Stretch Reflex and Clonus PDF

Monosynaptic reflexes: The stretch reflex ○ The simplest reflex arc is the one with a single synapse between the afferent and efferent neurons (ie, monosynaptic reflexes), ○ Reflex arcs in which interneurons are interposed between the afferent and efferent neurons are called polysynaptic reflexes ○...

Monosynaptic reflexes: The stretch reflex ○ The simplest reflex arc is the one with a single synapse between the afferent and efferent neurons (ie, monosynaptic reflexes), ○ Reflex arcs in which interneurons are interposed between the afferent and efferent neurons are called polysynaptic reflexes ○ There can be anywhere from 2 to hundreds of synapses in a polysynaptic reflex arc ○ When a skeletal muscle with an intact nerve supply is stretched, it contracts. This response is called the stretch reflex or myotatic reflex. The stimulus that initiates this reflex is stretch of the muscle, and the response is contraction of the muscle being stretched ○ The sense organ is a small encapsulated spindle like or fusiform shaped structure called the muscle spindle, located within the fleshy part of the muscle ○ The impulses originating from the spindle are transmitted to the CNS by fast sensory fibers that pass directly to the motor neurons that supply the same muscle ○ The neurotransmitter at the central synapse is glutamate. ○ The stretch reflex is best-known and studied monosynaptic reflex and is typified by the knee jerk reflex. Clonus ○ Clonus is characteristic of states in which there is increased y motor neuron activity ○ This neurologic sign is the occurrence of regular, repetitive, rhythmic contractions of a muscle subjected to sudden, maintained stretch. ○ Only sustained clonus with 5 or more beats is considered abnormal ○ Ankle clonus is initiated by brisk, maintained dorsiflexion of the foot; the response is rhythmic plantar flexion at the ankle. ○ The stretch reflex inverse stretch reflex sequence may contribute to this response. However, it can occur on the basis of synchronized motor neuron discharge without Golgi tendon organ activation. The spindles of the tested muscle are hyperactive, and the burst of impulses from them activates all the motor neurons supplying the muscles at once. ○ The consequent muscle contraction stops spindle discharge. However, the stretch has been maintained, and as soon as the muscle relaxes it is again stretched and the spindles stimulated. ○ There are numerous causes of abnormal clonus including traumatic brain injury, brain tumors, strokes, and multiple sclerosis. Clonus may also occur in spinal cord injury that disrupts the descending cortical input to a spinal glycinergic inhibitory interneuron called the Renshaw cell ○ This cell receives excitatory input from alpha motor neurons via axon collaterals (and in turn it inhibits the same alpha motor neuron). ○ In addition, cortical fibers activate ankle flexors synapse on renshaw cells (as well as type Ia inhibitory interneurons) that inhibit the antagonistic ankle extensors. This circuitry prevents reflex stimulation of the extensors when flexors are active. Therefore, when the descending cortical fibers are damaged (upper motor neuron lesion), the inhibition of antagonists is absent ○ The result is repetitive, sequential contraction of ankle flexors and extensors (clonus). ○ Clonus may be seen in patients with amyotrophic lateral sclerosis (ALS), stroke, multiple sclerosis, spinal cord damage, epilepsy, liver or kidney failure, and hepatic encephalopathy Decerebrate and Decorticate postures ○ Damage to lower midbrain and upper pons causes decerebrate posturing in which lower extremities are extended with toes pointed inward and upper extremities extended with fingers flexed and forearms pronate with the neck and head extended ○ Damage to the upper midbrain may cause decorticate posturing in which upper limbs are flexed, lower limbs are extended with toes pointed slightly inward and head is extended. ○ Function of principle afferent systems to the cerebellum ○ Afferent tracts transmits vestibulocerebellar vestibular impulses from labyrinths, direct and via vestibular nuclei ○ Dorsal spinocerebellar: proprioceptive and exteroceptive impulses from muscle spindles, Golgi tendon organ, and joint receptors of lower limbs and trunk ○ Ventral spinocerebellar: proprioceptive and exteroceptive impulses from muscle spindles, golgi tendon organ, and joint receptors of upper limbs and lower limbs ○ Cuneocerebellar: Proprioceptive impulses from muscles spindles, golgi tendon organ, and joint receptors of upper limb and upper thorax ○ Tectocerebellar: auditory and visual impulses via inferior and superior colliculi, respectively