Understanding the Reflex Arc

• A reflex arc facilitates a simple, involuntary reaction to a stimulus.

Reflex Arc Components

• Sensory receptors detect the stimulus.
• Sensory neurons transmit the signal.
• The spinal cord segment processes the information.
• Motor neurons carry the response signal.
• Neuromuscular junction facilitates signal transmission to the muscle.
• Muscle or other end organ executes the response.

Monosynaptic Reflex

• Involves a small segment of the nervous system, testing a very specific area.
• An example is the patella reflex.
• Characterized as simple, uncommon, and rapid.
• Known as the stretch reflex.
• The Process includes the stretch on the tendon stimulating muscle spindles
• Information passes via the afferent sensory neuron to the spinal cord and the impulse transmitted to the efferent motor neuron innervating the quadriceps muscle.
• There is no interneuron involved.

Patellar Reflex

• It is a monosynaptic reflex.
• A test of the femoral nerve.
• Striking the patellar ligament should result in the contraction of the quadriceps muscle.

Polysynaptic Reflex

• Involves a broad area and multiple segments.
• An example is the withdrawal reflex.
• There may be involvement of motor neurons to several muscles and involve inhibition of muscle contraction.
• An interneuron is utilized in the spinal cord.
• The skin receptor detects a stimulus, passing it to the sensory neuron, which synapses on an interneuron; this then synapses at a motor neuron, reaching a muscle via the spinal cord.

Withdrawal Reflex

• It is a polysynaptic reflex.
• Pinching a limb results in flexion of all joints in the limb.
• Coordinated contraction of many flexor muscles occurs across several joints.
• A message is sent to the opposite leg to engage extensor muscles.
• If too much force is used, the test assesses pain, not the reflex.

Reflex vs Response

• A reflex only involves the spinal cord and doesn't require conscious thought.
• Reflexes are present even if the spinal cord is severed cranial to the reflex.
• A response carries sensory information to the brain for interpretation.

Lower Motor Neuron (LMN)

• Motor neuron in the peripheral nervous system that connects directly to the muscle.
• It originates in the grey matter of the spinal cord within the reflex arc area
• Damage is more likely to be permanent.
• Results in flaccid muscle tone (loose and floppy) and weak or absent reflexes.

Upper Motor Neuron (UMN)

• Located totally in the CNS
• Communicates with the LMN to initiate motor movement.
• It originates in the brain and travels within the spinal cord to communicate with the LMN in the gray matter.
• Resulting in some tone in muscles, where muscle tone is often exaggerated.
• Damage is more likely to recover as the UMN has the ability to find a new neuron connection.

Spinal Functional Segments

• Higher Cervical C1-C5: UMN
• Lower Cervical C6-T2,: LMN to front and UMN to the rear
• Thoracolumbar T3-L3: UMN to the rear
• Lumbosacral L4-S3: LMN to the rear legs
• Sacrocaudal S1: caudal; LMN to the bladder, anus, and tail

Menace Response

• Blinking in response to a potentially harmful object moving toward the eye
• The opposite eye must be covered during testing.
• A positive response is blinking.
• Not seen in puppies/kittens under 16 weeks old.
• It is a learned response.

Pupillary Light Reflex

• It is a polysynaptic reflex.
• It involves cranial nerve II (optic nerve) and cranial nerve III (oculomotor nerve).
• It does not assess vision.
• The pupil will constrict with bright light.

Panniculus Reflex

• Polysynaptic reflex.
• Light touch is applied along the dorsal surface of the back.
• The muscle along the back is the cutaneous trunci.
• A stimulus to the trunk causes the cutaneous trunci muscle to twitch.
• Ascending sensory tracts extend up the spinal cord to the cell body of the lateral thoracic nerve in spinal cord segment C8.
• The lateral thoracic nerve supplies motor innervation to the cutaneous trunci.

Palpebral Reflex

• Polysynaptic reflex.
• Touching the eyelid sends sensory information over the trigeminal nerve (CN V) to synapse with the facial nerve (CN VII) in the brain stem.
• Motor fibers of the facial nerve cause the orbicularis oculi muscle to contract.
• Often used to evaluate the depth of anesthesia.

Perineal Reflex

• Polysynaptic reflex.
• It is tested by touching the sides of the perineal area.
• A positive test results in contraction of the anal sphincter and flexion of the tail.
• Mediated by the pudendal nerve, with roots from S1-S3.