Somatic Motor System, Stretch Receptors & Reflexes

Somatic motor output

Skeletal muscle fibers contract only when stimulated by somatic motor neurons.

Upper motor neurons

Neurons in the CNS that initiate movement; originate in motor areas such as the primary motor cortex and relay signals toward lower motor neurons.

Lower motor neurons

Multipolar neurons that directly innervate skeletal muscle fibers and cause them to contract.

Lower motor neuron cell bodies

Located in the CNS in the anterior horn of the spinal cord or in the brainstem.

Lower motor neuron axons

Large, myelinated axons that travel in the PNS.

Upper-to-lower motor neuron pathway

CNS initiates movement through upper motor neurons → signals are relayed to lower motor neurons → lower motor neurons stimulate skeletal muscle fibers.

Lower motor neuron neurotransmitter

Acetylcholine (ACh), which is released onto skeletal muscle fibers to initiate contraction.

Motor neuron pool

Group of lower motor neurons that innervate the same muscle.

Alpha (α) motor neurons

Lower motor neurons that stimulate skeletal muscle fibers to contract.

Gamma (γ) motor neurons

Lower motor neurons that innervate specialized stretch receptors called intrafusal fibers.

Muscle spindle

Stretch receptor embedded among regular extrafusal skeletal muscle fibers that monitors muscle stretch.

Extrafusal muscle fibers

Regular contractile skeletal muscle fibers surrounding muscle spindles.

Intrafusal muscle fibers

Specialized muscle fibers located within a muscle spindle.

Number of intrafusal fibers in a muscle spindle

Approximately 2–12 intrafusal muscle fibers are found within each spindle.

Central region of intrafusal fibers

Region of intrafusal fibers where contractile filaments are absent.

Contractile poles of muscle spindles

Contractile ends of intrafusal fibers that are innervated by gamma motor neurons.

Primary afferent neurons of muscle spindles

Respond to stretch of intrafusal fibers when the stretch is first initiated.

Secondary afferent neurons of muscle spindles

Respond to the static length of a muscle and the position of the limb.

Muscle spindles and fine movement

Muscles that produce fine movements contain more muscle spindles.

Golgi tendon organs

Mechanoreceptors located within tendons near the muscle-tendon junction that monitor tension produced by muscle contraction.

Structure of a Golgi tendon organ

Encapsulated bundle of collagen fibers attached to about 20 extrafusal muscle fibers.

Golgi tendon organ sensory axon

A single somatic sensory axon registers contraction of the muscle.

Golgi tendon organ firing rate

The greater the muscle tension, the more rapidly the sensory neuron fires.

Muscle spindle vs. Golgi tendon organ

Muscle spindle monitors muscle stretch/length; Golgi tendon organ monitors muscle tension.

Reflex

Programmed, automatic response to selected sensory input; most reflexes are protective and help prevent tissue damage.

Monosynaptic reflex

Reflex involving a single synapse within the spinal cord between the sensory neuron and motor neuron.

Polysynaptic reflex

Reflex involving multiple synapses.

Visceral reflex

Reflex in which neurons innervate internal organs; associated with the autonomic nervous system.

Somatic reflex

Reflex in which neurons innervate skeletal muscles and structures of the head.

Simple stretch reflex

Monosynaptic reflex that shortens a stretched muscle to return it to its optimal length.

Simple stretch reflex sequence

External force stretches muscle → muscle spindles detect stretch → sensory afferents carry action potential to spinal cord → sensory afferents synapse on α-motor neurons → α-motor neurons stimulate the muscle to contract.

Muscle spindle role in stretch reflex

Detects stretching of the muscle and sends sensory information to the spinal cord.

Alpha motor neuron role in stretch reflex

Stimulates the stretched muscle to contract and return toward its optimal length.

Antagonist inhibition in stretch reflex

Some simple stretch reflexes also require inhibition of the antagonist muscle while the stretched muscle contracts.

Patellar (knee-jerk) reflex

Simple stretch reflex in which tapping the patellar tendon stretches the quadriceps femoris and produces rapid leg extension.

Jaw-jerk reflex

Simple stretch reflex in which tapping the chin causes reflexive contraction of the masseter and temporalis muscles.

Flexion (withdrawal) reflex

Protective spinal reflex that causes a limb receiving a painful stimulus to flex and withdraw.

Flexion reflex pathway

Nociceptive afferents transmit a painful stimulus to spinal cord interneurons → interneurons communicate with α-motor neurons → α-motor neurons stimulate muscles that flex the affected limb.

Crossed-extension reflex

Reflex occurring with the flexion reflex in which muscles of the opposite limb extend to maintain balance.

Crossed-extension reflex purpose

Maintains balance while the limb receiving the painful stimulus is withdrawn.

Flexion vs. crossed-extension reflex

Stimulated side flexes and withdraws; opposite side extends to support the body.

Golgi tendon reflex

Protective reflex that causes a muscle to relax when muscle and tendon tension increases dramatically.

Purpose of the Golgi tendon reflex

Protects muscles and tendons from damage caused by excessive tension.

Golgi tendon reflex pathway

Golgi tendon organs signal the spinal cord and cerebellum → motor neurons supplying the contracting muscle are inhibited → motor neurons supplying antagonist muscles are activated.

Golgi tendon reflex during strenuous activity

May cause a muscle to “give out” during strenuous activities such as weightlifting.

Cranial nerve reflexes

Polysynaptic reflexes involving afferent and efferent cranial nerves.

Gag reflex

Cranial nerve reflex in which visceral sensory endings of glossopharyngeal nerve IX detect stimulation in the posterior throat and somatic motor neurons of vagus nerve X produce pharyngeal contraction.

Afferent nerve of the gag reflex

Glossopharyngeal nerve (IX).

Efferent nerve of the gag reflex

Vagus nerve (X).

Corneal blink reflex

Cranial nerve reflex in which stimulation of somatic sensory receptors of the trigeminal nerve in the cornea results in contraction of the orbicularis oculi muscle.

Sensory nerve of the corneal blink reflex

Trigeminal nerve (V).

Peripheral neuropathies

Disorders affecting sensory and motor neurons of the PNS.

Sensory neuron disorders

Peripheral neuropathies whose symptoms and severity depend on which spinal or cranial nerve is affected.

Lower motor neuron disorders

Usually result from injury to a spinal or cranial nerve or lower motor neuron cell bodies in the spinal cord and may cause paralysis or paresis.

Paresis

Weakness of a muscle resulting from impaired motor function.

Upper motor neuron disorders

Disorders involving CNS neurons along pathways from the motor cortices to the spinal cord; technically not peripheral neuropathies.

Spinal shock

Initial response to upper motor neuron damage characterized by paralysis after normal upper motor neuron input to spinal cord circuits is removed.

Spasticity

Condition developing after the initial spinal shock period characterized by increased stretch reflexes and increased muscle tone.

Clonus

Alternating contraction and relaxation of muscles that may occur with spasticity.

Cause of spasticity

Likely results from loss of normal inhibition normally mediated by upper motor neurons.

Babinski sign

Abnormal extension of the hallux and fanning of the other toes when the bottom of the foot is stroked.

Babinski sign in infants

Babinski response is normal in infants up to approximately 18 months of age.

Babinski sign in adults

Presence of the Babinski response in an adult may indicate an upper motor neuron disorder.