Muscles

Question 1

What are the 3 types of muscles?

Answer 1

1. Involuntary (smooth) muscle: Innervated by the autonomic nervous system and are found in the gut, bladder, eye… responsible involuntary reflex actions like pupils dilating, bladder tension, peristalsis… Contract very slowly but also difficult to fatigue.
2. Cardiac muscles: 3 types found in the heart (atrial, ventricular, specialised excitatory and conductive) and are responsible for cardiac contractions. Some are myogenic, in the sense that they contract without nervous stimulation, e.g. the muscle in the SAN setting the rhythm for whole heart. These muscles are striated and contractions can be sped up by noradrenaline (from sympathetic nerves) or slowed down by acetylcholine (from parasympathetic nerves). Despite continuous contractions, cardiac muscle does not fatigue.
3. Voluntary (striated) muscle: Innervated by the somatic nervous system and is responsible for all voluntary movements, making up all of the skeletal muscles.

Question 2

What is the structure of involuntary muscles?

Answer 2

Each muscle fibre is one spindle-shaped, unstriated cell containing one nucleus and bundles of actin/myosin fibres.

Question 3

What is the structure of cardiac muscles?

Answer 3

Each muscle fibre is one highly branched, striated, cylindrical cell containing one nucleus. Each fibres can be connected to multiple other fibres by a intercalated disc offering very low electrical resistance. This creates a highly branched network of fibres which allows for action potentials to spread through quickly.

Question 4

What is the structure of voluntary muscles?

Answer 4

Each muscle fibre is striated and contains multiple nuclei, surrounded by a plasma membrane called the sarcolemma. Cytoplasm inside each fibre called the sarcoplasm. Inside the fibres are many mitochondria, sarcoplasmic reticulum (specialised endoplasmic reticulum containing Ca2+ ions) and myofibrils (consisting of actin and myosin filaments) responsible for contractions. Membrane tubes, T-tubules, extend down from the sarcolemma to sarcoplasmic reticulum.

Question 5

What is the macrostructure of skeletal muscle?

Answer 5

• Each muscle consists of bundles of muscle fibres.

- Each bundle of muscle fibres contains muscle fibres held together by connective tissue.

Question 6

What are the different types of skeletal joints present in the body?

Answer 6

1. Ball and socket joints (e.g. shoulder) allow for movement in multiple.
2. Gliding joints (e.g. wrist) consist of many small bones gliding over each other, allowing for wide range of movements in multiple planes.
3. Hinge joints (e.g. elbow) allow movement in only one plane.

Question 7

What is the action of skeletal muscle about joints in order to bring about skeletal movement?

Answer 7

• Muscles are connected to bones via tendons.
• Bones are held together by ligaments.
• Skeletal muscles can only actively contract (shorten), they lengthen when relaxed, which is passive process.
• Movement in one plane usually requires the action of an antagonistic pair of muscles; with movement clockwise requiring the contraction of one and the relaxation of the other, and vice versa anticlockwise.
• Movement in multiple planes usually require the action of groups of muscles called synergists.

Question 8

What is the structure of an elbow joint?

Answer 8

• Elbow joint is a hinge joint.
• Bicep muscle connected to the humerus and radius via tendons.
• Tricep muscle connected to the humerus and ulna via tendons.
• Humerus connected to ulna + radius via ligaments in a synovial joint.

Question 9

What is the structure of a synovial joint?

Answer 9

• Ligaments physically hold bones together.
• Cartilage on ends of bones in joint reduce friction when 2 bones meet during movement.
• Synovial fluid in between 2 bones act as lubrication, surrounded by synovial membrane producing fluid.

Question 10

What is the mechanism of action of an elbow joint?

Answer 10

• When the triceps relax and the biceps contract, the elbow bends.
• When the triceps contract and the biceps relax, the elbow straightens.

Question 11

What is the structure of a myofibril?

Answer 11

• Consists of units called sarcomeres, containing actin and myosis filaments, each being divided by Z-lines.
• A-band: Width of myosin filament, including overlaps with actin filaments. Seen as the dark band.
• I-band: Actin only, excluding overlap with myosin. Seen as the light band.
• H-zone: Length of myosin filaments; excluding overlap with actin filaments.
• M-line: Line going down middle of H-zone.

Question 12

What is the structure of actin (thin) filaments?

Answer 12

• Consists of 2 strands of F (fibrous)-actin made from G (globular)-actin ‘beads’. These are twisted around each other like double strand of beads.
• Rod-like protein called tropomyosin raps around actin fibres as reinforcement.
• Each tropomyosin molecule binds to troponin, which i made from 3 polypeptides, so have 3 binding sites; one for actin, one for tropomyosin and one for Ca2+ ions.

Question 13

What is the structure of myosin (thick) filaments?

Answer 13

• Myosin molecules bundled into thick filament structures.

- Each molecule of myosin has 2 heads, each sticking out from opposite ends of the thick filament.

Question 14

What is the structure of a neuromuscular junction?

Answer 14

Similar to the structure of a synapse between nerves, consisting of a pre-synaptic knob containing vesicles of acetylcholine separated from the sarcoplasm by a synaptic cleft. Complementary acetylcholine receptors on surface of sarcolemma bind to ACh when released to bring about muscular contraction.

Question 15

What is the sequence of events that trigger muscular contraction?

