spinal reflexes Flashcards
reflex vs somatic reflexes
reflex = rapid involuntary yet stereotyped (same every time) and coordinated response to sensory stimulus
usually muscle contraction - can include glandular response e.g. lactation
reflexes can be learnt - Pavlov
somatic reflexes = spinal reflexes - spinal cord circuitary involved
somatic reflexes are unlearned - can also be visceral
spinal reflexes
require stimulation - sensory input
quick - few synapses involved
involuntary and automatic - only aware as they happen
little input from higher centres, remain intact with a severed spinal cord
stereotyped - same way each time
patellar reflex
knee jerk reaction
example of a myotatic reflex (stretch reflex)
1. knee is hit (sensory stimulus)
2. travel down primary afferent neuron to dorsal root ganglion
3. direct synapse to motor neuron (monosynaptic)
4. travels down motor neuron to extensor - causes muscle contraction and the leg to kick
extensor = muscle causing limb to extend
flexor = muscle causing limb to flex (bend)
myotatic reflexes
stretch reflex e.g. knee jerk
used to maintain body posture - proprioceptive system (knowledge of where body is in space)
myotatic reflex example - pouring a drink
as the drink is poured the glass gets heavier
- sensory fibres send muscle stretch signals to spinal cord
- monosynaptic connection to motor neuron fires action potential to contract bicep
- simultaneously, a distinct connection to an inhibitory interneuron inhibits firing of motor neurons to triceps – relaxes the antagonistic muscle – reciprocal inhibition
muscle spindle
sensory receptor which detects muscle stretch
innervated by Ia sensory fibres - provide feedback to (alpha) motor neurons innervating surrounding muscles about muscle stretch
found in most striated muscle - especially muscles for fine motor control e.g. hands
muscle spindle is innervated by axons from gamma motor neurons
stimulate intrafusal muscle fibres to adjust tension in spindle as extrafusal muscle fibres (bulk of muscle, surrounds intrafusal fibres) of surrounding muscle contract - so spindle is never slack
what is proprioception
perception of the location, movement, and action of body parts
example - Waterman - viral infection caused loss of proprioception/kinaesthesia - learnt to move by consciously controlling movements using visual information
golgi tendon organ
aka GTO
type of proprioceptor
detects muscle tension due to contraction (opposite of muscle spindle which detects muscle stretch)
activation of GTO sensory (Ib) afferents = activation of inhibitory interneurons = inhibits alpha motor neurons which innervate the same muscle = stops muscle contracting too much
negative feedback loop protects tendons and muscles against large forces
maintains muscle length (same role as muscle spindle)
flexor reflex
withdrawal reflex
quick contraction of flexors to withdraw from injurious stimulus e.g. heat, cuts
activation of nociceptive sensory receptors (pain receptors)
polysynaptic reflex - more than one synapse in a parallel after-discharge circuit (other flashcard)
inhibitory interneurons also activated to relax extensor muscles (reciprocal inhibition)
rapid withdrawal of limb could cause imbalance - contralateral element to flexor reflexes
crossed extensor reflex = provides postural support with limb withdrawal
ipsilateral vs contralateral
ipsilateral = same side
contralateral = opposite side
parallel after-discharge circuit
activation of multiple excitatory interneurons
each have different number of synapses
fewer synapses = faster
more synapses = slower
therefore initial signal is sustained over extended period
don’t want to stop and then restart harmful behaviour
locomotion
extensors and flexors are alternating, coordination between limbs
basic alteration of activity and coordination between legs continues even if spinal connections to the brain are severed (e.g. headless chicken walking around) = presence of local circuits that can generate pattern of alternating flexion and extension
central pattern generators
patterns of action potential firing
cellular basis of pattern generators has been worked out in simple organisms (not humans)
inhibition across the midline is critical (Iv and Id) for patterns
proprioception at molecular level - nematode worms
every cell in nematode worms body is known
can mutate to change its swimming pattern
take out TRP-4 stretch receptor which detects body bending - needed for negative feedback loop to alternate muscle contraction
