Reflexes

What is the reflex arc (spinal reflex)?

A patterned motor response to a sensory stimulus Shaped by nature, intensity and location of the sensory stimulus May be monosynaptic or polysynaptic Usually have reciprocal effects on antagonist muscles Can function as long as all of its local components are intact

What is a dermatome?

A region of skin that is innervated by a specific sensory nerve, that can be tested with the panniculus reflex

What is the panniculus reflex pathway?

Afferents go to the spinal cord and run cranially until C8-T1 --> reflex arc activates lateral thoracic nerve BILATERALLY --> skin twitch on both sides

What does the deep pain response test?

Deep pain (nociceptive pathways) cranial to the segments that innervate the limb you are testing

Where do the afferents in a crossed-extensor reflex come from?

From the limb contralateral to the reflex via nociceptor activation

What are the afferents in the perineal reflex?

From the pudendal nerve Only need a very light touch to the skin of the anus/perineum to activate touch/pressure receptors and possibly nociceptors

What action is seen in the muscle spindle reflex?

Increased tone to the muscle being stretched

What does the perineal reflex test?

Integrity of segments S1 - S3 and pudendal nerve

What are some examples of responses?

Menace response Deep pain response

Where are the afferents for the muscle spindle reflex from?

Muscle spindle receptors, sensing muscle stretch

What are other names for the muscle spindle reflex?

Myotatic reflex Tendon tap reflex Stretch reflex

Do reflexes imply consciousness?

No

Do reflexes require participation from the brain?

No Although it can be modified by the cerebrum to some degree

Are all reflexes present in normal animals?

No, some are but others you would only see in an animal with a lesion to the system it is testing (crossed-extensor reflex)

When should the crossed-extensor reflex be observed?

Normal when weight bearing Should not be present when not weight bearing (unless there is severe damage to UMNs cranial to the limb showing the reflex)

What are some examples of reflexes?

PLR Corneal reflex Palpebral reflex Paniculus reflex Withdrawal reflex Crossed-extensor reflex Perineal reflex Myotatic reflex

How do you perform a deep pain response test?

Pinch toe just proximal to the nail to activate periosteum of bone (more force than just light pinching) Pinch increasingly harder until you see a behavioral response from the animal, without causing permanent damage Make sure to test all limbs, but remember there is much bilateral decussation of the afferent pathways

How do you perform a panniculus reflex test?

Pinching the skin over the dermatomal areas in the dorsal thoracolumbar area Should get efferent contraction of cutaneous trunci m. bilaterally Test all dermatomes (up to L4) until there is no reaction seen

What are the afferents for the withdrawal reflex?

Proprioceptors and superficial pain receptors Some nociceptors and thermoreceptors can be involved

What things do you look for when evaluating limbs in a neuro exam?

Reflexes- decreased, normal or increased (hyper-reflexia) Muscle tone- decreased, normal or increased (hypertonicity) Muscle mass- decreased, normal or increased

What does the panniculus reflex test the integrity of?

Sensory receptors Spinal cord Efferent nerve (lateral thoracic n. to cutaneous trunci m.)

What happens if you don't get a deep pain response?

Severe spinal cord injury! Usually an emergency situation to relieve pressure on the spinal cord ASAP

When will a deep pain response be seen?

Should be seen in normal animals Absent in animals with lesions cranial to the limb *May also elicit a withdrawal response if the pathways are intact

How does the withdrawal reflex occur?

Stimulation of plantar/palmar surface of the paw --> activates the reflex arc at this level of the spinal cord to excite the LMNs --> signal is shared by nearby spinal segments via interneurons to activate all flexors of the limb to withdraw it from the stimulus --> different interneurons also inhibit the extensors of the limb so flexion may occur

How does the crossed-extensor reflex occur?

Stimulus --> activation of flexors to initiate withdrawal of ipsilateral limb (with simultaneous antagonist inhibition via interneurons) + activation of contralateral interneurons --> inhibition of flexors and activation of extensors on contralateral limb

What are the afferents for the panniculus reflex?

Superficial pain receptors in the thoracolumbar area

What do you need to understand to determine what causes decreased or increased responses or reflexes?

The difference between upper motor neurons and lower motor neurons

What is the panniculus reflex used for?

The only way to detect damage between T2-L4 (thoracolumbar) spinal cord segments

What action is seen in the perineal reflex?

Tightening of the anal sphincter muscle Should be seen in normal animals Absent with spinal cord disease or damage along the pudendal nerve pathway

What is the function of the crossed-extensor reflex?

To allow balancing by increasing extensor tone of the other limb, when one leg is withdrawn in response to a stimulus

What two different types of stretch can be tested in the muscle spindle reflex pathway?

Tonic stretch reflex- manipulate limbs (flex and extend) to determine resistance to passive movement Phasic stretch reflex- use reflex hammer to strike tendon and rapidly stretch muscle

What is fictive walking?

Type of reflex, common with thoracolumbar damage When hind limbs are weight bearing, they will reflexively move (looks like walking, but not voluntary)

What is the reflex arc influenced by?

Upper motor neurons

What is the deep pain reaction?

Varies with the patient to include an actual behavioral response: head turn, biting, whining or attempts to escape after a severe toe pinch

What is a reaction?

Voluntary behavior that requires cortical perception of a noxious stimulus

What action is seen with the withdrawal reflex?

Withdrawal of limb being stimulated Should be present in normal animals Altered in animals with spinal cord, spinal root, peripheral nerve or muscular injury

What is the withdrawal reflex?

Withdrawal of the limb after a toe pinch (all joins flex) Pinch does not need to be severe to see this reflex

Can a reflex occur without a reaction?

Yes, but depends on the animal Need to distinguish between the two