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
a. Located in the anterior horn of the spinal cord
b. 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.)
a. Responsible for most of the integrative functions of
the spinal cord
b. All types of neuronal circuits are found in the
interneuronal pool
Muscle Sensory Receptors
• Muscles and Tendons with Two Types of Sensory
Receptors
a. Muscle spindles-distributed throughout the belly
of the muscle; information about muscle length
or rate of change of length
b. 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
c. Signals are for intrinsic muscle control
d. 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
a. Structure and motor innervation- 3-12 intrafusal fibers
surrounded by large extrafusal fibers
1. 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
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
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
1. Nuclear bag muscle fibers
2. Nuclear chain fibers
d. 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
a. Neuronal circuitry
Fig. 54.5 Neuronal circuit of the stretch reflex
Muscle Sensory Receptors
• Muscle Stretch Reflex- monosynaptic pathway
b. 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
c. 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 Contractionsignal 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 of
a contraction
c. Maintains the proper damping function of the muscle
spindle
Muscle Sensory Receptors
• Brain Areas for Control of the Gamma Motor System
a. Excited specifically by signals from the bulboreticular
facilitatory region
b. Secondarily by impulses transmitted into the bulboreticular 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
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 the
cerebellum 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 tension
on the muscle
Muscle Sensory Receptors
• Golgi Tendon Reflex
f. Tendon reflex equalizes contractile forces among
the 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
Flexor Reflex and the Withdrawal Reflexes
• Neuronal Mechanism of the Flexor Reflex
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
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
a. Positive supportive reaction
b. Stepping and walking movements