spinal reflexes Flashcards
define lower motor neuron
motor neuron that has cell body in ventral horn
define upper motor neuron
motor neuron that has cell body in the brain
nerve cells that are higher in the brain and send axons down the spinal cord in descending tracts, synapsing with dendrites of the LMNs
what do upper motor neurons synapse with?
dendrites of the LMNs
define a motor unit
a motor neurone together with its cell body in the dorsal horn, its motor axon and set of muscle fibres that it innervates
what is the smallest unit of contraction we can produce?
contraction of one motor unit
how does the size of the motor unit affect the size of the force of contraction?
activation of a small motor unit = small force
large motor unit = large force
when is a twitch generated?
when a single AP contracts one motor unit
what is a tetanus/tetanic contraction?
smooth contraction initiated by a train of action potentials at high frequency to produce a fused contraction
what is the tetanus fusion frequency?
the frequency at which action potentials are fired to induce tetanus in its motor unit
what is the usual tetanus fusion frequency
10/second
explain how the all or nothing applies in motor axon frequencies?
a higher frequency then there’s no increase in tension, but lower frequency –> jerky contraction
how can the force of contraction in a muscle be increased/decreased?
recruiting more or less motor units
for fine control, what motor units are activated?
small motor units
for full power, what motor units are activated?
large motor units
what happens when there’s a random death of motoneurons?
remaining motor axons grow peripherally to innervate the denervated muscle fibres –> increase in motor unit size and decreased fine control
are afferent fibres sensory or motor?
sensory
are efferent fibres sensory or motor?
motor
what are the synaptic inputs of the LMNs?
- Descending tracts in spinal cord from UMN
- Input from local interneurons (cells with their processes inside the CNS)
- Input from local sensory nerve fibres via reflexes
explain pyramidal systems?
UMN cell bodies in the cortical frontal lobe
Axons travel to spinal cord via corticospinal tract (aka pyramidal tract) to synapse with LMN for fine motor control
what type of control are pyramidal systems used for?
fine motor control
explain extrapyramidal systems
- Upper motor neurone cell bodies in the brainstem project to spinal cord
- Brainstem neurons are regulated by input from motor cortex
what movements are extrapyramidal systems used to control?
regulates reflexes
more primitive
where are the LMNs in the spinal cord?
in the ventral horn
what is a reflex?
involuntary motor action triggered by sensory input
tiny motor programs stored in synaptic connections between sensory inputs, outputs and interneurones in the grey matter of the spinal cord
what is another name for a reflex?
myotactic reflex
where can most reflexes be modulated from?
cortex and brainstem
what are monosynaptic reflexes?
reflexes that contain no interneurons between muscle spindle afferent and motorneuron efferent -does not happen anywhere else in the NS, only in tendon jerk reflexes
name examples of monosynaptic reflexes
myotactic knee, patella, ankle, biceps, triceps and supinator reflexes
why are monosynaptic reflexes hard to supress?
Direct connection between input sensory nerve fibre and the motor neuron makes it harder for the brain to suppress this reflex
what does tapping of the patella cause?
contraction of quadriceps muscle: knee jerk
flexors (hamstrings) relax
what is a homonymous reflex?
the muscle sending the sensory information is the one that contracts
what is reciprocal inhibition?
when the extensor contracts, the flexor relaxes
explain how reciprocal inhibition occurs?
same tendon tap that activates the extensor reflex also inhibits the antagonist flexor muscle through an inhibitory interneuron
does the extensor reflex involve an interneuron?
no
does the reciprocal inhibition reflex involve an interneuron?
yes
what is a muscle spindle?
the receptor that mediates tendon reflexes (also known as a proprioceptor): responds to movements of body rather than external stimuli
what is a muscle spindle made up of?
• Consists of a complex encapsulated stretch receptor inside a connective tissue sheath
what are the main sensory afferents in muscle spindles?
1a afferents
what muscle fibres are found in muscle spindles?
intrafusal muscle fibres
what happens when you activate a muscle spindle?
