C H A P T E R 2 5
Muscle relaxants
387
to help move the blood towards the heart. Other spinal
reflexes may involve synapses with interneurons within
the spinal cord, which adjust movement and response
based on information from higher brain centres to coor-
dinate movement and position.
Brain control
Many areas within the brain influence the spinal motor
nerves. Areas of the brainstem, the basal ganglia and
the cerebellum modulate spinal motor nerve activity
and help to coordinate activity among various muscle
groups, thereby allowing coordinated movement and
control of body muscle motions. Nerve areas within
the cerebral cortex allow conscious, or intentional,
movement. Nerves within the cortex send signals down
the spinal cord, where they cross to the opposite side
of the spinal cord before sending out nerve impulses to
cause muscle contraction. In this way, each side of the
cortex controls muscle movement on the opposite side
of the body.
Different fibres control different types of move-
ments. Those fibres that control precise, intentional
movement make up the
pyramidal tract
within the CNS.
The
extrapyramidal tract
is composed of cells from the
cerebral cortex, as well as those from several subcortical
areas, including the basal ganglia and the cerebellum.
This tract modulates or coordinates unconsciously con-
trolled muscle activity, and allows the body to make
automatic adjustments in posture or position and
balance. The extrapyramidal tract controls lower-level,
or crude, movements.
NEUROMUSCULAR ABNORMALITIES
All of the areas mentioned work together to allow for a
free flow of impulses into and out of the CNS to coor-
dinate posture, balance and movement. When injuries,
diseases and toxins affect the normal flow of informa-
tion into and out of the CNS motor pathways, many
clinical signs and symptoms may develop, ranging from
simple muscle spasms to spasticity—or sustained muscle
spasm—and paralysis.
Muscle spasm
Muscle spasms often result from injury to the musculo-
skeletal system—for example, overstretching a muscle,
wrenching a joint or tearing a tendon or ligament.
These injuries can cause violent and painful involuntary
muscle contractions. It is thought that these spasms are
caused by the flood of sensory impulses coming to the
spinal cord from the injured area. These impulses can
be passed through interneurons to spinal motor nerves,
which stimulate an intense muscle contraction. The con-
traction cuts off blood flow to the muscle fibres in the
injured area, causing lactic acid to accumulate, resulting
in pain. The new flood of sensory impulses caused by
the pain may lead to further muscle contraction, and a
vicious cycle may develop (see Figure 25.2).
Muscle spasticity
Muscle spasticity is the result of damage to neurons
within the CNS rather than injury to peripheral
Interneuron
Synapse
Ventral
horn
Dorsal
horn
Spinal cord
cross section
Ventral root
Dorsal root
Sensory
neuron (axon)
Relaxed muscle fibre
Spindle (stretch sensor)
stretches (in response
to gravity)
Stimulates sensory nerve
Spinal cord
Stimulates gamma nerve
Muscle fibre contracts
No stretch
Sensory nerve not
stimulated
Gamma nerve not
stimulated
Muscle fibre relaxes
Motor
neuron (axon)
Dorsal root
ganglion
Cell body
Spinal
nerve
Spindle
(dendrite)
Muscle fibre
Nerve ending
in muscle fibres
FIGURE 25.1
Reflex arc showing the
pathway of impulses. The spindle
gamma loop reflex arc: The relaxing
and contracting of muscle fibres causes
muscle tone and ability to stand upright
and promotes venous return.
