Lecture Reflexes and Motor System_Dr E Marais.ppt

PHYSIOLOGY OF THE MOTOR SYSTEM Dr Erna Marais Division Medical Physiology e-mail: [email protected]  Chapter 13 Integrative Physiology I: Control of Body Movement Lectures by Paul Findell University of Texas, Austin Copyright ©...

PHYSIOLOGY OF THE MOTOR SYSTEM Dr Erna Marais Division Medical Physiology e-mail: [email protected]  Chapter 13 Integrative Physiology I: Control of Body Movement Lectures by Paul Findell University of Texas, Austin Copyright © 2009 Pearson Education, Inc. About Chapter 13 Neural reflexes Autonomic reflexes Skeletal muscle reflexes Reflexes  Integration of sensory info into involuntary response  Key feature of reflex path = negative feedback Reflexes Some reflexes = feedforward (allows body to anticipate) i.e. Before food even arrives, digestive activity in the stomach begins Neural Reflexes Table 13-1 Neural Reflexes Table 13-1 Integration by CNS Sensory information Spinal cord to brain by ascending pathways Directly to brain stem via cranial nerves Visceral reflexes integrated in brain stem or spinal cord usually subconscious Lateral view of brain stem Neural Reflexes Table 13-1 Neural Reflexes Learned/conditioned reflexes: Neural Reflexes Table 13-1 Reflexes Reflex path in nervous system: consist of chains of neurons that link sensory receptor to muscles/glands Sensory Stimulus Receptor neuron Spinal cord Integrating center Skeletal muscle Target cell Efferent Response effector neuron Figure 13-1a Somatic Motor Reflexes Monosynaptic somatic motor reflexes (only somatic motor reflexes can be monosynaptic) (a) A monosynaptic reflex has a single synapse between the afferent and efferent neurons. Sensory Stimulus Receptor neuron Spinal cord Integrating center Skeletal muscle Somatic motor neuron Response Target cell effector Efferent One neuron synapse Figure 13-1a Somatic Motor Reflexes Somatic Motor Reflexes Polysynaptic somatic motor reflexes (b) Polysynaptic reflexes have two or more synapses. Synapse 1 Sensory Spinal Stimulus Receptor cord neuron Integrating center Interneuron Efferent Synapse 2 Target cell Response neuron effector Figure 13-1b Autonomic Reflexes All autonomic reflexes are polysynaptic At least, one synapse in CNS and one in ganglion Sensory Stimulus Receptor neuron CNS integrating center Preganglionic autonomic Response neuron Postganglionic autonomic neuron Target Autonomic cell ganglion Figure 13-2 Autonomic Reflexes Autonomic Reflexes Also known as visceral reflexes Temperature control (involve internal organs) Water balance Some (i.e. urination and defecation) Eating behavior Hypothalamus are spinal reflexes (without input from brain), but modulated by CNS (i.e. potty training = learned reflex ) Pons Integrated in hypothalamus, Urinary bladder thalamus and brain stem (incl. control Secondary Medulla medulla, pons, midbrain) respiratory center Blood pressure Emotion-linked (i.e. “I was so control Respiratory center scared my hair stood on end!” = piloerection) Salivating, Vomiting, Sneezing, Coughing, Swallowing, Gagging Tonic activity (i.e. continuous control of blood vessels) Antagonistic control Autonomic Reflexes: Tonic control A continuous stream of action potentials create ongoing activity in the effector (blood vessel) Autonomic system: Antagonistic control Skeletal Muscle Reflexes Excitation of somatic motor neurons always causes contraction of skeletal muscle Skeletal Muscle contraction: Skeletal Muscle Reflexes Excitation of somatic motor neurons always causes contraction of skeletal muscle There is no inhibitory neuron on skeletal muscle for relaxation Relaxation results from absence of excitatory input by somatic motor neuron (a) A monosynaptic reflex has a single synapse between the afferent and efferent neurons. Sensory Stimulus Receptor neuron Spinal cord Integrating center Skeletal muscle Somatic motor neuron Target cell Efferent One Response effector neuron synapse Figure 13-1a Skeletal Muscle Reflexes Excitation of somatic motor neurons always causes contraction of skeletal muscle There is no inhibitory neuron on skeletal muscle for relaxation Relaxation results from absence of excitatory input by somatic motor neuron Inhibitory interneurons