somatic reflexes - lecture

what are the features of a reflex?

- requires stimulation

- rapid processing

- involuntary

- stereotyped action (same response every time)

what do spinal reflexes involve?

somatic nervous system

can you have visceral (autonomic) reflexes?

yes

what is the main mechanism of the knee jerk reflex?

tapping the patella tendon causes stimulation of the muscle spindles which causes contraction of the quadriceps and relaxation of the hamstrings

what type of reflex is the knee jerk reflex?

monosynaptic stretch

what are muscle spindles?

sensory receptors within muscles

what are the differences between muscle spindles and normal muscle cells?

muscle spindles have...

- nuclei that can be in a chain (spread out) or bag (all in the middle)

- sarcomeres only at the ends so have contractile outer portions and a non-contractile inner portion

- motor innervation by gamma motor neurones instead of alpha

- named intrafusal fibres rather than extrafusal

why do muscle spindles need a non-contractile portion?

to only send signals when the muscle actually contracts

what is wrapped around the non-contractile portion of the muscle spindles?

type Ia primary afferent fibres (in blue)

what is located in the contractile portion of the muscle spindles?

type IIa secondary afferent fibres (in green)

what are the functions of type Ia and IIa afferent fibres?

detect changes in the length of the muscle - stimulated by stretching of the mid-portion of the muscle spindle

what happens to type Ia and IIa afferent fibres when they are stimulated?

increase firing

what is the motor innervation of the muscle spindle?

gamma motor neurones in the contractile portions (red)

what happens in the monosynaptic stretch reflex?

- stretching of the muscle stimulates the muscle spindles

- sends signals from type Ia afferent fibres to dorsal horn

- synapses with alpha motor neurone of same muscle

- stimulates muscle to contract

what else needs to happen alongside contraction of the muscle group?

relaxation of the opposite muscle group

is the relaxation pathway monosynaptic?

no, it is polysynaptic, including an inhibitory interneurone

what is the relaxation pathway for the stretch reflex?

- stretching of muscle stimulates muscle spindles

- sends signals to dorsal horn of spinal cord

- synapses with inhibitory interneurone

- synapses with motor neurone and stimulates the relaxation of muscle group

why is a distraction sometimes needed when trying to initiate a reflex?

to override conscious awareness - Jendrassik manouevre

what can cause hyperreflexia? (4)

upper motor neurone problems:

- spinal cord injury

- brain injury

- MS

- pre-eclampsia (causes cerebral oedema)

what causes hyporeflexia?

lower motor neurone problems:

- peripheral neuropathy (diabetes, alcohol etc)

- trauma

- guillian barre syndrome

what is the golgi tendon relfex also called?

reverse stretch reflex

what is the function of the golgi tendon relfex?

protective reflex to stop muscles from damaging themselves from too much tension

what is a golgi tendon organ?

sensory receptor located at the junction between a muscle and its tendon

what stimulates the golgi tendon body?

muscle tension stimulates type II B fibres

what happens in the golgi tendon reflex?

- muscle tension stimulates type II B fibres

- type II B fibres synapse with inhibitory interneurones in the dorsal root

- inhibitory interneurones synapse with alpha motor neurones

- inhibition of alpha motor neurones causes muscle to relax