Muscle tissue has 4 unique properties that allow it to function.
1. ELECTRICAL EXCITABILITY is the ability to receive and respond to stimuli (something that causes a response).
2. CONTRACTILITY is the ability to shorten and thicken in response to a stimulus.
3. EXTENSIBILITY is the ability to stretch(extend).
4. ELASTICITY is the ability to return to its original shape after contracting or extending.
Contractions allow the muscles to perform 3 important functions.
1. When muscles are integrated with bones and joints, they contribute to body movements like walking, running, grasping etc. Contraction of muscles helps the body stay in a stationary position and maintain posture.
2. Other muscles control movement without any attachment to bones or joints, like the heartbeat or contractions of the digestive tract.
3. A BYPRODUCT of muscle contractions is HEAT. This heat helps to maintain the body’s temperature. Involuntary contractions of skeletal muscles(SHIVERING) can increase the rate of heat production.
Humans have 3 major types of muscle: SKELETAL, SMOOTH and CARDIAC.
1. SKELETAL MUSCLE
– Skeletal muscle accounts for between 40% and 50% of the body’s weight.
– Skeletal muscles are usually attached to the bones of the body. Some however are not attached to bone, like the muscles that allow you to open and close your eyelids.
– The muscle tapers at the end, forming a dense connective cord (TENDON) which attaches the muscle to the bone.
– There are about 600 skeletal muscles that mobilize the skeletal system.
– They control the voluntary actions of the body, like walking. They move the arms, legs, back, face, jaw etc.
– Skeletal muscles are categorized as either FLEXOR or EXTENDOR.
– When a FLEXOR muscle (like the biceps) contracts, it causes a joint to bend.
– When an EXTENDOR muscle (like the triceps) contracts, the joint straightens.
– Muscles shorten and pull.
– The biceps/triceps relationship is called an ANTAGONISTIC MUSCLE PAIR, because one muscle reverses the effects of the other.
– Skeletal muscles are known for the speed of their contractions.
– The muscle cell is cylinder shaped and can be long in length. It is often referred to as a MUSCLE FIBER because it runs the entire length of the muscle.
– Because of its length there are thousand of nuclei per cell.
– Skeletal muscle consists of bundles of MUSCLE FIBERS.
– Skeletal muscle has STRIPES or STRIATIONS (many dark lines) going across the cell surface when viewed under the microscope.
– Together, many skeletal muscle fibers form BUNDLES, which are wrapped in connective tissue to form muscles.
2. SMOOTH MUSCLE
– Smooth muscle are found in the organs of the body.
– The most common function the smooth muscle is to squeeze (exert pressure) on the space inside the tube or organ it surrounds. Food is moved down the esophagus by the squeezing action of smooth muscle. Blood is pumped through the arteries with the aid of smooth muscle.
– They are found in the digestive tract, blood vessels and other internal organs.
– These muscles are usually not under voluntary control, the contract INVOLUNTARYLY. They are stimulated by the AUTOMATIC NERVOUS SYSTEM.
– Smooth muscle consist of spindle-shaped cells that contain their own nuclei.
– The fibers of smooth muscle are smaller that skeletal muscle.
– The cells form thin broad sheets of tissue.
– Smooth muscle cells do not have any striped or striations.
– Smooth muscle allow electrical impulses to travel from one smooth muscle to another, without the aid of nervous stimulation.
– Because of their structure, smooth muscle takes longer than skeletal muscle to trigger a contraction and longer for the contraction to subside.
3. CARDIAC MUSCLE
– Cardiac muscle is only found in the heart. It controls the beating of the heart.
– The muscle has striations, like skeleton muscle.
– Like smooth muscle it is under involuntary control and has a single, centrally located nucleus.
– The cells form rows of fibers.
– Cardiac muscle tissue needs a constant supply of oxygen, therefore the cytoplasm is more abundant and the cell’s mitochondria are larger and more plentiful.
– Unlike other muscle tissue, cardiac muscle cells are lined up end to end and are joined to each other by a dense band (INTERCALATED DISK).
– The intercalated dicks strengthen cardiac muscle tissue and help conduct the electrical impulse from one muscle fiber to another.
– When a single fiber is stimulated, all are stimulated, and they contract in a synchronous manner.
– Cardiac tissue contract independently of a nerve supply. Each heartbeat is started by self-activating electrical activity of the heart’s PACEMAKER, called the SINOATRAIL NODE (S-A NODE), located in the wall of the right atrium.
– Cardiac muscle remains contracted 10 to 15 times longer than skeletal muscle.
Muscle cells contract either entirely or not at all.
When a muscle receives a stimulus, the response or how strong the muscle contracts depends on the number of muscle cells stimulated. And this all depends on the strength of the initial stimulus.
By increasing the stimulus, more muscle fibers will contract, which is the MAXIMAL STIMULUS. After reaching a maximal stimulus, any increase will not cause a stronger muscle contraction.
When a muscle is stimulated, a certain base level of electricity is necessary to produce a contraction, after this contraction, it takes a brief break for the muscle to contract and then relax before it can contract again.
The time taken to contract is the CONTRACTION PERIOD and the time taken to relax before the muscle can contract again is the RELAXATION PERIOD.
The amount of time from when the initial stimulus is administered until the contraction begins is called the LATENT PERIOD.
The latent, contraction and relaxation periods together make up a muscle twitch.
When a muscle is not allowed to relax completely before being stimulated again, the next contraction stimulate by the same electrical input will have a stronger response.
The energy required for muscle contraction is ATP (adenosine triphosphate), which is stored in the muscles until needed.
The storage of ATP in muscles can be depleted in seconds.
A high-energy compound (Phosphocreatine) in muscle fibers converts ADP to ADT and provides additional energy for muscle contractions.
Once that is used up, GLYCOGEN in the skeletal muscle and liver is broken down to GLUCOSE, which will provide energy for several minutes of activity.
Once the glycogen supply has been used up, muscles break down fat to provide the energy to resynthesize ATP.
THE SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION
The fine structure of a muscle fiber controls contractions.
A muscle fiber is made up of a bundle of finer fibers called MYOFIBRILS.
A single myofibril is composed of smaller units called SARCOMERES, which are the pieces that contract in muscle tissue. Sarcomeres are arranged in a single file along the length of the myofibrils.
In the sarcomeres are alternating rows of thin (ACTIN PROTEIN) and thick (MYOSIN PROTEIN) FILAMENTS.
The thin filaments are attached to two vertical bands of thick protein called the Z LINES. A Z line marks the boundary between sarcomeres.
Muscles contract due to a SLIDING FILAMENT MECHANISM. When the thick and thin filaments slide past each other, the Z lines of the sarcomeres are pulled together.
When the sarcomeres contract (pull closer together), the myofibrils also contract, causing the contraction of the muscle finer.