in CNS inhibit activity of somatic motor neuron Sensory Spinal Stimulus Receptor cord neuron Integrating center Inhibitory Skeletal muscle interneuron Somatic motor neuron Target cell Efferent Response neuron effector Skeletal Muscle Reflexes Proprioceptors are located in skeletal muscle, joint capsules, and ligaments Proprioceptors carry input sensory neurons to CNS CNS integrates input signal Somatic motor neurons carry output signal = Alpha motor neurons Effectors are (a) A monosynaptic normal reflex has contractile a single synapse between the afferentSkeletal muscle fibers and efferent neurons. or Extrafusal muscle fibers Sensory Stimulus Receptor neuron Spinal cord Integrating center Skeletal or Extrafusal muscle Somatic or Alpha motor neuron Target cell Efferent Response effector neuron  Proprioceptors 1) Muscle spindle 2) Golgi tendon organ Joint receptors: type III (in the ligament) ligament type IV 3) Joint receptors: found in capsules and ligaments around joints fast-adapting type I the joint signal joint angle capsule) type II the joint capsule) Proprioceptors Muscle spindles and Golgi tendon organs are sensory receptors in muscle Extrafusal muscle fibers are normal contractile fibers Alpha motor neuron or somatic motor neuron from CNS innervate extrafusal fibers Muscle spindle (among and parallel to Golgi tendon organ extrafusal muscle fibers) (links muscle to tendon) Tendon Figure 13-3a Muscle Spindles Stretch receptors involved in Stretch Reflex Muscle Spindle Intrafusal fibers Extrafusal fiber Figure 13-3b Muscle Spindles Stretch receptors involved in Stretch Reflex Contractile end Non- Central region contractile lacks myofibrils centre Contractile end Muscle Spindle Intrafusal fibers Extrafusal fiber Figure 13-3b Muscle Spindles Stretch receptors involved in Stretch Reflex Gamma motor neurons from CNS innervate intrafusal fibers To CNS Contractile end Tonically active sensory neurons (stimulated by stretch) Non- Central region contractile lacks myofibrils centre Gamma motor neurons Contractile end Muscle Spindle Intrafusal fibers Extrafusal fiber Figure 13-3b Muscle Spindles: maintain muscle tone (a) Spindles are firing even when muscle is relaxed. They maintain muscle tone 1 1 Extrafusal muscle fibers at resting length Sensory neuron 3 endings 2 2 Sensory neuron is tonically active. Intrafusal fibers Sensory of muscle spindle neuron 3 Spinal cord integrates function. Alpha motor neuron Spinal cord 4 Alpha motor neurons to extrafusal fibers receive tonic 4 input from muscle spindles. 5 5 Extrafusal fibers maintain a certain level of tension even at rest. Figure 13-4a Muscle Spindles: prevent overstretching Muscle spindles monitor muscle length and prevent overstretching Too much or too little overlap of thick and thin filaments in resting muscle results in decreased tension: Muscle Spindles: prevent overstretching (b) Muscle stretch can trigger a Stretch Reflex, which Contracts the muscle to avoid overstretching. Figure 13-4b Muscle Spindle Reflexes (a) Load added to (b) Muscle and (c) Reflex Contraction muscle. muscle spindle initiated by muscle stretch as arm spindle restores arm extends. position. Figure 13-6a-c Alpha-Gamma Coactivation: maintains spindle function (a) Muscle contracts and shortens when alpha motor neuron fires. Simultaneously, gamma motor neuron innervates muscle fibers at the ends of muscle spindles. Thus spindles remain stretched and active. The parallel pulling keeps muscle spindles taut and readily able to detect minute changes in stretch. (a) Alpha-gamma coactivation maintains spindle function when muscle contracts. Alpha 1 1 Alpha motor neuron fires Muscle shortens Gamma 1 2 and gamma motor neuron Muscle fires. length 2 Both Muscle contract. 3 Intrafusal fibers do not slacken so 2 firing rate remains constant. 3 Stretch on centers of Action potentials intrafusal fibers unchanged. of spindle Firing rate of afferent sensory neuron Muscle shortens 1 neuron remains constant. Time Figure 13-5a Without Gamma Motor Neurons (b) Muscle contracts and shortens when alpha motor neuron fires. Without, gamma motor neurons. Release of tension on spindles and firing of afferent neurons will slow down or stop. (b) Alpha 1 1 Alpha motor neuron fires. No Gamma Muscle shortens Muscle 3 length 2 Only extrafusal muscle contracts. 