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Two types ofsensory (afferent) nerves innervate intrafusal fibers:■■Group Ia afferent nerves innervate both nuclear bagfibers and nuclear chain fibers and transmit informationregarding the rate of change in muscle length (contraction). Group Ia fibers form spiral endings around intrafusal fibers.Group II afferent nerves mainly innervate nuclear chainfibers and detect the length of muscle fibers; they formspraylike endings on the fibers.Golgi tendon organs, like muscle spindles, are stretch receptors and are found at the insertion of skeletal muscles to■■The stretch reflex is the most basic, in which sensoryafferent nerves synapse directly with α-motor neuronsin the spinal cord.
Stretch reflexes are therefore referredto as monosynaptic reflexes. In the knee jerk reflex, atype of stretch reflex, a tap on the patellar tendon of theknee stretches muscle spindles within the quadricepsmuscle. This initiates the reflex arc, in which type Iaafferent nerves depolarize and conduct the signal to thespinal cord, where they synapse directly with α-motorneurons. The α-motor neurons conduct the signal backto the quadriceps, producing contraction (the “kneejerk” response). Simultaneously, activation of interneurons results in relaxation of opposing muscles(see Fig. 6.4A).The Golgi tendon reflex (also known as the inversemyotactic reflex), is a bisynaptic reflex. It is a mechanism for prevention of muscle damage when muscletension is excessive.
Stretch of Golgi tendon organs activates type Ib afferent sensory nerves that synapse in thespinal cord with interneurons that subsequently inhibitα-motor neurons to the muscle, causing relaxation. Thereflex also causes antagonistic muscles to contract, coordinating the response (see Fig. 6.4B).78The Nervous System and MuscleAlpha motor neurons to extrafusalstriated muscle end platesGamma motor neurons to intrafusalstriated muscle end platesIa (A␣) fibers from annulospiralendings (proprioception)II (A) fibers from flower sprayendings (proprioception),from paciniform corpuscles (pressure),and pacinian corpuscles (pressure)III (A␦) fibers from free nerve endingsand from some specialized endings(pain and some pressure)IV (unmyelinated) fibers from freenerve endings (pain)Alpha motor neuron to extrafusalmuscle fiber end platesIb (A␣) fibers from Golgi tendonorgans (proprioception)A␣ fibers fromGolgi-type endingsGamma motor neuron to intrafusalmuscle fiber and platesA fibers from paciniformcorpuscles and RuffiniterminalsA␦ and C fibers fromfree nerve endingsII (A) fiber from flower spray endingsExtrafusalmuscle fiberIa (A␣) fiber from annulospiral endingsIntrafusalmuscle fibersSheathLymph spaceNuclear bag fiberNuclear chain fiberDetail of muscle spindleEfferent fibersAfferent fibersFigure 6.1 Muscle and Joint Receptors Muscle spindles are specialized sensory receptorscomposed of intrafusal fibers of two types, the nuclear chain fiber and the nuclear bag fiber.
They functionin coordinating fine movements, returning muscles to normal resting length, and proprioception. Intrafusalfibers are innervated by γ-motor neurons that regulate their sensitivity. Information about rate of change inmuscle length is conveyed by group Ia afferent sensory nerves innervating both nuclear chain fibers andnuclear bag fibers. These group Ia nerves form spiral endings around intrafusal fibers. Group II sensoryafferent nerves convey information on the length of muscle. Golgi tendon organs are located at the insertionof skeletal muscles of tendons; when stretched, they produce a reflexive relaxation of the skeletal muscle.A number of other joint receptors are also illustrated.■The Flexor withdrawal reflex occurs in response topainful or otherwise noxious stimuli. When cutaneouspain receptors are stimulated, for example by touchinga hot object, afferent signals are conducted throughsensory nerve fibers to the spinal cord, where activation of multiple interneurons produces simultaneousflexion and relaxation of the appropriate musclesto withdraw the limb.
