Muscle/Contractile Tissue - Histology (L1)

Question 1

Action potentials

Answer 1

The cells of muscle tissues are usually elongate, and have a greater capacity for contraction than do other cells. Contractions are preceded by action potentials in these cells, flows of ions along their cell surfaces. This action potential stimulates the cell to contract. Muscles are responsible for most movements in higher animals.

Question 2

Three principal types of muscle are recognized

Answer 2

1. Skeletal muscle, AKA striated muscle. Composed of very long, unbranched, cylindrical cells (fibers). Responsible for most voluntary movement;
2. Smooth muscle, which is involved in most involuntary movements of internal organs; and
3. Cardiac muscle, the type found exclusively in the heart.

Muscle tissue accounts for c. 40% of the total human body by weight.

Question 3

Skeletal muscle

Answer 3

AKA striated muscle. composed of very long, unbranched, cylindrical cells, or fibers. Each fiber may be up to several centimeters in length, while their diameter is approximately 20-100 μm. Because these cells develop
from a syncitium of myoblast cells joining together to become one cell, each fiber is multinucleate, containing a great many flattened, peripherally-located nuclei and mitochondria at fairly regular intervals.

Most skeletal muscle is attached by tendons to bones and can move the bones by voluntary contractions. Each skeletal muscle fiber is controlled by a synapse with a motor neuron originating in the central nervous system.

Question 4

Sarcomeres

Answer 4

AKA Striations. Each skeletal muscle fiber is also crossed perpendicularly by numerous regularly spaced alternating light and dark bands called sarcomeres, or striations, providing this tissue with its alternate name. These bands are the result of the arrangement of the contractile proteins actin and myosin into myofibrils, which are responsible for the contractile capability of the cells, and the peripheral displacement of the nuclei.

Question 5

Actin and myosin

Answer 5

Contractile proteins which organize into myofibrils

Question 6

Smooth muscle

Answer 6

AKA visceral muscle. Smooth muscle is found encapsulating arteries, and on the outside of larger structures as the intestinal tract, the uterus, ureters and the bladder. Note the absence of striations, the actin and myosin contractile proteins are not organized into sarcomeres in this muscle cell type. Each cell is teardrop shaped: elongate, pointed at each end, and contains a single centrally located rod-shaped nucleus. Smooth muscle fibers interlace to form sheets of muscle rather than bundles.

Question 7

Cardiac muscle

Answer 7

The cells of cardiac muscle branch and interdigitate, forming a complex three dimensional network. Like skeletal muscle, the sarcomeres of cardiac muscle form visible striations across the cell, perpendicular to their length, but unlike skeletal muscle, cardiac muscle cells rarely fuse together and generally have only one nucleus. You’ll see in the slide that where adjacent cardiac muscle cells meet there are darker staining bands.

Question 8

Intercalated discs

Answer 8

These bands which look like extra thick sarcomeres are actually not sarcomeres at all, but specialized intercellular junctions called intercalated discs. The intercalated discs have gap junctions that permit the rapid flow of ions between cells, spreading contractile stimuli directly from one cell to another, resulting in nearly simultaneous contractions of large portions of the heart. Intercalated discs also have anchorage sites for the actin proteins in the nearest sarcomeres.

Question 9

Gap junctions

Answer 9

The intercalated discs have gap junctions that permit the rapid flow of ions between cells, spreading contractile stimuli directly from one cell to another, resulting in nearly simultaneous contractions of large portions of the heart.