Spinal reflexes Flashcards
What does a motor unit consist of?
motor neuron with axon and axon branches and muscle fibres
The fewer the number of fibres in a motor unit…
the smaller the twitch and greater the control of movement
Define tetanic contraction
Twitches frequent enough to give a smooth contraction
Define tetanus
fused twitches
Define tetanus fusion frequency
The frequency at which motor axon must fire at to induce tetanus in motor unit
What are the 3 main types of synaptic inputs present in the lower motor neuron
1- descending tracts in spinal cord from UMN
2- input from local interneurons
3- input from local sensory nerve fibres via reflexes
What does the pyramidal system refer to?
the upper motor neuron cell bodies that project from the frontal lobe to spinal cord via corticospinal tract
What does the extrapyramidal system refer to?
Cell bodies in the brainstem that project down to the spinal cord
Define reflex
Involuntary motor action triggered by sensory input
Where are reflexes ‘stored’
Dorsal and ventral grey matter of spinal cord in pattern on synaptic connections between inputs, output and interneurons
Why is the knee-jerk reflex called a homonymous reflex?
Because stretch of quadriceps muscle activates the same (quads) muscle to contract
What are the 5 main monosynaptic reflexes?
knee ankle biceps triceps supinator
Why are monosynaptic reflexes so named?
No interneurons are involved in the reflex
Why is it hard to suppress tendon jerk reflexes?
There are no interneurons involved, so direct connection makes it hard for brain to suppress.
What is meant by reciprocal inhibition in terms of reflexes?
When tendon is tapped, extensor reflex also inhibits antagonist flexor via inhibitory interneuron
What receptor mediates all tendon reflexes?
Muscle spindle
Define proprioceptor
A receptor that responds to movement of body, not external stimulus
What does muscle spindle consist of?
Complex stretch receptor inside connective tissue sheath
What is the main sensory nerve fibre from muscle spindle?
1a afferent
What activates muscle spindle?
Changes in muscle length causes increase in firing rate
Why does tapping tendon cause reflex?
Muscle stretched a small amount, sufficient to briefly increase firing of spindles
What is spatial summation?
Two or more synapses active simultaneously make a motor neuron fire
What is temporal summation?
Single la nerve fibre fires high frequency burst of action potentials which add together to trigger action potential
Describe structure of muscle spindle (3)
Consists of skeletal muscle fibres that have become modified inside capsule
Contractile tissue is at the end
Centre of fibre is without actin or myosin but full of cell bodies, so is very easily stretched
Where do la afferents coil around muscle spindle?
Around central region which doesn’t have connective tissue
What motor neuron supplies muscle spindle?
Gamma motor neuron
What is the effect of a gamma motor neuron on muscle spindle?
Causes end of intrafusal fibres to contract and therefore stretch the centre, increasing its response
How are gamma motor neurons activated?
driven from descending pathways such as corticospinal tract
Effect of pathological increases in gamma activity?
Hyperactive tendon reflexes
Function of muscle spindles (other than reflex)
provides continuous feedback to motor neurons and continually adjust the motor neuron output during normal movement
Function of monosynaptic reflex
Negative feedback system to maintain constant muscle length despite fatigue
How does monosynaptic reflex keep muscle length constant despite fatigue?
When fatigue sets in, arm begins to droop. This stretches the muscle, causing an increase in muscle spindle activity.
Increased stretch of muscle means increased frequency of action potentials to spinal cord
Increases synaptic input of motor neurons- higher frequency of firing increased contraction
Why are monosynaptic reflexes important in leg extensor muscles?
Fatigue in leg muscles would result in falling if not for reflexes
Continuous feedback from muscle spindles plus high speed action potential conduction gives tendon jerk reflex shortest possible delay
What type of receptor is golgi tendon organ? What is it activated by?
Proprioceptor
Activated by muscle tension
Does the golgi organ have an interneuron in its connection?
Yes
What type of interneuron is present in golgi tendon organ?
glycinegic inhibitory neuron
How does the golgi tendon reflex act when muscle contracts strongly
Muscle contracts strongly enough to increase tension in tendon to extent it may damage muscle, GTO switches off by inhibitory action
What nerve fibres mediate reflex withdrawal from painful stimulus?
A-delta (small myelinated nociceptor afferents)
What do A-delta act on?
Act on interneurons in spinal cord
Why can flexion reflex be suppressed by the brain?
Because flexion reflex relies on several interneurons, pain can be overcome in certain circumstances
What is meant by crossed extensor reflex?
Activation of extensors in other leg during a flexion reflex
How is crossed extensor reflex produced?
Branches of afferent nerve fibres cross from stimulated side of body to contralateral side of spinal cord
Synapse with interneurons which excite or inhibit alpha motor neurons to muscles of contralateral leg
What is meant by muscle tone?
Small amounts of contraction in muscles when passively moved
What happens to muscle tone when there is lower motor neurone damage?
Tone becomes reduced or absent
Flaccid paralysis
What happens to muscle tone when there is upper motor neurone damage?
Exaggerated reflexes
Pathological increase in muscle tone
Spasticity
