Muscular System

Three types of muscle exist in mammals. They include:

- Skeletal

- Visceral

- Cardiac

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Skeletal/Striated/Voluntary Muscle

This is the familiar flesh (meat) of the domestic animal. The individual cells appear striated when viewed under the microscope. Each muscle cell or fibre is covered by a cell membrane or sheath known as sarcolemma. This covering acts a link between muscle fibres and tendons and gives elasticity to the muscle fibres.  The nuclei and other organelles of skeletal muscle cells are found next to the sarcolemma and the majority of the Sarcoplasm is filled with the contractile machinery of the cell, the myofibrils. Skeletal muscle cells are derived from individual myocytes which fuse to produce a mature multinucleated muscle fibre. There are few if any of the precursor myocytes found in a mature muscle, and so muscles produce no new cells after maturity. Individual cells respond to training by enlarging and building myofibrils and other components.

Smooth /Involuntary/Visceral/Unstriated Muscle

This has no visible striations and is usually found in systems that are automatic in their functioning such as the wall of the digestive tract, the walls of the urogenital system and the blood vessels. Contraction of the smooth muscles is inherent, i.e. requires no nerve stimulus although its contraction can be regulated by the autonomic nervous system and is affected by certain drugs. The individual muscle cells are spindle shaped with a centrally located nucleus. Smooth muscle cells connect to form single-unit syncytia similar to cardiac muscle. But impulses and contractions occur much more slowly in smooth than in cardiac muscle.

There are two types of smooth muscles:

Single unit smooth muscle: Here the cells connected to function as a single unit (syncytium) e.g. in Gastrointestinal Tract (GIT). About 99% of the smooth muscle in the animal body is of this type.

Multi unit smooth muscle: Here the cells grouped into many contractile units controlled by the nervous system, e.g. in blood vessel walls and sphincters in GI tract.

Cardiac Muscle

Cardiac is only found in the heart. Cardiac muscle cells are much shorter than cells in skeletal muscle and they branch to connect to neighboring cells through specialized membranes called intercalated disks to form a network called a syncytium. The cells are striated and the nuclei are centrally located. 

Skeletal Muscle

Muscle fibres are arranged in bundles surrounded by fibrous connective tissue. The connective tissue between individual muscle fibres is called endomysium. The sheath surrounding bundles of muscle fibres is called perimysium and the connective tissue around the entire muscle is known as epimysium.

The proportion of connective tissue to muscle tissues and the amount of marbling (fat lying between muscle bundles) largely account for the relative toughness or tenderness of a meat cut. Thus a cut from the rump or loin of an animal will be expected to be tenderer than cuts from the shank end.

Muscle fibres may be arranged in a parallel manner in sheets as in the abdominal muscles or bands as in Sartorius muscle located on the medial side of the thigh. Other arrangements of muscle fibres include spindle shape and various (penniform) feather-like arrangements. A parallel arrangement of muscle fibres gives the greatest distance of shortening but is relatively weak arrangement, while the feather like arrangement increases the power of a muscle but at the expense of distance and contraction.

Muscle Contraction

Muscles are highly specialized tissue that is capable of converting chemical energy into mechanical through its contraction. Muscles are positioned and attached to the skeleton in such a way that their contraction and relaxation lead to movement and locomotion. This ability to contract and relax is lost when the muscles is converted to meat.

Muscle is stimulated to contract in response to stimulus that arrives at the surface of the muscle fibre. This stimulus is coming from the brain and spinal cord and is transmitted to muscle via a nerve. Each skeletal muscle is made up of a large number of fibres. These lie parallel to one another and are bound to each other and to the tendons at each end of the muscle by a connective tissue. Each fibre is bounded by a membrane the sarcolemma. Within the sarcolemma, there are at least five (5) important type of material namely the nuclei, sarcoplasm, the sarcoplasmic reticulum, the mitochondria and the contractile fibrils.

Muscles also contain stores of glycogen and the energy system necessary for its complete oxidation. They also contain the red pigment myoglobin which like haemoglobin can bind oxygen and thus acts as oxygen store.

Muscles are made up of muscle fibres. Within each muscle fibre, sub units of muscles (called sarcomeres) line up end to end in a long chain.

The end of each sarcomere pulls towards each other during contraction such that sarcomere gets shorter during the contraction process. Thus, the alignment of sarcomeres end to end allows the muscle to contract in a coordinated fashion in a specific direction as shown below

Before Contraction

Arrows depict the ends of the sarcomeres pulling towards each other

After contraction

Note how each sarcomere is shorter following contraction, and how the length of the muscle fiber is shorter.

The two main constituents of the fibrils are the protein Actin and Myosin. The interaction of these proteins within each sarcomere causes the sarcomere to shorten. When the muscle is stimulated, bridges are formed between the actin and myosin filaments. When these bridges shorten, the actin and myosin filaments are pulled past each other. The shortened bridges then break and new ones are formed which in turn shorten and pull the filaments a little further. The energy required for muscle contraction is derived from the energy rich compound ATP when it splits into ADP and phosphoric acid (H₃PO₄).

ATP → ADP + (H₃PO₄) + Energy for immediate use in contraction

Thin filaments are composed of primarily of actin. These filaments are attached to the end walls of the sarcomere. The thin filament do not extend the entire way across the sarcomere, instead they run towards the entire way across the sarcomere, leaving a gap in the middle.

