Flashcards in 5 - Spinal Reflexes Deck (45):
Define a motor unit
= motor neuron together with its cell body in the dorsal horn, its motor axon and the set of muscle fibres it innervates
Contraction of 1 motor unit by a single action potential generates what?
For smooth contraction, MU is activated by what to produce a type of which contraction?
MU is activated by a train of action potentials at a frequency high enough to produce a smooth fused contraction = tetanus/tetanic contraction (how healthy muscles normally contract, not a pathological condition)
What does a large activation of large motor unit produce?
How are the force of contraction in muscle increase/decrease by?
Increase/decrease the force of contraction in muscle by recruiting more/less MU
Each motor axon fire at its tetanus fusion frequency or not at all.
Most muscles = Mixture of small and large motor unit.
Fine control of force = activate more or less small MU.
Full power = recruit the large MU
What is tetanus?
Contraction of a motor unit, smooth contraction, how normal healthy muscles work
Define a lower motor neuron
= A neuron with its cell body in the ventral horn of the spinal cord
What are the 3 main synaptic inputs to a lower motor neuron
Upper motor neurones from descending tracts in the spinal cord
Local interneurons (cells with all processes inside the CNS)
Local sensory nerve fibres via reflexes
What is the pathological condition if LMN is damaged?
Reduced tone or complete absence of tone = flaccid paralysis
Define an upper motor neuron
A neuron with its cell body in the brain that projects down the spinal cord in a descending tract to synapse with the cell bodies of the lower motor neurones
Some have their cell bodies in the brainstem, some in the motor cortex, basal ganglia or the cerebellum
What are the 2 systems of upper motor neurons and how do they relate to the spinal cord?
Pyramidal system – upper motor neurone cell bodies projecting directly from the motor cortex that travel via the corticospinal tract (pyramidal tract) to lower motor neurones
Extrapyramidal system – upper motor neurones with cell bodies in the brainstem that are regulated by the motor cortex
Spinal cord - lower motor neurones in ventral horn that project out via peripheral nerve to muscles
What is the pathological condition if the upper motor neuron is damaged?
= pathological increases in muscle tone and hyperactive reflexes = spasticity
Pathological types of reflexes indicating upper motor neuron injury include = cogwheel rigidity, exaggerated tendon reflexes, clonus reflexes, pendular reflexes etc.
What is meant by the babinski response?
Vagenskies response = (when scratching a baby's foot, the toe will curl up but in normal adult, it curls down) ⇒ plantarflexion reflexion (downward push). Examination for spinal cord lesion in the foot will include scratching the base of the foot and if lesion is present in an adult, the toes will curl upwards.
Define muscle tone
Muscle tone (residual muscle tension or tonus) = the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state. It helps maintain posture and declines during REM sleep
Spasticity = a condition in which certain muscles are continuously contracted. This contraction causes stiffness or tightness of the muscles and can interfere with normal movement, speech, and gait. Spasticity is usually caused by damage to the portion of the brain or spinal cord that controls voluntary movement.
What is meant by a monosynaptic reflex arc?
involuntary motor action triggered by sensory input
What are myotatic reflexes and what are they also called?
Myotatic reflexes are those that are elicited by tapping the tendon of a muscle. They are also called tendon jerk reflexes
Where are the reflexes stored and in what form?
Reflexes are stored in the dorsal and ventral grey matter of the spinal cord in the form of patterns of synaptic connections between inputs, outputs and interneurons
Where are most reflexes modulated?
Most reflexes can be modulated from the cortex and brainstem, e.g. the flexion reflexes can be partially or completely inhibited by the forebrain acting on the spinal cord. Tendon reflexes cannot and are the most useful.
Give an example of myotatic reflex and what is meant by homonymous reflex?
Knee-jerk (patellar) reflex is a classic myotatic reflex – the stretch of the quadriceps muscle activates the same muscle muscle to contact = homonymous reflex. The myotatic knee, ankle, biceps, triceps and supinator reflexes are called monosynaptic reflexes as no interneurons are involved in the reflex
What is meant by reciprocal inhibition?
