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The muscular system - Coggle Diagram
The muscular system
Muscle Types
There are three types: skeletal, smooth and cardiac.
- First, skeletal and smooth muscles are elongated. For this reason, these types of muscle cells (but not cardiac muscle cells) are called muscle fibres.
Second, the ability of muscle to shorten, or contract, depends on two types of myofilaments, the muscle cell equivalents of the microfilaments of the cytoskeleton.
Skeletal Muscle
Skeletal Muscle Fibres are packaged into organs called skeletal muscles that attach to the skeleton. As the skeletal muscles cover our bones and cartilage framework, they help form the smooth contour of the body.
- Skeletal muscle fibres are large, cigar-shaped, multinucleate cells.
- Skeletal muscles are also known as striated muscle ( because their fibres have stripes) and also known as voluntary muscle (because it is the only muscle type subject to conscious control).
- The reason that skeletal muscles are not ripped apart as they exert force is that connective tissue bundles thousands of their fibres together, which strengthens and supports the muscle as a whole.
- Each muscle fibre is enclosed in a delicate connective tissue sheath called endomysium. Several sheathed muscle fibres are then wrapped by a coarser fibrous membrane called perimysium to form a bundle of fibres called a fascicle. Many fascicles are bound together by an even tougher overcoat of connective tissue called an epimysium which covers the entire muscle. The ends of the epimysium that extend beyond the muscle (like wrapper on a piece of candy) blend either into a strong cordlike tendon or a sheetlike aponeurosis which indirectly attaches the muscle to bone, cartilage or another connective tissue covering.
- The most important functions of the tendons include durability and conserving space. Tendons are mostly tough collagen fibres so they can cross rough bony projections, which would tear the more delicate muscle tissue.
Smooth Muscles
Smooth muscles has no striation and are involuntary, which means that we cannot consciously control it. Found mainly in the walls of hollow (tubelike) organs such as the stomach, urinary bladder, and respiratory passages, smooth muscle propels substances along a pathway. Think of smooth muscle as visceral, nonstriated and involuntary.
- Smooth muscle fibres are spindle-shaped, uninucleate and surrounded by scant endomysium. They are arranged in layers and most often there are two such layers, one running circularly and the other longitudinally. As the two layers alternately contract and relax, they change the size and shape of the organ.
- Smooth Muscle contraction is slow and sustained.
Cardiac Muscle
- Found only in one part of the body- the heart, where it forms the bulk of the heart walls.
The heart serves as a pump, propelling blood through blood vessels to all body tissues. Like skeletal muscle, cardiac muscle is striated and like a smooth muscle, it is uninucleated and under involuntary control.
- The cardiac cells are cushioned by small amounts of endomysium and are arranged in spirals or figures.
- When the heart contracts, its internal chambers become smaller, forcing blood on a one-way path through the chambers and into large arteries leaving the heart. Cardiac muscle fibres are branching cells joined by special gap junctions called an intercalated disc. These two structural features and the spiral arrangement of the muscle bundles in the heart allow heart activity to be closely coordinated.
The cardiac muscle usually contracts at a steady rate set by the heart "in-house" pacemaker.
Muscle Function
Produce Movement:
- Skeletal muscles are responsible for our body's mobility including all locomotion (walking, swimming and cross-country skiing for instance) and manipulating things with your agile upper limbs. They enable us to respond quickly to changes in the external environment. They also allow us to express our emotions with the silent language of facial expression. They are distinct from the smooth muscle of blood vessel walls and cardiac muscle of the heart, which work together to circulate blood and maintain blood pressure and the smooth muscle of other hollow organs which forces fluid (urine, bile) and other substance(food, a baby) through internal body channels.
Maintain Posture and Body Position:
- Skeletal Muscles that maintain body posture yet they function almost continuously making one tiny adjustment after another so that we maintain an erect or seated posture, even when we slouch, despite the never-ending downward pull of gravity.
Stabilise Joints:
- As skeletal muscles pull on bones to cause movements, they also stabilise the joints of the skeleton. Muscles and tendons are extremely important in reinforcing and establishing joints that have poorly articulating surfaces, such as the shoulder and knee joints.
Generate Heat:
- Muscle activity generates body heat as a by-product of contraction. As ATP is used to power muscle contraction, nearly three-quarters of its energy escapes as heat. This heat is vital in maintaining normal body temperature.
Additional Functions:
- Smooth muscles form valves that regulate the passage of substances through the internal body openings, dilate and constrict the pupils of our eyes and make up the arrector pili muscles that cause our hairs to stand on end. Skeletal muscles form valves that are under voluntary control, and they enclose and protect fragile internal organs.