Thick filaments are made of myosin. Thick filaments also do not run the entire length of sarcomere. They transverse the centre of the sarcomere but do not extend to the end.

Several biochemical reactions occur consecutively to cause the mechanical shortening of muscle fibres. The action potential arrives at the end of the myoneural junction. It causes a chemical transmitter acetycholine to be released. Acetycholine acts on sarcolemma then make it more permeable to Na⁺. Sodium ion (Na⁺) rushes in while potassium ion (K⁺) trickles out through the sodium pump located in the sarcolemma. However because Na is more permeable muscle fibre membrane become positively charged triggering an action potential. This action potential on the muscle fibre causes the sarcoplasmic recticulum to release Ca²⁺. These Ca²⁺ binds to troponin present in the thin filament. The troponin modulates the tropomyosin. Normally the tropomyosin physically obstruct the binding sites of the cross bridges. Once Ca²⁺ binds to troponin, the troponin forces the tropomyosin to move out of the way unblocking the bonding site. This shift allows the myosin head to form cross bridges between the myosin and actin filaments.

The actin and myosin bridges bind using ATP as energy. Unfortunately the concentration of ATP in skeletal muscle is relatively small supplying enough energy to maintain contraction for a brief period, the initial ATP has been used up the resulting ADP is phoshorylated again from another source creatine phosphate (CP).  There is more CP in the skeletal muscle than the ATP, so that transphosphorylation of CP to the resulting ADP will result in the formation of ATP again.

 CP + ADP → C +ATP

 This replenishment reaction occurs almost as fast as ATP is being used. Therefore the ATP level does not change much until the concentration of CP gets low but the concentration of CP is also limited thus if muscle contraction continues for any length of time, the CP and the new ATP have to be replaced eventually.  Meanwhile as long as muscle continues to be used, lactic acid will start build up in the active muscle. The glucose, which supplies the needed energy, is obtained from the blood supply in addition to that from the glycogen that is stored in the muscle after its breakdown by the process of glycogenolysis. Relaxation occurs when stimulation of the nerve stops. Ca is then pumped back into the sarcoplasmic reticulum, thereby breaking the link between the actin and myosin. Actin and myosin returns to the unbound state causing the muscle to relax. Relaxation will also occur when ATP is no longer in the system.

Muscle Fatigue

Fatigue is a decrease in work capacity caused by work itself. Muscle fatigue is therefore the drop in the force of contraction of muscle following prolonged stimulation or a decreased capacity to perform a maximum voluntary muscle action or a series of repetitive muscle actions. The length of time that muscle contraction can be maintained depends on the ability to supply energy in the form of ATP and Ca²⁺ to the contractile protein filaments. As the total available ATP supply is decreased, the force of contraction decrease and the muscle gets progressively weaker and weaker.  Fatigue can occur at any state of muscle contraction that is reasonably prolonged but its onset will vary considerably with the type of muscle involved. Muscle contractions increase the mean arterial blood pressure which consequently decreases the net blood flow to the working muscles. This produces Ischemia (lack of blood) and induces fatigue. The occlusion of blood flow to a working muscle substantially decrease the time to exhaustion and increase the magnitude of the decline in force of contraction. Lack of blood and the buildup of lactic acid, lead to increased acidity level in the muscle, which in turn causes muscle pain, soreness, spasm, and cramps. Anaerobic breakdown of glycogen leads to intracellular accumulation of inorganic acids of which lactic acid is the most important. Since lactic acid is a strong acid, it dissociates into lactate and H⁺. Lactate ions have no effect in muscle contraction however the increased H⁺ reduces pH and it’s the classic cause of skeletal muscle fatigue.

Rigor Mortis

Rigor mortis, commonly known as rigor, is the post-mortem muscle stiffening of the body. It consists of a sustained contraction of the muscles of the body, which begins at 2–6 hours after death, persists for 24–84 hours, and is then followed by gradual relaxation until the muscles again become flaccid (Gill-King, 1997). It is created because the lack of oxygen to the cells results in lower levels of ATP in the body. ATP is needed for muscle cells to maintain their delicate balance of various chemicals, including calcium used for muscle contraction. If most of the available ATP becomes depleted in the muscle, the calcium ion can no longer be sequestered back into the sarcoplasmic reticulum by the calcium pump and this causes the body cells to leak calcium around muscle fibril bundles. The body becomes stiff and rigid because the actinomyosin filaments cannot be separated because of the absence of ATP in the body

The muscle bundles will maintain the contraction for a period of time until the muscle fibrils begin to deteriorate, caused by proteolytic enzymes within the muscle cells which disrupt the myosin/actin units, causing the cross linkages to break down and the muscles to relax.

Onset of rigor mortis is influenced by many factors including:

- Temperature - the higher the temperature, the shorter was the onset of rigor and the faster the resolution.

- Condition of the animal - in very thin or emaciated animals, onset of rigor mortis is usually slow or virtually absent, but in fat animals, onset is fast.

Other extrinsic factors which affect the course of rigor mortis are electrocution causing death, which accelerates the onset of rigor and shortens the duration, possibly because the violent cramps experienced cause a rapid fall in ATP.