Neurophysiology Part 16 - Reflex Arc and Motor Neurons Summer 2024 PDF

Andre Azevedo, DVM, MSc Locum Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: List the 5 components of the reflex arc 5 Understand the classification of the reflexes 6 Describe the stretch reflex and inver...

Andre Azevedo, DVM, MSc Locum Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: List the 5 components of the reflex arc 5 Understand the classification of the reflexes 6 Describe the stretch reflex and inverse stretch reflex 10 11 13 IS Understand the basics of withdrawal reflex, patellar8reflex, and extensor tone Localize and differentiate upper and lower motor neurons and their clinical signs Localize a spinal cord lesion based on UMNs and LMNs signs, and give a prognosis based on nociceptive findings  The spinal cord has a variety of functions, including: Receiving and distributing information from the PNS Sensory system Relaying afferent/sensory information to the brain centers Local integration of sensory and motor functions for reflex activity Reflexes Both within a limb and between limbs Relaying efferent/motor information from motor management centers via specific tracts (upper motor neurons - UMN) to connect with lower motor neurons (LMN) Motor system Contain interneurons and complex neural circuits for motor control CENTRAL PATTERN GENERATORS  The spinal cord has a variety of functions, including: Receiving and distributing information from the PNS Sensory system Relaying afferent/sensory information to the brain centers Local integration of sensory and motor functions for reflex activity Reflexes Both within a limb and between limbs Relaying efferent/motor information from motor management centers via specific tracts (upper motor neurons - UMN) to connect with lower motor neurons (LMN) Motor system Contain interneurons and complex neural circuits for motor control CENTRAL PATTERN GENERATORS  A reflex always returns some condition of the body back to the desired state 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 REFLEX ARC 2.0 A reflex arc contains 5 fundamental components 1. A receptor 2. A sensory neuron 3. One or more synapses in the CNS 4. A motor neuron 5. A target organ Clinically, the reflexes are tested to evaluate the integrity of the different components of the reflex arc a and the influence of descending UMN motor pathways  Monosynaptic x Polysynaptic L 2 Reflexes can be classified in different ways: Monosynaptic X Polysynaptic Mono = Reflex arc involving synaptic contact directly between sensory and motor neurons Poly = involves sensory neurons, interneurons, and motor neurons (more common) Ipsilateral X Contralateral Segmental x Intersegmental Segmental X Intersegmental Segmental = a reflex arc passes through only a small segment of the spinal cord or a small region of the brainstem Intersegmental = the reflex arc travels through many segments of the spinal cord or several major brain divisions (ex: medulla to midbrain) examplepanniculusreflex Somatic X Autonomic Somatic = effector organ is skeletal muscle Autonomic = effector organ is cardiac or smooth muscle, or a gland  1.0 3 STRETCH REFLEX aka TENDON REFLEX aka MYOTATIC REFLEX (e.g. Patellar reflex) Primarily a monosynaptic reflex Ipsilateral, segmental, somatic Afferent axons from a muscle stretch receptor (MUSCLE SPINDLE) directly synapse with spinal cord alpha motor neurons, which cause contraction of that same muscle  4 go 1) PATELLAR REFLEX (QUADRICEPS, KNEE-JERK REFLEX) Elicited by tapping the tendon of the quadriceps femoris muscle located immediately distal to the patella Tests the stretch reflex mediated by the femoral nerve This is the most reliable test of the stretch reflex Cord segment involved: L4,L5, L6  THE STRETCH REFLEX ILLUSTRATED AS THE “PATELLAR REFLEX” Secondary action: reciprocal inhibition inactivationofantagonistmuscle Main action: contraction Stretch reflexes and other muscle contractions