Spinal Reflexes Flashcards
where does the motor neurone have its cell body in
- in the ventral horn of the spinal cord
where are the upper motor neurones having their cell bodies
- they have there cell bodies in the brainstem, motor cortex, basal ganglia or cerebellum
How do upper neruones connect to lower motor neurones
- the upper motor neurones send axons down the spinal cord in descending tracts to synapse on the cell bodies and dendrites of the lower motor neurone
what is a motor unit
- this is a motor neurone together with its cell body in the dorsal horn, its motor axon and the set of muscle fibres it innervates
what it is the smallest unit of contraction that we can produce
contraction of one motor unit
activation of a small motor unit ….
produces a small force, activation of a large motor unit produces a large force
what generates a twitch
Contraction of one motor unit by a single action potential generates a twitch
what generates a tetanus contraction
- in order to produce a tetanus contraction (smooth contraction) the motor unit has to be activated by a train of action potentials high enough to produce a smooth fused contraction
what is the tetanus fusion frequency
- this is the idea that in order to produce a smooth contraction the motor axon must fire at sufficiently high frequency
- usually about 10s-1
describe how motor neurones fire their fusion frequency
motoneurons fire at their fusion frequency or not at all: there is an all-or-none frequency code in motoneurons.
how can you decrease or increase the force of contraction
- can decrease or increase the force of contraction in a muscle by recruiting more or less motor units
what are the mixture of small or large motor units
- for fine control of force we activate small motor units
- for full power we recruit the large motor units
what is the difference between sensory and motor neurones
- motor neurones fires its tetanus fusion frequency or not at all
- whereas sensory nerve fibres are where frequency in a single nerve fibre codes for intensity of stimulation
what happens if there is random death of motor neurones
- then the remaining motor axons sprout peripherally to innervate the denervated muscle fibres
- this leads to an overall increase in motor unit size and decreased fine control
what are the threes types of synaptic inputs into lower motor neurones
- descending tracts in the spinal cord from the upper motor neurones
- input from local interneurons (cells with all their processes inside the CNS)
- input from local sensory nerve fibres and activating reflexes
what are the two systems of the upper motor neurone
- the pyramidal system
- the extrapyramidal system
compare the extrapyramidal system and the pyramidal system
Extrapyramidal
- upper toro neurones cell bodies are in the brainstem and they project into the spinal cord
- they are regulated by the motor cortex
- this is the more primitive system
- found in animals
pyramidal
- upper motor neurone cell bodies are in the cortical frontal lobe which is mainly in the motor cortex
- the axons travel to the spinal cord
- they travel via the corticospinal tract and synapse on the lower motor neurones
- more precise movements
- fine motor control
describe the extrapyramidal system
- upper toro neurones cell bodies are in the brainstem and they project into the spinal cord
- they are regulated by the motor cortex
- this is the more primitive system
- found in animals
describe the pyramidal system
- upper motor neurone cell bodies are in the cortical frontal lobe which is mainly in the motor cortex
- the axons travel to the spinal cord
- they travel via the corticospinal tract and synapse on the lower motor neurones
- more precise movements
- fine motor control
where are the lower motor neurones in the spinal cord
ventral horn
what is a reflex
- it is an involuntary motor action that is triggered by a sensory input classic
what are the inputs into reflexes
1) Descending axons of upper motor neurones,
2) sensory inputs,
3) interneurones in other parts of spinal cord
where are reflexes stored
Reflexes are stored here in the dorsal and ventral grey matter of the spinal cord in the form of patterns of synaptic connections between inputs, outputs and interneurones
where can reflexes be modulated from
- can be modulated from the cortex and brainstem
- they can be partially or completely inhibited by the forebrain acting on the spinal cord
describe how the knee jerk reflex works
- myotactic reflex
- homonymous reflex -the stretch of the quadriceps muscle activates the same muscle to contract
- there is a receptor in the muscle when tapping the tendon the tendon is pulled down a tiny bit and this stretches the muscle and activates a sensory ending in the muscle,
- the afferent goes up through the dorsal root and makes a synaptic connection with the lower motor neurone,
- there is a direct connection between the afferent synapse and the lower motor neurone, the same muscle that is stretched contracts
what is a homonymous reflex
when the stretch of a muscle activates the same muscle to contract
what is a monosynaptic reflex
There is a direct synaptic connection between the input sensory nerve fibre (a muscle spindle afferent) and the motor neurone.
- there is no interneurone
where do monosynaptic reflexes happen
only in the tendon jerk reflexes
what do monosynaptic reflexes make hard
This direct connection makes it hard for the brain to suppress this reflex
how does reciprocal inhibition work
normally the same tendon tap that activates the reflex also inhibits the antagonist flexor muscle by the action of an inhibitory interneuron when the extensor contracts the flexor relaxes this is called reciprocal inhibition
what is a proprioreceptor
a receptor that responds to movements of the body rather than external stimuli and determines the position of body parts
what does the muscle spindle do
they are stretch receptors that detect the changes in the length of the muscle .
