Muscle Blanks

Muscle is a tissue made up of cells which are specialised to enable [blank_start]movement[blank_end] of other [blank_start]tissues[blank_end] or [blank_start]organs[blank_end]. Skeletal muscle is also called [blank_start]striated[blank_end] or [blank_start]voluntary[blank_end]. Smooth muscle is also called [blank_start]visceral[blank_end] or [blank_start]involuntary[blank_end]. There is also [blank_start]cardiac[blank_end] muscle.

SKELETAL MUSCLE Each individual muscle is composed of many cells held together by [blank_start]connective tissue[blank_end]. The cells are also called [blank_start]muscle fibres[blank_end] because of their size and shape. They are extremely long (up to 10cm), cylindrical, and un-branched. Each fibre has a connective tissue covering called the [blank_start]endomysium[blank_end]. The fibres are grouped into bundles of [blank_start]fascicule[blank_end] which are surrounded by a connective tissue layer called the [blank_start]perimysium[blank_end]. The whole muscle is then surrounded by a connective tissue layer called the [blank_start]epimysium[blank_end]. The connective tissue layers merge at the end of a muscle belly and continue as the [blank_start]tendon[blank_end]. Each fibre is made up of many [blank_start]myofibrils[blank_end] lying parallel to each other, which are separated by [blank_start]sarcoplasm[blank_end] and numerous [blank_start]mitochondria[blank_end]. Each fibre has several nuclei which are found at the surface of the fibre just under the sarcolemma. Myofibrils are made up of contractile units called [blank_start]sarcomeres[blank_end]. Most of a sarcomere is made up of the proteins [blank_start]actin[blank_end] and [blank_start]myosin[blank_end] which are arranged in longitudinal filaments. When a myofibril contracts, bridges are formed between the actin and myosin filaments. these bridges pull the filaments across one another so that the overlap is increased. The muscle therefore shortens. The actin and myosin filaments stay the same length. The bridges are formed spontaneously but energy is needed for detachment. This process of bridge forming, sliding and bridge-breaking happens over and over again; [blank_start]50-100[blank_end] times a second, and is known as he [blank_start]sliding filament theory[blank_end].

SMOOTH MUSCLE Smooth muscle is found in the walls of blood vessels and in viscera such as the gut, bladder, glands and uterus. Smooth muscle is not striped or [blank_start]striated[blank_end] when examined under a microscope. It has a spontaneous rhythm and contraction, although it can be controlled by [blank_start]nerves[blank_end] which slow down or speed up the rate of contraction. Smooth muscle contracts and fatigues slowly. The muscle cells or fibres are [blank_start]slender[blank_end] and [blank_start]spindle-shaped[blank_end] with a central nucleus and are much [blank_start]shorter[blank_end] than the skeletal muscle fibres. The muscle fibres are usually arranged in sheets or layers. In many tubular visceral structures, e.g. [blank_start]oesophagus[blank_end], the smooth muscle is arranged in layers. The cells are on layer lie at [blank_start]right angles[blank_end] to the cells of the adjacent layer. This allows a wave of contraction ([blank_start]peristalsis[blank_end]) to pass down the tube propelling the contents forwards. In some place, e.g. [blank_start]the skin[blank_end], smooth muscle fibres occur singly or in small groups. In the [blank_start]pyloric sphincter[blank_end], at the exit of the stomach into the small intestine, the muscle fibres are arranged in a [blank_start]ring[blank_end].

CARDIAC MUSCLE Cardiac muscle is found in the [blank_start]heart[blank_end] and is made up of a network of [blank_start]interconnective[blank_end] muscle fibres. The fibres are divided into cells which have a single [blank_start]nucleus[blank_end] and contain fine contractile [blank_start]myofibrils[blank_end]. The cells are joined by [blank_start]intercalated[blank_end] discs which are reinforced plasma membranes. Between the fibres is connective tissue which has an extremely good [blank_start]blood[blank_end] supply. The fibres show the same kind of cross banding as skeletal muscle and the mechanism of contraction is roughly the same. The interconnections between the fibres ensure the rapid and uniform spread of [blank_start]excitation[blank_end] throughout the wall of the heart which ensures [blank_start]uniform[blank_end] contraction of the heart. Cardiac muscle is able to contract rhythmically and without fatigue. It has a [blank_start]spontaneous[blank_end] rhythm of contraction. The region in the heart with the fastest spontaneous rhythm sets the overall pace and acts as a natural [blank_start]pacemaker[blank_end]. As with smooth muscle the rate of contraction can be altered by the [blank_start]involuntary[blank_end] nervous system.