This is a polysynaptic reflex (seeFig. 6.4C).ROLE OF HIGHER CENTERS IN MOTORCONTROLCNS areas involved in motor system control of balance,posture, and movement include the spinal cord, brainstem,cerebellum, basal ganglia, and cerebral motor cortex. Formany types of voluntary movement, lower centers regulatepatterns of skeletal muscle activity when they are activated bythe motor cortex. In the case of fine motor movements, par-The Somatic Motor System79A. Spinal effector mechanismsDorsal horninterneuronTo skeletal muscleProprioceptive fibersDorsal horn interneuronFlexor reflex interneuronDorsal horn interneuronFromcutaneousreceptorFrommusclespindleDorsal rootganglionFigure 6.2 Proprioception: Spinal Effector Mechanisms A, Spinal effector mechanisms for proprioceptioninvolve afferent input from cutaneous receptors and musclespindles.
Monosynaptic reflex pathways at one spinal cordsegment and polysynaptic reflex pathways at multiple spinalcord segments activate α-motor neurons, producing musclecontraction. B, Motor neuron cell bodies producing flexion andextension of limb muscles are distributed somatopically in theventral horn.To skeletal muscleVentral rootTo skeletal muscle␣-motor axonB. Schematic representation of motor neuronsIn cervicalenlargement ofspinal cordo rsExtensIn lumbarenlargementof spinal cordsFlexororsFlexo rssExtenticularly of the distal muscles of the limbs (for example, in thefingers and hands), the cerebral cortex regulates muscle activity more directly.Corticospinal TractThe corticospinal tract, also known as the pyramidal tract(because it passes through the medullary pyramids), is themost important descending pathway for control of fine motoractivity originating in the cortex (Fig.
6.5). Nerve fibers in thistract originate in the primary motor cortex, as well as in theadjacent premotor and supplementary motor areas andsomatosensory areas posterior to the motor cortex. Most ofthe fibers cross in the lower medulla to form the lateral corticospinal tract; others descend through the anterior corticospinal tract. At various levels of the spinal cord, some fibersfrom these tracts synapse directly with second order motorneurons (anterior horn cells). Secondary motor neuronsinnervated by axons of the lateral corticospinal tract aremainly those controlling distal limb muscles, whereas thoseinnervated by the anterior corticospinal tract are mainly thosecontrolling axial muscles.
The motor homunculus is a graphicrepresentation of the location and approximate relative sizeof areas of the motor cortex controlling motor activity ofvarious regions of the body (see Fig. 6.5).BrainstemThe brainstem (medulla, pons, and mesencephalon) has animportant role in the motor system, especially in the controlof balance and posture:■Pontine reticular nuclei send projections through thepontine reticulospinal tract of the anterior column of80The Nervous System and MuscleIb fibersIa fibers ⫹⫹⫹⫹Extrafusal muscle fiberIntrafusal muscle fiberAlpha motor neurons ⫹⫹⫹Gamma motor neuronsGolgi tendon organA. Passive stretch. Both intrafusal and extrafusal muscle fibers stretched; spindles activated.Reflex via Ia fibers and alpha motor neurons causes secondary contraction (basis of stretchreflexes, such as knee jerk).
Stretch is too weak to activate Golgi tendon organs.Ib fibers ⫹⫹Alpha activation from brainIa fibersExtrafusal muscle fiberIntrafusal muscle fiberInhibitory interneuronAlpha motor neurons ⫹⫹Gamma motor neuronsGolgi tendon organB. Active contraction. Central excitation of alpha motor neurons only causes contractionof extrafusal muscle fibers with consequent relaxation of intrafusal fibers; spindles notactivated. Tension is low; does not adjust to increased resistance.
Tendon organ activated,causing relaxation.Alpha and gamma activation from brainIb fibers ⫹⫹⫹Ia fibers ⫹⫹⫹⫹Extrafusal muscle fiberIntrafusal muscle fiberAlpha motor neurons ⫹⫹⫹⫹Gamma motor neurons ⫹⫹⫹⫹Golgi tendon organC. Active contraction with gamma coactivation. Intrafusal as well as extrafusal fiberscontract; spindles activated, reinforcing contraction stimulus via Ia fibers in accord withresistance. Tendon organ activated, causing relaxation if load is too great.Figure 6.3 Proprioceptive Reflex Control of Muscle Tension A, During passive stretch of skeletal muscle, both intrafusal (as well as extrafusal) fibers are stretched, producing reflex contraction via Iafibers and α-motor neurons, for example, during a knee jerk reflex.
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