Muscle Tissue Types and Functions

Muscle Tissue and Types

• About half of the human body's weight comes from muscles.
• Muscles are responsible for moving structures and internal organs.
• Muscle tissue facilitates body functions by changing shape and length.
• Contractions cause movements that facilitate body functions.
• Types of muscle tissue:
  • Skeletal muscle: Moves the body by responding to signals from the nervous system; voluntary.
  • Smooth muscle: Found in many organs; responds to hormones and involuntary signals from the nervous system. Contracts and relaxes to contain and move substances.
  • Cardiac muscle: Responds to involuntary signals from its own conduction system. Heartbeats are contractions that pump blood.

Skeletal Muscle

• Striated and responds to voluntary commands from the nervous system.
• Each skeletal muscle is protected by a layer of connective tissue called the epimysium.
• Skeletal muscle tissue is composed of cells called muscle fibers grouped into bundles called fascicles.
• Capillaries of the circulatory system and motor neurons of the nervous system supply these cells.
• Proteins within a muscle cell are organized into units called myofibrils.
• Each myofibril is formed by long chains of thick filaments (myosin) and thin filaments (actin), organized into repeating segments called sarcomeres.
• Contraction relies on the structure of muscle fibers and surrounding nerves. A nerve impulse reaching a motor neuron causes a chemical message to pass into the muscle fiber.
• Thick and thin filaments of a sarcomere slide across each other to shorten the muscle, creating contraction.
• Myocytes: Long, cylindrical, striated, and multinucleate cells that compose skeletal muscle fibers.
  • Composed of sarcoplasm and numerous rod-like myofibrils and enclosed in sarcolemma.
• Neuromuscular Junction: Somatic motor neurons transmit action potentials through the transverse tubules of the sarcolemma to skeletal muscle fibers, causing voluntary contraction that moves the attached bones and skin.
• Each myofibril is enclosed in a sarcoplasmic reticulum and arranged in contractile sarcomeres that contain thick and thin myofilaments.
• Dark and light bands appearance when viewed with a microscope are due to thick and thin myofilaments.

Smooth Muscle

• Non-striated and contracts involuntarily in peristaltic waves that move substances through the digestive, respiratory, urinary, and reproductive tracts.
• Myocytes: Spindle-shaped, non-striated, and uninucleate cells composed of thick, thin, and intermediate filaments.
• Single-unit smooth muscle: Two or three layers of myocytes connected by gap junctions, forming muscle sheaths. Allows action potentials from autonomic motor neurons to spread among the myocytes.
  • Produces involuntary, coordinated contraction that propels blood through small blood vessels, food & fecal matter through the stomach and intestines, urine through the bladder, and offspring through the uterus.
• Multi-unit smooth muscle: Individual myocytes with their own motor neuron terminals, so they contract independently.
  • Propels blood through large arteries and air through the trachea and bronchi.

Smooth Muscle in Digestive System

• Stomach wall has three layers of smooth muscle tissue: circular, longitudinal, and oblique.
• Contraction of these muscle layers creates motion, which aids digestion and movement of food through the stomach.
• Inner oblique layer churns and breaks down food.
• Outer longitudinal layer moves the bolus toward the pylorus.
• Middle circular layer moves chyme through the pylorus into the duodenum.

Smooth Muscle in Blood Vessels

• Artery and vein walls have a layer of smooth muscle and elastic fibers called the tunica media, between the epithelial tunica intima and connective tunica externa (or tunica adventitia).
• Smooth muscle fibers are arranged circularly around the lumen and contract and relax to constrict and dilate the lumen, regulating blood flow and blood pressure.
• Tunica media is thicker in arteries than in veins because arteries maintain blood pressure and circulation.
• Since blood pressure is low in veins, they have thinner walls including a much thinner tunica media with less smooth muscle and elastic tissue and larger lumens than arteries.
• Veins hold more blood volume and offer less resistance to blood flow.

