Chapter 32 Control of Motor Function by Nervous System
Contents Motor Unit and Final Common Pathway Spinal Reflexes Function of Brain Stem Function of the Basal Ganglion Function of Cerebellum Function of the Cortex
Every striated muscle has encapsulated muscle fibers scattered throughout the muscle called muscle spindles. • Extrafusal and intrafusal fibers
The extrafusal muscle fibers are innervated by Alpha motor neuron The intrafusal muscle fibers are innervated by Gamma motor neurons
Motor units • A single motor neuron (a motor) and all (extrafusal) muscle fibers it innervates • The physiological functional unit in muscle (not the cell) • All cells in motor unit contract synchronously
Motor units and innervation ratio Innervation ratio Fibers per motor neuron Extraocular muscle 3:1 Gastrocnemius 2000:1 Purves Fig. 16.4
The muscle cells of a motor unit are not grouped, but are interspersed among cells from other motor units • The coordinated movement needs the activation of several motors
Spinal Cord Overview - organization of motor systems Motor Cortex Brain Stem -motor neuron Final common pathway Skeletal muscle
muscle fibers (+) Transmitter? Schwann cells motor nerve fiber (-) axon hillock Receptors? acetylcholine esterase (+) NM junction Final common path - -motor neuron (-)
Final Common Pathway, a motor pathway consisting of the motor neurons by which nerve impulses from many central sources pass to a muscle in the periphery.
Section 2. Spinal Reflexes • Somatic reflexes mediated by the spinal cord • May occur without the involvement of higher brain centers • Was facilitated or inhibited by brain • For example • Stretch reflex • Deep tendon reflex • Crossed extensor reflex • Superficial reflex
1 Anatomy of Muscle Spindle • 3-10 intrafusal muscle fibers • detect change in the length of the muscle -- stretch receptors that report the stretching of the muscle to the spine. • The central region and peripheral region of the intrafusal fibers
Anatomy of Muscle Spindle • Intrafusal fibers are wrapped by two types of afferent endings • Primary sensory endings • Type Ia fibers • Innervate the center of the spindle • Secondary sensory endings • Type II fibers • Associated with the ends of the nuclear chain fiber
Components of muscle spindle Primary ending } Ia Afferent axons II Secondary ending } Nuclear Bag Fiber Nuclear Chain Fiber
Anatomy of Muscle Spindle • Primary sensory endings • Type Ia fibers • Stimulated by both the rate and amount of stretch (dynamic response)
Anatomy of Muscle Spindle • Secondary sensory endings • Type II fibers • stimulated only by degree of stretch (static response)
Anatomy of Muscle Spindle • The contractile region of the intrafusal muscle fibers are limited to their ends • only these areas contain actin and myosin filaments • are innervated by gamma () efferent fibers
Muscle stretch reflex Definition: Whenever a muscle is stretched, excitation of the spindles causes reflexive contraction of the same muscle from which the signal originated and also of closely allied synergistic muscle. The basic circuit: Spindle Ia or II nerve fiber dorsal root of the spinal cord synapses with anterior motor neurons -motor N. F. the same M. from whence the M. spindle fiber originated.
The Stretch Reflex • Exciting a muscle spindle occurs in two ways • Applying a force that lengthens the entire muscle • Activating the motor neurons that stimulate the distal ends of the intrafusal fibers to contact, • thus stretching the mid-portion of the spindle (internal stretch)
The Stretch Reflex • Whatever the stimulus, when the spindles are activated • their associated sensory neurons transmit impulses at a higher frequency to the spinal cord
The Stretch Reflex • The reflexive muscle contraction resists further stretching of the muscle
The Stretch Reflex • Branches of the afferent fibers also synapse with inter- neurons that inhibit motor neurons controlling the antagonistic muscles
Inhibition of the antagonistic muscles is called reciprocal inhibition • causes the antagonists to relax
The types of the Stretch Flex • 1) Tendon reflex (dynamic stretch reflex) • Caused by rapid stretch of the muscle, as knee-jerk reflex • Transmitted from the IA sensory ending of the M. S. • Causes an instantaneous, strong reflexive contraction of the same muscle; • Opposing sudden changes in length of the M • A monosynaptic pathway • being over within 0.7 ms
The types of the Stretch Flex 2)Muscle tonus (static stretch reflex): Caused by a weaker and continues stretch of the muscle, Transmitted from the IA and II sensory ending of the M. S. Multiple synaptic pathway, continues for a prolonged period. Non-synchronized contraction, M. C. for at least many seconds or minutes, maintaining the posture of the body.
The Stretch Reflex most important in large extensor muscles which sustain upright posture Contractions of the postural muscles of the spine are almost continuously regulated by stretch reflexes (Muscle tonus )
Muscle spindle: motor innervation • Gamma motoneurons: • Innervate the poles of the fibers.
Descending influence (UMN) 1a a g-LOOP Muscle spindle g Activation of the g-loop results in increased muscle tone MUSCLE
Functional significance of gamma impact on spindle activity • The tension of intrafusal fibers is maintained during active contraction by gamma activity. • The system is informed about very small changes in muscle length.
Structure and Innervation of Golgi Organ • Located in the muscle tendon junction. • Connective tissue encapsulating collagen fibers and nerve endings. • Attached to 10-20 muscle fibers and several MUs. • Ib afferent fiber. • sensitive to tension
Golgi tendon organ: response properties Less frequent than muscle spindle Sensitive to the change of tension caused by the passive stretch or active contraction
The Deep Tendon Reflex • When muscle tension increases moderately during muscle contraction or passive stretching, • GTO receptors are activated and afferent impulses are transmitted to the spinal cord
The Deep Tendon Reflex • motor neurons in the spinal cord supplying the contracting muscle are inhibited • antagonistic muscle are activated
The Deep Tendon Reflex • cause muscle relaxation and lengthening in response to the muscle’s contraction • opposite of those elicited by stretch reflexes • help ensure smooth onset and termination of muscle contraction • important in activities involving rapid switching between flexion and extension such as in running