B3.3 Muscle and Motility PDF

# B3.3 Muscle and motility ## All living organisms are adapted for movement - Movement is one of the functions of life and all organisms have adaptations for it. - In every organism there are internal movements such as peristalsis in the gut, or ventilation of the lungs. - There are movements...

# B3.3 Muscle and motility ## All living organisms are adapted for movement - Movement is one of the functions of life and all organisms have adaptations for it. - In every organism there are internal movements such as peristalsis in the gut, or ventilation of the lungs. - There are movements in the cytoplasm of unicellular organisms. - **Motile organisms** move their entire body from one place to another. This is locomotion, with each motile organism adapted to its method of locomotion. For example, bar-tailed godwits (***Limosa lapponica***) have wings for flight. They migrate 10,400 km from Siberia to New Zealand in a week, doubling their body weight with fat reserves before the journey. - **Sessile organisms** remain in a fixed position. Most plants are sessile, with roots growing into the soil. Most animals are motile, but some are sessile. For example, a coral consists of a colony of sessile polyps. In hard corals, the polyps construct a rigid skeleton around themselves. They can extend their tentacles into the water when they are filter-feeding, but cannot move to a new location. ## Muscle contraction - The structure of muscle fibres is introduced in Section A2.2.9. Each fibre contains many parallel, cylindrical myofibrils. | | | |---|---| | one sarcomere | dark band | | | | | | | | → Z-disc between sarcomeres | | - Each myofibril consists of a series of sarcomeres linked end-to-end at Z-discs. There are light bands at either end of a sarcomere and a dark band in the centre. - Two types of protein filament are arranged in a regular pattern within sarcomeres - thin actin filaments and thick myosin filaments. - The dark band in the centre of a sarcomere contains many parallel myosin filaments. - The ends of these myosin filaments overlap with six equidistant actin filaments. - The light bands at the ends of sarcomeres contain actin filaments but not myosin. - Each actin filament is attached to a Z-disc at one end and overlaps with myosin filaments at the other end. | | | |---|---| | **light band** | **dark band** | | **light band** | | | z-disc | | | sarcomere relaxed | | | | | | contraction is due to actin being pulled towards the centre of the sarcomere by myosin | dark band remains the same length during contraction (but light bands shorten) | | sarcomere contracted | | | transverse section (TS) of sarcomere where actin and myosin filaments overlap | | - The contraction of sarcomeres is due to the sliding of actin and myosin filaments. - Myosin has "heads" that can attach to binding sites on actin. - These heads undergo a cycle of binding to form a cross-bridge, pulling the actin molecule towards the centre of the sarcomere by about 10 nm and then detaching and swivelling to the next binding site on actin. This is a molecular ratchet mechanism. | | |---|---| | **cross-bridge formed** | | | | **power stroke** | | | | **head cocked** | | **myosin** | | | | **actin** | - Because of the many heads per myosin and many myosin filaments, in many sarcomeres, in many myofibrils, in many muscle fibres, the small force exerted by each myosin head is multiplied up and muscles can exert very powerful forces. ## Muscle relaxation - When muscles relax, potential energy is stored by titin, an elastic protein that has the largest polypeptides so far discovered (over 34,000 amino acids long in humans). - Titin releases potential energy when it recoils during muscle contractions. - This increases the amount of force that a muscle can exert. - Titin has two other roles. - It connects the end of myosin filaments in sarcomeres to the Z-disc and holds each myosin filament in the correct position in the centre of six parallel actin filaments. - It also prevents overstretching of the sarcomere. | | | | |---|---|---| | **light band** | | | | | | Z-disc | | | | | | **actin** (thin filaments) | **myosin titin** (thick filaments)| | | | | | **dark band** | | | **light band** | | - Energy is needed to stretch titin and therefore to lengthen a muscle. - Lengthening of muscles happens when they relax. - Muscles can only exert force when they contract, so a muscle cannot supply the energy it needs to lengthen. - The energy has to be provided by another muscle that is known as the antagonist. - Despite the name, an antagonistic pair of muscles work together, with the contraction of each member of the pair providing the energy needed to lengthen the titin molecules in the other as it relaxes. ## Motor units - Skeletal muscles are composed of striated muscle fibres, which contract when stimulated by a motor neuron. - The synapse between a motor neuron and a muscle fibre is a neuromuscular junction. - The neurotransmitter used is acetylcholine. - Each motor neuron has branches that form neuromuscular junctions with different muscle fibres, usually hundreds. - A nerve impulse conveyed by the motor neuron therefore stimulates simultaneous contraction in a group of muscle fibres. - A motor unit is one motor neuron together with all the muscle fibres that it stimulates. This pattern helps to achieve coordinated contraction of a muscle with as few motor neurons as possible. ## Skeletons facilitate movement - A skeleton is a hard framework that supports and protects an animal's body. - Arthropods such as spiders, crustaceans and insects have exoskeletons consisting of tough plates of chitin that cover most of the body surface. Vertebrates have endoskeletons consisting of bones. - Skeletons facilitate movement by providing an anchorage for muscles and acting as levers. - Typically, a