C h a p t e r 3 6
Disorders of Neuromuscular Function
883
If the skeletal muscles of the body are to perform nor-
mally, the brain must be continually informed of the cur-
rent state of contraction, and the muscles must exhibit
healthy tone (resistance to active and passive stretch at
rest). The first requirement depends on the transmission
of information regarding the sense of body position,
movement, and muscle tone to the CNS. Information
from sensory afferent neurons is relayed to the cerebel-
lum and cerebral cortex and is experienced as
proprio-
ception
or the sense of body movement and position,
independent of vision. To provide this information, the
muscles and their tendons are supplied with two types
of receptors: muscle spindles and Golgi tendon organs.
The
muscle spindles
, which are distributed throughout
the belly of a muscle, relay information about muscle
length and rate of stretch. The
Golgi tendon organs
are found in muscle tendons and transmit information
about muscle tension or force of contraction at the junc-
tion of the muscle and the tendon that attaches to bone.
Muscle Spindle and the Stretch Reflex.
The muscle
spindles consist of a group of specialized miniature skel-
etal muscle fibers called
intrafusal fibers
that are encased
in a connective tissue capsule and attached to the extra-
fusal fibers of a skeletal muscle. In the center of the
receptor area, a large sensory neuron spirals around the
intrafusal fiber, forming the so-called
primary
or
annu-
lospiral ending
(see Understanding the Stretch Reflex
and Muscle Tone).
The intrafusal muscle fibers function as stretch recep-
tors that increase their firing when the muscle is stretched
and decrease their firing when the muscle is relaxed.
3
Axons of these spindle fiber neurons enter the spinal
cord through several branches of the dorsal root. Some
FIGURE 36-3.
Upper motor neuron
pathways: corticospinal (red) and
corticobulbar tracts (blue). (Modified
from Bickley LS. Bates’ Guide to Physical
Examination and HistoryTaking. 8th ed.
Philadelphia, PA: Lippincott Williams &
Wilkins; 2003:543.)
Knee
Hip
Trunk
Hand
Face
Tongue
Basal
ganglia
Thalamus
Internal
capsule
MIDBRAIN
LOWER MEDULLA
SPINAL CORD
Corticospinal tracts
Lateral corticospinal
tract
Lower motor neuron
Anterior corticospinal
tract
Corticobulbar tract
Pyramids
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