Understanding the Reflex Arc

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Questions and Answers

Which component is not part of the reflex arc?

  • Sensory receptor
  • Neuromuscular junction
  • Motor neuron
  • Cerebral cortex (correct)

What is the primary function of a sensory (afferent) neuron in a reflex arc?

  • To cause muscle contraction directly
  • To carry impulses from the brain to the muscle
  • To connect a sensory neuron to a motor neuron
  • To carry sensory impulses to the central nervous system (correct)

What characterizes a monosynaptic reflex?

  • It always results in the inhibition of muscle contraction.
  • It requires conscious thought.
  • It involves multiple interneurons within the spinal cord.
  • It involves a single synapse between a sensory and a motor neuron. (correct)

Which of the following is an example of a monosynaptic reflex?

<p>Patellar reflex (A)</p> Signup and view all the answers

What role does an interneuron play in a polysynaptic reflex arc?

<p>It connects the sensory neuron to the motor neuron. (C)</p> Signup and view all the answers

Which of the following reflexes is polysynaptic?

<p>Withdrawal reflex (B)</p> Signup and view all the answers

How does a reflex differ from a response?

<p>A reflex is an automatic reaction involving the spinal cord, while a response involves brain interpretation. (B)</p> Signup and view all the answers

If the spinal cord is severed cranial to the reflex arc, which of the following would still be present?

<p>Reflex (D)</p> Signup and view all the answers

Damage to the lower motor neurons (LMN) typically results in which of the following?

<p>Flaccid muscle tone and weak or absent reflexes (A)</p> Signup and view all the answers

What is characteristic of upper motor neuron (UMN) damage, compared to lower motor neuron (LMN) damage?

<p>UMN damage often leads to exaggerated muscle tone. (C)</p> Signup and view all the answers

If the spinal cord is damaged in the C6-T2 region, which limb would be affected?

<p>Only the front limbs (A)</p> Signup and view all the answers

Which nerves are involved in the pupillary light reflex?

<p>CN II and CN III (A)</p> Signup and view all the answers

The menace response involves which action?

<p>Blinking (C)</p> Signup and view all the answers

What nerve mediates the perineal reflex?

<p>Pudendal nerve (D)</p> Signup and view all the answers

Which functional segment of the spinal cord controls the bladder, anus, and tail function?

<p>S1-Cd (A)</p> Signup and view all the answers

The cutaneous trunci muscle is innervated by what?

<p>Lateral thoracic nerve (A)</p> Signup and view all the answers

In the stretch reflex, where does the afferent (sensory) neuron send information?

<p>To the spinal cord (D)</p> Signup and view all the answers

Which of the following is the correct sequence of events in a reflex arc?

<p>Receptor, sensory neuron, interneuron, motor neuron, effector (D)</p> Signup and view all the answers

If you are testing the depth of anesthesia, which reflex would you MOST likely assess?

<p>Palpebral reflex (C)</p> Signup and view all the answers

Which reflex relies on coordinated contraction of many flexor muscles across several joints?

<p>Withdrawal reflex (D)</p> Signup and view all the answers

Following a spinal cord injury at L2, what deficits would you primarily expect to observe?

<p>Weakness in the rear limbs (C)</p> Signup and view all the answers

What is the functional consequence of damaging the sensory neuron in a monosynaptic reflex arc?

<p>The ability to sense the stimulus is lost, eliminating the reflex (A)</p> Signup and view all the answers

Which statement accurately describes how the menace response differs from a reflex mediated solely by the spinal cord?

<p>The menace response is a learned behavior that requires processing in the brain. (A)</p> Signup and view all the answers

How does the absence of the panniculus reflex caudal to the mid-thoracic region relate to the localization of a spinal cord lesion?

<p>It suggests a lesion cranial to the level of the observed sensory deficit. (B)</p> Signup and view all the answers

A patient presents with exaggerated patellar reflexes bilaterally. What could this indicate about the location of the lesion?

<p>The lesion is located in the upper motor neurons of the brain or spinal cord. (A)</p> Signup and view all the answers

Why is use of excessive force inappropriate when eliciting a withdrawal reflex?

<p>Because if too much force is used, you are testing pain and not the reflex. (C)</p> Signup and view all the answers

Damage confined to the interneuron within a reflex arc will most directly affect which aspect of that reflex? Assume a typical 3-neuron arc.

<p>The integration of sensory input and modulation of the motor response (A)</p> Signup and view all the answers

You are examining a dog with suspected spinal cord trauma. When performing the patellar reflex test, you notice an absence of the reflex in the right hind limb and an exaggerated reflex in the left hind limb. Given this information, where is the MOST likely location and nature of the spinal cord lesion?

