Neurophysiology of Spinal Reflexes and Clinical Applications

Functions of the Spinal Cord

• The spinal cord performs a variety of critical functions within the nervous system:
  • Receiving and distributing information from the Peripheral Nervous System (PNS).
  • Relaying afferent (sensory) information to centers within the brain.
  • Local integration of sensory and motor functions specifically for reflex activity.
  • Relaying efferent (motor) information from motor management centers via specific tracts; these involve Upper Motor Neurons (UMN) connecting with Lower Motor Neurons (LMN).
  • Containing interneurons and complex neural circuits responsible for motor control, specifically referred to as central pattern generators.

The Reflex and the Reflex Arc

• A reflex is defined as an involuntary motor response of the nervous system to a specific stimulus.
• Reflexes serve to return a condition of the body back to its desired state.
• The neuronal circuit directing this motor response is called a reflex arc.
• There are five fundamental components of a reflex arc:
  1. A receptor.
  2. A sensory neuron.
  3. One or more synapses located in the Central Nervous System (CNS).
  4. A motor neuron.
  5. A target organ.
• Evaluating a reflex provides a clinical assessment of the integrity of all five components within that specific reflex arc.

Classification of Reflexes

• Reflexes are classified according to four primary criteria:

• Synaptic Complexity:

  • Monosynaptic: A reflex arc that involves direct synaptic contact between a sensory neuron and a motor neuron.
  • Polysynaptic: A reflex arc that involves sensory neurons, interneurons, and motor neurons; this is the more common type of reflex.

• Side of Response:

  • Ipsilateral: The stimulus and the response occur on the same side of the body.
  • Contralateral: The response occurs on the opposite side of the stimulus.

• Anatomical Distribution:

  • Segmental: The reflex arc passes through only a small segment of the spinal cord or a limited region of the brainstem.
  • Intersegmental: The reflex arc travels through many segments of the spinal cord or several major divisions of the brain (e.g., from the medulla to the midbrain).

• Effector Organ:

  • Somatic: The effector organ is skeletal muscle.
  • Autonomic: The effector organ involves cardiac muscle, smooth muscle, or a gland.

The Stretch Reflex and the Muscle Spindle

• The stretch reflex is characterized as a monosynaptic, ipsilateral, segmental, and somatic reflex. It is also referred to as the tendon reflex or the myotatic reflex.
• The main effect of the stretch reflex is contraction in response to the stimulus of muscle stretching.
• Muscle Spindle: This receptor responds to stretching. In a simple muscle stretch reflex arc, stretching the muscle spindle causes a reflex contraction of that muscle.
• Reciprocal Inhibition: The stretch reflex often identifies a secondary effect where the antagonist muscle is inhibited. This phenomenon prevents muscles from working against each other during the reflex movement.

Inverse Stretch Reflex and Golgi Tendon Organ

• The inverse stretch reflex (also known as the Golgi tendon organ reflex) is triggered by muscle contraction rather than stretching.
• Golgi Tendon Organ: This receptor responds to muscle contraction. In a simple inverse stretch reflex arc, the stretch of the Golgi tendon organ causes reflex inhibition (relaxation) of the muscle-tendon complex.
• Reciprocal Stimulation: Unlike the standard stretch reflex which causes reciprocal inhibition, the inverse stretch reflex triggers reciprocal stimulation of the antagonist muscle. The normal stimulation of the contracting muscle is reduced or inhibited at the level of the spinal cord.

The Patellar Reflex

• The patellar reflex is a stretch reflex of the quadriceps muscle, also known as the quadriceps reflex or the knee-jerk reflex.
• It is considered the most reliable test of the stretch reflex.
• Elicitation: It is elicited by tapping the tendon of the quadriceps femoris muscle, located immediately distal to the patella.
• Response: Tapping the tendon causes contraction of the quadriceps, resulting in the extension of the limb.
• Innervation: The reflex is mediated by the femoral nerve.
• Spinal Cord Segments: The involved cord segments are $L4$, $L5$, and $L6$.

