Spinal Reflexes

Question 1

what is a reflex?

Answer 1

a rapid, involuntary, and coordinated response to a sensory stimulus.

Question 2

what are some factors of spinal reflexes?

Answer 2

they are quick,
involuntary,
automatic,
difficult to supress,
stereotyped (occur the same way each time).

Question 3

what is the Patellar / knee-jerk reflex?

Answer 3

involves a sensory stimulus (tapping the knee),
causes primary afferent sensory neuron to conduct and action potential,
relayed on to the dorsal horn of the spinal cord,
sensory afferent synapses on to motor neurons in the ventral horn.

Question 4

what type of reflex is the patellar reflex?

Answer 4

monosynaptic – only one synapse is involved.

Question 5

what happens when the motor neurons in the ventral horn are activated during the Patellar reflex?

Answer 5

the motor neurons fire,
project back on to the muscle,
stimulating the extensor muscle to contract.

Question 6

what are extensor muscles?

Answer 6

muscles causing limbs to extend.

Question 7

what are flexor muscles?

Answer 7

muscles causing limbs to flex.

Question 8

what happens when extensor muscles contract?

Answer 8

the limb pulls away / bends.

Question 9

what is the Myotatic / stretch reflex?

Answer 9

refers to the contraction of a muscle in response to its passive stretching, such as pouring a drink and the glass getting heavier.

Question 10

what does the Myotatic / stretch reflex involve?

Answer 10

involves sensory fibres sending signals to the spinal cord,
a monosynaptic connection on to motor neurons,
fires action potential,
causing bicep contraction and the glass is held in place.

Question 11

explain the reciprocal inhibition that occurs during the Myotatic / stretch reflex:

Answer 11

simultaneously, the sensory fibre also synapses on to an inhibitory interneuron,
inhibits the firing of motor neurons connected to the triceps,
it relaxes the antagonistic muscles.

Question 12

what does this reciprocal inhibition ensure?

Answer 12

it ensures that the resistance to the stretch is not then further resisted by the tricep.

Question 13

what is the name of the sensory receptor that detects muscle stretch?

Answer 13

the muscle spindle.

Question 14

what type of sense organ are muscle spindles?

Answer 14

proprioceptors - monitor the position and movement of body parts.

Question 15

where are muscles fibres found?

Answer 15

striated muscle.

Question 16

what does this mean?

the muscle fibres are innervated by Ia sensory fibres.

Answer 16

they provide feedback to the motor neurons innervating the surrounding muscle on the amount of stretch that is occuring.

Question 17

what are muscle spindles made up of?

Answer 17

intrafusal fibres and extrafusal fibres.

Question 18

what happens when the extrafusal fibres of muscles contract?

Answer 18

the motor neurons stimulate intrafusal fibres to adjust tension in the spindle.

Question 19

what is the Ian Waterman, 1972 case?

Answer 19

a viral infection destroyed nerves providing sense of touch and nerves attached to muscles that provided limb perception,
lost the ability to coordinate movement unconsciously.

Question 20

what is the Golgi tendon organ?

Answer 20

a kind of proprioceptor that detects muscle tension due to muscle contraction.

Question 21

what does the activation of the Golgi tendon organ sensory Ib afferents lead to?

Answer 21

the activation of inhibitory interneurons that inhibit motor neurons innervating the same muscle.

Question 22

the Golgi tendon organ reflex is a negative feedback circuit which does what…

Answer 22

regulates muscle tension and protects the muscle and tendons from damage when large forces are generated.

Question 23

what is the flexor / withdrawal reflex?

Answer 23

a quick contraction of flexor muscles to withdraw a limb from an injurious stimulus.

Question 24

what sensory receptors does the flexor / withdrawal reflex activate?

Answer 24

nociceptors.

Question 25

regardless of the speed, what type of reflex is the flexor / withdrawal reflex?

Answer 25

a polysynaptic reflex - activation of multiple excitatory interneurons.

Question 26

there is also reciprocal inhibition occurring in the flexor / withdrawal reflex:

Answer 26

inhibitory interneurons are also activated to relax extensor muscles.

Question 27

the flexor / withdrawal reflex also has a contralateral element. what for?

Answer 27

because the rapid withdrawal of a limb could lead to imblance.

Question 28

what is the contralateral element in the flexor / withdrawal reflex?

Answer 28

there is a projection of interneurons across the midline causing excitation of the extensor muscles and contraction to counteract a loss of balance.

Question 29

what happens to extensors and flexors when an animal walks?

Answer 29

they are alternately activated or inhibited,

and the activity of these muscles in different limbs is coordinated between the limbs so the legs alternate.

Question 30

alternation of activity and coordination in legs continues even when the connections of spinal cord to the brain are severed…

Answer 30

implies the presence of local circuits that can generate the pattern of alternating flexion and extension.