Answer 15

1. Nerve impulse travels down axon of motor neurone, along dendrites, to the synaptic knob.
2. Nerve impulse causes voltage-gated Ca2+ channels to open, resulting in influx of Ca2+ ions by diffusion, down concentration gradient.
3. Ca2+ ions cause vesicles of acetylcholine (ACh) to fuse with pre-synaptic membrane.
4. ACh released into synaptic cleft by exocytosis.
5. ACh diffuses across synaptic cleft and binds to complementary receptors on surface of sarcolemma.
6. Na+ channels in sarcolemma open.
7. Large influx on Na+ ions into sarcoplasm by diffusion, causing depolarisation.
8. Depolarisation spreads along membrane of T-tubules to sarcoplasmic reticulum.
9. Voltage-gated Ca2+ channels on sarcoplasmic reticulum open.
10. Ca2+ ions diffuse out of sarcoplasmic reticulum.
11. Ca2+ ions bind to troponin, causing a conformational change to tertiary structure of troponin.
12. Troponin moves tropomyosin to expose myosin binding sites on F-actin.
13. Power strokes between actin and myosin filaments occur. Causing muscle fibres to shorten and the muscle to contract.

Question 16

What is the sequence of events in a power stroke?

Answer 16

1. ATP binds to myosin head group, causing a former actin-myosin cross-bridge to break and the head group moves backwards.
2. Cross-bridge are formed between myosin head and binding site on actin fibre.
3. As ATP is hydrolysed to ADP and Pi, head group bend to original positions, pulling thin filaments with it, increasing overlap with thick filaments.
4. New ATP molecule binds to myosin head, breaking current cross-bridge and bending head groups, allowing it to bind to the actin fibre further along.

Question 17

What is the role of ATP during muscle contractions?

Answer 17

Energy from hydrolysis of ATP required to break actin-myosin cross-bridge and also to move the myosin head backwards so it can bind to actin fibre further along.

Question 18

What are the 3 methods by which ATP supply is maintained during muscle contractions?

Answer 18

1. Aerobic respiration: Occurs in mitochondria and ATP production dependent on the amount of oxygen available as well as the type of respiratory substrate being used.
2. Anaerobic respiration: When oxygen is not supplied to muscle quick enough for aerobic respiration to meet demands, anaerobic respiration occurs in sarcoplasm of muscle fibres, generating toxic lactic acid. Lactic acid carried way by blood and stored in liver. Blood supply to muscles increased to quickly remove excess lactic acid.
3. Phosphorylation via creatine phosphate: Phosphate group from creatine phosphate used to phosphorylate ADP to ATP. This process occurs in the sarcoplasm and is very quick, used as a last resort when both previous methods fail.

Question 19

How is the system reset after contraction?

Answer 19

1. Acetylcholine broken down by acteylcholinesterase on surface of sarcolemma, stopping action potential being generated.
2. Ca2+ ions unbind from troponin.
3. Tertiary structure of troponin changes back to original shape, moving tropomyosin to again, cover myosin binding sites on actin fibres.
4. Ca2+ ions are pumped back into sarcoplasmic reticulum by active transport.
5. Na+ ions pumped out of sarcoplasm by active transport, so it repolarises.
6. Lack of power strokes result in actin filaments sliding passively back to original positions, resulting in muscle fibres lengthening again.

Question 20

What are the changes that occur to elements of a sarcomere during contraction?

Answer 20

• A-band: No change.
• I-band: Shortens.
• Z-line: No change.
• H zone: Shortens.
• M line: No change.
• Overlap region: Lengthens.

Question 21

What is the overall effect of contraction on length of sarcomeres?

Answer 21

Length of sarcomeres decrease during muscular contractions.

Question 22

How are strengths of contractions controlled by the CNS through graduation of response?

Answer 22

• Cluster of muscle cells controlled by one neuromuscular junction is called a motor unit.
• One motor neurone usually branches off to control many motor units.
• A muscle is usually innervated by many motor neurones.
• By controlling how many motor neurones are stimulated, the number of motor units stimulated are also controlled.
• The more motor units that are stimulated to contract, the greater the strength of contraction.
• CNS controls number of motor units stimulated to control strength.
• This is called graduation of response.

Question 23

How are strengths of contractions controlled by the CNS through temporal summation?

Answer 23

• One action potential down motor neurone causes the muscle to twitch.
• Frequent action potentials down motor neurone causes smooth contraction called tetanus.
• This process is called temporal summation because frequent action potentials cause continuous release of ACh at neuromuscular junctions.
• The greater frequency of action potentials, the stronger the contraction.
• CNS able to adjust frequency to control strength.

Question 24

Which systems in mammals coordinate all responses to environmental stimuli?

Answer 24

1. Nervous system.

2. Endocrine system.

Question 25

What are the physiological changes associated with detecting environmental threat?

Answer 25

• Pupils dilate.
• Heart rate increases.
• Blood pressure increases.
• Blood flow to digestive system decreased and blood flow to liver and muscle increased (via arteriole action).
• Blood glucose increases.
• Rate of metabolism increases.
• Hairs on skin surface erect.
• Breathing rate and tidal volume increases.
• Sweat production increases.

Question 26

What is the sequence of events associated with the coordination of threat response?

Answer 26

1. A stimuli, e.g. visual or auditory, detected by a receptor and information passed to cerebrum.
2. Cerebrum associates stimuli with threat.
3. Cerebrum signals hypothalamus.
4. Hypothalamus uses sympathetic nervous system to stimulate adrenal medulla (in adrenal glands) to release hormone adrenaline into bloodstream.
5. Hypothalamus releases corticotropin-releasing factor (CRF) into pituitary gland, stimulating anterior pituitary gland to release adreno-corticotropic hormone (ACTH) into bloodstream.
6. ACTH stimulate adrenal cortex to release a number of corticosteroid hormones into bloodstream that have variety of effects on body.