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The essential function of muscle is to contract or shorten. As a result of this ability, muscles are responsible for all body movement and can be viewed as the machine of the body.
Special functional properties of skeletal muscles:
- The first is irritability also called responsiveness which is the ability to receive and respond to a stimulus.
- The second, contractility, which is the ability to forcibly shorten when adequately stimulated.
- Extensibility is the ability of muscle fibers to stretch whereas elasticity is their ability to recoil and resume their resting length after being stretched.
The Nerve stimulus and action potential:
- To contract, skeletal muscle fibres must be stimulated by nerve impulses. One motor neuron may stimulate a few muscle fibres or hundreds of them, depending on the particular muscle and the work it does.
- A motor unit consists of one neuron and all the skeletal muscle fibres it stimulates. When the long, threadlike extension of a neuron called the axon reaches the muscle, it branches into several axon terminals, each of which forms junctions with the sarcolemma of different muscle fibers.
- These junctions called neuromuscular junctions, contain synaptic vesicles filled with chemicals referred to as a neurotransmitter. The specific neurotransmitter that stimulates skeletal muscle fibres is acetylcholine.
- Although the nerve endings and the muscle fibre membranes are very close, they never touch. The gap between them, the synaptic cleft, is filled with interstitial fluid.
Events at the neuromuscular junction
Step 1: Nerve impulse reaches the axon terminal of the motor neuron
Step 2: Calcium channels open, and calcium ions enter the axon terminal
Step 3: Calcium ion entry causes some synaptic vesicles to release acetylcholine
Step 4: ACh diffuses across the synaptic cleft and attaches to receptors on the sarcolemma of the muscle cell.
Step 5: If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium ions. More sodium ions enter than potassium ions leave. The entry of sodium ions produces an imbalance in which the interior has more positive ions (depolarisation), thereby opening more sodium channels.
Step 6: Depolarisation opens more sodium channels that allow sodium ions to enter the cell. An action potential is created. Once begun, the action potential is unstoppable. Conducts the electrical impulse from one end of the cell to the other.
Step 7: Acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid and choline. AChE ends muscle contraction. A single nerve impulse produces only one contraction.
What causes the filaments to slide?
- The formation of a cross-bridge- when the myosin heads attach to actin- requires calcium ions and ATP (to energise the myosin head).
- Action potential passes deep into the muscle fibre along membranous tubules that fold inward from the sarcolemma. Inside the cell, in a process called excitation-contraction coupling, the action potential stimulates the sarcoplasmic reticulum to release calcium ions into the cytoplasm. The calcium ions trigger the binding of myosin to actin, initiating filament sliding. When the action potential ends, calcium ions are immediately returned to the SR storage areas, the regulatory proteins return to their resting shape and block myosin-binding sites, and the muscle fibre relaxes and settles back to its original length. This whole series of events take a few thousandths of a second. When the nervous system activates muscle fibers, the myosin heads attach to binding sites on the thin filaments and the sliding begins. Each cross-bridge attaches and detaches several times during a contraction, generating tension that helps pull the thin (actin) filaments towards
The Sliding Filament Theory:
- In a relaxed muscle fibre, the regulatory proteins forming part of the actin myofilaments block and prevent myosin binding. When an action potential sweeps along its sarcolemma and a muscle fibre is excited, calcium ions are released from the intracellular storage area (the sacs of the sarcoplasmic reticulum).
- The flood of calcium acts as the final trigger for contraction because as calcium binds to the regulatory proteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thin filaments. This action exposes myosin-binding sites on the actin, to which the myosin heads can attach and the myosin heads immediately begin seeking out binding sites. The binding of myosin to actin constitutes cross-bridge formation.
- Using energy from ATP, free myosin heads are cocked much like an oar ready to be pulled on for rowing. Myosin attachment to actin causes the myosin heads to snap (pivot) towards the center of the sarcomere in a rowing motion. When this happens, the thin filaments are pulled slightly towards the center of the sarcomere. ATP provides the energy needs to release and recock each myosin head so that it is ready to attach to a binding site farther along the thin filament.
Contraction of a skeletal muscle as a whole
Graded response:
The whole muscle reacts to stimuli with graded responses, or different degree of shortening, which generates different amount of force. In general, graded muscle contractions can be produced two ways: one by changing the frequency of muscle stimulation and two by changing the number of muscle fibres being stimulated at one time.
Muscle fibre contraction is "all or none" meaning the muscle fibre (not the whole muscle) will contract to its fullest extend when stimulated adequately.
