Neurophysiology: Reflexes

Spinal Cord Function

• The spinal cord receives and distributes sensory information to the peripheral nervous system (PNS) and relays afferent/sensory information to brain centers.
• The spinal cord integrates sensory and motor functions for reflex activity, both within a limb and between limbs.
• The spinal cord relays efferent/motor information from motor management centers to connect with lower motor neurons (LMN) and contains interneuron and complex neural circuits for motor control.

Reflex Activity

• A reflex is an involuntary, qualitatively unvarying motor response of the nervous system to a stimulus.
• A reflex arc contains five fundamental components: a receptor, sensory neuron, one or more synapses in the CNS, a motor neuron, and a target organ.
• A reflex always returns some condition of the body back to the desired state.

Reflex Classification

• Reflexes can be classified as monosynaptic or polysynaptic, based on the presence or absence of interneurons.
• Reflexes can be classified as ipsilateral or contralateral, based on the location of the reflex arc.
• Reflexes can be classified as segmental or intersegmental, based on the extent of the spinal cord or brainstem involved.
• Reflexes can be classified as somatic or autonomic, based on the type of effector organ involved.

Stretch Reflex

• A stretch reflex is a monosynaptic reflex that responds to muscle stretching, causing muscle contraction.
• The stretch reflex involves afferent axons going from a muscle stretch receptor to directly synapse with spinal cord alpha motor neurons, which cause contraction of that same muscle.
• The patellar reflex is an example of a stretch reflex, testing the stretch reflex mediated by the femoral nerve.

Inverse Stretch Reflex

• The inverse stretch reflex, also known as the golgi tendon organ reflex, has the major goal of preventing excessive tension on the muscle, in response to muscle contraction.
• The receptor is sensitive to muscle tension, increasing its firing rate when the muscle contracts, which can excite an inhibitory interneuron in the spinal cord, inhibit the alpha motor neuron, and lead to muscle relaxation.

Flexor Reflex

• The flexor reflex is a coordinated polysynaptic reflex in which all the flexor muscles of the limb contract in response to a noxious stimulus.
• The flexor reflex involves alpha motor neuron stimulation, while those to the extensor muscles are inhibited, due to reciprocal innervation.

Upper Motor Neurons (UMN)

• UMN's initiate, regulate, modify, and terminate the activity of lower motor neurons (LMN).
• UMN's may inhibit or facilitate LMN.
• Loss of inhibitory UMN results in increased muscle tone and spinal reflexes.
• Loss of facilitatory UMN results in paresis or paralysis.

Lower Motor Neurons (LMN)

• LMN have their cell bodies in the CNS (brainstem or spinal cord), while their axons project into the PNS via cranial or spinal nerves, connecting with skeletal muscle at the neuromuscular junction.
• When stimulated, LMN's induce muscle contraction.
• Muscle tone and bulk depend on LMN function.
• Loss of LMN's results in paresis/paralysis, with decreased to absent muscle tone and reflexes.

UMN vs LMN: Signs

• Clinically, an intact reflex, be it normal or exaggerated, tells the clinician that the lesion does NOT involve that area of the CNS or PNS.

Spinal Cord Lesion Localization: Exception

• Schiff-Sherrington posture occurs due to interruption of inhibitory neuron input from the lumbar spinal cord, characterized by rigid forelimb extension with normal function, with concurrent hindlimb paresis and paralysis.
• This posture is NOT associated with prognosis of the underlying disease.