Reflexes Flashcards
What is a reflex
Stereotyped - predictable
Involuntary - autonomic response
Rapid
Clinical relevance of reflexes
Testing reflexes
Pupillary reflex - construction of pupil in response to bright light under ANS
deep tendon reflex - SNS
General functions of reflexes
Protective - limb withdrawal/ cough reflex
Postural control - walking. No conscious activity
Homeostasis - blood glucose/ pressure
Components of reflex arc
Sensory receptors
Afferents
Integration centre CNS
Efferent neurones
Effectors
Importance of sensory receptors in reflex arc
Detect stimuli
Generate AP
Importance of CNS in reflex arc
Interneurones in CNS - where info from a sensory neurone is passed to motor neurone
Can be excitatory and inhibitory
Where are interneurones found
Spinal cord
Brain stem nuclei
Enteric NS - control of gut
Does reflex arc work in isolation
No works with the rest of ns
Impulse sent to muscle to remove it from hot object but impulses also sent to the brain which cause u to feel pain
Brain can also override reflexes e.g. choose to hold hand close to hot
Where do somatic/ autonomic reflexes normally operate from
most somatic reflexes operate via spinal cord whereas most autonomic reflexes operate via the brain stem nuclei
What is modulation
Interneurones produce output depending on the inputs they receive
Importance of efferent neurones in reflex arc
Called motor neurones - somatic reflexes
Called efferent neurones - ANS
Innervate effectors by passing on AP from interneurones/ sensory neurones
What are simple stretch reflexes
Required for adjusting posture
Adjust degree of skeletal muscle contraction
Sensory receptors - proprioceptors (mechanoreceptors)
Monitor state of skeletal muscle
Two examples of proprioceptors
Muscle spindles
Golgi tendon organs
Where are muscle fibres found
Within all skeletal muscles
What are muscle spindles
Specialised set of muscle fibres
Made up of nerve endings of sensory axons wrapped around muscle fibres
If there is change in muscle fibres, the nerve endings can detect change
There is a direst synapse between the sensory neuron and the motor neurone: no interneurons and no modulation potentials in between
What is a monosynaptic reflex
One neurone to neurone synapse
No NMJ
Mechanism 1 of simple stretch reflex mediated via muscle spindle when lifting heavy weights
Opposes sudden change in muscle length
(Refer to diagram if confused)
Muscle is stretched and elongated which stretches the muscle spindle
Generates AP which travels down sensory neurone to spinal cord
At synapse the sensory neurone will release excitatory NT
Excites the motor neurone - AP travel along the motor neurone via ventral root
AP gets to NMJ the alpha motorneurone releases ACh
Leads to skeletal contraction
Contraction prevents damage
Mechanism allows us to pick up heavy objects
Mechanism 2: knee jerk reflex
Myotatic reflex
Antagonistic muscle pairs - quadriceps and hamstrings
Tendon being hit causes stretching of the quadriceps muscle - stretching - activates muscle spindle
Cause AP to be sent to spinal cord via sensory neurone
Sensory neurone releases excitatory NT (ACh)
Alpha motor neurone stimulated causing contraction of quadricep
Excitatory NT - interneurone - inhibitory NT causing hamstring to relax
Simultaneous contraction and relaxation allow extension of the knee
Where is the muscle spindle fibre (sensory receptor) present in the knee jerk reflex
In quadricep
Sensory neurone in knee jerk reflex connects to
Motor neurone
Interneurone - which then connects to another motor neuroen
How to stimulate knee jerk reflex
Tapping below the patella
Hit tendon
What are Golgi tendon organs
Receptors in tendons (muscle to bone)
Amongst collagen fibres are nerve endings of sensory neurones which form Golgi tendon organ
Interneurone btw afferent and efferent neurone
Described as polysynpatic
Polysnypatic reflex
withdrawal reflex is polysynaptic, meaning that, in addition to the sensory and motor neurons, this response utilizes interneurons which pass signals between the sensory and motor neurons, ultimately creating multiple synaptic connections.
Difference between monosynaptic and polysynpatic
Monosynaptic reflex: ↑ A reflex that only contains one space for an action potential to travel between a sensory and motor neuron.
Polysynaptic reflex: ↑ A complex reflex that contains many connections between neurons.
Mechanism of Golgi tendon organ
Muscle stimulated by heavy weight so muscle contracts
Tendon pulled due to increase in muscle contraction
Collagen fibres in tendon straighten and lightly compress the nerve ending and GTO activated
Reaches threshold - AP down sensory axons
Excitatory NT - interneurone - inhibitory output
Prevents activation of motor neurone
Decreases muscle activity so decreases less
Reverse myotatic reflex - more muscle activity results in less muscle activity after reflex
Importance of Golgi tendon organ reflex
Prevents damage due to overwork of muscles - stops contraction e.g. weightlifting
Fine control of muscle tension - tiny adjustments to muscle activity e.g. when writing
Mechanism when someone walking and steps on smth sharp (crossed extensor flexor)
Causes leg to lift:
Nociceptors activated in skin
AP to dorsal horn of spinal cord via sensory neurone
Sensory neurone branches
One branch to brain allows pain experience
Other branches:
Excitatory NT - interneurone - excitatory output - activates alpha motor neurone - causes right hamstring contraction
Excitatory NT - interneurone - inhibitory output - deceases right quadricep activity (relaxation)
If person walking lifting leg will cause them to fall so need stabilisation in the other leg:
Excitatory NT - interneurone - excitatory output - alpha motorneurone - left quadriceps contraction
Excitatory NT - interneurone - inhibitory output - left hamstring relaxation
Refer to diagram on lecture