Within a whole skeletal muscle, not all fibres may be stimulated during the same interval. Different combinations of muscle fibre contractions may give different responses . Muscle Response to Increasingly rapid stimulation:
- Although muscle twitches (single, brief, jerky contractions) sometimes results from certain nervous system problems, this is not the way our muscles normally operate. In most type of muscle activity nerve impulses are delivered to the muscle at a very rapid rate - so rapid that the muscle does not get a chance to relax completely between stimuli. As a result, the effect of the successive contractions are summed (added) together and the contractions of muscle get stronger and smoother.
- The muscle exhibits unfused tetanus or incomplete tetanus. When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained, the muscle is in fused tetanus, or complete tetanus or in tetanic contraction.
Muscle response to stronger stimuli
- Tetanus produces stronger (more forceful) muscle contractions but its primary role is to produce smooth and prolonged muscle contractions. How forcefully a muscle contracts depends to a large extent on how many of its cells are stimulated. When all the motor units are active and all muscle fibers are stimulated, the muscle contractions is as strong as it can get.
Providing Energy for Muscle Contraction
- As a muscle contract, the bonds of ATP molecules are hydrolysed to release the needed energy.
ATP:
- An only energy source that can be used to directly power muscle contractions.
- Stored in muscle fibres in small amounts that are quickly used up
- After this initial time, other pathways must be utilised to produce ATP.
- Must generate ATP continuously during muscle contraction.
Working muscles use three pathways to regenerate ATP.
- Direct Phosphorylation of ADP by creatine phosphate. This unique high energy molecule, creatine phosphate (CP), is found in muscle fibres but not in other cell types. As ATP is depleted, interactions between CP and ADP result in transfers of a high energy phosphate group from CP to ADP, thus regenerating more ATP in a fraction of a second.
- Aerobic pathway. At rest and during light to moderate exercise, 95 per cent of the ATP used for muscle activity comes from aerobic respiration. Aerobic respiration occurs in the mitochondria and involves a series of metabolic pathways that uses oxygen. These pathways are collectively referred to as oxidative phosphorylation. During aerobic respiration, glucose is broken down completely to carbon dioxide, and water and some of the energy released as the bonds are broken is captured in the bonds of ATP molecules. Although aerobic respiration provides a rich ATP harvest, it is relatively slow and requires continuous delivery of oxygen and nutrient fuels to the muscle to keep it going.
- Anaerobic glycolysis and lactic acid formation. The initial steps of glucose breakdown occur via a pathway called glycolysis which does not use oxygen and hence is anaerobic. During glycolysis, which occurs in the cytosol, glucose is broken down to pyruvic acid, and small amounts of energy are captured in ATP bonds. As long as enough oxygen is present, the pyruvic acid then enters the oxygen-requiring aerobic pathway that occurs within the mitochondria to produce more ATP. However, when muscle activity is intense, or oxygen and glucose delivery is temporarily inadequate to meet the needs of working muscles, the sluggish aerobic pathway cannot keep up with the demands for ATP. Under these conditions, the pyruvic acid generated during glycolysis is converted to lactic acid, and the overall process is referred to as anaerobic glycolysis.
Types of Body Movement
- Every skeletal muscle is attached to bone, or to other connective tissue structures at no fewer than two points. One of these points, the origin is attached to the immovable or less movable bone. Think of the origin as the anchor or leverage point. Another point, the insertion is attached to the moveable bone. When the muscle contracts, the insertion moves towards the origin. Some muscles have interchangeable origins and insertions, depending on the action being performed.
- Body movements occur when muscles contract across joints. The type of movement depends on the mobility of the joint and the location of the muscle in relation to the joint.
The common types of body movements:
- Flexion: Flexion is a movement, generally in the sagittal plan, that decreases the angle of the joints and brings two bones closer together. Flexion is typical of hinge joints (bending the knew or elbow), but it is also common at ball and socket joints.
- Extension: Extension is the opposite of flexion so it is a movement that increases the angle or distance, between two bones or parts of the body (straightening the knee or elbow). Extension beyond 180 degrees is hyperextension.
- Rotation: Rotation is the movement of a bone around its longitudinal axis. Rotation is a common movement of ball and socket joint and describes the movement of the atlas around the dens of the axis.
- Abduction: Abduction is moving a limb away (generally in frontal plane) from the midline or median plane of the body. The terminology also applies to the fanning movement of you fingers or toes when they are spread apart.
- Adduction. Adduction is the opposite abduction, so it is the movement of a limb towards the body midline.
