8- Neurophysiology (Reflexes).docx

- **Spinal Cord: Function** - The functions of the spinal cord include: - **Sensory System** - Receiving and distributing information to the PNS (peripheral nervous system) - Relaying afferent/sensory information to the brain...

- **Spinal Cord: Function** - The functions of the spinal cord include: - **Sensory System** - Receiving and distributing information to the PNS (peripheral nervous system) - Relaying afferent/sensory information to the brain centers - **Reflexes** - Local integration of sensory and motor functions for reflex activity (both within a limb and between limbs) - **Motor System** - Relaying efferent/motor information from motor management centers via specific tracts (UMN) to connect with LMN (lower motor neurons) - Contain interneuron and complex neural circuits for motor control - Central pattern generators - **Reflex Activity** - A **reflex** is an involuntary, qualitatively unvarying motor response of the nervous system to a stimulus. - The neuronal circuit that directs this motor response is called a **reflex arc**. - A reflex arc contains 5 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. - Clinically, the reflexes are tested to evaluate the integrity of different components of the reflex arc and the influence of descending UMN (upper motor neurons) motor pathways. - **Spinal Cord: Reflexes** - [Reflexes] can be [classified] in these different ways: - Monosynaptic vs. Polysynaptic - **Monosynaptic** has a reflex arc involving contact directly between sensory and motor neurons- they [lack interneurons]. - **Polysynaptic** involves sensory neurons, interneurons, and motor neurons- this is the most common type. - Ipsilateral vs. Contralateral - Segmental vs. Intersegmental - **Segmental** has a reflex arc that passes through only a [small segment] of the spinal cord or a small region of the brainstem. - **Intersegmental** has a reflex arc that travels through [many segments] of the spinal cord or several major brain divisions (ex: medulla to midbrain) - Somatic vs Autonomic - **Somatic** is when the effector organ is skeletal muscle - **Autonomic** is when the effector organ is cardiac or smooth muscle, or a gland. - **Spinal Cord: Stretch Reflex** - A **stretch reflex** is primarily a monosynaptic reflex that is normally ipsilateral, segmental, and somatic, and responds to [muscle stretching] (causing muscle contraction). - The stretch reflex involves afferent axons going from a muscle stretch receptor (muscle spindle) 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 (AKA: tendon reflex or myotatic reflex). - The **patellar reflex** is elicited by tapping the tendon of the quadriceps femoris muscle located immediately distal to the patella, and it tests the stretch reflex mediated by the femoral nerve. - The patellar reflex involves quadriceps and knee-jerk reflexes. - This is the most reliably test of the stretch reflex, and it involved the following cord segments: L4, L5, L6. - The [main action] of the patellar reflex is contraction, and the [secondary action] is reciprocal inhibition. - **Reciprocal inhibition** involves the antagonistic muscle relaxing while the other muscle is contracting. - Stretch reflexes and other muscle contractions often accompanies reciprocal inhibition- a reflex phenomenon that prevents muscles from working against each other by inhibiting the antagonists. - **Spinal Cord: Inverse Stretch Reflex** - The inverse stretch reflex is also known as the "golgi tendon organ reflex" or "autogenic inhibition" and has the major goal of preventing excessive tension on the muscle, in response to [muscle contraction] (inhibiting muscle contraction). - The duration and force of muscle contraction associated with the stretch reflex are mitigated to some degree by the golgi tendon organ reflex. - 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 alfa motor neuron causing contraction - Lead to muscle relaxation - The [main action] of the inverse stretch reflex is that the normal stimulation is reduced, and inhibited at the level of the spinal cord. - The [secondary action] of the inverse stretch reflex is that instead of reciprocal inhibition, there is reciprocal stimulation. - **Spinal Cord: 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 going to the flexor limb muscles, being stimulated, while those to the extensor muscles are inhibited- this is due to **reciprocal innervation**. - The force and duration of the withdrawal reflex are proportional to the intensity of the noxious stimulus applied. - An example of a flexor reflex is the withdrawal reflex. - The **withdrawal reflex** assess the integrity of sensory and motor reflex pathways that mediate flexion of the thoracic and pelvic limbs. - This reflex is elicited by applying pressure to the base of the toenail, and the response to pinching the toe pad is flexion of the stimulated limb. - The thoracic limb has the following spinal cord segments involved: C6-T2 - The pelvic limb has the following spinal cord segments involved: L4-S1 - **Extensor Tone** - **Muscle tone** is important for maintaining normal posture and providing support for the joints to stabilize their position. - **Extensor tone** refers to the muscles continuous resistance to being stretched (resisting tension). - Extensor tone is regulated by the: - Local spinal cord reflexes (example: reflexes using muscle spindle or golgi tendon organ) - Higher levels of the brain (example: gamma loop) - Muscle tone (extensor tone) is evaluated during neurological examination and is one of the [most reliable spinal evaluations]. - It is tested by gently applying pressure to the plantar and palmar surface of the pelvic and thoracic limbs. - In normal animals, they should easily flex the limb when pressure is applied. - **Upper Motor Neurons (UMN)** - UMN are completely contained within the CNS (central nervous system), with the cell body being located in the motor nucleus of the brainstem or in the motor cortex of the forebrain. - The axons of the UMN connect to LMN, either synapsing on them directly or indirectly via interneurons. - UMN's exert their effects by stimulating or inhibiting LMN's, which have neurons that innervate the muscles. - The UMN system is responsible for: - Initiation of voluntary movement - Maintenance of muscle tone for support against gravity - Regulation of posture - UMN's initiate, regulate, modify, and terminate the activity of LMN- they 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. - Paresis is the weakness of the voluntary movement, causing an animal to drag its limb. - Paralysis is the loss of motor function, causing the animal to be unable to move the limb. - Ataxia is when an animal has uncoordinated movements. - UMNs typically originate in the brain and project to and control LMNs. - UMN usually belong to the corticomedullary (neuron 3), corticospinal (neuron 4), or descending brainstem motor (neuron 5) pathways. - **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, LMNs induce muscle contraction. - Muscle tone and bulk depends on LMN function. - Loss of LMNs results in paresis/paralysis, with decreased to absent muscle tone and reflexed. - LMNs innervating the limbs are confined to the cervical and lumbar intumescences. - **Cervical intumescence**: C6-T2 - **Lumbar intumescence:** L4-S3 - LMNs typically originate in the ventral horn of the spinal cord (neuron 1) or in cranial nerve nuclei (neuron 2), and synapse within skeletal muscle. - **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. - It will be present even if the neuroaxis cranial or caudal to the reflex circuit has been severely damaged. - **Exaggerations** of spinal reflexes and/or extensor tone (rigid/hypertonic) may occur with lesions affecting UMNs [cranial to the reflex], due to lack of overall inhibitory influence on LMNs. - **Diminished or absent** spinal reflexes and/or extensor tone (flaccid/hypotonic) usually occur with lesion affecting LMNs at the spinal cord [segments involved in the reflex] (intumescences), due to direct control from LMN being lost. - **UMN signs** include: - Paresis/paralysis of motor function - Normal to increased reflexes - Normal to increased extensor muscle tone - Mild/chronic/slow muscle atrophy - **LMN signs** include: - Paresis/paralysis of motor function - Decreased to absent reflexes - Decreased to absent extensor muscle tone - Severe/fast muscle atrophy - If **all 4 limbs** are affected, the problem must be on: - **C1-C5** if there are signs of UMN in both limbs - **C6-T2** if there are signs of LMN in the thoracic limb and signs og UMN in the pelvic limb. - If the just the **pelvic limbs** are affected, the problem must be on: - **T3-L3** if there are signs of UMN in the pelvic limb - **L4-S3** if there are signs of LMN in the pelvic limb - If **all 4 limbs have signs of LMN**, this means that they have flaccid paralysis or diffuse neuromuscular presentation. - This involves a diffuse problem in the PNS (at the nerve or neuromuscular junction) - **Flaccid paralysis** can be caused by polyradiculoneuritis (coonhound paralysis), tick paralysis, botulism, or fulminant myasthenia gravis. - **UMN vs LMN: Clinical Examples** - Lesion at **C1-C5** - Thoracic Limb: UMN affected- normal to increased muscle tone - Pelvic Limb: UMN affected- normal to increased muscle tone - Lesion at **C6-T2** - Thoracic Limb: LMN affected- reduced to absent muscle tone - Pelvic Limb: UMN affected- normal to increased muscle tone - Lesion at **T3-L3** - Thoracic Limb: Fully normal (no UMN or LMN affected) - Pelvic Limb: UMN affected- normal to increased muscle tone - Lesion at **L4-S3** - Thoracic Limb: Fully normal (no UMN or LMN affected) - Pelvic Limb: LMN affected- reduced to absent muscle tone - Lesion at **CD1-CD5** - Thoracic Limb: Fully normal (no UMN or LMN affected) - Pelvic Limb: Fully normal (no UMN or LMN affected) - Tail issues will be present - **Spinal Cord Lesion Localization: Exception** - **Schiff-Sherrington posture** occurs due to interruption of inhibitory neuron input from the lumbar spinal cord. - The posture is characterized by rigid forelimb extension with normal function, with concurrent hindlimb paresis and paralysis. - Trauma to the spinal cord between T3-L3 results in inhibitory pathway interruption, causing neurons caudal to the cervical intumescence to be released, further causing excessive extensor tone in the forelimbs. - This posture is NOT associated wit prognosis of the underlying disease. - Schiff-Sherrington posture should be considered when the doctor sees a combination of forelimb extensor rigidity and hind limb paralysis in an animal with spinal trauma, like being hit by a car or acute intervertebral disc disease.