sensory+motor nerves and muscle+ motor units Flashcards
how do neurons conduct current?
- by electron flow in metal
- current flow is impeded by resistance
what are the two parameters of decay in a neuron?
- space constant
- time constant
what is space constant?
- how far the voltage travels
what is time constant?
- how fast the voltage travels
explain decay across a neuron
- 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
what is conduction velocity?
- speed an electrical impulse travels down a neural pathway
how do you work out the space constant in a neuron?
- 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
how do you work out the time constant in a neuron?
- medial resistance multiplied by medial capacitance
- time for voltage to reach 37% of original value
how do you get a big space constant?
- high Rm and low Rl
why do we need amplification in a neuron?
because when there is a voltage travelling through a neuron passively just ionic flow, it will decay within 2-4mm
do we want a high or low time constant?
- low
increasing the space constant or reducing the time constant will…
increase conduction velocity
what are two factors that increase the space constant?
- myelination
- axon diameter
what is membrane potential?
- voltage across a cell membrane
- determined by a balance of electrical and chemical forces (electrical charge and osmotic force)
explain chemical and electrical gradients
- 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
what does hyperpolarising a neuron do?
- increases potential
- makes action potential less likely and to fire
what does depolarising a neuron do?
- reduces potential
- action potential more likely
what is the energy dependent process of a neuron: sodium-potassium pump:
- 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
what is action potential mediated by?
- 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
what is the 7 steps of the action potential positive feedback cycle?
- cell membrane becomes depolarised
- once depolarisation reach a threshold, Na+ channels open
- causes influx of Na+ = more depolarisation
- adjacent Na+ channels opened causing chain reaction
- K+ channels open causing outflow of K+
- outflow of K+ depolarises membrane potential
- Na+ channels close and temporarily deactivated
what are the dynamic difference in K and Ma channels?
- Na channels open quickly
- K channels open more slowly
this difference is crucial for action potential
how is action potential unidirectional?
- the refractory period prevents the positive feedback cycle continuing
- ensures action potential only travels in one direction
what happens to the action potential at different temperatures?
- high temperatures facilitate the Na/K ion channels opening and closing
- at high temperatures action potential becomes faster and shorter
are are the effects of the neurons diameter increasing?
- 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
how does myelin affect conduction?
- 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)
what’re the differences of diameter and velocity for different neurons?
- alpha motor neurons are large, myelinated and fast (diameter or 8-20)
- pain fibres are small, unmyelinated and very slow (diameter less than 1)
how do you measure conduction velocity in a human nerve?
- stimulate motor neuron at 2 sites
- measure latency of evoked responses
- calcolar velocity based upon time and distance
what are the effects/diseases of demyelination?
- MS
- guillan-Barre syndrome
- reduces amplitude and increases latency
who is Luigi Galvani?
- father of neurophysiology for work on frogs legs 1791
- showed electrical stimulation of muscular tissue produces contraction and force
what is the history of an EMG
- Erlanger, gasser and newcomer 1920 - first EMG recordings using cathode ray oscilloscope
- amplified signals form pair of electrodes connected to muscle - signal now displayed
what is the chain of events in a muscle contraction?
- action potential stimulates release of neurotransmitter across neuromuscular junction
- action potential spreads across muscle membrane and into fibre along T-tubules
- causes calcium release from sarcoplasmic reticulum
- calcium binds to muscle and causes cross-bridge recycling
define the motor unit
- one alpha motor neuron and all of the muscle fibres it activates
what do action potentials generate?
- 1 generates a single twitch
- multiple generate a continuous tetanic force
describe twitch fusion
- a fusion of multiple twitches is generated by a continuous smooth muscle contraction
- increases EMG activity by increasing electrical activity generated
describe motor unit recruitment
- recruiting more motor units and increasing frequency of firing generates muscles extra force
- increases EMG activity by increasing generated electrical activity
how do you measure a single motor units action potential (MUAP)
- subject presses transducer by abducting index finger against gauge
- wire electrodes inserted into first dossal interosseous - records action potentials from motor neurons near tip of electrode
- sampan records no. of single motor neurons with different shapes and sizes
what do twitch properties depend on?
- fibre type
- fatigue
- temp
- this requires changes in motor unit firing rates to compensate
how does the amount of motor units affect movement precision?
- 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)
how to measure muscle activity
- 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
what is the relationship between muscle activity and force?
- 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
fast fibre characteristics
- higher resting membrane potentials
- greater density of sodium channels
- faster action potentials
- generates larger electrical responses
eccentric vs concentric muscle contraction
- 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)
EMG/force relationship depends on what 3 things?
- muscle lengthening and shortening
- muscle contraction
- joint angle
how does the length-tension relationship affect the EMG/force relationship?
- 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
how does muscle fatigue affect force?
- no further recruitment of motor units so force cannot be maintained
- fatigue causes prolonged twitch duration due to muscles biochemical changes
explain muscle wisdom
- frequency of motor units firing falls over time to compensate for prolonged twitch time
- causes drop in EMG amplitude
what mechanisms sense a reduction in motor neuron firing (muscle wisdom)
- peripheral detection the build of metabolites and pain via receptors
- spinal suppression of motor neuron firing rates
- changes in voluntary activation
what does NS reduce firing rate?
-reduce likelihood of neuronal fatigue
- failure of neuromuscular transmission
- more likely to maintain a fine motor control
how is high frequency fatigue caused?
- continuous high frequency stimulation
- muscle wisdom helps prevent
- caused by failure of transmission along muscle membrane
what causes loss of force during a fatiguing contraction?
- biochemical changes in muscle itself
- neural mechanisms
what is the twitch interpolation technique?
- 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
what is central fatigue?
- neural drive to muscle is reduced as fatigue develops
- central fatigue lies upstream of the motor cortex
