Locomotion and Movement | ISC Class 11 Biology Notes

This note covers movement and locomotion, muscle properties and types, skeletal muscle organisation, contractile proteins, muscle contraction and relaxation, red and white fibres, the human skeleton, joints, summation, tetanus, rigor mortis, and disorders of muscles and bones.

How do movement and locomotion differ?

Definition: Locomotion is voluntary movement that changes an animal's place or location. All locomotions are movements, but all movements are not locomotions.

Movement includes changes in the position of body parts without a change in the location of the whole organism. Moving the eyelids, tongue or jaws illustrates movement. Walking, running, climbing, flying and swimming are forms of locomotion.

The same structures can serve both purposes. Human limbs help change body posture and also bring about locomotion. In Hydra, tentacles capture prey and also help in locomotion. In Paramoecium, cilia move food through the cytopharynx, its feeding passage, and help in locomotion.

What produces the different movements?

Cilia and flagella are outgrowths of the cell membrane. Ciliary movement occurs in most internal tubular organs lined by ciliated epithelium, a covering tissue bearing cilia. In the trachea, or windpipe, coordinated ciliary movements help remove dust particles and some inhaled foreign substances.

Ciliary movement also facilitates passage of ova, the female reproductive cells, through the female reproductive tract. Flagellar movement helps spermatozoa, the male reproductive cells, swim. It also maintains water currents in the canal system of sponges and enables locomotion in Euglena.

Amoeboid movement occurs through pseudopodia, temporary projections formed by streaming of the cell's living contents. Some specialised human cells, including macrophages, cells that engulf material, and leucocytes, white blood cells, show it. Microfilaments, fine components of the cell's supporting framework, also participate.

Muscular movement uses the ability of muscles to contract. It moves limbs, jaws and the tongue. Human locomotion requires coordinated activity of the muscular, skeletal and neural systems, meaning muscles, the supporting skeleton and nervous control working together.

Locomotion is generally associated with finding food, shelter, a mate, suitable breeding grounds or favourable climatic conditions, or escaping enemies. The method varies with habitat and circumstances; a change of place does not require a separate structure used exclusively for locomotion.

What properties distinguish muscles and their main types?

Muscle is a specialised tissue derived from mesoderm, the middle embryonic tissue layer. About 40 to 50 per cent of an adult human's body weight is contributed by muscles. Their properties explain how stimulation can produce useful movement.

• Excitability: the ability to respond to a stimulus, such as a nerve signal.
• Contractility: the ability to develop tension and bring about contraction.
• Extensibility: the ability to be stretched.
• Elasticity: the ability to return towards the original form after stretching or contraction.

How do skeletal, visceral and cardiac muscles compare?

Muscles can be classified by location, appearance and control. Striated means having a striped appearance under the microscope. Voluntary means under voluntary nervous control; involuntary means that activity is not under such control. These are separate criteria.

Skeletal muscles are primarily involved in locomotory actions and changes of posture. Visceral muscles assist the transport of food through the digestive tract and gametes, or reproductive cells, through the genital tract. Their location explains the name visceral.

Cardiac muscles consist of cells assembled in a branching pattern. They are striated but involuntary: the nervous system does not directly control their activities. A striped appearance therefore cannot, by itself, establish whether a muscle is voluntary.

How is a skeletal muscle organised into sarcomeres?

A skeletal muscle contains bundles called fascicles. A common layer of collagenous connective tissue, called fascia, holds the bundles together. Collagen is a structural protein of connective tissue. Each bundle contains muscle fibres, which are muscle cells.

The sarcolemma is the muscle fibre's cell membrane. It encloses the sarcoplasm, its cellular contents outside the nuclei. A skeletal muscle fibre is a syncytium, a shared mass of cytoplasm containing many nuclei. Its sarcoplasmic reticulum is a membrane system that stores calcium ions.

Numerous parallel myofibrils, the contractile fibrils within the fibre, show alternate light and dark bands. Their appearance reflects the arrangement of actin and myosin, the principal contractile proteins. Actin forms thin filaments; myosin forms thicker filaments.