2 4 Less stretch on Action potential 3 Less stretch on center intrafusal fibers of intrafusal fibers Action potentials of spindle 4 Firing rate of spindle sensory neuron Muscle shortens sensory neuron decreases. Time Fine motor skills (such as movements with the fingers and eyes) are affected most since any lack of tautness within the muscle spindle hinders its ability to detect a precise amount of stretch through the sensory endings, since it is so limp. Therefore parallel pulling keeps muscle spindles taut and readily able to detect minute changes in stretch. Upper motor neuron lesion (UML) SIMULTANEOUS STIMULATION OF INTRA- AND EXTRA- FUSAL FIBRES Upper motor neuron lesion (UML) Upper motor neuron lesion (UML) Symptoms: 1. Skeletal muscle paralysis (no voluntary control over muscle activity)* 2. Affects large parts of body (ie whole of one side), rather than isolated muscles* 3. Paralysed muscles are spastic 4. Muscle-stretch reflexes are present, and exaggerated, in paralysed muscles 5. Paralysed muscle do not atrophy 6. Babinski footsole reflex response *Comment: This is very variable, because it depends on how much of the descending motor pathways are affected by a lesion. The impression that an UML causes complete paralysis is not correct. Upper motor neuron lesion (UML) Reasons for Symptoms: 1.Skeletal muscle paralysis Voluntary control is collated in cerebral motor cortex, before conveyed to alpha motor neurons (which supply the extrafusal muscle fibres) via corticospinal tracts. These tracts are severed in UML, therefore there can be no voluntary control over muscle activity below the lesion. (Comment: as noted, this depends on the severity of the lesion. Most lesions are not complete and therefore some degree of voluntary control being preserved is probably the rule rather than the exception.) 2.Affects large parts of body A corticospinal tract conveys all voluntary control to all skeletal muscles on one side of the body. All muscles on one side of the body, below the point at which a corticospinal tract is severed are therefore without voluntary control. (Comment: this is very variable, for example a stroke can just give rise to weakness of the hand and nothing else. Some version of a hemiplegic is certainly quite common, but there is considerable variability) Upper motor neuron lesion (UML) Reasons for Symptoms (cont.): 3. Paralysed muscles are spastic* The corticospinal tracts synapse with: 1) the alpha motor neurons going to the extrafusal fibres 2) the gamma efferents going to the intrafusal fibres 1) the alpha motor neurons are NOT spontaneously active (ie. they do not spontaneously generate APs) descending corticospinal fibres which synapse with them have a stimulatory effect on them when the muscles are “willed” into activity in UML this source of stimulation is missing *Comment: This is not typically correct. In fact, as recovery occurs and the muscle becomes spastic, strength tends to improve. The term paralysis generally implies a very severe degree of weakness, and the most severe forms of weakness are noted at the onset of the illness when spasticity is not present. Upper motor neuron lesion (UML) Reasons for Symptoms (cont.): 3. Paralysed muscles are spastic The corticospinal tracts synapse with: 1) the alpha motor neurons going to the extrafusal fibres 2) the gamma efferents going to the intrafusal fibres 1) the alpha motor neurons are NOT spontaneously active (ie. they do not spontaneously generate APs) descending corticospinal fibres which synapse with them have a stimulatory effect on them when the muscles are “willed” into activity in UML this source of stimulation is missing 2) the gamma efferents are spontaneously active if not inhibited by the corticospinal tracts cause intrafusal fibre contraction (shortening) in UML inhibiting influence is lacking – thus intrafusal fibres are chronically contracted, stimulating the corresponding stretch receptors - This causes increased AP along spindle afferent fibres, which reflexly stimulate the alpha motor neurons serving the same muscle. This in turn causes chronic contraction of the