Skeletal muscle control Flashcards
a-motorneurones
Motor neurones of the spinal cord
Cell bodies in the ventral horn
Axons project out to skeletal muscles
Synapse with muscle fibres = neuromuscular junction
Motor unit
single α motoneurone and all the muscle fibres it innervates
Focal innervation
Each muscle fibre receives input from a single a-motorneurone
Safety feature of nerve terminals
Nerve terminals always release 8-10x more ACh than necessary so we always get muscle contraction with a single action potential
Smooth contraction
Each α-motoneurone innervates muscle fibres that are spread throughout the muscle
α-motoneurones fire asynchronously
Innervation ratio
number of muscle fibres innervated by each individual α-motoneurone
Innervation ratio is inversely correlated with contractile precision
Type I muscle fibres features
Slow twitch
Low force of contraction
High resistance to fatigue
Oxidative energy source
Red in colour
Function: posture
Type II muscle fibre features
Fast twitch
High force of contraction
Low resistance to fatigue
Glycolytic energy source
White in colour
Function: Rapid movements
Increasing the force of contraction via a-motorneurones
Recruit more α-motoneurones
Increase the action potential firing rate of α-motoneurones
Summation of muscle contractions
Maximum summation (tetanus) = 3.5kg/cm2 force in all muscles
3 ventral horn neurones
a-motorneurones
renshaw cells
y-motorneurones
Renshaw cells
Inhibitory interneurons
Release glycine
Synapse with α-motoneurones (inhibit their activity)
Lateral inhibition = prevents overactivity and enables fine control of movement
Target site of tetanus bacteria toxin
y-motorneurones
Innervate muscle spindles
Mechanoreceptors
Stretch receptors
Sensory neurones
Innervate middle section of intrafusal muscle
Types of intrafusal muscle
Ia = absolute stretch and change in stretch
II = absolute stretch
Knee jerk reflex steps
Strike patellar tendon with a hammer
Stretches the muscle spindle in the quadricep
Action potentials project along Ia afferents to spinal cord
Synapse (excitatory, glutamate) with α-motoneurone in ventral horn that innervates the quadricep
α-motoneurone releases Ach at NMJ causing contraction of quadricep
Collateral synapse (excitatory, glutamate) with Renshaw cell in ventral horn
Renshaw cell (inhibitory, glycine) synapses with α-motoneurone that innervates hamstring
Reciprocal inhibition of hamstring
Leg raises
The gamma (y) loop
Monitors muscle contraction and corrects when required
Golgi tendon organs
Mechanoreceptors
Tendons (between the muscle and bone)
Innervated by Ib sensory neurones
When the muscle stretches so do the Golgi tendon organs
Golgi tendon reflex
Protects the muscle from overloading - inverse stretch reflex