Cardiac Muscle

• Found only in the myocardium of the heart.
• Short, branched, striated, and uninucleate cells.
• Contain rod-like myofibrils composed of myosin and actin myofilaments arranged into contractile sarcomeres.
• Connected via intercalated discs, which allow action potentials from the heart's conduction system to spread among the fibers.
• Involuntary, coordinated, and rhythmic contraction maintains a constant heart rate and pumps blood into circulation.
• Cardiomyocytes: Short, branched, and striated cells, each with a central nucleus.
• Connected by intercalated discs: specialized junctions that contain:
  • Desmosomes: Anchor the cardiomyocytes together.
  • Gap junctions: Allow electrical signals to travel between the cardiomyocytes.

Muscles of the Body by Region

• The muscles of the body are classified by region, including the: abdomen, back, head, hip and gluteal region, lower limbs, upper limbs, neck, pelvis, thorax, and shoulder girdle.

Skeletal System

• The bones of the human skeletal system are divided into an axial region and an appendicular region.
• Axial skeleton: Bones of the vertebral column, thoracic cage, and skull and head region.
• Appendicular skeleton: 126 bones of the upper and lower limbs, shoulder girdle, and pelvic girdle.

External Intercostals

• Muscles of the thorax that extend from the tubercles in the posterior part of the ribs to the cartilages in the anterior part of the ribs.
• On the posterior thorax, their fibers run obliquely downward and laterally, and on the anterior thorax, their fibers run downward, forward, and medially.
• These muscles assist in elevating the ribs during forced inspiration.

Alimentary Canal

• The alimentary canal is a musculomembranous tube that extends from the mouth to the anus, where food is digested, nutrients are extracted, and waste is expelled.
• Approximately nine meters long, beginning with the orifice of the mouth, which opens into the oral cavity, where food is ingested and physical digestion through mastication with the teeth and chemical digestion from the secretions of salivary glands occurs.
• Food then passes to the pharynx, through the esophagus, and into the stomach.
• Stomach muscles perform mechanical digestion by churning ingested food as gastric secretions further break it down chemically into chyme, which is passed into the duodenum of the small intestine.
• In the duodenum, bile from the liver and exocrine secretions from the pancreas eventually pass through the common bile duct and are secreted into the duodenum, which aids digestion and neutralizes acid.
• Chyme then passes through the jejunum and the ileum, where the majority of digestion and nutrient absorption occurs, into the pouch-like cecum of the large intestine.
• In the colon, the majority of water reabsorption occurs, and stools form, these pass into the rectum to be expelled through the anal canal at the anus.

Bronchi

• The bronchi and its subdivisions are the major airways of the lower respiratory system.
• The bronchi begin at the right and left primary bronchus and divide into secondary bronchi of smaller diameter that enter the lungs. In the lungs, further branching results in still narrower secondary and tertiary bronchi that, in turn, subdivide into bronchioles.
• Inhaled air moves from the trachea into the right or left bronchi and passes into the branching smaller airways in the lungs secondary bronchi, tertiary bronchi, and higher order branches and bronchioles.
• These progressively smaller airways deliver oxygen-rich air to the lungs, where gas exchange occurs in tiny air sacs called alveoli.
• Exhaled oxygen-poor and carbon dioxide-rich air leaves the body by the reverse route.

Systemic Circulation

• The systemic circulation of the cardiovascular system manages the passage of blood from the heart through the blood vessels, veins and arteries, that distribute blood to and from the body.
• Systemic circulation delivers oxygenated blood from the left ventricle throughout the body and deoxygenated blood from the body into the right atrium of the heart.
• Oxygenated blood in the left atrium is the result of gas exchange during pulmonary circulation.
• This blood passes through the bicuspid mitral valve into the left ventricle, where it is then pumped out through the aortic valve into the aorta.
• At the arch of the aorta, the carotid and subclavian arteries diverge to supply blood to the head, neck, and upper limbs, and the descending aorta delivers blood to the organs of the abdomen and the lower half of the body.
• After the blood passes through capillary beds and gas and nutrient exchange occurs, the oxygen depleted venous blood is delivered back to the right atrium of the heart through the superior and inferior vena cava, completing the circuit.

