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control of human movement > sensory+motor nerves and muscle+ motor units > Flashcards

sensory+motor nerves and muscle+ motor units Flashcards

1
Q

how do neurons conduct current?

A
  • by electron flow in metal
  • current flow is impeded by resistance
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2
Q

what are the two parameters of decay in a neuron?

A
  • space constant
  • time constant
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3
Q

what is space constant?

A
  • how far the voltage travels
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4
Q

what is time constant?

A
  • how fast the voltage travels
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5
Q

explain decay across a neuron

A
  • each segment of a neuron has a resistance
  • this resistance causes an applied voltage to decay to zero when travelling along the neuron
  • conduction velocity = space constant/time constant
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6
Q

what is conduction velocity?

A
  • speed an electrical impulse travels down a neural pathway
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7
Q

how do you work out the space constant in a neuron?

A
  • square root of the medial resistance (Rm) divided by the longitudinal resistance (Rl)
  • distance for voltage to reach 37% of original value
  • determined by axonal resistance
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8
Q

how do you work out the time constant in a neuron?

A
  • medial resistance multiplied by medial capacitance
  • time for voltage to reach 37% of original value
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9
Q

how do you get a big space constant?

A
  • high Rm and low Rl
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10
Q

why do we need amplification in a neuron?

A

because when there is a voltage travelling through a neuron passively just ionic flow, it will decay within 2-4mm

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11
Q

do we want a high or low time constant?

A
  • low
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12
Q

increasing the space constant or reducing the time constant will…

A

increase conduction velocity

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13
Q

what are two factors that increase the space constant?

A
  1. myelination
  2. axon diameter
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14
Q

what is membrane potential?

A
  • voltage across a cell membrane
  • determined by a balance of electrical and chemical forces (electrical charge and osmotic force)
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15
Q

explain chemical and electrical gradients

A
  • positive change coating surface of phospholipid bilayer of neuron and negative coating on the inside (in resting state)
  • for the neurone to fire/send massage using an action potential depends on the voltage/potential different of the outside and inside
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16
Q

what does hyperpolarising a neuron do?

A
  • increases potential
  • makes action potential less likely and to fire
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17
Q

what does depolarising a neuron do?

A
  • reduces potential
  • action potential more likely
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18
Q

what is the energy dependent process of a neuron: sodium-potassium pump:

A
  • for every ATP molecule, 2 potassiums are pumped inward and 3 sodiums are pumped outward
  • high potassium concentration on inside and high sodium on outside
  • sets baseline of cell so they are capable of transmitting action potentials
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19
Q

what is action potential mediated by?

A
  • voltage-gated ion channels embedded in cell membrane
  • open when threshold voltage is reached
  • NA and K channels are the primary mediators
  • NA channels can be open, closed or deactivated
  • K channels can open by open or closed
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20
Q

what is the 7 steps of the action potential positive feedback cycle?

A
  1. cell membrane becomes depolarised
  2. once depolarisation reach a threshold, Na+ channels open
  3. causes influx of Na+ = more depolarisation
  4. adjacent Na+ channels opened causing chain reaction
  5. K+ channels open causing outflow of K+
  6. outflow of K+ depolarises membrane potential
  7. Na+ channels close and temporarily deactivated
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21
Q

what are the dynamic difference in K and Ma channels?

A
  • Na channels open quickly
  • K channels open more slowly
    this difference is crucial for action potential
22
Q

how is action potential unidirectional?

A
  • the refractory period prevents the positive feedback cycle continuing
  • ensures action potential only travels in one direction
23
Q

what happens to the action potential at different temperatures?

A
  • high temperatures facilitate the Na/K ion channels opening and closing
  • at high temperatures action potential becomes faster and shorter
24
Q

are are the effects of the neurons diameter increasing?

A
  • increased medial capacitance
  • decreased medial resistance
  • both effect cancel out os time constant is unaffected
  • but space constant is affected = diameter affects the longitudinal resistance more than the medial creating a larger space constant and faster conduction velocity
25
Q

how does myelin affect conduction?

A
  • thick layer of insulation increase space constant like diameter
  • ion current can travel further before decaying
  • passive ionic conduction only happens between nodes of ranvier and is amplified at nodes by Ma channels hence it jumps between nodes (saltare)
26
Q

what’re the differences of diameter and velocity for different neurons?

A
  • alpha motor neurons are large, myelinated and fast (diameter or 8-20)
  • pain fibres are small, unmyelinated and very slow (diameter less than 1)
27
Q

how do you measure conduction velocity in a human nerve?

A
  • stimulate motor neuron at 2 sites
  • measure latency of evoked responses
  • calcolar velocity based upon time and distance
28
Q

what are the effects/diseases of demyelination?

A
  • MS
  • guillan-Barre syndrome
  • reduces amplitude and increases latency
29
Q

who is Luigi Galvani?

