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Download Cell Bio 5- SDL Spinal Reflexes Circuits A neuron never works
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Transcript
Cell Bio 5- SDL Spinal Reflexes • • • • • • Circuits A neuron never works alone Even in primitive nervous systems all neurons participate in synaptically interconnected networks, or pathways – Neuronal circuits Reflexes Reflexes are an example of neuronal circuits Reflexes are reactions of glands or muscles to a stimulus Reflexes are defined by specific properties • They require stimulation • They are quick • They are involuntary • They are stereotyped Reflexes prevent us from having to think about all the little details required from day to day living • Posture, locomotion, visceral activity Reflex Categories Reflexes may be categorized on the basis of four general properties 1. Development • Innate reflexes result from connections that form during development • Acquired reflexes are learned – Not preestablished 2. Response • In terms of motor response there are two types of reflexes 1. Somatic reflexes • Provide a mechanism for the involuntary control of the skeletal muscle system 2. Visceral (autonomic) reflexes • Controlled by the autonomic nervous system 3. Complexity of Circuit • The simplest reflex circuits (arcs) consist of two neurons that synapse together (monosynaptic) – Stretch reflex • Polysynaptic reflexes involve more than two neurons 4. Processing Site • Integration of reflex information processing may occur in two areas 1. Spinal cord: Spinal reflexes (Integrated in gray matter) 2. Brain: Cranial reflexes These categories are not mutually exclusive Cell Bio 5- SDL Spinal Reflexes • • • • • • • • • Spinal Reflexes Spinal (cord) reflexes are relatively simple reflexes that allow control of muscle function – Mundane, repetitive actions Without neuronal circuits of the cord purposeful muscle movement would not be possible – Brain can only issue command – Additionally the brain is advised of spinal reflex activity and continually facilitates this activity Spinal reflexes occur without immediate conscious awareness Subsequently signals are sent to higher centers – Sensation of pain Spinal reflexes are at the lower end of the hierarchy of complexity in CNS function – Spinal cord level • Simple, automated functions – Lower brain/subcortical level • Blood pressure, breathing – Higher brain/cortical level • Thought, precise control of lower level function The brain has a controlling function with regard to reflexes Spinal reflexes can be suppressed by conscious thought – Higher centers Testing of spinal (somatic) reflexes is of clinical importance in assessing the condition of the nervous system Exaggerated, distorted, or absent reflexes indicates pathology before other signs are apparent Organization of Cord Gray Matter Each segment of the spinal cord has several million neurons in its gray matter These neurons are organized into circuits – Local circuits • Spinal reflex circuits are a type of local circuit Local circuits generally have three elements 1. Input • The main input to the spinal cord is through afferent sensory axons in the dorsal root • Sensory signals travel to two destinations 1. One branch of the sensory nerve synapses locally for cord reflex 2. Another branch transmits signals to higher levels of the nervous system • There are three types of sensory fibers supplying skeletal muscle 1. Type Ia sensory fibers 2. Type Ib sensory fibers 3. Type II sensory fibers 2. Local processing • Local processing is performed by interneurons—which are small, numerous (30X motor neurons), and highly excitable (fire 1500 times/second) – They are present in all areas of the cord gray matter • The vast majority of sensory neurons synapse on interneurons – Some synapse directly on motor neurons 3. Output • Skeletal muscle fibers are innervated by (anterior) motor neurons • There are two types of motor neurons 1. Alpha motor neurons Type A alpha (Aα) motor neurons are the larger of the two motor neurons 14 µm in diameter Each nerve fiber branches repeatedly to innervate large skeletal muscle fibers A motor unit • • Cell Bio 5- SDL Spinal Reflexes 2. Gamma motor neurons Type A gamma (Aγ) motor neurons are the smaller of the two motor neurons 5 µm in diameter These neurons innervate special skeletal muscle fibers known as intrafusal fibers Intrafusal fibers are part of the muscle spindle Muscle Sensory Receptors Proper control of muscle function requires continuous sensory feedback at each instant – Length – Tension Muscle sensory feedback is accomplished with two types of receptors (proprioceptors) 1. Muscle spindles • Muscle spindles monitor the length of muscles and changes in this length • Muscle spindles consist of ~3-12 intrafusal muscle fibers • The spindles are embedded in parallel with extrafusal fibers – Up to ~100 spindles/per one gram of muscle • The spindle consists of two main parts – Peripheral contractile areas – Central sensory region • Free of actin and myosin • There are two classes of intrafusal fibers – Nuclear bag fibers • Clustered nuclei • 1-3 per spindle – Nuclear chain fibers • Single-file nuclei • Two types of nerve fibers supply the sensory region of the spindle – Primary ending • Type Ia nerve fiber