Urinary System

• The urinary system consists of the kidneys, the ureters, the urinary bladder, and the urethra.
• The urinary system filters impurities and other unwanted or unneeded substances from blood and excretes these as fluid waste called urine.
• Urine, secreted by the kidneys passes through the ureters into the urinary bladder, where the urine is temporarily retained until it is discharged from the body through the urethra.
• Urine includes water, salts, acids and potentially toxic nitrogenous compounds formed during protein metabolism.
• Hormonal adjustments to the volume of water excreted in urine have a role in maintaining proper blood volume and, by extension, blood pressure.
• Blood filtering by the kidneys may also remove residues of antibiotics and other foreign substances.
• Blood enters the urinary system through the renal artery.
• The urinary system of the average adult filters about 200 quarts of blood a day and excretes approximately two quarts of waste.

Female Reproductive System

• The internal genitalia of the female reproductive system are situated mainly within the pelvis.
• They consist of the ovaries, the uterine tubes, the uterus, and the vagina. A fold of peritoneum called the broad ligament helps anchor all these structures.
• The uterine tubes, also called fallopian tubes or oviducts, convey ova from the ovaries to the cavity of the uterus. Projections called fimbriae direct ova into the funnel-shaped end of each tube.
• The uterine tubes open, one on either side, into the broad, upper part of the uterus, which is situated between the bladder and the rectum.
• The lower part of the uterus opens into the cavity of the vagina, the only part of the internal female genitalia that is outside the pelvis.

Pectoralis Major

• A thick, fan-shaped muscle that extends from the anterior shoulder girdle and covers the upper chest.
• It assists in movements of the shoulder and arm and is composed of three heads: the clavicular, sternocostal, and abdominal heads.
• Clavicular head, origin: medial.

Brachial Plexus

• A complex intertwining of nerves that is responsible for cutaneous and muscular innervation of the shoulder and upper limb, with the exception of the trapezius muscle and an area of skin near the axilla.
• Derives from the ventral rami of the lower four cervical spinal nerves (C05-C08) and a small contribution from the first thoracic spinal nerve (T01)
• These structures form part of the roots of the brachial plexus and give rise to the trunks of the brachial plexus middle trunk and lower trunk. As they pass beneath the clavicle, each trunk splits into an anterior division and a posterior division
• The anterior divisions of the trunks unite to form three cords of the brachial plexus, named for their position relative to the axillary artery, the lateral cord of the brachial plexus formed by the union of the anterior divisions of the upper and middle trunks, the medial cord of the brachial plexus that is the anterior division of the lower trunk, and the posterior cord of the brachial plexus formed by the union of the posterior divisions of all three trunks.
• The cords produce non-terminal branches and terminal branches that become the nerves in the arms, terminating in the nerves of the hands.

Thoracoacromial Trunk

• A short trunk that branches from the axillary artery.
• It pierces the coracoclavicular fascia and divides into the pectoral acromial deltoid and clavicular branches.
• The pectoral branch descends between the pectoralis major and minor, anastomosing with the intercostal branches of the internal thoracic and the lateral thoracic arteries.
• The acromial branch runs laterally over the coracoid process and under the deltoid muscle, to which it gives branches.
• The deltoid branch, often arising with the acromial, crosses over the pectoralis minor and passes in the same groove as the cephalic vein between the pectoralis major and deltoid, branching to both muscles.
• The clavicular branch runs medially along the pectoralis minor towards the sternal end of the clavicle.

Smooth Muscle Tissue Function

• Smooth muscle tissue is in the walls of hollow organs throughout the body.
• The tissue contracts and relaxes to contain substances and move substances through the body.
• Smooth muscle contractions are involuntary actions managed by impulses that travel through the autonomic nervous system to the smooth muscle tissue.
• The arrangement of cells in smooth muscle tissue provides for contraction and relaxation with great elasticity.
• The smooth muscle in the digestive tract, contracts and relaxes in peristaltic waves that move swallowed food and nutrients from the mouth through the stomach and the intestines.
• Artery walls include smooth muscle that relaxes and contracts as pressure changes move changing volumes of blood through the vessel.
• The smooth muscle of the bladder and uterus allow for contraction of those organs.

