5: Physiology - Motor units, muscle spindles, Golgi tendon organs and joint receptors Flashcards
The somatic motor system is made up of upper motor neurons and lower motor neurons.
What are their functions?
LMNs - initiate contraction of skeletal muscle
UMNs - modulate activity of LMNs
Where in the spinal cord do LMNs emerge from?
Anterior horn
What is the function of an alpha motor neuron?
Initiates contraction of skeletal muscle
What is the function of a gamma motor neuron?
Which specific structure of skeletal muscle does it supply?
Acts as a proprioceptor to monitor muscle length and contraction
Muscle spindle
How many spinal nerves are there at each level and in total?
30
C8, T12, L5, S5
What is a muscle spindle?
Specialised stretch receptor which detects the degree of stretch of a skeletal muscle fibre
What is a motor unit?
Alpha motor neuron and all the muscle fibres it supplies
In terms of alpha motor neurons, what is a motor neuron pool?
All the motor units supplying a single named muscle
What is the relationship between the firing rate of an alpha motor unit and the force of contraction?
Direct proportion
the greater the firing rate, the greater the force of contraction (up to a limit)
What is the relationship between the number of activated motor units and force of contraction?
Direct proportion
the greater x, the greater y
Where in the spinal cord are motor neurons supplying
a) proximal muscles
b) distal muscles
found?
a) Medial in anterior horn
b) Lateral in anterior horn
Where are motor neurons supplying
a) flexor muscles
b) extensor muscles
found relative to each other in the anterior horn?
a) Posterior
b) Anterior
Name some
a) alpha motor neuron
b) muscle fibre
factors which influence of rate and force of muscle contraction?
a) Frequency of impulses, summation of impulses
b) Size (hypertrophied, atrophied?), type (slow or fast twitch?)
What word is used to describe the contraction of skeletal muscle caused by a single action potential?
Twitch
What is required to produce sustained muscle contraction compared to a brief twitch?
Sustained firing of action potentials
The greater the firing rate of alpha motor neurons, the greater the time period of muscle contraction.
What happens if the firing rate is excessively fast?
Tetany
Involuntary muscle contractions and cramps
How many muscle fibres do
a) fine motor muscles
b) gross motor muscles have?
this is not a trick
a) Few muscle fibres (e.g extraocular muscles)
b) Many muscle fibres (e.g quadriceps)
How does the size of an alpha motor neuron change depending on the size of its motor unit?
Bigger alpha motor neurons for bigger motor units
How does the addition of small motor units affect the rate of increase of force of contraction?
If you add smaller motor units on, you add less force
opposite true for larger motor units
Which size of motor units tend to be recruited in slow fatiguing exercise?
Small motor units
Which size of motor units are activated by strenuous, fast fatiguing exercise?
Large motor units
to generate bigger increases in muscle contraction