The cutaway drawing shows bundles within a muscle and individual fibres projecting from a bundle. Labels identify a fascicle, muscle fibre, sarcolemma and blood capillary, a small blood vessel.

See Fig. 17.1 in your NCERT textbook

What do the bands and lines identify?

The letters I and A stand for isotropic and anisotropic, referring to different optical properties. Z, M and H identify the structural regions defined above. At rest, thin filaments partially overlap the ends of thick filaments while leaving the central H zone.

A sarcomere includes a central A band and half an I band at each end. Repeated sarcomeres form a myofibril. The muscle fibre is the anatomical unit of muscle, whereas the sarcomere is its functional unit of contraction.

The upper panel is a microscopic photograph showing alternating bands. The lower drawing labels the Z lines, A band, I band, H zone and the sarcomere extending between two Z lines.

See Fig. 17.2 in your NCERT textbook

How do actin and myosin support contraction?

What is present in a thin filament?

Each thin filament contains two F actin strands wound helically around each other. F means filamentous. Each strand is a polymer, a chain assembled from repeating units, of G actin. G means globular and describes the individual actin units.

Two strands of tropomyosin, another protein, run close to F actin along its length. Troponin is a complex protein distributed at regular intervals on tropomyosin. In the resting state, a troponin subunit masks the active sites where myosin can bind to actin.

What is present in a thick filament?

A thick filament is built from many myosin units called meromyosins. Each has a globular head with a short arm, together called heavy meromyosin, and a tail called light meromyosin. The projecting head and short arm form a cross arm.

The head has binding sites for actin and ATP, adenosine triphosphate, an energy-transferring molecule. It acts as ATPase, an enzyme that catalyses ATP breakdown. Hydrolysis, breakdown involving water, releases energy used during the cycle of interaction between myosin and actin.

ATP breakdown produces ADP, adenosine diphosphate, and inorganic phosphate, a phosphate group released from ATP. These names distinguish the energy-containing molecule that binds to myosin from the products released during the contraction cycle.

The thin-filament drawing labels F actin, tropomyosin and troponin. The myosin-unit drawing shows its head and cross arm, with separate actin-binding and ATP-binding sites indicated on the head.

See Fig. 17.3 in your NCERT textbook

The distinction between a cross arm and a cross bridge matters. The cross arm is a projecting part of myosin. A cross bridge forms when a myosin head binds to an exposed active site on actin.

How does a nerve signal initiate muscle contraction?

The central nervous system, comprising the brain and spinal cord, initiates skeletal muscle contraction through a motor neuron, a nerve cell carrying the signal to muscle. A motor neuron and the muscle fibres connected to it together form a motor unit.

The contact between the motor neuron and the sarcolemma is the neuromuscular junction, also called the motor-end plate. Here acetylcholine acts as a neurotransmitter, a chemical messenger carrying the signal across the junction to the muscle fibre.

What is the sequence from stimulation to binding?

1. A signal from the central nervous system reaches the neuromuscular junction through the motor neuron.
2. The arriving signal causes release of acetylcholine, which generates an action potential, an electrical change that travels along the sarcolemma.
3. This electrical excitation spreads through the muscle fibre and causes calcium ions to be released from the sarcoplasmic reticulum into the sarcoplasm.
4. The increased calcium level causes calcium to bind to a subunit of troponin on the thin filament.
5. This removes the masking of actin's active sites, enabling myosin heads to bind and form cross bridges.

Calcium ions are electrically charged calcium particles. The notation Ca²⁺ means a calcium ion with two positive charges. Calcium links electrical excitation to the exposure of binding sites; it is not the molecule supplying energy for the cross-bridge cycle.

The signal, messenger, calcium store and contractile proteins perform different roles. The neuron delivers the signal, acetylcholine initiates electrical excitation in the muscle membrane, and released calcium permits actin-myosin interaction. ATP supplies energy for the subsequent mechanical cycle.