Heart Wall

• The heart wall is composed of three distinct layers that help give the heart size, shape, and weight.
  • The outermost layer is the epicardium, a visceral layer of serous pericardium.
  • The middle layer is the myocardium, made up of muscular fibers attached to fibrous rings.
  • The innermost layer is the endocardium, a thin, smooth membrane
• Blood is distributed throughout the body as a result of the contractions of the muscles in the myocardium, which create the pumping action that distributes blood throughout the cardiovascular system.
• The contractions are triggered by electrical impulses delivered by the heart's conduction system.
• Some important structures of the interior of the heart include the conus arteriosus, a conical pouch formed by the upper and the interventricular septum The external surfaces of the heart can be designated as the diaphragmatic surface, the posterior portion directed downward and slightly backward that is formed by the ventricles, and the sternocostal surface, the anterior portion that is directed forward, upward, and to the left that is formed mostly by the right ventricle.

Coronary Vessels

• Coronary vessels, arteries, and veins, provide circulation throughout the heart.
• The coronary arteries provide blood supply to heart tissue.
• The coronary veins carry deoxygenated blood from the heart tissue into the right atrium.

Conduction System of the Heart

• A system within the heart that is controlled by the autonomic nervous system and that delivers electrical impulses and motivates the rhythmic contractions of the heart.
• Each contraction, or heartbeat, pumps blood throughout the cardiovascular system.
• The pathways for the electrical impulses are formed by a series of bundles of specialized muscle fibers within the heart: sinoatrial node, atrioventricular node, atrioventricular bundle of His, left and right bundle branches, and Purkinje fibers.
• There are distinct steps to each electrical impulse, these result in the contraction of the heart's upper chambers followed by the contraction of the heart's lower chambers.
• The steps are:
  1. Initiation of the electrical impulse at the sinoatrial node
  2. The pause of the impulse at the atrioventricular node
  3. The passage of the impulse into the bundle of His
  4. The branching of the signal into the bundle branches of each ventricle
  5. The culmination of the signal at the Purkinje fibers.
• Between steps one and two, the atria contract, pumping blood into the ventricles.
• Between steps four and five, the ventricles contract, pumping blood out of the heart.
• Each electrical impulse of the conduction system takes approximately 0.22 to complete this cycle.

Layers of the Heart Wall

• The heart wall is composed of three distinct layers that help give the heart size, shape, and weight.
  • The outermost layer is the epicardium, a visceral layer of serous pericardium the middle layer is the myocardium, made up of muscular fibers attached to fibrous rings, the innermost layer is the endocardium, a thin, smooth membrane of connective tissue and elastic fibers that lines the inner surface of the heart.
• Blood is distributed throughout the body as a result of the contractions of the muscles in the myocardium, which create the pumping action that distributes blood throughout the cardiovascular system.
• The contractions are triggered by electrical impulses delivered by the heart's conduction system.

Structures of the Interior of the Heart

• Some important structures of the interior of the heart include:
  • The conus arteriosus, a conical pouch formed by the upper and left portion of the right ventricle that gives rise to the pulmonary trunk,
  • The interventricular septum and the sternocostal surface, the anterior portion that is directed forward, upward, and to the left that is formed mostly by the right ventricle.

Superior Vena Cava

• Drains blood from the upper half of the body.
• This large vessel is formed by the junction of the two brachiocephalic veins on either side of the root of the neck.
• It begins immediately below the cartilage of the right first rib close to the sternum, and descends vertically behind the first and second intercostal spaces to end at its base in the upper part of the right atrium opposite the third right costal cartilage.
• Just before it pierces the pericardium, it receives the azygos vein and several small veins from the pericardium. The superior vena cava has no valves.

Myocardium

• The middle layer of the heart wall.
• It is made up of striated muscle fibers, similar to skeletal muscle tissue.
• The myocardium is responsible for the heart's pumping action, making powerful, continuous contractions possible.
• Its muscle fibers stretch as the atria and ventricles fill with blood, and contract involuntarily in response to signals from the heart's own conduction system.