Tag Archives: Skeletal Muscle

The Skeletal System – Coordination of the Bones and Muscle (part 2 of 2)

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
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 CONTRACTION
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.

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Types of Tissue

Tissues are groups of cells with a common structure and function.

They work together to perform a particular task.

ORGANS
– Organs are a collection of 2 or more of the basic body tissue types.
– Multiple tissues adapt as a group to perform specific functions and form structures called ORGANS.

ORGAN SYSTEMS
– A collection of 2 or more of the organs that together perform some complex body function.
– The human body is a cooperative of organ systems that are interdependent upon one another, either chemically or physically.

There are 4 main CATEGORIES OF TISSUE:
1.  EPITHELIAL TISSUE (covers and lines the body)
2.  CONNECTIVE TISSUE (binds and supports other tissue)
3.  MUSCLE TISSUE (is involved with movement)
4.  NERVOUS TISSUE (forms a communication network)

1. EPITHELIAL TISSUE

Epithelial tissue is covering and lining tissue, it covers body surfaces in general and lines cavities within the body.

It has little or no intercellular material between its cells.

The free surface of this tissue is exposed either to air or fluid.

The base of the cell is attached to a BASEMENT MEMBRANE (a dense layer of extra cellular material).

The cells are closely joined and may act as a barrier against injury, microbial invasion or fluid loss.

These cells may be specialized for absorption or secretion of chemical solutions.

Epithelial tissue is categorized by the number of LAYERS and the SHAPES of the free surface of the cells.

LAYERS can be:
1. SIMPLE: one layer of cells.
2. STRATIFIED: multiple tiers of cells.
3. PSEUDOSTRATIFIED: one layer that appears multiple because the layers vary in length.

SHAPES include:
1. SQUAMOUS
SIMPLE SQUAMOUS epithelial tissue is thin and leaky.
– These cells aid in the exchange of material by diffusion.
– They line blood vessels and air sacs in the lungs.
STRATIFIED SQUAMOUS tissue regenerates rapidly near the basement membrane.
– New cell are pushed to the free surface as replacements for the cells that are continually sloughed off.
– Stratified Squamous tissue is located on surfaces that are subject to abrasion, like the outer skin.

2. COLUMNAR
– They are like a cytoplasm filled water balloon.
– They are found where secretion or active absorption of substances is an important function, like the intestines, where they secrete digestive juices or absorb nutrients.
STRATIFIED COLUMNAR epithelial tissue line the inner surface of the urinary bladder.
PSEUDOSTRATIFIED CILIATED COLUMNAR epithelial tissue line the nasal passage.

3. CUBOIDAL
SIMPLE CUBOIDAL epithelial tissue is specializes for secretion.
– They can be found in the kidney tubules, the thyroid gland and the salivary glands.

2. CONNECTIVE TISSUE

Connective tissue binds and supports other tissue.

It has a sparse cell population scattered throughout an extensive extracellular matrix;  it has a lot of intercellular material between its cells.

The matrix contains long, slender rods and connective tissue fiber in a substance similar to soft-set gelatin.

This fiber helps connective tissue to do its job, to directly or indirectly connect body parts together.

3 types of FIBERS make up the various types of connective tissue:
1. COLLAGENOUS FIBERS
– They are bundles of fibers containing 3 collagen fibers each.
– These fibers are strong and resist stretching.
– The parallel lines on the palm of your hand are collagen bundles.

2. ELASTIC FIBERS
– They are long threads of the protein, elastin.
– If stretched, this tissue can return to its original shape.

3. RETICULAR FIBERS
– They are branched and tightly woven.
– They join connective tissue to neighboring tissue.

There are 6 CATEGORIES of connective tissue:
1. LOOSE CONNECTIVE TISSUE
– Contains all 3 fiber types; collagen, elastin and reticular.
– Holds organs in place and attaches the epithelium to underlying tissue.
– Contains 2 types of cells:
1. FIBROBLASTS: secrete proteins of extracellular fibers, like collagen.
2. MACROPHAGES: act as the “attack dogs” of the body’s immune system.

2. ADIPOSE TISSUE
– It is a loose connective tissue that is specialized to store fat.
– The fat is stored in adipose cells distributed throughout its matrix.
– Each adipose cell stores one fat droplet, which can vary in size.
– The stored fat insulates the body and is used for fuel when needed.

3. FIBROUS CONNECTIVE TISSUE
– Large numbers of collagenous fibers in parallel bundles makes this tissue very dense.
– This density gives it the great strength needed for tendons (to attach muscle to the bone) and ligaments (to attach bone together at joints).

4. CARTILAGE
– It is the strong and flexible connective tissue found in the skeleton of all vertebrate embryos.
– Most vertebrates convert the cartilage to bone, but they retain cartilage in the nose, ears and trachea.
– It is composed of collagenous fibers embedded in chondroitin sulfate (a protein-carbohydrate).

5. BONE
– Bone is hard, but not brittle or completely solid.
– It is mineralized connective tissue.
– OSTEOBLASTS (bone-forming cells) deposit a matrix of collagen and calcium phosphate that hardens into the mineral hydroxyapatite.

6. BLOOD
– Blood is the only liquid connective tissue in the body.
-It is a liquid extracellular matrix of plasma containing water, salt and proteins.
– Blood contains RED BLOOD CELLS (that transport oxygen), WHITE BLOOD CELLS (for the immune system) and PLATELETS (which are cell fragments that help in the clotting of blood).
– Blood cells are made in the red marrow of long bones.
– Blood vessels and nerve cells occupy slender canals in the bone tissue call HAVERSAIN CANALS.

3. MUSCLE TISSUE

There is more muscle tissue available in the human body than any other type of tissue.

It consist of long, slender muscle fibers, that contract or shorten to create body movement.

There are 3 types of muscle:
1. SKELETAL MUSCLE
– It is multinucleated and is usually attached to bones by tendons.
– Contractions are voluntary.
– This muscle appears striated under the microscope.

2. SMOOTH MUSCLE
– It is found in the walls of the internal organs and arteries.
– The spindle-shaped, uninucleated cells contract involuntarily.

3. CARDIAC MUSCLE
– It is located only in the wall of the heart.
– The cells are striated, uninucleated, and are joined by intercalated disks.
– Contractions are involuntary.

4. NERVOUS TISSUE
– It is the major tissue for communication and control within the body’s internal environment.
– It is designed to sense stimuli.
– It communicates by means of NEURONS (nerve cells).
– The neuron conducts impulses or bioelectric signals.
– It transmits signals from one part of the organism to another.