extrafusal fibres, making the muscle stiff/spastic Upper motor neuron lesion (UML) Reasons for Symptoms (cont.): 4.Muscle-stretch reflexes are present, and exaggerated, in paralysed muscles the muscle-stretch reflex is intact reflexes can therefore occur normally reflex responses are exaggerated however because the intrafusal muscle fibres are contracted, causing the spindle stretch receptors to be hypersensitive to stretch (Comment: this is a late phenomenon. Acute lesions of the central nervous system almost invariably result in floppy weakness, without any evidence of spasticity or exaggeration of muscle stretch reflexes. This is manifested most obviously in the case of spinal shock, but is the typical finding of a patient with an acute stroke affecting the hemisphere or brainstem.) 5.Paralysed muscle do not atrophy because the extrafusal muscle fibres are tonically contracted, and regularly undergo stretch reflex contractions, there is no inactivity atrophy in the paralysed muscles (Actually, some atrophy may be found, typically as a result of disuse of the limb.) 6.Babinski footsole reflex response is an innate spinal reflex, which is over-ridden by descending fibres in the corticospinal tracts Upper motor neuron lesion (UML) The plantar reflex is a reflex elicited when the sole of the foot is stimulated with a blunt instrument. The reflex response can take one of two forms. 1) In normal adults the plantar reflex causes a downward (flexor) response. 2) An upward (extensor) response is known as the Babinski response, The presence of Babinski's sign can identify disease of the spinal cord and brain in adults (ie spastic cerebral palsy), Upper motor neuron lesion (UML) The plantar reflex is a reflex elicited when the sole of the foot is stimulated with a blunt instrument. The reflex response can take one of two forms. 1) In normal adults the plantar reflex causes a downward (flexor) response. 2) An upward (extensor) response is known as the Babinski response, The presence of Babinski's sign can identify disease of the spinal cord and brain in adults (ie spastic cerebral palsy), and also exists as a primitive reflex in infants. Proprioceptors Muscle spindles and Golgi tendon organs are sensory receptors in muscle Extrafusal muscle fibers are normal contractile fibers Alpha motor neuron or somatic motor neuron from CNS innervate extrafusal fibers Muscle Golgi tendon organ spindle (links muscle to tendon) Tendon Figure 13-3a Golgi tendon organs Sense muscle tension involved in Relaxation Reflex Extrafusal muscle fibers Afferent neuron Capsule Sensory neuron Collagen fiber Tendon Figure 13-3c Golgi tendon organs: sense muscle tension Golgi Tendon Reflexes: protects muscle Neuron from Golgi tendon organ fires. Excites inhibitory interneuron in spinal cord. (d) Increasing the load. Golgi tendon organ will respond as muscle tension nears its maximum. (Golgi tendon organs respond to (e) Excessive load. tension in muscle during Golgi tendon reflex causes isometric contraction and is relaxation, preventing muscle relatively insensitive to damage. muscle stretch.) Figure 13-6d-e Stretch Reflex and Reciprocal Inhibition Movement around joints = controlled by groups of synergistic and antagonistic muscles Antagonistic muscles: move bones in opposite directions Stretch Reflex and Reciprocal Inhibition Movement around joints = controlled by groups of synergistic and antagonistic muscles These muscles act in coordinated fashion Sensory and efferent neurons are linked by pathways of interneurons within the spinal cord Collection of pathways controlling a single joint = myotatic unit (myo, muscle; tasis, stretching) Simplest reflex in myotatic unit = monosynaptic stretch reflex (involves only two neurons: sensory neuron from muscle spindle and somatic motor neuron to muscle) i.e. knee jerk reflex = monosynaptic stretch reflex (muscle stretch and contract) Reciprocal inhibition (antagonistic muscle relax) = polysynaptic reflex Stretch Reflex and Reciprocal Inhibition Patellar Tendon (Knee Jerk) Reflex = Monosynaptic Stretch reflex (muscle stretch and contract) Afferent path: Action Integrating potential travels through center: sensory neuron. Sensory neuron Receptor: Muscle