Chapter 54: Motor Functions of the Spinal Cord; the Cord Reflexes Guyton and Hall, Textbook of...

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


• Anterior Motor Neurons

a. Located in the anterior horn of the spinal cord

b. Two types: alpha and gamma; directly innervateskeletal muscle fibers

• Interneurons- present in all areas of the cordgray matter


Fig. 54.2 Peripheral sensory fibers and anterior motor neurons innervating skeletal muscle


• Interneurons (cont.)

a. Responsible for most of the integrative functions ofthe spinal cord

b. All types of neuronal circuits are found in theinterneuronal pool

Muscle Sensory Receptors

• Muscles and Tendons with Two Types of SensoryReceptors

a. Muscle spindles-distributed throughout the bellyof the muscle; information about muscle length or rate of change of length

b. Golgi tendon organs-located in the tendons andtransmit information about tendon tension orrate of change of tension


• Muscles and Tendons with Two Types of SensoryReceptors

c. Signals are for intrinsic muscle control

d. Operate at a subconscious level


• Receptor Function of the Muscle Spindle

Fig. 54.3 Muscle spindle



a. Structure and motor innervation- 3-12 intrafusal fiberssurrounded by large extrafusal fibers

1. The central portion does not contractwhen the ends do

2. End portions are excited by gamma motor fibers



b. Sensory innervation of the muscle spindle

1. Lengthening the whole muscle stretches the point of the spindle and excites the receptor

2. Even if the length of the muscle does not change, you get the receptor excited

3. Primary and secondary endings


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



c. Division of the intrafusal fibers

1. Nuclear bag muscle fibers2. Nuclear chain fibers

d. Static Response-when the receptor is stretched slowly, the number of impulses transmitted fromboth primary and secondary endings increasesin proportion to the stretching and can continue forseveral minutes



e. Dynamic Response- when the length of the spindlereceptor increases suddenly, only the primary endingis stimulated; the primary responds actively to a rapid rate of change

f. Gamma motor nerve control of static and dynamicresponses; gamma-s and gamma-d excite specific fibers

G. Continuous discharge of impulses to the spinal cord


Fig. 54.5 Neuronal circuit of the stretch reflex

• Muscle Stretch Reflex- monosynaptic pathway

a. Neuronal circuitry


• Muscle Stretch Reflex- monosynaptic pathway

b. Dynamic stretch reflex-responds to the primarysensory endings of the muscle spindles caused byrapid stretch or unstretch; functions to oppose sudden changes in muscle length

c. Static stretch reflex- weaker and elicited by continuous static receptor signals transmitted byprimary and secondary endings


• Damping Mechanism in Smoothing Muscle Contraction

Fig. 54.6


• Damping Mechanism in Smoothing Muscle Contraction-signal averaging function of the muscle spindles

• Role of the Muscle Spindle in Voluntary Motor Activity

a. Coactivation of alpha and gamma neurons

b. Keeps the length of the receptor portion of the muscle spindle from changing during the course ofa contraction

c. Maintains the proper damping function of the musclespindle


• Brain Areas for Control of the Gamma Motor System

a. Excited specifically by signals from the bulboreticularfacilitatory region

b. Secondarily by impulses transmitted into the bulbo-reticular area from the cerebellum, basal ganglia,and the cerebral cortex

• Muscle Spindle System Stabilizes Body PositionDuring Tense Action


• Clonus—Oscillation of Muscle Jerks

Fig. 54.7


• Golgi Tendon Reflex

a. Detects muscle tension not changes in muscle length

b. Has both primary and static responses

c. Transmits signals into the spinal cord and on to thecerebellum and cerebral cortex

d. Local cord signal stimulates a single inhibitory interneuron that inhibits the anterior motor neuron

e. Is a negative feedback that prevents too much tensionon the muscle


• Golgi Tendon Reflex

f. Tendon reflex equalizes contractile forces amongthe muscle fibers

g. 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



a. 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



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

a. Positive supportive reaction

b. Stepping and walking movements

Chapter 54: Motor Functions of the Spinal Cord; the Cord Reflexes Guyton and Hall, Textbook of...

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