often accompanies reciprocal inhibition – a reflex phenomenon that prevents muscles from working against each other – inhibit the antagonists  3 10 The duration and force of muscle contraction associated with the stretch reflex are mitigated to some degree by THE GOLGI TENDON ORGAN REFLEX aka INVERSE STRETCH REFLEX or AUTOGENIC INHIBITION The receptor is sensitive to muscle tension, increasing its firing rate when the muscle contracts. Excites an inhibitory interneuron in the spinal cord Inhibits the alfa motor neuron causing contraction Leads to muscle relaxation Prevents excessive tension on the muscle  3 INVERSE STRETCH REFLEX 2.0 Main action: this normal stimulation is reduced, inhibited at the level of the spinal cord Secondary action: now, instead of reciprocal inhibition, contracting we have reciprocal stimulation Responds to stretching Responds to contraction Causes contraction Inhibits contraction  1.0 4 FLEXOR REFLEX (aka WITHDRAWAL REFLEX) coordinated polysynaptic reflex in which all the flexor muscles of a limb contract in response to a noxious stimulus Alpha motor neurons to the limb flexor muscles are stimulated, while those to extensor muscles are inhibited Reciprocal innervation The force and duration of the withdrawal reflex are proportional to the intensity of the noxious stimulus applied  1O 4 2) WITHDRAWAL REFLEX Assess the integrity of sensory and motor reflex pathways that mediate flexion of the thoracic and pelvic limbs Elicited by applying pressure to the base of the toenail The response to pinching of the toe pads is flexion of the stimulated limb Thoracic limb – spinal cord segments involved: C6-T2 Pelvic limb – spinal cord segments involved: L4-S1  LO 4 3) EXTENSOR TONE Muscle tone is important for maintaining normal posture and providing support for the joints to stabilize their position It refers to the muscle’s continuous resistance to being stretched (resting tension) It is regulated by the local spinal cord reflexes and by higher levels of the brain Ex: Reflexes using muscle spindle / Golgi tendon organ; Gamma loop Muscle tone (EXTENSOR TONE) is evaluated during neurological examination One of the most reliable spinal evaluations!!! It is tested by gently applying pressure to the plantar or palmar surface of the pelvic or thoracic limbs In normal animals should be easy to flex the limb when pressure is applied  Motor system can be divided into 2 main components: UPPER MOTOR NEURON (UMN) LOWER MOTOR NEURON (LMN)  Cranial General organization of lower and upper motor nerve LMN neurons of the somatic motor system. Blue, Lower motor neurons typically originate in the ventral horn of the spinal cord (neuron 1) or in cranial nerve nuclei (neuron 2), and synapse within skeletal muscle. Green, Upper motor neurons typically originate in the brain and project to and control lower motor Spinal neurons. Upper motor neurons usually belong to nerve LMN the corticomedullary (neuron 3), corticospinal (neuron 4), or descending brainstem motor (neuron 5) pathways. Green arrowheads indicate that local synapses may be interposed between the upper and lower motor neurons.  LO S UPPER MOTOR NEURONS Completely contained within the CNS Cell body is located in a motor nucleus of the brainstem or the motor cortex of the forebrain Their axons connect to lower motor neurons Either synapsing on them directly or indirectly via interneurons Exert their effect by stimulating or inhibiting LMNs LMNs = neurons that innervate the muscles  LO S UPPER MOTOR NEURONS UMN system is responsible for: Initiation of voluntary movement Maintenance of muscle tone for support against gravity Regulation of posture Loss of inhibitory UMNs results in increased muscle tone and spinal reflexes UP UMNs initiate, regulate, modify and terminate the activity of the LMNs. they Loss of facilitatory UMNs results in may inhibit or facilitate LMNs. paresis or paralysis Paresis = weakness of the voluntary movement. The