Spinal reflexes

L25

In what way is the spinal cord structure related to physiological reflexes?

How do mono and polysynaptic reflexes differ?

Name examples of each kind of reflex

In what types of behaviours can simple reflexes be used?

Define a reflex

rapid, involuntary, yet stereotyped and co-ordinated response to a sensory stimulus

What types of physiological responses can be reflexes?

muscle contraction, glandular responses, e.g. lactation in response to suckling

What types of reflexes are learned?

Pavlovian ones ie visceral

Which nervous system is involved in unlearned reflexes? What are other names for these reflexes?

somatic nervous system

somatic reflexes/spinal reflexes

Are spinal reflexes spontaneous?

no, require stimulation ie sensory input

What is the speed of spinal reflexes and what explains this?

quick bc only a few synapses are involved

What does the stereotyped quality of spinal reflexes mean and suggest?

occur the same way every time - suggests underlying circuitry is very simple

What suggests that there is little input from higher centres during spinal reflexes?

involuntary and automatic reflexes; you often are aware of them only as they happen and they are difficult to suppress

What is the Patellar reflex?

leg kicks in response to a tap on the knee

Why does the leg kick in response to a tap on the knee? Which two structures are involved?

allows the correct extension of the limb/maintaining body posture - extensor part extends the limb, flexor bends/flexes it

What does monosynaptic mean?

only one synapse involved

What is an example of monosynaptic reflex?

Patellar reflex - direct synapse to motor neuron; Stretch reflex - direct connection to motor neuron

Is the Patellar reflex a response to pain in the knee? What is it?

no - corrects stretching of muscle during the process - avoids injury, maintains posture and allows proprioceptive system

What’s the muscle spindle?

special sensory receptor that detects muscle stretch

What are 3 steps of the stretch/myotactic reflex?

• sensory fibres sense muscle stretch and send signals to spinal cord

• monosynaptic connection to motor neuron fires AP = contracted biceps muscle

• reciprocal inhibition: connection to interneuron inhibits the firing of motor neurons connected to the triceps = relaxed antagonist muscle (triceps)

Why are muscle spindles called proprioceptors?

sense organs that monitor the position and movement of body parts

Where are muscle spindles most abundant?

found in most striated muscle and are particularly abundant in muscles involved in fine motor control (e.g. hand)

Which fibres innervate muscle spindles?

Ia sensory fibres

What are alpha motor neurons and what explains their name?

motor neurons innervating the surrounding muscle

conduction velocity

What do the I a sensory fibres innervating muscle spindles provide feedback to and on?

feedback to the alpha motor neurons innervating the surrounding muscle

on the amount of muscle stretch that is occurring like in the ‘knee-jerk’ reflex

What allows the muscle spindle to never be slack? How do these fibres achieve this?

intrafusal (muscle) fibres

adjust the tension in the spindle as the extrafusal (muscle) fibres of the surrounding muscle contract

What stimulates muscle spindle intrafusal muscle fibres?

innervation by axons from gamma motor neurons

Why did Ian Waterman’s viral infection lead to an inability to co-ordinate mvt unconsciously?

had lost proprioception ie sense of the position of the body relative to other parts

Etymology of proprioception

proprio = property

ception fron perception

so perception of one’s own property/body

What is kinaesthesia?

sense of your body moving in space

What is the golgi tendon organ (GTO)? What does it detect?

proprioceptor

muscle tension due to muscle contraction (not muscle stretch like muscle spindles!)

Which afferent neurons innervate the GTO and what does their activation lead to?

sensory (Ib) afferents

= activation of inhibitory interneurons = inhibit α motor neurons that innervate the same muscle

What is the purpose of the feedback mechanism of the GTO? Is it positive or negative?

regulates muscle tension and protects the muscle (and tendon) from damage when large forces are generated - as muscle tenses, inhibition is created

negative

How does the Golgi tendon Organ regulate muscle contraction?

prevents muscle from contracting too much (so works with muscle spindle that allows constant tension/tone)

What is the flexor (withdrawal) reflex?

quick contraction of flexor muscles to withdraw a limb from an injurious stimulus (e.g. heat or cut) - lasts bc need to sustain safety

Through the activation of which receptors is the flexor reflex triggered?

nociceptors (noci = hurt)

How quick is the flexor withdrawal reflex? Considering the number of synapses involved, why is this suprising?

very fast even though polysynaptic reflex

What sustains the response of the flexor reflex?

activation of multiple excitatory interneurons ie the parallel after-discharge circuit

What’s the structure of a parallel after-discharge circuit and how does this allow a sustained signal?

3 pathways with different numbers of synapses each of which takes the same amount of time = each pathway takes different amounts of time so signal over an extended period

Why are inhibitory interneurons activated as well during the flexor withdrawal reflex?

to relax extensor muscles (reciprocal inhibition again) = better balance when sudden withdrawal occurs

What type of balancing element do flexor reflexes often include?

contralateral element

What does the crossed extensor reflex provide during limb withdrawal?

support/balance

What does ipsilateral mean?

same side

What does contralateral mean?

opposite side

What effect does the crossed extensor reflex have on: muscles in the opposite leg to the one being lifted; extensor muscles at the front of the lifted leg; flexor muscles at the back of the lifted leg?

activates muscle movements in the opposite leg that will counteract the weight of the lifted one

relax

contract

How do patterns of activation and inhibition underlie locomotion in animals?

alternative activation and inhibition of extensors and flexors in individual limbs + co-ordination so limbs alternate

If a section is made between the spinal cord and the brain, how is activity and co-ordination in legs affected? What does this suggest?

continues

local circuits generating pattern of alternating flexion and extension

What are central pattern generators? What are they involved in?

local circuits generating pattern of alternating flexion and extension

correct locomotion

Do all neurons involved in locomotion have identical patterns of firing? In which organisms were these patterns worked out?

distinct for different neurons

simple ones like lobsters

What is a critical feature of neuron firing for locomotion?

inhibition across the midline

How is proprioception managed at a molecular level? In which organism was this studied?

stretch receptors act as mechanosensors detecting the body bending during mvt = inhibition of motor neurons = alternating muscle contraction ie correct locomotion

c. elegans

Name examples of monosynaptic vs polysynaptic reflexes?

stretch reflex vs flexor reflex

What are the 2 main processes dependent on reflexes in the body and receptors involved in these?

• maintaining body posture - proprioceptors within the somatic sensory system

• response to pain/noxious stimuli - nociceptors

Are higher motor centres or local reflex circuits involved in spinal motor output/locomotion etc?

both involved - reflex circuits allow simple and repetitive behaviours like locomotion but higher motor centres also important

In a dog called Ink, 2 parts of the spinal cord were compressed, what effect could this have on locomotion?

spinal cord involved in locomotion so compression/damage = loss of function like alternating limb mvts