SEE HANDOUT FOR ILLUSTRATION
INNERVATION OF SKELETAL MUSCLE
1) Every muscle fiber is supplied with a motor neuron
2) Generally, one motor neuron innervates several muscle fibers (Motor Unit = all
muscle fibers innervated by a single neuron)
3) Each motor unit has a different stimulus threshold, when the threshold stimulus is
attained get all-or-none contraction of all fibers within motor unit
4) Graded contraction possible by activating different numbers of motor units
Motor End Plates = synapse of motor neuron on sarcolemma
STIMULUS CONDUCTION AND TRANSFORMATION WITHIN MUSCLE CELLS
Transverse Tubules = tubular invaginations of sarcolemma extending deep
into muscle fiber. In humans, enters at A-I junction; in amphibians at Z-line.
Function is to conduct stimulus into entire fiber.
Sarcoplasmic Reticulum = muscle cell equivalent of smooth ER. Forms collar-
like complex around each myofibril, consisting of connecting tubules and flattened
terminal cisternae. A pair of terminal cisternae, in association with a single T-
tubule, occur at each A-I junction. SR function = regulation of Ca2+ concentration in
myofibrils. SEE HANDOUT.
CARDIAC MUSCLE - present in heart and walls of aorta as it leaves the heart
I. CELLS (Fibers)
1) elongate and cylindrical (80 micrometers long, 15 micrometers wide)
2) may be branched
3) single central nucleus
4) striations - same as in skeletal
5) Intercalated Discs = appear at junction betw. adjacent cells of muscle. Represent
points of cohesion between cells; composed of sarcolemma and cell junctions. Occur at
Z-lines.
6) Development: Mesenchymal cell ---> Myoblast ---> Cardiac muscle cell
II. ULTRASTRUCTURE
1) myofibrils branch, anastomose, and are of variable width; not discrete cylinders as
in skeletal muscle
2) T-tubules less well-developed (wider and fewer) and enter at Z-lines
3) SR smaller and less complex, more dependent on extracellular Ca2+
4) Only endomysium present - no perimysium or epimysium and no fascicles present
III. CARDIAC MUSCLE INNERVATION/IMPULSE CONDUCTION
1) Heart is Myogenic = demonstrates inherent spontaneous rhythmic activity
2) Sinoatrial Node = located near junction of vena cava with right atrium; consists of
specialized mass of cardiac muscle fibers innervated by Autonomic NS; functions
as pacemaker, each wave of excitation spreading over heart begins here -- spreads
via gap junctions between cardiac muscle cells
3) Atrioventricular Node = located near junction of right atrium and ventricle;
specialized cardiac muscle fibers innervated by Autonomic NS; as excitation wave
spreads over atria, it is momentarily delayed here before transmission to ventricles
4) A-V Bundle = Bundle of Histawara = bundle of specialized conducting cardiac
muscle fibers; function = passes impulse rapidly from AV node to ventricles
5) Purkinge Fibers = specialized cardiac muscle fibers extending from A-V septum
and lateral ventricle walls to supply ventricular muscle (base supplied after apex,
What does this order say about the spread of contraction?)
- Purkinge Fibers are larger than normal cardiac muscle cells; stain more lightly
and are swollen in nuclear region where no myofibrils or striations are present
SEE HANDOUT FOR REVIEW OF HEART INNERVATION
SMOOTH MUSCLE - present in walls of tubes (e.g., digestive tract, circulatory
system, etc.); contraction is slow process, fibers capable of sustaining partial
contraction indefinitely (= tonus)
