Spinal Cord Physiology

Question 1

What are the identifying characteristics of a spinal reflex?

Answer 1

Present when spinal cord is severed, short latency, with often known circuitry

Question 2

What is the stretch / muscle jerk flex? What is special about it?

Answer 2

Also called the myotatic reflex - it is reflex that makes your knee jump when you tap the patellar ligament. Causes muscle contraction via stimulation of intrafusal fibers.
It is the only monosynaptic reflex

Question 3

What is the tendon reflex?

Answer 3

Golgi tendon organ - “clasp knife” reflex - causes release of muscles when too much pressure is on a tendon - usually to protect an organ.

Question 4

What is the flexor flex?

Answer 4

Causes you to flex your leg when stepping on a noxious stimuli. It is paired with the crossed extensor reflex so you step onto the other leg

Question 5

What is the positive supportive reaction reflex?

Answer 5

In babies, will extend their legs to touch ground when it is near

Question 6

What is the grasp reflex?

Answer 6

Baby will grab something placed in palm

Question 7

What is the Babinski sign?

Answer 7

Fanning out of toes when brushing against foot. Should not be present in adults (suppressed by central command)

Question 8

What is clonus?

Answer 8

Pushing on foot causes stretch reflex oscillation of contraction -> indicative of spinal injury

Question 9

What is scratch reflex?

Answer 9

More present in frogs, organism will reflexively scatch at site of irritation

Question 10

What is the bulbocavernosus reflex?

Answer 10

reflex contraction of rectum when tip of penis or clitoris is squeezed. First indicator of spinal cord recovery after spinal shock

Question 11

What is meant by stimulus dependence?

Answer 11

The intensity of the reflex action is dependent on intensity of stimulus, position, and duration

Question 12

What is reciprocal innervation of reflexes?

Answer 12

In a reflex, synergistic muscles will feel similar output. Antagonistic muscles will use interneuron to have opposite output.

Question 13

What is meant by plasticity of the spinal cord reflexes? What can be used to reach threshold?

Answer 13

Spinal cord reflexes are adaptive and changing. To reach threshold, one can use temporal (multiple in a row) or spatial (simultaneous stimuli) summation

Question 14

What is habituation?

Answer 14

Responses to a repeated stimuli decrease

Question 15

What is sensitization? What neurotransmitter is involved?

Answer 15

Arousal stimuli can re-invigorate habituated responses, and even make them more powerful.

Serotonin mediates this effect

Question 16

What is the sensory element of the myotatic / stretch reflex? How does this coordinate with voluntary control?

Answer 16

Muscle spindles: Composed of intrafusal fibers with sarcomeres at either end, which are innervated by gamma motor neurons. When muscle length is decreased via voluntary contraction, gamma motor neurons also fire to keep the muscle spindles appropriately slack, so that the reflex can work properly at all muscle lengths. This is called alpha-gamma o-activation

Intrafusal fibers are innervated by stretch receptor neurons

Question 17

What is the functional reflex pathway of the stretch reflex?

Answer 17

Cell body of stretch receptor neuron in DRG. Synapses in ventral horn on alpha-motor neurons + interneurons. Transmission is brought via ventral root to cause contraction of extrafusal fibers. This holds the muscle in the proper state of contraction for the load.

Question 18

What is the reciprocal innervation for the stretch reflex?

Answer 18

Synergistic muscles are activated by weak, parallel innervation from central processes of sensory neurons.

Antagonistic muscles are inhibited by interneurons (disynaptic pathway)

Question 19

What is the function of interneurons?

Answer 19

They “flip the sign” of the original stimulus. For instance, in the stretch reflex, they are the means by which antagonist muscles are deactivated. The interneurons release glycine or GABA as neurotransmitters, which are inhibitory (hyperpolarizing), preventing antagonistic muscle contraction.

Question 20

How can the integrity of C7 spinal level be tested?

Answer 20

Triceps tendon reflex
Triceps and extensors of wrist and fingers will be affected
Radial nerve (C5-T1)

Question 21

How can the integrity of the L2-L4 spinal levels be tested?

Answer 21

Knee jerk reflex
Quadriceps / patellar tendon will be affected
(femoral nerve, L2-L4)

Question 22

How can the integrity of the S1 spinal level be tested?

Answer 22

Ankle jerk reflex
Plantar flexors including gastrocnemius will be affected
(tibial nerve, S1-S2)

Question 23

How can the integrity of C5 spinal level be tested?

Answer 23

Biceps reflex
Deltoid, biceps will be affected
Deltoid: Axillary nerve (C5, C6)
Biceps: Musculocutaneous nerve: (C5-C7)

Question 24

What is overactive in Parkinson’s disease?

Answer 24

Stretch reflex -> makes muscles more rigid

Question 25

How is clonus an example of reflex plasticity?

Answer 25

Subcutaneous nerve stimulation can suppress the reflex

Question 26

What type of reflex is the tendon reflex?

Answer 26

Disynaptic, uses an interneuron to inhibit alpha-motor neurons when there is too great a load on the tendon. Uses the Golgi tendon organ in series. Protective and adjustment function (along with stretch reflex).

Question 27

What is the generalized function of the flexor reflex work?

Answer 27

Works with the crossed-extensor reflex to withdraw the stimulated limb and extend the limb on opposite side of the body. This is the same pathway used for walking

Question 28

What is the flexor reflex pathway?

Answer 28

Noxious stimuli detected by ALS-type receptors in DRG (stronger nociception = stronger response). Interneurons are used, excites flexors and inhibits extensors on ipsilateral side.

Crossed-extensor: excites extensors and inhibits flexors on contralateral side

Question 29

What is the latency + long-lastedness of the crossed-extensor reflex?

Answer 29

Latency of 0.2-0.5 sec, indicating many interneurons in pathway. It is a long-lasting effect, indicating perseveration of signal in the spinal cord

Question 30

What is spinal shock and how does it differ in animals / humans?

Answer 30

State of transient reflex depression below the level of a spinal cord injury, due to loss of serotonin.

Days to months in humans, but in frogs it’s almost nonexistent

Question 31

What happens to reflexes during recovery from spinal cord injury? What does this explain?

Answer 31

They get stronger, and may even result in hyperreflexia as some reflexes are suppressed by central command: i.e. Babinski reflex. This explains why newborns lose spinal reflexes as they group up, and their central control exerts more positive and negative effects on spinal circuits.