- activates 1a afferents
- muscle spindles are spontaneously active at rest - when activated they increase firing rate so more APs are fired
- act on dendrites of motor neurons of muscles
- activates motor neurons –> twitch
how many contacts do motor neuron dendrites have from 1a afferents?
dozens/hundreds
how many synapses must a motor neuron have to fire through spatial summation?
at least 2 active synapses
when is an EPSP produced?
when a single AP in a 1a sensory afferent stimulates a motor neuron - it then doesnt produce an AP it produces an EPSP
explain spatial summation
Two synaptic inputs are active at the same time and the EPSPS are added together.
explain temporal summation
A single 1a nerve fibre fires high frequency burst of APs so EPSPs sum together to trigger an AP
describe the structure of a muscle spindle
intrafusal fibres
contractile tissue is at the ends - centre has no actin/myosin or connective tissue –> stretched easily
1a afferents around the centre region - fire APs when stretched
have gamma motor neurons
what do gamma motor neurons do and how?
increase sensitivity of the 1a afferents to stretch by contracting the end of the muscle spindles and therefore stretching the centre where 1a is found
what does pathological increase in gamma activity lead to?
hyperactive tendon reflexes
what do muscle spindles provide feedback to?
motor neurons
explain the negative feedback mechanism which stops muscles drooping during fatigue?
- when you hold something, there’s input to the motor neurons from the descending tracts and the 1a afferents to keep it in position
- muscle fatigues and contracts less strongly - starts to stretch
- stretch increases input to biceps motor neurons and spinal cord through higher AP frequency
- increases force of contraction
- arm moves back into place
why is it important that monosynaptic reflexes are the easiest to elicit in leg extensors?
Due to need for maximum stability –> damage to these reflex arcs makes patient more likely to fall/trip
explain how monosynaptic reflexes are easiest to elicit in the leg extensors?
- Continuous feedback from muscle spindles in the spinal cord
- High speed of AP conduction in afferent and efferent nerves gives tendon jerk reflex shortest possible delay
Faster feedback response, the quicker the corrective muscle action
how does the speed of the feedback response relate to the speed of corrective muscle action and balance and agility?
The faster the feedback response, the quicker the corrective muscle action –> improves balance and agility
what is the second major proprioceptor found in the muscle tendons?
golgi tendon organ
how are GTO activated?
tension not length
activated when muscle is passively stretched but switched off when it shortens to the original length
what is the function of the GTO?
Prevents muscles from contracting too strongly –> if a muscle contracts so strongly that it increases the tension in the tendon to the point it might get damaged, the GTO switches it off by inhibitory action
what type of connection does the GTO have to its own motor neuron?
disynaptic (interneurone) connection
what type of interneuron connects the GTO to its own motor neuron?
glycinergic inhibitory neuron
what mediates reflex withdrawal from painful stimulus?
small myelinated nociceptor afferents
what do small myelinated nociceptor afferents act on?
interneurons in the spinal cord
what does the flexion reflex arc involve?
involves two or three excitatory INs between pain afferent and flexor motor neuron
what type of reflex is the flexion reflex?
polysynaptic
what is the flexion reflex?
Reflex withdrawal from painful stimulus mediated by activation of small myelinated nociceptor afferents.
why can the flexion reflex be suppressed by the brain?
due to many interneurons in the reflex
describe the crossed extensor reflex
when you step on a nail, flexion reflex withdraws foot from the injury
extensors relax
extensors in the other leg contract to take body weight
explain how the crossed extensor reflex happens
signals travel up the spinal cord and cause contraction of contralateral muscles of the hip and abdomen - shift body’s centre of gravity over extended leg
done by afferent fibres crossing to the contralateral side of the spinal cord and synapsing with interneurons
what type of reflex is the crossed extensor reflex?
polysynaptic reflex
what is muscle tone?
small amount of contraction in muscles when these are moved passively
how is tone affected in LMN damage and what effect does this have?
tone is reduced, tone is absent –> muscles are paralysed (flaccid paralysis)
how is tone affected in UMN damage and what effect does this have?
tone is increased (spasticity)