encircles both classes of intrafusal muscle fibers – Secondary ending • Type II nerve fiber only supplies nuclear chain fibers • Two types of gamma motor fibers supply the intrafusal fibers – Dynamic (gamma-D) • Excites mainly nuclear bag fibers – Static (gamma-S) • Excites mainly nuclear chain fibers • Why are gamma fibers needed? • Cell Bio 5- SDL Spinal Reflexes There are two general responses of a muscle spindle to stretch 1. Static response When the muscle spindle is stretched slowly (or when it is under constant tension) impulses are transmitted in proportion to the degree of stretch In the static response both primary (type Ia) and secondary (type II) endings are active The static response is a measure of muscle length 2. Dynamic response When the length of the muscle spindle increases suddenly the primary (type Ia) ending is powerfully stimulated Burst of action potentials The dynamic response is a measure of the rate of change of muscle length • The most important motor response to sensory input from the spindles is that of the extrafusal fibers – Activation of α-motor neurons • The secondary concern is to modify the activity of intrafusal fibers – Activation of γ-motor neurons • The muscle spindles are primarily concerned with measuring the length of extrafusal fibers – Intrafusal fibers are inconsequential in terms of force production (or loss of force production due to injury) • The contribution of the γ-motor neurons is to prevent the sensory component of the spindles from going slack • To prevent slackness of spindles, α and γ motor fibers are coactivated by higher centers • Without such a system the spindles cease sending impulses to the CNS Muscle Stretch Reflex • The simplest manifestation of muscle spindle function is the muscle stretch (myotatic) reflex • A sudden stretch of a muscle elicits a reflexive contraction of that same muscle • The stretch reflex can be divided into two components • Dynamic stretch reflex • Opposes sudden changes in muscle length • Over within a fraction of a second • Static stretch reflex • Continues after dynamic stretch reflex • Ensures degree of muscle contraction remains relatively constant • There are two important functions of the stretch reflex • Opposition of stretch/lengthening helps maintain posture and equilibrium • Stretch reflex is key in postural muscles of spine • The stretch reflex dampens muscle function • Reduces jerkiness • There are two important functions of the stretch reflex • Opposition of stretch/lengthening helps maintain posture and equilibrium • Stretch reflex is key in postural muscles of spine • The stretch reflex dampens muscle function • Reduces jerkiness Extensors • The stretch reflex is particularly strong in gravity-opposing extensors • Walking depends on alternating stretch-contraction cycles Overriding the stretch reflex • To perform certain activities the stretch reflex must be suppressed • The stretch reflex depends on a reflex phenomenon known as reciprocal inhibition • In the patellar reflex, branches of type Ia sensory neurons also excite inhibitory interneurons innervating motor neurons of the flexors (hamstrings) • Hamstrings relax/quadriceps contract Cell Bio 5- SDL Spinal Reflexes 2. Golgi tendon organs • Golgi tendon organs monitor the force generated by muscles • The Golgi tendon reflex causes muscle relaxation in response to increased tension on the muscle – Inverse myotatic reflex • Equalization of contractile forces amongst groups of muscle fibers is likely the main function of the tendon organs • In cases of extreme tension the spinal cord may mediate instantaneous relaxation of the entire muscle – Lengthening reaction – This may prevent tearing of muscle or tendon avulsion • Tendon organs are an encapsulated tangle of nerve endings entwined in the collagen fibers of the musculotendinous junction – ~0.5 mm in length • Afferent fibers from the tendon organ are type Ib • Tension in the tendon forces collagen fibers together thus compressing the nerve endings of the organ • The type Ib sensory fibers synapse on two types of interneurons – Inhibitory • Innervate α-motor neurons of active muscle – Excitatory • Innervate α-motor neurons of antagonists Flexor and withdrawal reflexes • The flexor reflex is the quick contraction of flexor muscles to withdraw a limb from an injurious stimulus • Withdrawal reflexes refer more generally to other body parts – May not use flexor muscles • The neural pathway for the flexor reflex involves multiple types of interneurons – Diverging circuits • Activation of other muscles – Reciprocal inhibition circuits – Reverberating circuits • Reverberating circuits result in afterdischarge – Continued excitation of postsynaptic neuron Cell Bio 5- SDL Spinal Reflexes • Afterdischarge permits affected body part to be held away from the stimulus – Other reflexes, CNS, can move entire body away – The crossed extensor reflex has a longer after discharge than the flexor reflex Hold body away for longer period Crossed Extensor Reflex • About .2 to .5 seconds after the flexor reflex begins the contralateral limb starts to extend – Support for body – Push body away • Together, these receptors provide a full description of the dynamic state of each muscle – This function is performed intrinsically (subconsciously)