<p>Right-sided lesion affecting the lower motor neurons at the lumbar level with concurrent upper motor neuron signs on the contralateral side (D)</p> Signup and view all the answers

Which clinical scenario BEST illustrates a situation where the pupillary light reflex is present, but the menace response is absent?

<p>A dog exhibiting central blindness due to a lesion in the visual cortex. (D)</p> Signup and view all the answers

After administering a neuromuscular blocking agent, which component of the reflex arc is FIRST affected?

<p>Neuromuscular junction (B)</p> Signup and view all the answers

In performing a neurological exam on a horse, you note an absent withdrawal reflex but a normal patellar reflex in the same limb. Based on this finding, which of the following is the MOST likely location of the lesion?

<p>A lesion affecting the lower motor neurons within the spinal cord segment innervating the relevant flexor muscles, but sparing the femoral nerve. (A)</p> Signup and view all the answers

What type of nerve fiber is responsible for transmitting sensory information from the skin receptor to the spinal cord in a polysynaptic reflex?

<p>A-delta fiber (D)</p> Signup and view all the answers

After an injury to the spinal cord at the T3-L3 region, what reflex would a veterinarian MOST likely examine to evaluate bladder function in a small animal?

<p>Perineal reflex (C)</p> Signup and view all the answers

In a patient with a severed spinal cord at C7, what aspect of the panniculus reflex would be MOST likely observed?

<p>Normal panniculus reflex cranial to the level of the lesion, but absent caudal to it. (A)</p> Signup and view all the answers

Why is the pupillary light reflex considered a polysynaptic reflex arc?

<p>It relies on communication between cranial nerves through interneurons in the brainstem. (C)</p> Signup and view all the answers

A veterinarian is performing a neurological examination on a dog that experienced some trauma after being hit by a car including a fractured femur. What is a critical strategy that should be employed when assessing reflexes to ensure accurate interpretation?

<p>Compare reflex responses bilaterally and account for any concurrent orthopedic injuries. (C)</p> Signup and view all the answers

Flashcards

What is a reflex?

A simple, involuntary reaction to a stimulus.

What's included in a reflex arc?

Sensory receptors, sensory neurons, spinal cord segment, motor neurons, neuromuscular junction, and the target muscle or organ.

What is a monosynaptic reflex?

Small segment of the nervous system; tests a very specific area.

What is a polysynaptic reflex?

A broad area with multiple segments involved.

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What is a stimulus?

A change in the environment that initiates a response.

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What is a receptor?

The sensory neuron's ending that responds to a stimulus.

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What are sensory (afferent) neurons?

Neurons that send sensory impulses to the central nervous system (CNS).

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What is an interneuron?

Located in the spinal cord, connect sensory and motor neurons.

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What are motor (efferent) neurons?

Neurons that carry impulses to an organ/muscle.

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What is a monosynaptic reflex?

A reflex that involves a single synapse.

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What initiates the monosynaptic reflex?

The stretch on a tendon stimulating muscle spindles, sending information to the spinal cord.

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What muscle is involved in the monosynaptic reflex?

The efferent (motor) neuron's axon innervating the quadriceps muscle.

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What is the patellar reflex?

A stretch reflex that tests the femoral nerve.

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What is a polysynaptic reflex?

A reflex involving multiple synapses.

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What does the polysynaptic reflex involve?

A reflex that involves inhibition of muscle contraction and the involvement of an interneuron in the spinal cord.

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What does the withdrawal reflex involve?

Flexion of all joints of the pinched limb and coordinated contraction of many flexor muscles.

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What is a reflex?

A reflex that only involves the spinal cord, an automatic reaction.

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What is a response?

It carries sensory information to the brain for interpretation.

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What is a lower motor neuron (LMN)?

A motor neuron in the peripheral nervous system that connects directly to the muscle.

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What is an upper motor neuron (UMN)?

Located totally in the CNS that communicates with the LMN to initiate motor movement.

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What is the menace response?

Blinking in response to a potentially harmful object advancing toward the one eye.

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What is the pupillary light reflex?

It involves CN II (optic nerve) and CN III (oculomotor nerve) to constrict the pupil.

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What is the panniculus reflex?

Patient is lightly touched along the dorsal surface of the back causing the cutaneous trunci muscle to twitch.

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What is the palpebral 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.

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What is the perineal reflex?

Touching the sides of the perineal area causes contraction of the anal sphincter and flexion of the tail.

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Study Notes

  • 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.

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