How do muscle spindles wor k
- Ia sensory afferents coil around the
centre region and detect when it is
stretched. - when it is stretched the 1a afferents increase their firing rate
- these action potentials from the 1a afferents synapes with the lower motor neurones
- motor action by up to a dozen gamma motor neurones
what are the group of spinal muscle fibres in muscle spindles
- These special fibres are known as intrafusal muscle fibres
- in these fibres all the contractile apparatus is at the ends, this leaves the centre of the fibre without actin or myosin but full of cell bodies and mitochondria
- this makes it easily stretched
what are normal muscle fibres sometimes called
Normal muscle fibres are sometimes called extrafusal muscle fibres
describe spatial summation in a reflex
- Activity at a single synapse will not activate the motor neurone. There has to be at least two synapses active simultaneously to make the motor neurone fire. This is called spatial summation
describe temporal summation in a reflex
temporal summation can also activate a motor neurone, this occurs when a single la nerve fibres fires a high frequency burst of action potentails so that EPSPs sum together to trigger an action potential
Motor neurones only fire if
there is sufficient spatial and temporal summation at their inputs
what increases the sensitivity of a muscle spindle to stretch
a gamma motor neurone
- it also stimulates the contraction of the intramural fibres
what activates the gamma motor neurones
Gamma motoneurones are NOT reflexly activated by the muscle spindle afferents;
They are driven from descending motor pathways such as the corticospinal tract.
They act to contract the ends of the muscle spindles and thus stretch the centre region where the Ia sensory fibre is found.
This increases the sensitivity of the spindle to stretch.
what happens when the gamma motor neurone is pathological
hyperactive tendon reflexes
why do we have muscle spindles
They provide CONTINUOUS FEEDBACK to the motor neurones and CONTINUALLY ADJUST the motor neurone output during normal movement.
- this prevents fatigue
what is the monosynaptic reflex used by the brain as
The monosynaptic reflex is used by the brain as a form of negative feedback to maintain a constant muscle length despite ongoing muscle fatigue.
describe how muscles spindles prevent fatigue
1) Imagine you are holding a beer glass. Initially there is an input to the motor neurones from both the descending axons from the forebrain and also from the muscle spindle Ia afferents. The combined synaptic input keeps the arm in the desired position
2) However after a while fatigue in the biceps means it contracts less strongly and thus starts to stretch. The arm starts to droop. This stretch of the muscle increases the activity in the muscle spindles.
3) Increased stretch of muscle means stretched muscle spindles send increased frequency of action potentials to spinal cord. This Increases synaptic input to motor neurones which fire at higher frequency to increase force of contraction. Result: Arm moves back to desired (initial) position.
what are the two types of priorecetpors
Golgi tendon organ
muscle spindle
where are monosynaptic reflexes the easier to elicit
- leg extensor muscles
the faster the feedback response….
the quicker the corrective muscle action. This improves balance and agility.
what does damage to monosyantpic reflexes in the leg lead to
-Damage to these reflex arcs makes patient more likely to fall or trip.
what reflex has the shortest possible delay time
Continuous feedback from muscle spindles in the spinal cord plus the high speed of action potential conduction in the afferent and efferent nerves gives the tendon jerk reflex the shortest possible delay time
where is the golgi tendon organ found
- it is found in muscle tendons
what activates the golgi tendon organ
muscle tension
where are both the muscle spindle and golgi tendon organ activated
- both the spindle and GTO are activated when a muscle is passively stretched off if the muscle shorterns back to its original length
what kind of reflex is the golgi organ
disynatpic
- it has an interneurone
what is the internueones neurotransmitter int he golgi tendon organ
glycine
- inhibitor neurone
describe flexion reflex
- Reflex withdrawal from a painful stimulus is mediated by activation of small myelinated nociceptor afferents (Ad).
- pathway involves interneurones - can be 1-3 excitatory internuones between the pain afferent input and the flexor motor neurone.
- This flexion reflex is therefore known as a polysynaptic reflex
why can the brain suppress a flexion reflex
Because a flexion reflex involves several interneurones (which may be excitatory or inhibitory) it can be suppressed by the brain.
therefore We can overcome pain in extreme circumstances
what are crossed extensor reflex
Suppose you step on a nail. A flexion reflex withdraws you foot from the injury.
When this occurs the flexors in the withdrawing limb contract and the extensors relax.
Normally the extensors in the opposite leg will also contract, to take the extra weight of your body when you stand on one limb. (At the same time, signals travel up the spinal cord and cause contraction of the contralateral muscles of the hip and abdomen to shift the body’s center of gravity over the extended leg).
Activation of extensors in the other leg during a flexion reflex is the crossed extensor reflex
It is again a polysynaptic reflex.
what kind of reflex is a crossed extensor reflex
It is again a polysynaptic reflex.
how do you produce a crossed extensor reflex
- branches of the afferent nerve fibres cross the stimulated side of the body to the contralateral side of the spinal cord
- synapse with interneurones which can excite or inhibit the alpha motor neurones to the muscles of the contralateral limb
what is muscle tone
Normal muscles have a certain slight resistance to passive movement, even if the patient is co-operating fully. This is due to small amounts of contraction in muscles when they are passively moved. This slight resistance is known as muscle tone
what happens when lower motor neurones are damaged
When lower motor neurones are damaged tone is reduced or becomes completely absent. In the latter case the muscle is said to have flaccid paralysis.
what happens when upper motor neurones are damaged
Lesions of upper motor neurones produce syndromes where there are exaggerated reflexes and pathological increases in muscle tone. This condition is known as spasticity.
what are pathological types of reflexes that indicate upper motor neurone damage
cog wheel rigidity, lead pipe rigidity, exaggerated tendon reflexes, clonic reflexes, pendular reflexes etc.