The tendon tap activates the extensor reflex but also inhibits the antagonist flexor muscle by the action of an inhibitory interneurons
Draw a diagram of monosynaptic reflex arc
Describe the 4 steps of neuroanatomical pathways
①: stimulus (e.g. tendon tap) results in muscle stretch = muscle spindle stretch
②: muscle spindle stretch detected by afferent fibres. Initiates impulse firing.
③: impulses carried via afferent fibre to spinal cord (CNS)
④: afferent fibre synapse in ventral horn of spinal cord grey matter. Volley of impulses act on efferent dendrites. ⑤: each efferent fibre fires a single action potential resulting in the muscle twitch (contraction) The reflex arc The same tendon tap also has a RECIPRICOL INHIBITION. At the same time as the extensor contracts, there is a reciprocal inhibition of the antagonist flexor muscle via an inhibitory interneuron- inhibitory neurotransmitter is glutamate (extensor contracts and flexor relaxes). This multisynaptic pathway is separate from the stretch reflex which is monosynaptic (Crossman & Neary, 2014).
What is muscle spindle?
Muscle spindle (hollow middle narrowed end shaped)
= receptor that mediates the tendon reflexes = proprioceptor as it responds to movements of the body itself rather than external stimuli
What type of sensory afferent fibre are muscle spindles?
What is meant by muscle length detectors
Most spindles are spontaneously active at resting muscle lengths and they increase their firing rate when the muscle is stretched
Why does temporal and spatial summation takes place?
So that the motor neurones each fire a single action potential to produce a twitch
What is meant by an EPSP?
A single action potential in a Ia sensory nerve fibre does not produce an action potential in the motor neurone. Instead, there is a small depolarisation (5-15 mV) which is called an excitatory postsynaptic potential or EPSP.
Define spatial summation
2 synaptic inputs E1 and E2 on motor neurone - both active at the same time, the 2 EPSPs add together and may trigger an action potential
Define temporal summation
Activate a motor neuron and occurs when singe 1a nerve fibre fires a high frequency burst of action potentials so that EPSs sum together to trigger an action potential
What is meant by intrafusal fibres?
The muscle spindle consists of skeletal muscle fibres that are modified and within a capsule
What is meant by gamma motor neurons and which tract are they driven from?
= adjust sensitivity of the 1a afferents to stretch.
Gamma motor neurons are not reflexly activated by the muscle spindle afferents – they are driven from descending motor pathways such as the corticospinal tract
What happens if there
are pathological increases in gamma neuron activity?
Pathological increases in gamma neurone activity mimics the effects of motor neurone damage → leads to hyperactive tendon reflexes
What happens when Gamma S fibres are active?
= make precise changes to muscle length
What happens when Gamma D fibres are active?
hold muscle at a very precise length
What is meant by muscle fatigue? How is it controlled?
negative feedback loop to maintain a constant muscle length despite ongoing muscle fatigue
Muscle fibres detect drop and increase the drive in motor neurones to bring it back to desired length
Without this the limb would droop or collapse
What is the golgi tendon organ?
A tension receptor - prevents muscles from overcontracting by acting as a safety cut out switch
- Inhibitory reflex
What copuscles are related to the golgi tendon organ?
Which sensory fibres does the golgi tendon reflex use?
1b sensory fibres
Which neurotransmitter is involved in the golgi tendon reflex?
What is the mechanism of the golgi tendon reflex?
- Tension in the muscle is too high
- Inhibitory interneuone activated
- Inhibits motor neuron
Define the clasp knife reflex
massively increases firing of motor neurone - if forced enough, initiates golgi tendon organ and halts the firing of GTO by powerful inhibitory action.
What is meant by flexion reflexes?
polysynaptic reflex - several interneurons involved, not just one.
Reflex withdrawal from a painful stimulus is mediated by the activation of small myelinated nociceptor afferents – delta A fibres
These cells act on interneurons, not directly on motor neurones
These are polysynaptic reflexes as they often involve multiple excitatory interneurons
As interneurons can be controlled by the brain we can consciously inhibit flexor reflexes
What is meant by crossed extensor reflex? (also polysynaptic)
Activation of extensors in the other leg during flexion reflex