animal drags the limb. Paralysis = loss of motor function. The animal cannot move the limb. ine time Ataxia = uncoordinated movement. loseUMN loseabilitytoinhibit2mn  LO 9 LOWER MOTOR NEURONS Have their cell body in the CNS (brainstem or spinal cord) Their axons project into the PNS via cranial or spinal nerves Connect with skeletal muscle at the neuromuscular junction When stimulated, LMNs induce muscle contraction Muscle tone and bulk depend Loss of LMNs results in paresis/paralysis, with DOWN on LMN function decreased to absent muscle tone and reflexes  mend rewa.tk Clinically, an intact reflex, be it normal or exaggerated, tells the clinician that the lesion DOES NOT involve that area of the CNS or the 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 Lack of the overall inhibitory influence on LMNs Diminished or absent spinal reflexes and/or extensor tone (flaccid/hypotonic) usually occur with lesions affecting LMNs at the spinal cord segments involved in the reflex (intumescences) Direct control from LMNs was lost  Loss of inhibitory UMNs results in increased muscle tone and spinal reflexes UP UMNs initiate, regulate, modify and terminate the activity of the LMNs. they Loss of facilitatory UMNs results in may inhibit or facilitate LMNs. paresis or paralysis Muscle tone and bulk depend Loss of LMNs results in paresis/paralysis, with DOWN on LMN function decreased to absent muscle tone and reflexes UMN signs LMN signs Motor function Paresis or paralysis Paresis or paralysis Reflexes Normal to increased Decreased to absent Extensor muscle tone Normal to increased Decreased to absent Muscle atrophy Mild/chronic Severe/fast  all aft ginates a MY mn locatedinCNSneuronthatcommunicatesw LMN toinitiatemotormovementOriginatesinbrain itravelsw inspinalcordtocommunicatew MN ingraymatter C1-C5 C6-T2 T3-L3 L4-S3 LMNs innervating the limbs are confined to the cervical and lumbar intumescences Cd1-cd5 highercervicalo is umN  rewatch To 13 lesion reflex normal unipron A.Thoracic limbs B.Pelvic limbs C.All 4 limbs D.None Is it normal or abnormal? Which limbs are affected? (Tetra/Para/Hemi/Mono) What is the problem? (Ataxia/Paresis/Paralysis/Limping) A.Thoracic limbs B.Pelvic limbs C.All 4 limbs D.None Is it normal or abnormal? Which limbs are affected? (Tetra/Para/Hemi/Mono) What is the problem? (Ataxia/Paresis/Paralysis/Limping) a lesion reflexnormal itiiiis ftp.amafaiitiia A.Thoracic limbs B.Pelvic limbs C.All 4 limbs D.None Is it normal or abnormal? Which limbs are affected? (Tetra/Para/Hemi/Mono) What is the problem? (Ataxia/Paresis/Paralysis/Limping) Tois ifreflexisintact2  Lesions in the spinal cord will affect limbs caudal or related to the lesion UMN signs LMN signs Motor function Paresis or paralysis Paresis or paralysis Reflexes Normal to increased Decreased to absent Extensor muscle tone Normal to increased Decreased to absent Muscle atrophy Mild/chronic Severe/fast 4 limbs affected – problem must be on: C1-C5 - signs of UMN in both limbs C6-T2 - signs of LMN – front; UMN – hind Pelvic limbs affected – problem must be on: T3-L3 - signs of UMN in the pelvic limb L4-S3 - signs of LMN in the pelvic limb What if all limbs show signs of LMN? multiplelesionsofintumences botulism  ALL LIMBS WITH SIGNS OF LMN = FLACCID PARALYSIS or DIFUSE NEUROMUSCULAR PRESENTATION A diffuse problem in the PNS (nerve or neuromuscular junction) Common causes: polyradiculoneuritis (coonhound paralysis), tick paralysis, botulism, fulminant myasthenia gravis iiiiiiii VEA ALERT!!! A dog has a 1-week history of a severe lesion in the spinal cord at T6-7. Which of the following is correct? A. Reflexes in the thoracic limb are increased B. Reflexes in the pelvic limb are increased C. The thoracic limb has decreased reflexes and atrophic muscles D. The pelvic limb has decreased reflexes and atrophic muscles Exception to the rule when localizing spinal cord lesion Exception to the rule when localizing spinal cord lesion