Spinal Reflexes Flashcards
(34 cards)
What is a lower motor neurone?
A neurone which has its cell body in the ventral horn of the spinal cord
What is an upper motor neurone?
Neurones which send axons down the spinal cord in descending tracts to synapse on the cell bodies and dendrites of the lower motor neurones. These neurones have cell bodies in the brain which project down to the lower motor neurones and some also have their cell bodies
What are the three main types of synaptic input to lower motor neurones?
Descending tracts in the spinal cord from upper motor neurones, input from local inter neurones (cells with all their processes inside the CNS, grey matter) and Input from local sensory nerve fibres via reflexes
What are the two systems of upper motor neurones?
Pyramidal and extra-pyramidal systems
Describe the pyramidal system of upper motor neurones
Where upper motor neurone cell bodies project directly from the motor cortex of the brain to the spinal cord and travel via the corticospinal tract (pyramidal tract) to lower motor neurones
Describe the extra-pyramidal system of upper motor neurones
Where the upper motor neurone cell bodies in the brainstem project into the spinal cord and these motor neurones are regulated by the motor cortex
Describe the system of lower motor neurones
The spinal cord has lower motor neurones in the ventral horn that project out via peripheral nerves to muscles
What is the muscle spindle?
The muscle spindle is the receptor that mediates all of the tendon reflexes and is often called a proprioceptor as it responds to movements of the body rather than external stimuli
What is a reflex?
An involuntary motor action triggered by a sensory input. Can think about them as tiny ‘motor programmes’ which are stored in the synaptic connections between sensory inputs, outputs and interneurones in one or two spinal cord segments
Name the different reflex inputs
Descending axons of upper motor, sensory inputs, interneurones in other parts of the spinal cord
What are interneurons?
This may be excitatory or inhibitory to produce EPSPs or IPSPs
What are monosynaptic reflexes? Give examples
No interneurones are involved, and this makes it difficult for the brain to suppress these reflexes. Examples: myotactic knee, ankle, triceps, biceps and supination reflexes as
What is the evolutionary value of monosynaptic reflexes?
They are used by the brain as a form of negative feedback in order to maintain a constant muscle length despite ongoing muscle fatigue.
What is reciprocal inhibition in tendon reflexes?
Ordinarily, the same tendon tap that activates the (extensor) reflex will also inhibit the antagonist flexor through the action of an inhibitory neurones, so that when the extensor contracts in the reflex, the flexor relaxes to allow an unimpeded motion, and this is known as reciprocal inhibition.
Describe the basic structure of the muscle spindle
Spindle consists of a stretch receptor inside a connective tissue sheath, and the stretch of the muscle also stretches the muscle spindle within the sheath, activating sensory afferents (Ia sensory afferent predominantly)
How do muscle spindles act as muscle length detectors?
Spontaneously active at resting muscle lengths –> when muscle stretched –> increase firing rate on sensory afferents (Ia)
Describe the role of the muscle spindle in reflexes, using the example of the patella reflex
Tapping patellar tendon –> burst of action potentials in several muscle spindles in the muscle (sensory Ia afferents) –> burst of APs acts on dendrites of the motor neurones on the quadriceps (alpha motor neurones) –> spatial and temporal summation on the motor neurones–> motor neurones each fire a single action potential –> muscle twitch
Describe the detailed structure of the muscle spindle
Consists of modified skeletal muscle fibres within the capsule (intrafusal fibres), and all contractile tissue is at their ends, therefore the centre of the fibre doesn’t have actin or myosin but is full of cell bodies and mitochondria. Centre of fibre has no CT so can be easily stretched and Ia sensory afferents are located here
What is the role of gamma motor neurones to the muscle spindle?
When stimulated, these motor neurones cause the ends of the intrafusal fibres to contract and therefore stretch the centre –> adjusts the sensitivity of the Ia afferents to stretch (gamma motor neurones increase afferent’s sensitivity to stretch)
What does a pathological increase in gamma motor neurone activity lead to?
Hyperactive tendon reflexes
Where is the Golgi tendon organ found?
In the muscle tendons
What is the main role of the Golgi tendon organ?
Detect muscle tension
What is the interneurone between the GTO and muscle known as?
Glycinergic inhibitory neurone
How does the GTO respond when the tension in a tendon reaches a dangerous level?
Ib afferents detect this and send action potential to the glycinergic inhibitory neurone to act as a strong inhibitory of the alpha motor neurone to the muscle to inhibit muscular contraction –> switches of the muscle