Note: A motor unit includes a neuron and its connected muscle fibres. A neuromuscular junction is the contact at which a motor neuron communicates with a muscle fibre. Neither term means a sarcomere.

How do sliding filaments produce contraction and relaxation?

The sliding filament theory explains contraction as the sliding of thin filaments over thick filaments. The relative positions and overlap of the filaments change, bringing the Z lines closer together and shortening the sarcomere.

How does the cross-bridge cycle proceed?

1. Using energy from ATP hydrolysis, a myosin head binds to an exposed site on actin and forms a cross bridge.
2. The head pulls the attached thin filament towards the centre of the A band. The Z line attached to that filament is also pulled inwards.
3. ADP and inorganic phosphate are released as the myosin head completes its movement and returns towards its relaxed state.
4. A new ATP molecule binds to myosin, breaking the cross bridge between myosin and actin.
5. ATP is hydrolysed again by the myosin head, and repeated cross-bridge formation and breakage bring about further sliding.

The I bands shorten, while the A bands retain their length. The H zone narrows as overlap increases. The change is in filament overlap and sarcomere length; a shortening muscle does not require its thick filaments to become shorter.

Three drawings show two sarcomeres relaxed, contracting and maximally contracted. Z lines move closer together and the I bands and H zones diminish, while the A-band span is retained.

See Fig. 17.5 in your NCERT textbook

How is the resting arrangement restored?

1. Calcium ions are pumped back into the storage spaces of the sarcoplasmic reticulum.
2. The fall in available sarcoplasmic calcium restores the masking of active sites on actin.
3. Further cross-bridge cycling stops as the actin sites become unavailable to myosin.
4. The Z lines return to their original positions, restoring the relaxed sarcomere arrangement.

Relaxation therefore involves restoring calcium storage as well as ending productive actin-myosin interaction. The reaction time of fibres can vary between muscles. Contraction and relaxation should be traced through both the chemical events and the resulting positions of the filaments.

How do red fibres, white fibres and repeated stimulation differ?

Myoglobin is the red, oxygen-storing pigment in muscle. Some muscles contain much of it and appear reddish; their fibres are called red fibres. Other muscles contain very little myoglobin and appear pale or whitish; their fibres are called white fibres.

How do the two fibre types obtain energy?

Mitochondria are cellular structures involved in ATP production. Red fibres contain plenty of mitochondria, which can use their substantial stored oxygen for ATP production. They are also called aerobic muscles. Aerobic energy production uses oxygen; anaerobic energy production does not require oxygen.

White fibres also have a high amount of sarcoplasmic reticulum. Repeated activation of muscles can lead to accumulation of lactic acid through anaerobic breakdown of glycogen, a stored carbohydrate, causing fatigue. Fatigue is a reduction in the muscle's capacity to sustain its activity.

What are summation, tetanus and rigor mortis?

Summation is the increased force of contraction when a further stimulus reaches a muscle before it has completely relaxed from the previous response. The mechanical responses add together. A single brief contraction followed by relaxation is called a muscle twitch.

Tetanus, in muscle physiology, is a sustained contraction produced by rapidly repeated stimulation. When individual responses fuse completely, relaxation between successive responses is absent. It describes the result of stimulation frequency, whereas summation describes the addition of responses.

Rigor mortis is stiffness of muscles after death. ATP becomes unavailable for detaching myosin from actin, so attached cross bridges persist. It differs from tetanus, which depends on repeated stimulation of living muscle, and from tetany, a disorder involving muscle spasms.

What are the functions and divisions of the human skeleton?

The skeletal system is a framework of bones and a few cartilages. Bone and cartilage are specialised connective tissues, tissues that support and connect body structures. Their surrounding material is called the matrix. Bone has a very hard matrix because of calcium salts.

Cartilage has a slightly pliable matrix because of chondroitin salts. Pliable means capable of bending. The contrast between hard bone and slightly pliable cartilage helps distinguish the two tissues without treating cartilage as a liquid or as an unmineralised empty space.

How do the two skeletal divisions compare?

