1. 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

2. Organization of the Spinal Cord for Motor Functions
Fig. 54.1

3. 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

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

5. 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

6. 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

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

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

9. Muscle Sensory Receptors
Receptor Function of the Muscle Spindle
Structure and motor innervation 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

10. 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

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

12. 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

13. 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

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

15. 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

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

17. 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

18. 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

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

20. 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

21. 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

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

23. 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

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

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

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

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