1 Muscle Contraction: cardiac, smooth and skeletal
1. Types On the basis of structure, contractile properties and control mechanisms, three types of muscle can be identified: 1) skeletal muscle, 2) smooth muscle and 3) cardiac muscle. Although there are significant differences between these muscle types, the forcegenerating mechanisms are similar.
Skeletal Muscle: most skeletal muscle is attached to bone and its contraction is responsible for supporting and moving the skeleton.
The contraction of these muscles is initiated by action potential propagating down motoneurons to the muscle and can be under voluntary control. Smooth Muscle: sheets of smooth muscle surround various hollow organs and tubes (e.g., stomach, intestines, urinary bladder, uterus, blood vessels and airways).
Contraction of these cells may propel the luminal contents through the organ or regulate internal flows by changing tube diameters. Single and groups of smooth muscle cells are also found distributed throughout organs and perform various other functions: e.g., iris of the eye and attachment of hair. Smooth muscle contraction can be spontaneous or controlled by: the autonomic nervous system, hormones and other chemical signals. Cardiac Muscle: The muscle of the heart surrounds four pumping
NMJ Lecture Notes
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chambers. Contraction of cardiac muscle provides the impetus for the movement of blood through the pulmonary and systemic circulatory systems. Spontaneous cycling of an intrinsic pacemaker triggers each heartbeat (or contraction). However the autonomic nervous system and circulating hormones modulate the frequency of this activation. 2. Structure and Function of Skeletal Muscle
If one sections through a skeletal muscle, one can observed that it is organized into bundles of fibers call fascicles. The individual muscle fibers, multinucleated cells, contain long slender structures called myofibrils. These are made of myofilaments, which are organized into sarcomeres, the functional unit of contractions.
Both skeletal and cardiac muscle have a striated appearance under a light microscope, due to the organization of the myofilaments.
Each myofibril is composed of thick and thin filaments arranged in a repeating pattern along their length. thick filaments are composed primarily of the protein myosin and the thin filaments are made up the three proteins, troponin, tropomyosin and actin. It is the cyclic binding between myosin heads of the thick filament and actin of the thin filaments, crossbridge formation, that allows of force production or muscle shortening. It should be noted, that there exist other proteins within sarcomere which have recently been shown to have a role in contractile function, e.g., the elastic protein titan (also known as connectin). 3. The Motor Unit A single motor unit consists of one motor neuron and all of the muscle fibers it innervates. The cell bodies of motor neurons are located within the brainstem or spinal cord. The axons of these neurons are myelinated and large in diameter, and thus are able to propagate action potentials at high velocities. Once an alpha motor neuron is activated to produce an action potential, all of the fibers innervated by this neuron are activated and contract simultaneously. Each motor unit is made up of one type of muscle fibers: i.e., slow twitch, fast-twitch fatigable or
