Physiology of Reflexes 4 PDF

PL1400 and PL1001 Introduction to Physiology of Reflexes 1  Learning Outcomes Describe a components of reflex arc Describe the stretch reflex arc Outline the structure of a muscle spindle How is the muscle spindle innervated Outline the functi...

PL1400 and PL1001 Introduction to Physiology of Reflexes 1  Learning Outcomes Describe a components of reflex arc Describe the stretch reflex arc Outline the structure of a muscle spindle How is the muscle spindle innervated Outline the function of gamma motor neurones for muscle spindle function 2 Overview of Reflexes Reflexes classified as Somatic or COMPONENTS OF A REFLEX Autonomic A reflex is an involuntary, stereotyped but coordinated Centre (CNS or Spinal Cord) Involves an orderly sequence of events, called a reflex arc response to a stimulus v v Reflexes may be monosynaptic or polysynaptic Afferent Nerve (Sensory) Efferent Nerve (Motor) v Receptor Effector Response Stimulus 3 The Stretch Reflex Example of a simple reflex > the one shown here is the knee jerk When a muscle is stretched the result is reflex contraction. Muscle “fights back” Sensory receptor > Muscle spindle Afferent nerve fiber > IA afferents Synapse > Spinal cord Motor fibers > alpha motor neuron Effector organ > Skeletal Muscle contracts Reflex is an example of negative feedback 4 Muscle Spindle - Structure Mechanoreceptor > responds to stretch, and therefore responds to length changes in the muscle Composed of Intrafusal muscle fibers which lie parallel to Extrafusal fibers Intrafusal fibres have non- contractile (centre), and some contractile tissue at either end (poles) 2 types of intrafusal fibres – Nuclear bag and Nuclear chain 5 Spindle Structure Cont Spindle Innervation Motor innervation: gamma (γ) motor neurons innervate intrafusal fibers of the muscle spindle> know this definition! Sensory Innervation: Primary (annulospiral) nerve endings > Type 1a afferents > detect amount of muscle stretch (static response) and the rate of stretching (dynamic response) > Rapidly conducting myelinated neurons > larger than Type II Secondary flower spray endings > Type II afferents > Static response > Slower conducting neurons > also myelinated Primary sensory nerve endings innervate both nuclear bag and chain fibers > Type II mainly the nuclear chain fibers, 6 Importance of Gamma Motor neurones for MS Function Vertical red lines represent firing of spindle sensory neurons Thick red horizontal line represents muscle contraction A > represents steady stretch of the spindle At “B” spindle is “slack” following reflex contraction At “C” the gamma (γ) motor neurone is also stimulated causing contraction of spindle fibres > which takes the slack out of the spindle, even while the muscle is contracting > Co-activation > alpha and gamma motor neurones are stimulated simultaneously 7  Learning Outcomes 4b Outline the GTO reflex arc Pathway What is the function of a golgi tendon organ? Describe the GTO reflex Outline some of the important features of stretch and golgi tendon organ reflexes Outline the pathway for the flexor/withdrawal reflex Explain briefly what is meant by Flaccid Paralysis 8 A Summary/Reminder of How The MS works Stretch of muscle causes reflex contraction e.g knee jerk reflex But the contraction unloads the spindle which is now “Slack” > This is a problem!! > its no longer detecting muscle length! Solution? Gamma Motor Neurones > contraction of intrafusal (spindle) fibers > tightens (reload) the spindle again allowing it to continue to respond 9 GTO Reflex Arc Located at junction between tendon and muscle; very sensitive to muscle tension > GTO lie “in series” with the muscle Nerve terminals innervate collagen of tendon Sensory information via Ib afferents > large diameter rapidly conducting fibers 10 How Does The GTO Work Reflex work? During muscle contraction (e.g biceps muscle when holding an object) GTO detects tension in contracting muscle - This sensory information is carried into spinal cord and synapses with an interneuron If tension becomes excessive > results in inhibition of contracting muscle and therefore reflex relaxation This reflex (polysynaptic) may be a protective i.e by preventing excessive force in a muscle - 11 Summary Points on Muscle Spindles and Golgi Tendon Organs Both allow continuous feedback of Muscle spindles and stretch reflexes especially information to the brain from the contracting prominent in the anti-gravity muscles, also muscle about its status as the contraction neck muscles and in muscles where fine takes place control is required - This information provided by the muscle They prevent overstretching of a muscle by spindle and golgi tendon organ initiating reflex contraction - Above are stretch and tension/force GTO reflex prevents excessive force generation detectors respectively proprioceptors in a muscle - Stretch reflexes are important in overall Important in the feedback control of muscle maintenance of normal posture and balance length in response to a load and so help as reflexes allow for rapid automatic prevent jerky movements and thus have a adjustments in the muscle, as required damping or smoothing effect 12  Flexion (withdrawal ) Reflex most powerful flexion reflex, takes precedent over all others (even those involved in locomotion!) Also called Crossed-Extensor Reflex Pathway for this reflex shows considerable divergence of 1 afferent. All major joints of limb may be involved in strong flexor withdrawal reflex. Can occur in areas other than limbs. In abdominal pain get contraction of abdominal muscles and limitation of trunk movement 13  Summary of LMN Also called final common pathway – Most Cell bodies located in the spinal cord from where the axon travels out directly to the skeletal muscle stimulating it to contract - LMN previously stimulated by UMN (originating in the brain) - LMN supplies from 1 to perhaps hundreds of muscle fibres (motor unit) - Impulses from UMN either facilitate or inhibit LMN Damage to LMN leads to reduced/absent stretch reflexes and a Flaccid Paralysis 14