Reflexes

Reflexes

A reflex, in its most restrictive definition, is a rapid, predictable motor response to a stimulus – it is unlearned, unpremeditated, and involuntary

Reflexes can be put into learned and basic categories

Learned reflexes include things like driving a car

Basic reflexes would be jumping back from touching a hot iron

There is no clear-cut line between learned and basic – if you spilled boiling water on your arm, and there was a child next to you, instead of just dropping the pot and jumping away, you’d probably set the pot down and try to hold back the obscenities

Reflex arcs are highly specific neural pathways that carry the reflex reaction; they have five essential components

Receptor

Sensory neuron

Integration center (always within the CNS)

Motor neuron

Effector

Reflexes are classified functionally as somatic (skeletal muscle is effector) or autonomic (visceral) (smooth or cardiac muscle and/or glands)

Somatic reflexes mediated by the spinal cord are called spinal reflexes

Stretch reflex

Stretch reflexes preserve or cause changes in muscle length and help to preserve muscle tone
While the reflex is occurring, information on the muscle length and shortening is being sent to the brain, allowing muscle tone to be kept
Stretch reflexes are most important in postural muscles of the trunk and legs (such as the quadriceps)
The stretch reflex is always accompanied by the gamma motor neuron reflex arc, in order to prevent crude, jerky muscle contractions
The patellar or knee-jerk reflex is the most familiar example of a stretch reflex – the sudden jolt to the patellar tendon causes stretching of the quadriceps which in turn causes the stretch reflex reaction of contracting the quads.
Stretch reflexes as monosynaptic and ipsilateral
If there is peripheral nerve damage or ventral horn injury, the stretch reflex will be suppressed or absent. The reflex is also absent in those suffering from chronic diabetes mellitus, neurosyphilis and coma.
If there is no inhibitory effect from the brain (or it is suppressed), the stretch reflexes are hyperactive, such as seen in polio and stroke.

Deep tendon reflexes

Opposite of the stretch reflex – muscle relaxation and lengthening in response to contraction.
Deep tendon reflexes are polysynaptic and ipsilateral

Flexor reflexes

Also called the withdrawal reflex
Initiated by a painful stimulus (actual or perceived) and causes automatic withdrawal of the threatened body part
Flexor reflexes are polysynaptic and ipsilateral

Crossed extensor reflex

A complex spinal reflex consisting of the withdrawal reflex with a contralateral extensor reflex
The best example is the balancing act that occurs if you step on a piece of broken glass in bare feet – the withdrawal reflex lifts the damaged foot, while the extensor reflex balances you on the unaffected foot

Superficial reflexes

Elicited by gentle cutaneous stimulation
The plantar reflex tests the integrity of the spinal cord from L₄ to S₄ and is elicited by drawing a blunt object downward along the lateral aspect of the sole of the foot. The normal response is curling of the toes, but if the primary motor cortex or corticospinal tract is damaged, Babinski’s sign occurs (the big toe dorsiflexes and the smaller toes fan laterally).
The abdominal reflex occurs when the skin of the lateral abdomen is stroked to the side, above or below the umbilicus. The umbilicus will move towards the stimulated site. This checks the integrity of the spinal cord from T₈ to T₁₂.