Muscle 401a. Most of these fibers cross to the contralateral side in
the brain stem, forming structures called the pyramids;
therefore, this system is called the pyramidal system.
b. The left side of the brain thus controls the
musculature on the right side, and vice versa.
2. Other descending motor tracts are part of the
extrapyramidal system.
a. The neurons of the extrapyramidal system make
numerous synapses in different areas of the brain,
including the midbrain, brain stem, basal nuclei, and
cerebellum.
b. Damage to the cerebellum produces intention tremor,
and degeneration of dopaminergic neurons in the
basal nuclei produces Parkinson’s disease.12.6 Cardiac and Smooth Muscles 391
A. Cardiac muscle is striated and contains sarcomeres.
- In contrast to skeletal muscles, which require neural
stimulation to contract, action potentials in the heart
originate in myocardial cells; stimulation by neurons is
not required. - Also unlike the situation in skeletal muscles, action
potentials can cross from one myocardial cell to another.
B. The thin and thick filaments in smooth muscles are not
organized into sarcomeres.
1. The thin filaments extend from the plasma membrane
and from dense bodies in the cytoplasm.
2. The myosin proteins are stacked perpendicular to the
long axis of the thick filaments, so they can bind to actin
all along the length of the thick filament.
3. Depolarizations are graded and conducted from one
smooth muscle cell to the next.
a. The depolarizations stimulate the entry of Ca^2 1 ,
which binds to calmodulin; this complex then
activates myosin light-chain kinase, which
phosphorylates the myosin heads.
b. Phosphorylation of the myosin heads is needed
for them to be able to bind to actin and produce
contractions.
4. Smooth muscles are classified as single-unit, if they are
interconnected by gap junctions, and as multiunit if they
are not so connected.
5. Autonomic neurons have varicosities that release
neurotransmitters all along their length of contact
with the smooth muscle cells, making synapses en
passant.Test Your Knowledge
- A graded whole muscle contraction is produced in vivo
primarily by variations in
a. the strength of the fiber’s contraction.
b. the number of fibers that are contracting.
c. both of these.
d. neither of these. - The series-elastic component of muscle contraction is
responsible for
a. increased muscle shortening to successive twitches.
b. a time delay between contraction and shortening.
c. the lengthening of muscle after contraction has ceased.
d. all of these. - Which of these muscles have motor units with the highest
innervation ratio?
a. Leg muscles
b. Arm muscles
c. Muscles that move the fingers
d. Muscles of the trunk - The stimulation of gamma motoneurons produces
a. isotonic contraction of intrafusal fibers.
b. isometric contraction of intrafusal fibers.
c. either isotonic or isometric contraction of intrafusal
fibers.
d. contraction of extrafusal fibers.Review Activities
- In a single reflex arc involved in the knee-jerk reflex, how
many synapses are activated within the spinal cord?
a. Thousands
b. Hundreds
c. Dozens
d. Two
e. One - Spastic paralysis may occur when there is damage to
a. the lower motor neurons.
b. the upper motor neurons.
c. either the lower or the upper motor neurons. - When a skeletal muscle shortens during contraction, which
of these statements is false?
a. The A bands shorten. c. The I bands shorten.
b. The H bands shorten. d. The sarcomeres shorten. - Electrical excitation of a muscle fiber most directly causes
a. movement of tropomyosin.
b. attachment of the cross bridges to action.
c. release of Ca^2 1 from the sarcoplasmic reticulum.
d. splitting of ATP. - The energy for muscle contraction is most directly obtained
from
a. phosphocreatine. c. anaerobic respiration.
b. ATP. d. aerobic respiration.