Chapter 54: Motor Functions of the Spinal Cord; the Cord Reflexes Unit Eleven: The Nervous System: C. Motor and Integrative Neurophysiology Chapter 54: Motor Functions of the Spinal Cord; the Cord Reflexes Guyton and Hall, Textbook of Medical Physiology, 12 edition

Organization of the Spinal Cord for Motor Functions Fig. 54.1

Organization of the Spinal Cord for Motor Functions Anterior Motor Neurons Located in the anterior horn of the spinal cord Two types: alpha and gamma; directly innervate skeletal muscle fibers Interneurons- present in all areas of the cord gray matter

Organization of the Spinal Cord for Motor Functions Fig. 54.2 Peripheral sensory fibers and anterior motor neurons innervating skeletal muscle

Organization of the Spinal Cord for Motor Functions Interneurons (cont.) Responsible for most of the integrative functions of the spinal cord All types of neuronal circuits are found in the interneuronal pool

Muscle Sensory Receptors Muscles and Tendons with Two Types of Sensory Receptors Muscle spindles-distributed throughout the belly of the muscle; information about muscle length or rate of change of length Golgi tendon organs-located in the tendons and transmit information about tendon tension or rate of change of tension

Muscle Sensory Receptors Muscles and Tendons with Two Types of Sensory Receptors Signals are for intrinsic muscle control Operate at a subconscious level

Muscle Sensory Receptors Receptor Function of the Muscle Spindle Fig. 54.3 Muscle spindle

Muscle Sensory Receptors Receptor Function of the Muscle Spindle Structure and motor innervation- 3-12 intrafusal fibers surrounded by large extrafusal fibers The central portion does not contract when the ends do 2. End portions are excited by gamma motor fibers

Muscle Sensory Receptors Receptor Function of the Muscle Spindle b. Sensory innervation of the muscle spindle Lengthening the whole muscle stretches the point of the spindle and excites the receptor Even if the length of the muscle does not change, you get the receptor excited 3. Primary and secondary endings

Muscle Sensory Receptors Fig. 54.4 Details of nerve connections from the nuclear bag and nuclear chain muscle spindle fibers

Muscle Sensory Receptors Receptor Function of the Muscle Spindle c. Division of the intrafusal fibers Nuclear bag muscle fibers Nuclear chain fibers Static Response-when the receptor is stretched slowly, the number of impulses transmitted from both primary and secondary endings increases in proportion to the stretching and can continue for several minutes

Muscle Sensory Receptors Receptor Function of the Muscle Spindle e. Dynamic Response- when the length of the spindle receptor increases suddenly, only the primary ending is stimulated; the primary responds actively to a rapid rate of change f. Gamma motor nerve control of static and dynamic responses; gamma-s and gamma-d excite specific fibers G. Continuous discharge of impulses to the spinal cord

Muscle Sensory Receptors Muscle Stretch Reflex- monosynaptic pathway Neuronal circuitry Fig. 54.5 Neuronal circuit of the stretch reflex

Muscle Sensory Receptors Muscle Stretch Reflex- monosynaptic pathway Dynamic stretch reflex-responds to the primary sensory endings of the muscle spindles caused by rapid stretch or unstretch; functions to oppose sudden changes in muscle length Static stretch reflex- weaker and elicited by continuous static receptor signals transmitted by primary and secondary endings

Muscle Sensory Receptors Damping Mechanism in Smoothing Muscle Contraction Fig. 54.6

Muscle Sensory Receptors Damping Mechanism in Smoothing Muscle Contraction- signal averaging function of the muscle spindles Role of the Muscle Spindle in Voluntary Motor Activity Coactivation of alpha and gamma neurons Keeps the length of the receptor portion of the muscle spindle from changing during the course of a contraction Maintains the proper damping function of the muscle spindle

Muscle Sensory Receptors Brain Areas for Control of the Gamma Motor System Excited specifically by signals from the bulboreticular facilitatory region Secondarily by impulses transmitted into the bulbo- reticular area from the cerebellum, basal ganglia, and the cerebral cortex Muscle Spindle System Stabilizes Body Position During Tense Action

Muscle Sensory Receptors Clonus—Oscillation of Muscle Jerks Fig. 54.7

Muscle Sensory Receptors Golgi Tendon Reflex Detects muscle tension not changes in muscle length Has both primary and static responses Transmits signals into the spinal cord and on to the cerebellum and cerebral cortex Local cord signal stimulates a single inhibitory interneuron that inhibits the anterior motor neuron Is a negative feedback that prevents too much tension on the muscle

Muscle Sensory Receptors Golgi Tendon Reflex Tendon reflex equalizes contractile forces among the muscle fibers Apprise higher motor control centers of instantaneous changes occurring in the muscles

Flexor Reflex and the Withdrawal Reflexes Neuronal Mechanism of the Flexor Reflex Fig. 54.9 Flexor reflex, crossed extensor reflex, and reciprocal inhibition

Flexor Reflex and the Withdrawal Reflexes Neuronal Mechanism of the Flexor Reflex Involves the following basic types of circuits 1. diverging circuits to spread the reflex to the necessary muscles for withdrawal 2. circuits to inhibit the antagonistic muscles (reciprocal inhibition) 3. circuits to cause afterdischarge lasting after the stimulus stops

Flexor Reflex and the Withdrawal Reflexes Neuronal Mechanism of the Flexor Reflex Fig. 54.10 Myogram of the flexor reflex showing rapid onset, an interval of fatigue, and finally, afterdischarge when the stimulus is over

Crossed Extensor Reflex Neuronal Mechanism Fig. 54.11 Myogram of a crossed extensor reflex showing slow onset but prolonged afterdischarge

Crossed Extensor Reflex Reciprocal Inhibition and Reciprocal Innervation Fig. 54.12 Myogram of a flexor reflex showing reciprocal inhibition

Reflexes of Posture and Locomotion Reciprocal Inhibition and Reciprocal Innervation Positive supportive reaction Stepping and walking movements