A
  • father of neurophysiology for work on frogs legs 1791
  • showed electrical stimulation of muscular tissue produces contraction and force
30
Q

what is the history of an EMG

A
  • Erlanger, gasser and newcomer 1920 - first EMG recordings using cathode ray oscilloscope
  • amplified signals form pair of electrodes connected to muscle - signal now displayed
31
Q

what is the chain of events in a muscle contraction?

A
  1. action potential stimulates release of neurotransmitter across neuromuscular junction
  2. action potential spreads across muscle membrane and into fibre along T-tubules
  3. causes calcium release from sarcoplasmic reticulum
  4. calcium binds to muscle and causes cross-bridge recycling
32
Q

define the motor unit

A
  • one alpha motor neuron and all of the muscle fibres it activates
33
Q

what do action potentials generate?

A
  • 1 generates a single twitch
  • multiple generate a continuous tetanic force
34
Q

describe twitch fusion

A
  • a fusion of multiple twitches is generated by a continuous smooth muscle contraction
  • increases EMG activity by increasing electrical activity generated
35
Q

describe motor unit recruitment

A
  • recruiting more motor units and increasing frequency of firing generates muscles extra force
  • increases EMG activity by increasing generated electrical activity
36
Q

how do you measure a single motor units action potential (MUAP)

A
  1. subject presses transducer by abducting index finger against gauge
  2. wire electrodes inserted into first dossal interosseous - records action potentials from motor neurons near tip of electrode
  3. sampan records no. of single motor neurons with different shapes and sizes
37
Q

what do twitch properties depend on?

A
  • fibre type
  • fatigue
  • temp
  • this requires changes in motor unit firing rates to compensate
38
Q

how does the amount of motor units affect movement precision?

A
  • more motor neurons = finer force modulation = better control
    innervation number = fibres/motor neurones
  • lower innervation number = more control
  • e.g., calfs are large but have poor degree of control true to high innervation (1,000,000+ fibres so innervation 1,800)
39
Q

how to measure muscle activity

A
  • action potentials propagate long sarcolemma - starting at neuromuscular junction to ends of muscle fibre
  • this single can be recorded by inserting a needle electrode into muscle or a surface electrode at level of skin
  • EMG
40
Q

what is the relationship between muscle activity and force?

A
  • amplitude of the sEMG signal is proportional to the force produced by the muscle
  • neural input signal is high frequency
  • force output is low frequency
  • filtered and integrated EMG is generally proportional to force but some muscles do show a non linear relationship
41
Q

fast fibre characteristics

A
  • higher resting membrane potentials
  • greater density of sodium channels
  • faster action potentials
  • generates larger electrical responses
42
Q

eccentric vs concentric muscle contraction

A
  • movements depends on ratio of muscle and lad torques
  • ratios produce isometric, concentric or eccentric contractions
  • muscle torque/load torque = 1 (isometric)
    > 1 (concentric)
    < 1 (eccentric)
43
Q

EMG/force relationship depends on what 3 things?

A
  1. muscle lengthening and shortening
  2. muscle contraction
  3. joint angle
44
Q

how does the length-tension relationship affect the EMG/force relationship?

A
  • how the force generate capacity is effected by muscles length
  • excessive lengthening or shortening reduces force production capacity
  • a specific joint angle = maximum force production
45
Q

how does muscle fatigue affect force?

A
  • no further recruitment of motor units so force cannot be maintained
  • fatigue causes prolonged twitch duration due to muscles biochemical changes
46
Q

explain muscle wisdom

A
  • frequency of motor units firing falls over time to compensate for prolonged twitch time
  • causes drop in EMG amplitude
47
Q

what mechanisms sense a reduction in motor neuron firing (muscle wisdom)

A
  1. peripheral detection the build of metabolites and pain via receptors
  2. spinal suppression of motor neuron firing rates
  3. changes in voluntary activation
48
Q

what does NS reduce firing rate?

A

-reduce likelihood of neuronal fatigue
- failure of neuromuscular transmission
- more likely to maintain a fine motor control

49
Q

how is high frequency fatigue caused?

A
  • continuous high frequency stimulation
  • muscle wisdom helps prevent
  • caused by failure of transmission along muscle membrane
50
Q

what causes loss of force during a fatiguing contraction?

A
  • biochemical changes in muscle itself
  • neural mechanisms
51
Q

what is the twitch interpolation technique?

A
  • studies degree of motor unit activation during voluntary effort
  • contraction - a twitch imposed onto muscle or nerve evoking a interpolated twitch torque or twitch force
  • measures amount of motor units not recruited during contraction
52
Q

what is central fatigue?

A
  • neural drive to muscle is reduced as fatigue develops
  • central fatigue lies upstream of the motor cortex

control of human movement (10 decks)

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