The axial skeleton lies along the body's main axis. The appendicular skeleton comprises the limbs and the girdles connecting them to the axial framework. The 126 appendicular bones are the remainder when 80 axial bones are subtracted from 206.

How does the framework support movement?

The skull protects the brain. The vertebral column supports the head, protects the spinal cord and provides attachment for ribs and back muscles. Limb bones and the jaw provide skeletal components needed for movements such as walking and chewing.

Muscles generate force, bones provide the framework, and joints permit movement at the contacts between skeletal elements. A joint is a point of contact between bones or between bone and cartilage. Movement depends on these parts working together under nervous control.

Which bones form the axial skeleton?

How are the skull and vertebral column arranged?

The skull comprises 22 bones: eight cranial bones form the cranium, the brain's protective covering, and 14 facial bones form the front of the skull. A U-shaped hyoid bone lies at the base of the buccal cavity, or mouth cavity.

The eight cranial bones are one frontal, two parietal, two temporal, one occipital, one sphenoid and one ethmoid. These names identify individual bones or paired bones of the protective cranial region.

The 14 facial bones are two maxillae, two zygomatic, two nasal, two lacrimal, two palatine, two inferior nasal conchae, one vomer and one mandible. Maxillae form the upper jaw; the mandible is the lower jaw bone.

The side-view drawing labels frontal, parietal, temporal, occipital, sphenoid and ethmoid bones, together with lacrimal, nasal and zygomatic bones, maxilla and mandible. The occipital condyle and hyoid bone are also labelled.

See Fig. 17.6 in your NCERT textbook

Each middle ear contains three small ear ossicles: malleus, incus and stapes. The skull articulates, or forms a joint, with the vertebral column through two occipital condyles, rounded joint-forming projections. A skull with these two condyles is described as dicondylic.

The vertebral column consists of 26 serially arranged adult units. Each vertebra, an individual unit of the column, has a central neural canal through which the spinal cord passes. The first vertebra is the atlas, which articulates with the occipital condyles.

There are seven cervical vertebrae in almost all mammals, including humans. The fused sacral and coccygeal units must be counted as presented above when describing the 26 adult units.

How do the ribs attach?

The sternum is a flat bone on the ventral midline of the thorax, meaning the front centre of the chest. There are 12 pairs of ribs. A rib is bicephalic because its dorsal end, towards the back, has two articulation surfaces.

• True ribs: pairs 1 to 7 attach dorsally to thoracic vertebrae and ventrally to the sternum through hyaline cartilage, the cartilage forming these rib connections.
• False ribs: pairs 8 to 10 connect to the seventh rib through hyaline cartilage instead of joining the sternum directly. They are also called vertebrochondral ribs.
• Floating ribs: pairs 11 and 12 lack a ventral connection.

The thoracic vertebrae, ribs and sternum together form the rib cage. Its components belong to the axial skeleton. The different rib attachments must be retained when explaining the cage: a general statement that every rib attaches directly to the sternum would be incorrect.

Which bones form the appendicular skeleton?

Each limb contains 30 bones. The bones of the upper and lower limbs follow comparable arrangements, but they have distinct names. A girdle is the skeletal arrangement that helps connect a limb to the axial skeleton.

The femur, the thigh bone, is the longest bone. The patella is a cup-shaped bone covering the knee ventrally. Digits are fingers or toes, and their bones are called phalanges. The patella is included in the lower limb's total.

How does the pectoral girdle connect an upper limb?

Each half of the pectoral girdle contains a clavicle and a scapula. The clavicle, or collar bone, is long and slender with two curvatures. The scapula is a large, triangular, flat bone on the back of the thorax between the second and seventh ribs.

The scapula bears a slightly elevated ridge called its spine. This projects into the acromion, a flat expanded process that articulates with the clavicle. Below it, the glenoid cavity is a depression receiving the humeral head to form the shoulder joint.

How does the pelvic girdle connect a lower limb?

The pelvic girdle consists of two coxal bones, the hip bones. Each is formed by fusion of the ilium, ischium and pubis. At their fusion lies the acetabulum, the cavity with which the femur articulates.

