning it hits max force output at lower frequency

describe how different motor units fire and work tgt to produce smooth net force

each MU produces unfused tetanus. But because each MU fire asynchronously, the net force is smooth

what are the 2 discharge patterns that can be altered to affect force production

double discharge

synchrony

human MU usually discharges btw ___ Hz, aka ___ ms btw consecutive action potentials

7-35 Hz

30-140ms

describe double discharge

firing another action potential within 10ms » get a big jump in force production

when does double discharge happen? consequences?

at the onset of a rapid movement

easier to get fatigue

describe and contrast these 3 graphs

a= double discharge, cause increase in force

b = double discharge at the beginning of contraction, even larger force than A

c= double discharge at first, but missed an impulse, decrease in force

describe synchrony

the degree to which 2 MUs within a muscle fire at the same time

when does synchrony occur

rarely occur during smooth contraction

but during large contraction or state of fatigue, synchrony happen more frequently (to produce more force)

what does more/less muscle synchronization imply

less synchronization = smoother contraction = musician’s dominant hand

more synchronization = more unsmooth contraction = weight trainers (need larger force output)

Why does synchrony happen

due to common presynaptic input of two MUs —> causing Short term synchronization

what clinical populations have ppl with synchronization

ALS and Klippel-Feil syndrom (high level of MU synchronization)

how does Howell et al.’s study contradict with Henneman’s size principle

Compare MU activity during isometric vs concentric vs eccentric contraction

Finding:

• although MU D has a higher recruitment threshold than MU ABC, it only come one during the eccentric phase of the contraction

• » suggesting that MU recruitment during eccentric phase may be diff from concentric and isometric (task dependent?)

what is a neuromuscular compartment? what does it suggest about muscle function?

compartment= contains a unique population of MUs, which are confined to a certain region of a muscle.

» MUs within a muscle could be sorted by task/functions

describe the cat study about neuromuscular compartmentalization

researcher recorded muscle activity of cats lateral gastroc (has 4 compartment) during different activities like standing, walking, paw shake, jumping » each compartment has its own function!!

describe the immobilization experiment

design: test participant max and 15% submax elbow flexion performance, then cast their arms for 4 weeks, test max contraction and then perform 15% sub max to observe their EMG and how long they can perform

Participant: 6 man/6 woman

result:

• different muscle activation pattern after casting among most women

• 100-200% longer contraction time in women

what did the immobilization experiment reveal:

there may be gender differences btw submaximal MU recruitment pattern

not activating muscles for 4 weeks may have altered whatever the size principle was based on

describe the muscle EMG pre-casting vs post-casting in the immobilization study

pre: increase in muscle activation overtime due to fatigue

post men: same pattern

post women: MUs turn on and off throughout, lasted much longer than pre-cast

Bawa et al. studied how MUs behave during prolonged submax contraction and observed:

after gently flexing MU1 for 10min, as force output slightly increase, MU2 turn on and MU1 rest. after another 10min, force output increase a bit more, and MU2 turned off and MU1 turned on.

» MU1 and MU2 took turns sustaining force output

what did Bawa et al.’s study suggest:

another interpretation of size principle:

There is a hierarchy to recruitment threshold, but MU with similar thresholds take turns activating before triggering the next group of MU with a much higher threshold

how does sensory input affect MU selection/activation

a low threshold MU could get a threshold boost through stimulation, while a higher threshold MU could get a threshold downgrade through stimulation

» stimulation could modify MU selection/activation threshold

describe the imagined muscle training by Yue and Cole

objective: effects of 4 weeks of mental training on force production

subjects:

• group 1= repeated isometric maximal contraction training

• group 2 = imagined making contraction (EMG make sure they are relaxed)

control group= tested for force production but no training

measured dominant vs non-dominant muscle » look at crossed effects

describe the results of the imagined muscle training:

both G1 and G2 improved force output on the trained and untrained side

untrained side also improving » suggest that the strength increase is due to neural adaptation instead of muscle hypertrophy

imagine group also improved a lot » mentally practice affects central motor programming » helped improve the ability to activate and recruit muscle» improved strength

what implication on clinical population does the imagined muscle training experiment have

cross-training effect and mental training can both help improve neural control in populations yet able to lift weights

why was the imagined training designed to be done in 4 weeks and measures both dom and non-dom hand’s obscure hand

4 weeks = ensure that any gain in strength is not due to hypertrophy

dom vs non-dom = show cross effect

train obscure non-dom muscle= minimize potential skill/training in that muscle

while Henneman’s size principle holds for most movements, what factors that could impact recruitment threshold

sensory input, task(compartmentalization), type of movement (con vs ecc)

3 classes of receptors:

1. interoceptors (transduce into from within body)

2. exteroceptors (transduce info from environment)

3. proprioceptors (transduce info about relative configuration of body segments)

how can receptors be stimulated

chemically or mechanically or both

describe the role and pathway of afferent/sensory inputs

periphery receptors project info centrally> dorsal root ganglion > grey matter of spinal cord > higher brain and nervous system

what is the receptor cell body shaped like, nickname, location?

shape = T

name= pseudounipolar neuron

location = dorsal root ganglions

what is a ganglion

a collection of cell bodies in the periphery of nervous system

whats special about muscle spindle during sensory input pathway

muscle spindle > dorsal root ganglion > grey matter > (skip interneuron)> motor neurons > muscle fibers

how are afferent fibers labeled

based on cross section size/diameter

I = largest

IV= smallest

based on Rheobase, which afferent fibers would conduct the fastest (I - IV)

I = largest = fastest conduction

IV = smallest = slowest conduction

describe the distribution of afferent fibers based on size

bimodal distribution, most afferent fibers are either type Ia or II, some Ib, with little of III and IV

where does afferent fibers Ia, Ib, II go?

