Spinal Cord Physiology Flashcards
What are the identifying characteristics of a spinal reflex?
Present when spinal cord is severed, short latency, with often known circuitry
What is the stretch / muscle jerk flex? What is special about it?
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
What is the tendon reflex?
Golgi tendon organ - “clasp knife” reflex - causes release of muscles when too much pressure is on a tendon - usually to protect an organ.
What is the flexor flex?
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
What is the positive supportive reaction reflex?
In babies, will extend their legs to touch ground when it is near
What is the grasp reflex?
Baby will grab something placed in palm
What is the Babinski sign?
Fanning out of toes when brushing against foot. Should not be present in adults (suppressed by central command)
What is clonus?
Pushing on foot causes stretch reflex oscillation of contraction -> indicative of spinal injury
What is scratch reflex?
More present in frogs, organism will reflexively scatch at site of irritation
What is the bulbocavernosus reflex?
reflex contraction of rectum when tip of penis or clitoris is squeezed. First indicator of spinal cord recovery after spinal shock
What is meant by stimulus dependence?
The intensity of the reflex action is dependent on intensity of stimulus, position, and duration
What is reciprocal innervation of reflexes?
In a reflex, synergistic muscles will feel similar output. Antagonistic muscles will use interneuron to have opposite output.
What is meant by plasticity of the spinal cord reflexes? What can be used to reach threshold?
Spinal cord reflexes are adaptive and changing. To reach threshold, one can use temporal (multiple in a row) or spatial (simultaneous stimuli) summation
What is habituation?
Responses to a repeated stimuli decrease
What is sensitization? What neurotransmitter is involved?
Arousal stimuli can re-invigorate habituated responses, and even make them more powerful.
Serotonin mediates this effect
What is the sensory element of the myotatic / stretch reflex? How does this coordinate with voluntary control?
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
What is the functional reflex pathway of the stretch reflex?
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.
What is the reciprocal innervation for the stretch reflex?
Synergistic muscles are activated by weak, parallel innervation from central processes of sensory neurons.
Antagonistic muscles are inhibited by interneurons (disynaptic pathway)
What is the function of interneurons?
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.
How can the integrity of C7 spinal level be tested?
Triceps tendon reflex Triceps and extensors of wrist and fingers will be affected Radial nerve (C5-T1)
How can the integrity of the L2-L4 spinal levels be tested?
Knee jerk reflex
Quadriceps / patellar tendon will be affected
(femoral nerve, L2-L4)
How can the integrity of the S1 spinal level be tested?
Ankle jerk reflex
Plantar flexors including gastrocnemius will be affected
(tibial nerve, S1-S2)
How can the integrity of C5 spinal level be tested?
Biceps reflex
Deltoid, biceps will be affected
Deltoid: Axillary nerve (C5, C6)
Biceps: Musculocutaneous nerve: (C5-C7)
What is overactive in Parkinson’s disease?
Stretch reflex -> makes muscles more rigid
How is clonus an example of reflex plasticity?
Subcutaneous nerve stimulation can suppress the reflex
What type of reflex is the tendon reflex?
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).
What is the generalized function of the flexor reflex work?
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
What is the flexor reflex pathway?
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
What is the latency + long-lastedness of the crossed-extensor reflex?
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
What is spinal shock and how does it differ in animals / humans?
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
What happens to reflexes during recovery from spinal cord injury? What does this explain?
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.