The two girdle halves meet ventrally at the pubic symphysis, a junction containing fibrous cartilage. The pectoral and pelvic girdles therefore connect upper and lower limbs respectively, with the glenoid cavity receiving the humerus and the acetabulum receiving the femur.

How are joints classified by structure and movement?

Joints are essential for movements involving bony parts. Muscular force produces movement through them, with the joint acting as a fulcrum, the supporting pivot for movement. The amount of movement varies with the structure of the joint.

What distinguishes the three structural forms?

Fibrous joints allow no movement. Dense fibrous connective tissue joins the flat skull bones end to end as sutures, the seams forming the cranium. The word suture here refers to a skull joint, rather than a surgical stitch.

Cartilaginous joints join bones through cartilage. The joints between adjacent vertebrae are of this pattern and permit limited movement. Their limited mobility contrasts with both the immobility of skull sutures and the considerable mobility of synovial joints.

Synovial joints contain a fluid-filled synovial cavity between the articulating bone surfaces. This arrangement permits considerable movement and contributes to locomotion as well as other movements. Different shapes permit different patterns of motion.

A joint's structural category and its named subtype answer different questions. A knee joint is synovial by its fluid-filled cavity and is a hinge joint by its form and movement. The atlas-axis joint is another synovial joint, but its movement is rotational.

What disorders affect the muscular and skeletal systems?

Disorders may affect communication between nerves and muscles, the muscle tissue itself, joints or bone mass. The affected structure helps distinguish conditions that all interfere with movement. A shared symptom such as difficulty moving does not identify the underlying disorder.

Which disorders chiefly involve muscle?

Myasthenia gravis is an autoimmune disorder, meaning the immune system acts against the body's own components. It affects the neuromuscular junction and leads to fatigue, weakening and paralysis of skeletal muscle. Paralysis means loss of the ability to move the affected muscles.

Muscular dystrophy involves progressive degeneration of skeletal muscle, mostly due to a genetic disorder. Degeneration means deterioration of tissue. The condition affects the muscle tissue itself, whereas myasthenia gravis affects communication at the neuromuscular junction.

Tetany consists of rapid muscle spasms, or uncontrolled contractions, caused by low calcium-ion levels in body fluid. It must be distinguished from physiological tetanus, the sustained contraction produced by repeated stimulation.

Which disorders involve joints or bones?

Arthritis means inflammation of joints. Gout is joint inflammation caused by accumulation of uric acid crystals. Uric acid is a body waste substance; in gout, its crystalline deposits provide the specific cause of inflammation.

Osteoporosis is an age-related disorder marked by decreased bone mass and increased chances of fractures, or breaks in bones. Decreased levels of oestrogen, a sex hormone, are a common cause. A common cause is not necessarily the cause in every case.

Keep the distinctions precise: low calcium in body fluid is linked to tetany; reduced bone mass characterises osteoporosis; uric acid crystals cause gout. Arthritis names joint inflammation more generally, while myasthenia gravis specifically involves the neuromuscular junction.

Glossary

• Locomotion — Voluntary movement that changes the place or location of the whole animal.
• Fascicle — A muscle bundle containing several fibres within the organisation of a skeletal muscle.
• Sarcolemma — The plasma membrane surrounding a muscle fibre and enclosing its sarcoplasm.
• Sarcoplasmic reticulum — The membrane system in a muscle fibre that stores calcium ions for contraction.
• Sarcomere — The functional contractile unit of a myofibril between two successive Z lines.
• Troponin — A regulatory protein whose subunit masks myosin-binding sites on actin in the resting state.
• Cross bridge — The connection formed when a myosin head binds to an exposed actin site.
• Motor unit — A motor neuron together with all the muscle fibres connected to it.
• Myoglobin — The red pigment in muscle that stores oxygen and is abundant in red fibres.
• Summation — Addition of contractile responses when another stimulus arrives before the preceding relaxation is complete.
• Tetanus — A sustained muscle contraction caused by rapidly repeated stimulation of living muscle.
• Rigor mortis — Muscle stiffness after death associated with ATP depletion and persistent actin-myosin attachment.
• Synovial joint — A joint with a fluid-filled cavity between articulating bone surfaces, permitting considerable movement.
• Osteoporosis — An age-related disorder involving decreased bone mass and increased chances of fractures.

