Muscle Stretch Reflex Notes

Muscle receptors and spinal reflexes

-Continuous feedback of sneosry information from each muscle to the spinal cord-this indicates the functional status of each muscle at each instant

-What is the length of the muscle

-What is the instantaneous tension

-How rapidly is its length or tension changing

-To provide this information the muscles and tendons are supplied abundantly with 2 speical types of sensory receptors

-Muscle spindles-distributed throughout the belly of the muscle and send information to the nervous system about muscle length or rate of change of length

-Golgi tendon organs-located in the muscle tendons and transmit information about tension or rate of change of tension

-Signals from these two receptors are entirely for intrinsic muscle control-operate completely at a subconscious level

1) Muscle spindles/stretch receptor

Well answered on the whole, but some essays lacked diagrams of the muscle spindle and most

confined themselves to the phasic stretch reflex without mentioning its tonic component. Few got

to grips with the role of the stretch reflex in voluntary movements and few discussed the role of

muscle spindles in position sens

-So when bulk of muscle is stretched by joint movement-muscle spindle is also stretched- Receptors signal the length, rate of change of length of muscle

Components of the muscle spindles:

i)-Specialised intrafusal muscle fibres (don’t contribute in any significant way to tensile force that the muscle can exert)

-Each spindle is 2-4mm long- contains 3-12 tiny intrafusal muscle fibres (specialised skeletal muscle fibres) encapsulated (fibrous capsule) spindle shaped (fusiform)

- found in fleshy parts of muscles in parallel with extrafusal -Either ends connected to glycocylyx of the surrounding large extrafusal skeletal muscle fibres

- so intrafusal fibres are stretched or shortened correspondingly when extrafusal fibres change length

-central part of is the receptor portion which is non contractile (no myosin +actin) and contains the nuclei

-Nuclear bag fibres-non contractile central part is expanded and nuclei are clustered within it

-Nuclear chain fibres-central region is not expanded and nuclei are arranged longitudinally

-Typical muscle spindle has 2-3 nuclear bag fibres and a variable number of chain fibres

Types

-Dynamic nuclear bag fibres- adapt very rapidly (though not completely) and report the rate of change of stretch. Contractile ends are more viscous-so stretching occurs in the central part

-Static nuclear bag and nuclear chain fibres- almost non adapting-so mirror precisely the length of the muscle at any instant

ii) Sensory nerve fibres

-Stimulated by stretchin of the midportion of the spindle- so can be exicted by

-Lengthening the whole muscle stretches the midportion of the spindle

-Contraction of the end portion of the spindle’s intrafusal fibres stretches midportion of the spindle

-Two types of sensory endings: Primary and secondary

a) Single primary 1a sensory nerve ending-encircles the central portion of each intrafusal fibre-annulospiral ending-thickest myelinated axons in the body

-highly sensitive to small changes in muscle length. Their responsiveness coupled with large diameter of their axons allows CNS to respond very rapidly to unexpected changes-firing rate increases when there iss change in muscle length

-Rapidly adapting response-so provide information about the velocity and direction of the muscle stretch

-Excited by both nuclear bag and nuclear chain intrafusal fibres

b) Group 2 afferent fibres-Secondary ending-innervate receptor region on one or both sides of the primary ending

-Sometimes it encircles the intrafusal fibres but often it spreads like branches on a bush

-myelianted and thinner-afferents produce sustained response to constant muscle length-so provide information about static muscle position

-Excited only by nuclear chain fibres

Response to length of receptor-static response: When receptor portion of the muscle spindle is stretched slowly the number of impulses transmitted from primary and secondary endings increases directly in proportion to degree of stretching and endings continue to transmit impulses for several minutes

-Occurs in primary and secondary endings

Rate of change of receptor Length-Dynamic reponse

-When length of spindle receptor increases suddenly-primary ending is stimulated powerfully

iii) Motor neurons

a) Dynamic gamma motor neurons- innervate the contractile polar regions of dynamic bag fibres

-This causes the dynamic responses of group 1a neurons of the muscle spindle to become increased whereas the static response is hardly affected

b) Static gamma motor neurons innervate the contractile polar regions of static bag and nuclear chain fibres-Increased activity increases tonic level oof activity of both primary and secondary endings

thought to be part of spinal reflex network that regulates precise limb movement such as grasping delicate object

ROLE 1: PROPRIOCEPTION

Sensory information for proprioception

-Proprioception(sense of position and movement of one’s own body parts)-static (joint-position sense) and dynamic (kinaesthesia-sense of limb in movement)

-Changes in muscle length- associated with changes in angles of joints over which they act: Muscle spindles-provide information about that CNS uses to determine relative positions of the body-proprioception

-Joint mechanoreceptors-also provide proprioceptive info- Larger fibres signal joint position, smaller Aδ fibres are active at extremes of movement-primarily protective

-Skin mechanoreceptors-signal postural information-important in speech and facial expression

Propriocetive afferents used by CNS in 3 different ways

-afferent info from muscle spindles, Golgi tendon organs, joint receptors, cutaneous mechanorecptors is processed in the CNS

-Afferent input for spinal reflexes

-For unconscious control of movement in cerebellum-thoracic nucleus, external cuneate nucleus-spinocerebellar tracts-output modulates the...

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