Ia » muscle spindle (aka primary afferent spindle)

Ib » golgi tendon organ

II » muscle spindle (aka secondary afferent spindle)

what is synaptic potential

how presynaptic neuron changes (closer or further) how close a post-synaptic neuron is from generating an AP

describe cell-to cell communication at the synapse

AP arrive at pre-synaptic cell axon terminal > Ca2+ channel open, Ca2+ influx > vesicle containing NT travel to membrane > release NT to the synaptic cleft > create synaptic potential in post-synaptic neuron

define EPSP

Excitatory post-synaptic potentials= a synaptic input that depolarizes the post-synaptic neuron, making it closer to generating an AP

define IPSP

inhibitory post-synaptic potentials = a synaptic input that hyperpolarizes, makes it harder for post-synaptic neuron from generating an AP

characteristic of IPSP, EPSP, and AP

the synaptic potentials are local and graded, they are also not followed by a refractory period (can summate)

AP are global (propagates) and not one size (all or none), they have a refractory period

what is the magnitude of EPSP and IPSP correlated to

correlated to the amount of NT released

decribe the relationship btw EPSP and IPSP

they interplay and determine if the post-synaptic neuron will fire an AP or not

describe divergence vs convergence

Divergence= a single neuron synapses multiple neurons (ex: MN)

convergence = many neurons converge on fewer neurons (ex: sensory neuron)

describe how spatial summation occurs (when 1 EPSP is not enough to reach threshold)

via convergence, where multiple EPSPs from different neurons converge at the same post-synaptic axon » graded potentials summate » AP generated

describe how temporal summation occurs

when a neuron fires repeatedly before the last EPSP graded potential dissipate » summate > generate AP

describe muscle spindle shape and orientation in muscle

fusiform shaped receptors, lie in parallel with skeletal / extrafusal muscles

describe how different muscles have diff number/density of spindles

hand muscle have high spindle density, more proximal muscles have relatively lower spindle density (bc larger in size)

Describe the components of a muscle spindle and its length and shape

length = 1cm

several intrafusal fibers (bag or chain type)

equatorial region covered by connective tissue capsule

polar regions have myofilament contractile components (like myosin n actin)

how are intrafusal fibers classified

based on nuclei distribution:

bag = nuclei bundle tgt in a bag

chain = nuclei line up after each other

bags are usually longer than chain too

describe typical intrafusal fiber distribution per muscle spindle

2-20 intrafusal fibers per spindle (lots of variability btw spindles)

• 2-3 bag fibers, 3+ chain

describe the 2 types of sensory afferent innervation in a spindle

primary (Ia) afferent — wrap around and innervate the central area of bag1,2, and all chains

secondary (II) afferent— innervate bag2 and chains, but not bag1

distinguish bag1 vs bag2 intrafusal fibers

bag1 = dymanic

bag 2 = static

what is the gamma (y) system

gamma system aka fusimotor

its a special efferent/motor system that are special to muscle spindles

what is the gamma system consist of

dynamic and static gamma motor neurons (y-MN)

contrast alpha and gamma system

both efferent systems

alpha innervate skeletal muscle

gamma innervate muscle spindles, specifically, gamma dynamic innervate bag1 intrafusal fiber, gamma static innervate bag2& chains

describe how a stretch in muscle lead to muscle spindle AP

the Ia and II sensory nerve endings that innervates the intrafusal fibers have stretch sensitive mechanoreceptors

a stretch in muscle > stretch sensory receptor axon endings membrane > channel open, increase leakiness > ion movement > graded potential

Enough graded potential » AP!

Ia afferents are sensitive to ___, II afferents are sensitive to ___

Ia = velocity of muscle stretch & length of stretch

II = length of stretch

which afferent is sensitive to taps and vibrations

primary/Ia

what happens to primary afferent on the unloading phase of a muscle stretch

Ia stop firing when tension is being released

what does Ia and II afferent code for

Ia mainly code velocity of muscle stretch (+ some length)

II code for length of muscle (+ some velocity)

what is the dynamic index

its related to the velocity of the stretch. Its the difference btw peak and steady state at 0.5 sec after the stretch

what can we infer from response of primary and secondary afferent to different dynamic index of a contraction

primary afferent firing rate is more proportional to the increase in muscle stretching velocity than secondary

describe microneurography

put tungsten electrode in the muscle fiber and stimulate it to observe its activity

why does the gamma need to work with the alpha system work tgt during voluntary contraction

so the spindles maintain its sensitivity during a shortening contraction

how does primary afferent fiber respond to static vs dynamic gamma stimulation

primary become more velocity sensitive during dynamic stim

become more length sensitive during static stim

if alpha and gamma don’t coactivate during a voluntary contraction, what will happen

spindles will become unloaded and stop firing when the muscle shortens » making them ineffective as a afferent feedback system during the movement

how does the fusimotor system control the level of activation

depending on the task, fusimotor system changes the activity of gamma dynamic/static

ex: during slow walking and standing » activate more gamma static

Ex: during running or unexpected movements » activate more gamma dynamic

describe how gamma static/dynamic stimulation affect the dynamic index of Ia afferent

doing the same stretch, stiming gamma static decreased the dynamic index of Ia (less sensitive to velocity of stretch)

stimming gamma dynamic increased the dynamic index of Ia afferent (more sensitive to velocity of stretch)