Common errors and misconceptions

• Misconception: Every movement is locomotion. Correct: Locomotion changes the animal's location; movement of a body part need not do so.
• Misconception: All striated muscles are voluntary. Correct: Cardiac muscle is striated but involuntary.
• Misconception: Thick filaments shorten during contraction. Correct: Thin filaments slide over thick filaments, and the A band retains its length.
• Misconception: Calcium provides the energy of contraction. Correct: Calcium exposes myosin-binding sites on actin through troponin; ATP supplies energy for the cross-bridge cycle.
• Misconception: Every rib directly joins the sternum. Correct: False ribs connect through the seventh rib, and floating ribs lack a ventral connection.
• Misconception: Tetanus and tetany are interchangeable terms. Correct: Physiological tetanus results from repeated stimulation; tetany involves spasms due to low calcium in body fluid.
• Misconception: Every mammal has seven cervical vertebrae. Correct: Almost all mammals, including humans, have seven.
• Misconception: Muscular dystrophy is invariably genetic. Correct: It is progressive skeletal-muscle degeneration mostly due to a genetic disorder.

Exam-style questions with model answers

Q1. Define locomotion and distinguish it from movement of the eyelids without a change in the person's location. [2 marks]

1. Locomotion is voluntary movement that changes the place or location of the whole animal.
2. Eyelid movement changes a body part's position without relocating the person, so it is movement but not locomotion.

Q2. Compare skeletal, visceral and cardiac muscles by stating the striation and voluntary-control status of each type. [3 marks]

1. Skeletal muscle has a striped or striated appearance. Its activity is under voluntary nervous control, and it is primarily involved in locomotion and changes of posture.
2. Visceral muscle lacks striations and appears smooth. It is involuntary and occurs in the walls of hollow internal organs.
3. Cardiac muscle is striated but involuntary. Its cells assemble in a branching pattern to form the muscle of the heart.

Q3. Explain four stages by which a motor-neuron signal makes actin available for myosin binding in a skeletal muscle fibre. [4 marks]

1. The signal arrives at the neuromuscular junction and causes acetylcholine, a chemical neurotransmitter, to be released.
2. Acetylcholine generates an action potential in the sarcolemma, and electrical excitation spreads through the muscle fibre.
3. The excitation causes calcium ions stored in the sarcoplasmic reticulum to be released into the sarcoplasm.
4. Calcium binds to a troponin subunit, removing the masking of actin's active sites so that myosin heads can attach.

Q4. Explain the sliding filament mechanism in five points, including ATP use, bridge breakage and the changes in the I and A bands. [5 marks]

1. After actin sites are exposed, a myosin head uses energy from ATP hydrolysis to attach to actin, forming a cross bridge.
2. The attached head pulls the thin filament towards the centre of the A band. Its associated Z line moves inwards, shortening the sarcomere.
3. ADP and inorganic phosphate are released during the cycle. A fresh ATP molecule binds to myosin and breaks the cross bridge.
4. The myosin head hydrolyses ATP again. Repeated attachment, pulling and detachment cause further sliding of thin filaments over thick filaments.
5. During shortening, the I bands become smaller while the A bands retain their length. The change reflects increased filament overlap rather than shortening of thick filaments.

Q5. Classify the joints at these five locations and state the movement each permits: shoulder between humerus and pectoral girdle; knee; atlas and axis; adjacent carpals; carpal and metacarpal of the thumb. [5 marks]

1. The shoulder connection between humerus and pectoral girdle is a ball-and-socket joint. It permits movement in several directions, including rotation.
2. The knee is a hinge joint. Its principal movements are bending and straightening, unlike the multidirectional movements at a ball-and-socket joint.
3. The joint between atlas and axis is a pivot joint. It permits rotational movement around an axis at this vertebral connection.
4. The joints between adjacent carpals are gliding joints. Their adjoining bone surfaces can slide relative to one another during movement.
5. The carpal-metacarpal joint of the thumb is a saddle joint. It permits movement in two planes; like the other listed joints, it is synovial.