synapses in spindle stretches and fires. spinal cord. Stimulus: Tap to tendon stretches muscle. One synapse Efferent path 1: onto Somatic motor neuron Effector 1: Quadriceps muscle (extensor) Response: Quadriceps contracts, swinging lower leg forward. Figure 13-7 Stretch Reflex and Reciprocal Inhibition Patellar Tendon (Knee Jerk) Reflex: Reciprocal inhibition (antagonistic muscle relax) = Polysynaptic Reflex Afferent path: Action Integrating potential travels through center: sensory neuron. Sensory neuron Receptor: Muscle synapses in spindle stretches and fires. spinal cord. Stimulus: Tap to tendon stretches muscle. Two synapses Efferent path 1: onto Somatic motor neuron Effector 1: Quadriceps muscle (extensor) Efferent path 2: Interneuron inhibiting somatic motor neuron Response: Quadriceps contracts, swinging lower leg forward. Effector 2: Hamstring muscle (flexor) Response: Hamstring stays relaxed, allowing extension of leg (reciprocal inhibition). Figure 13-7 Flexion Reflex and the Crossed Extensor Reflex Flexion Reflex and the Crossed Extensor Reflex Flexion reflexes (polysynaptic reflex) to pull away from painful stimulus Polysynaptic reflex - rely on divergent pathways in spinal cord Flexion reflexes in one limb causes extension of opposite limb through crossed extensor reflex Crossed Extensor reflex = postural reflex that helps maintain balance Flexion Reflex and the Crossed Extensor Reflex Spinal cord Sensory neuron 1 Painful stimulus activates nociceptor. Nociceptor Painful stimulus 1 Figure 13-8, step 1 Flexion Reflex and the Crossed Extensor Reflex Spinal cord Gray matter 2 Spinal White cord Sensory matter neuron 1 Painful stimulus activates nociceptor. - 2 Primary sensory neuron - enters spinal cord and diverges. Nociceptor Painful stimulus 1 Figure 13-8, steps 1–2 Flexion Reflex and the Crossed Extensor Reflex Spinal cord Ascending pathways to brain Gray 3a matter 2 Spinal White cord Sensory matter neuron 1 Painful stimulus activates nociceptor. - 2 Primary sensory neuron - enters spinal cord and diverges. 3a One collateral activates ascending pathways for sensation (pain) and Nociceptor postural adjustment (shift in center of gravity). Painful stimulus 1 Figure 13-8, steps 1–3a Flexion Reflex and the Crossed Extensor Reflex Spinal cord Ascending pathways to brain Gray 3a matter 2 Spinal White cord Sensory matter neuron 1 Painful stimulus activates nociceptor. 3b - 2 Primary sensory neuron - enters spinal cord and diverges. 3a One collateral activates ascending pathways for sensation (pain) and Nociceptor postural adjustment (shift in center of gravity). 3b Withdrawal (Flexion) reflex pulls Painful Alphamotor foot away from painful stimulus. stimulus neurons 1 Extensors inhibited Flexors contract, moving foot away from painful stimulus. Flexion reflex Figure 13-8, steps 1–3b Flexion Reflex and the Crossed Extensor Reflex Spinal cord Ascending pathways to brain Gray 3a matter 2 Spinal White cord Sensory matter neuron 1 Painful stimulus activates nociceptor. 3b 3c - 2 Primary sensory neuron - enters spinal cord and diverges. 3a One collateral activates ascending pathways for sensation (pain) and Nociceptor postural adjustment (shift in center of gravity). 3b Withdrawal (Flexion) reflex pulls Painful Alphamotor foot away from painful stimulus. stimulus neurons 1 3c Crossed extensor reflex supports body as weight shifts away from painful stimulus. Extensors inhibited Extensors contract as weight shifts to left leg. Flexors contract, moving foot away Flexors inhibited from painful stimulus. Crossed extensor reflex Figure 13-8, steps 1–3c Summary Neural reflexes Somatic reflexes, autonomic reflexes, spinal reflexes, cranial reflexes, monosynaptic reflex, and polysynaptic reflex Autonomic reflexes Polysynaptic, visceral and spinal reflexes, tonic activity, antagonistic control Skeletal muscle reflexes Proprioceptors, inhibitory interneurons Extrafusal muscle fibers, alpha motor neurons, muscle spindles, intrafusal fibers, gamma motor neurons, stretch reflex and muscle tone, Alpha-gamma coactivation Golgi tendon organs, isometric contraction, relaxation reflex Myotatic unit, stretch reflex, reciprocal inhibitions, flexion reflexes, crossed extensor reflex