Muscle Types

Major functions

1. Movement - all 3 types of muscle
2. Maintain posture - skeletal muscle
3. Heat production - skeletal muscle

Characteristics of muscle tissue

1. Excitability - ability to respond to stimuli
2. Contractility - ability to shorten forcefully
3. Extensibility - ability to stretch
4. Elasticity - ability to return to original shape and length

In the following lectures, identify the structures that contribute to these four characteristics of muscle tissue.

Muscle tissue composes 40-50% of total body weight.  There are three types of muscle: smooth muscle found in the walls of the viscera, cardiac muscle found only in the heart, and skeletal muscle associated with bones.  We will compare the three types of muscle according to the following categories:

1. Gross muscle arrangement - indicates how contractile force is applied
2. Connective tissue framework and vascular supply - indicates activity level
3. Innervation - affects type and speed of contraction
4. Fiber structure and cell shape - determines physiology

Smooth Muscle

1. Occurs in small groups (ex. arrector pili) or sheets of overlapping cells tightly bound together (ex. digestive tube, uterus, bladder, respiratory tract, vessels), can regenerate.
2. Sparse reticular and collagenous c.t. network, no tendons, sparse capillary network
3. Two types of innervation:
   1. Multiunit innervation - motor units with each muscle cell innervated by one or more motor neurons, produces rapid coordinated contraction.  ex. ductus deferens, ciliary body and iris, arrector pili, large arteries
   2. Visceral innervation - one motor neuron innervates several cells, muscle cells connected by gap junctions, produces a wave of contraction (peristalsis).  Contraction can also be caused by hormones, stretching, and some chemicals.  In absence of stimulation smooth muscle shows rhythmic cycles of contraction.
4. Spindle-shaped cells with one central nucleus; unstriated, no myofibrils, scattered myosin filaments, actin filaments attached to dense bodies in network of intermediate fibers (desmin), adjacent cells connected by dense bodies.  Contraction produces a cellular twisting motion.  Action is slower but longer lasting, aerobic, resist fatigue.

Cardiac Muscle

1. Branching network of cells forming layers that wind in overlapping spirals to form heart.  Cells connected by intercalated discs containing desmosomes and gap junctions.
2. Loose c.t. with extensive capillary beds.  A c.t. layer separates atria from ventricles.
3. Inherent rhythmic contraction, pacemaker cells, gap junction connections form functional syncytium of the two atria and of the two ventricles, heart rate moderated by ANS.
4. Branching cells with one or more central nuclei, striated, actin and myosin in myofibrils, aerobic metabolism of lipids and carbohydrates, resist fatigue.

Skeletal Muscle

1. Bundles of cylindrical fibers (cells) run in parallel (fascicle); in various arrangements such as spindles, bands, or sheets; typically between two bones and across a joint
2. Connective tissue subdivided into three parts which together form part of the deep fascia, also merge to form tendons and aponeurosis.  Very vascular.
   1. Endomysium - thin c.t. layer with capillaries surrounding fiber, includes satellite cells (stem cells) involved in muscle repair
   2. Perimysium - c.t. layer of collagenous and reticular fibers with vessels surrounding fascicle
   3. Epimysium - dense c.t. layer surrounding groups of fascicles and entire muscle
3. Motor unit - functional unit of muscle, consists of a motor neuron and all muscle fibers it innervates.  Degree of muscle contraction depends on number of active motor units or recruitment
4. Long multinucleate cylinders with peripheral nuclei; striated, actin and myosin arranged in myofibrils.