The Withdrawal Reflex

• The withdrawal reflex, also known as the flexor reflex, is a coordinated polysynaptic reflex where all flexor muscles of a limb contract.
• Elicitation: It is elicited by applying a noxious stimulus, such as pinching the toes.
• Response: Flexion of the stimulated limb. While alpha motor neurons to the limb flexor muscles are stimulated, those to the extensor muscles are inhibited via reciprocal inhibition.
• Clinical Significance: This reflex assesses the integrity of sensory and motor reflex pathways mediating flexion in both thoracic and pelvic limbs.
• Thoracic Limb Segments: The involved spinal cord segments are $C6$ through $T2$.
• Pelvic Limb Segments: The involved spinal cord segments are $L4$ through $S1$.
• Intensity: The force and duration of the withdrawal response are directly proportional to the intensity of the noxious stimulus applied.

The Perineal Reflex

• The perineal reflex occurs in response to a light touch or stroke applied to the perineal region.
• Response: Contraction of the anal sphincter and flexion of the tail.
• Procedure: Both the left and right sides must be tested independently.
• Clinical Assessment: It assesses the integrity of the sacral ($S1$ to $S3$) and caudal spinal cord segments (responsible for tail flexion).
• Innervation: It evaluates the perineal and caudal rectal nerve branches, both of which are branches of the pudendal nerve.

The Cutaneous Trunci Reflex

• The cutaneous trunci reflex is a response to a light pinch of the skin just lateral to the spine.
• Response: A bilateral contraction of the cutaneous trunci muscle, appearing as a twitch of the skin over the thorax and abdomen.
• Procedure: The test should start just cranial to the iliac crests and proceed cranially one vertebral level at a time. Both sides are tested.
• Clinical Assessment:
  • It assesses the sensory integrity of all dermatomes over the thoracolumbar vertebral column.
  • It assesses the motor integrity of the lateral thoracic nerve ($C8$ to $T1$).
• Variability: This reflex is not always elicited, especially in cats.
• Lesion Localization: An obvious cutoff point in the reflex response suggests a spinal cord lesion located approximately two vertebral bodies cranial to the level of the cutoff.

The Crossed Extensor Reflex

• The crossed extensor reflex occurs when the flexion of one limb induces a reflex extension in the other limbs.
• Timing: Approximately $0.2$ to $0.5$ seconds after a stimulus triggers a withdrawal reflex in one limb, the opposite limb begins to extend.
• Distribution: This can occur in the same limb girdle or across girdles (e.g., front versus hind).
• Physiological Function: It is normal during walking to allow balance and posture maintenance when a limb stumbles or is injured.
• Clinical Relevance: This reflex is not normally elicited during a standard neurological examination in a stationary animal. If it is present during examination, it is a pathological reflex indicating an Upper Motor Neuron (UMN) lesion.

The Positive Support Reflex

• The positive support reflex occurs when pressure is applied to the footpad or sole.
• Mechanism: Sensory receptors in the skin and deeper tissues send signals to the spinal cord when pressure is felt, causing the contraction of extensor muscles.
• Response: The limb extends against the applied pressure.
• Function: This occurs naturally during walking and involves complex circuits of interneurons similar to those in the flexor and crossed extensor reflexes. It helps prevent the animal from falling toward the side where pressure is applied.

Extensor Tone and Spinal Evaluation

• Extensor tone is the muscle's continuous resistance to being stretched, also known as resting tension.
• Regulation: Tone is regulated by local spinal cord reflexes including the stretch reflex, Golgi tendon organ reflex, and positive support reflex.
• Testing: It is tested by gently applying pressure to the plantar or palmar surface of the pelvic or thoracic limbs.
• Clinical Findings:
  • In normal animals, some resistance is present, though the limb should flex if more pressure is applied.
  • Increased Tone: Associated with Upper Motor Neuron (UMN) lesions.
  • Decreased Tone: Associated with Lower Motor Neuron (LMN) lesions.
• Extensor tone evaluation is considered one of the most reliable methods for spinal evaluation. If the reflex is functioning, all components of the arc (receptor, sensory neuron, spinal cord intumescence, lower motor neuron, and skeletal muscle) are intact.