Q6. A human skeleton has 206 bones, including 80 axial bones. There are two upper and two lower limbs, each with 30 bones; the two pectoral-girdle halves each have a clavicle and scapula, and the pelvic girdle has two coxal bones. Calculate the appendicular total and reconcile it in four points. [4 marks]

1. The appendicular skeleton contains 206 minus 80, giving 126 bones when the axial division is excluded from the whole skeleton.
2. The two upper and two lower limbs give four limbs, so their contribution is four times 30, or 120 bones.
3. Each pectoral-girdle half has two bones, giving four altogether; adding the two coxal bones gives six girdle bones.
4. The appendicular total is therefore 120 limb bones plus six girdle bones, or 126, agreeing with the subtraction.

Q7. Define summation, physiological tetanus and rigor mortis, distinguishing the circumstance responsible for each. [3 marks]

1. Summation is the addition of contractile responses when a new stimulus arrives before the muscle has fully relaxed from its preceding response.
2. Physiological tetanus is sustained contraction produced by rapidly repeated stimulation. In complete fusion, there is no relaxation between the successive responses.
3. Rigor mortis is muscle stiffness after death. ATP depletion prevents detachment of attached myosin heads from actin, allowing cross bridges to persist.

Q8. Distinguish tetany, gout and osteoporosis by giving each condition's defining change and its associated cause or risk factor. [3 marks]

1. Tetany involves rapid spasms of muscle. It results from low calcium-ion levels in body fluid, rather than repeated stimulation producing physiological tetanus.
2. Gout involves inflammation of joints caused by accumulation of uric acid crystals. This specific cause distinguishes it from the general term arthritis.
3. Osteoporosis is age-related loss of bone mass with increased chances of fractures. Decreased oestrogen levels are a common cause, not an inevitable explanation for every case.

Key takeaways

• Locomotion changes the animal's location, while movement includes actions of body parts that need not relocate the whole animal.
• Skeletal muscles are striated and voluntary, visceral muscles smooth and involuntary, and cardiac muscles striated and involuntary.
• A sarcomere lies between successive Z lines; thin actin and thick myosin filaments form its contractile arrangement.
• Calcium binding to troponin exposes actin sites, while ATP supports repeated myosin attachment, pulling and detachment.
• During contraction, Z lines approach each other and I bands shorten, while A bands retain their length.
• The human skeleton contains 206 bones divided between the axial framework and the limbs with their girdles.
• Fibrous joints are immovable, cartilaginous joints permit limited movement, and synovial joints permit considerable movement.
• Summation, physiological tetanus, rigor mortis and tetany describe different conditions and should be distinguished by their defining circumstances.

Test yourself

What structures bound a sarcomere?

Two successive Z lines bound the sarcomere, the functional unit of muscle contraction.

Which muscle band retains its length during contraction?

The A band retains its length while the I bands shorten.

What event breaks an actin-myosin cross bridge?

Binding of a new ATP molecule to myosin breaks the cross bridge.

Why do red fibres appear reddish?

They contain a high amount of myoglobin, the red oxygen-storing pigment in muscle.

Which ribs are floating ribs?

The eleventh and twelfth pairs are floating ribs because they lack a ventral connection.

Which bones unite to form a coxal bone?

The ilium, ischium and pubis fuse to form each coxal bone.

Where are the glenoid cavity and acetabulum, and what articulates with each?

The scapula's glenoid cavity articulates with the humeral head. The pelvic girdle's acetabulum articulates with the femur.

What distinguishes tetany from physiological tetanus?

Tetany consists of muscle spasms due to low calcium in body fluid. Physiological tetanus is sustained contraction produced by rapidly repeated stimulation.