- Circumduction. Circumduction is a combination of flexion, extension, abduction and adduction commonly seen in ball and socket joints, such as the shoulder. The proximal ends of the limb is stationary and its distal end moves in a circle.
The special movements:
- Dorsiflexion and plantar flexion. Up-and-down movement of the foot at the ankle are given special names. Lifting the floor so that its superior surface approaches the skin (pointing your toes towards your head) is dorsiflexion, whereas pointing the toes away from your head is plantar flexion. Dorsiflecion of the foot corresponds to extension and hyperextension of the hand at the wrist whereas plantar flexion of the foot corresponds to flexion of the hand.
- Inversion and eversion. Inversion and eversion are also special movements of the foot. To invert the foot, turn the sole medially, as if you were looking at the bottom of your foot. To evert the foot, turn the sole laterally.
- Supination and pronation. The terms supination and pronation refer to movements of the radius around the ulna. Supination occurs when the forearm rotates laterally so that the palm faces anteriorly (or up) and the radius and ulna are parallel as in anatomical position. Pronation occurs when the forearm rotates mediallt so that the palm faces posteriorly (or down). Pronation brings the radius across the ulna so that two bones form an X.
- Opposition: In the palm of the hand, the saddle joints between metacarpal 1 and carpals allows opposition of the thumb. This is the action by which you move your thumb to touch the tips of the other fingers on the same hands.
The Five Golden Rules of Skeletal Muscle Activity
- With a few exceptions, all skeletal muscles cross at least one joint.
- Typically, the bulk of a skeletal muscle lies proximal to the joint crossed
- All skeletal muscles have at least two attachments; the origin and the insertion.
- Skeletal muscles can only pull; they never push
- During contraction, a skeletal muscle insertion moves towards the origin.
Interactions of skeletal muscles in the body:
- Muscles can't push- they only pull as they contract- so most often body movements result from two or more muscles acting together or against each other. Group of muscles that produce opposite movements lie on opposite sides of a joint.
- The muscle that has the major responsibility for causing particular movement is called the prime mover. Muscles that oppose or reverse a movement are antagonist. When a prime mover is active, its antagonist is stretched and relaxed. Antagonists can be prime movers in their own right, but for different actions.
- Synergists help prime movers by producing the same movement or by reducing undesirable movements. When a muscle crosses two or more joints, its contraction will cause movement in all the joints crossed unless synergists are there to stabilise them.
- Fixators are specialised synergists. They hold a bone still or stablise the origin of a prime mover so all the tension can be used to move the insertion bone. The postural muscles that stabilise the vertebral column are fixators, as are the muscles that anchor the scapular to the thorax.
Naming Skeletal Muscles:
-Direction of the muscle fibres: Some muscles are named in reference to some imaginary line, usually the midline of the body or the long axis of a limb bone. When a muscle's name includes the term rectus( straight), its fibres or whole structure run parallel to that imaginary line.
- Relative size of the muscle. Such terms as maximum (largest), minimums (smallest) and longus (long) are sometimes used in the name of muscles.
- Location of the muscle. Some muscles are named for the bone with which they are associated.
- Number of origins. When the term biceps, triceps or quadriceps forms part of a muscle name, you assume that the muscle has two, three or four origins, respectively.
- Location of the muscle's origin and insertion. Occasionally muscles are named for their attachment sites.
- Shape of the muscle. Some muscles have a distinctive shape that helps to identify them.
- Action of the muscle. When muscles are named for their actions, terms such as flexor, extensor, and adductor appear in their names.
Arrangement of Fascicles
- Skeletal muscles consist of fascicles, but fascicle arrangement varies, producing forces with different structures and functional properties.
Common patterns of fascicle arrangement:
- In a circular pattern, the fascicles are arranged in concentric rings. Circular muscles are typically found surrounding external body openings, which they close by contracting, creating a valve. Examples are the orbicularis muscles surrounding the eye and mouth.
- In convergent muscle, the fascicles converge towards a single insertion tendon. A convergent muscle is triangular or fan-shaped, such as the pectoralis major muscle of the anterior thorax.
- In parallel arrangement, the length of the fascicles run parallel to the long axis of the muscle as in the sartorius of the anterior thigh. These muscles are straplike. A modification of the parallel arrangement called fusiform, results in a spindle-shaped muscle with an expanded belly (midsection) and tapered ends.
- In a pennate pattern, short fasicles attach obliquely to a central tendon. In the extensor digitorum muscle of the leg, the fascicles insert into only one side of the tendon, and the muscle is unipennate. If the fascicles insert into opposite sides of the tendon, the muscle is bipennate. If the fascicles insert from several different sides, the muscle is multipennate.