muscle is attached to two parts of the skeleton. - One attachment is the insertion, where muscle contraction causes movement. - The other is the origin and is fixed, so contraction does not cause movement. - By acting as levers, bones can change the size and/or direction of a force. - Levers have a fixed point called the fulcrum, which is the pivot point. - The force applied to the lever is the effort. - It is converted by the lever into a resultant force. - The diagrams show how limb bones and muscles can be adapted either for maximum rapidity of movement (as in the limbs of gazelles), or for maximum force (e.g. a mole's front limb that it uses for digging). | | | | | |---|---|---|---| | origin of muscle | large movement, small force | insertion close to fulcrum | | | | | | | | | | | | | | long limb bone so load is far from fulcrum | | fulcrum | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | short limb bone so load is close to fulcrum | fulcrum | ## Synovial joints - Bones meet at joints. - Most joints allow bones to move in relation to each other (articulation). - Synovial joints are self-lubricating articular joints that have the following parts: - **Bones** - an anchorage for muscles and ligaments, with the bones shaped to allow a specific range of movements - **Muscles** - cause movement at a joint because the origin and insertion of a muscle are on opposite sides of the joint - **Tendons** - tough collagen-rich cords of tissue that attach muscle to bone and transmit force from contractions - **Cartilage** - tough, smooth tissue covering bone at joints to prevent friction and absorb shocks, preventing fractures - **Synovial fluid** - fluid that fills the cavity between the cartilages on the ends of the bones, lubricating the joint - **Ligaments** - tough cords of tissue with much collagen that prevent movements that would dislocate the joint - **Joint capsule** - tough ligamentous covering that seals the joint, holds in the synovial fluid and prevents dislocation. | | | | |---|---|---| | pelvis | ligament connecting pelvis and femur | | | | | | | | | cartilage covering the end of the femur and lining the socket in the pelvis | | | synovial membrane | | | | | | | | | | | femur | | ligament | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | joint capsule, with synovial membrane on its inner surface | | | | | cartilage covering the head of the femur | ## Application of skills: measuring the range of motion at a joint - The elbow and knee are hinge joints allowing movements in one plane: flexion (bending) and extension (straightening). - The hip is a ball-and-socket joint that moves in three planes: protraction/retraction, abduction/adduction, and rotation. - The range of movement at a joint can be measured with a goniometer or by computer analysis of images. | | |---|---| | 0° | | | | 60° | | | | 120° | | | | | | | | 180° | | | | ▲ Goniometer | ## Importance of locomotion - Animals have many reasons for moving from place to place. The table shows four reasons with examples of each. | Reason | Examples in honey bees | Examples in salmon | |---|---|---| | Foraging for food | Bees fly from flower to flower searching for nectar and pollen | Salmon in the sea swim to catch their prey of smaller fish and large invertebrates | | Escaping from danger | Bees fly back to their colony when a storm is approaching because heavy rain is a danger | Salmon in the sea swim to escape from predators such as bluefin tuna and swordfish | | Searching for a mate | Male bees (drones) fly at a height of 10-40 m and mate with a virgin queen if they find one | Male salmon search for a female laying eggs in spawning grounds, then shed sperm on the eggs | | Migration | A swarm of bees is a migrating colony containing a queen and many workers | Young salmon migrate from rivers to the sea and then as adults migrate back to the river to breed | ## Intercostal muscles - Ventilation of the lungs is explained in Section B3.1.5. - It is an example of internal body movement and is due to the antagonistic action of two layers of intercostal muscle, external and internal. - Muscle fibres in each layer are attached to adjacent ribs in the ribcage and cause the ribcage to move when they contract. - The diagram shows the orientation of muscle fibres in the two layers (with only some of the fibres shown). - External and internal intercostal muscle fibres are orientated differently so they cause the ribcage to move in different directions. | | | |---|---| | sternum | vertebra | | | | | rib | | | | | | external intercostal muscle | | | | | | internal intercostal muscle | | | | | | ribcage up and out with external intercostal muscles contracted and internal intercostals relaxed | | | | | | | | | | | | | ribcage down and in with internal intercostal muscles contracted and external intercostals relaxed | - When the external intercostal muscles contract, the ribcage is moved up and out, increasing the volume of the thorax and drawing air into the lungs (inhalation). - When the internal intercostal muscles contract the ribs are pulled down and in, decreasing the volume of the thorax and forcibly expelling air from the lungs (exhalation). - Contraction in either layer stretches muscle fibres in the other layer, which will be relaxing. - Titin molecules in the sarcomeres of the relaxing muscle are elongated by forces from the contracting muscle, generating a store of potential energy. - This energy is released when the relaxing muscle starts to contract and there is elastic recoil of the titin molecules. ## Marine mammals are adapted for swimming - Water is about 1,000 times denser than air and much more viscous. - Swimming therefore requires different adaptations from those needed for locomotion on land or in the air. | | | |---|---| | blowhole | flipper| | | | | dorsal fin | flukes | | | | | | | |◄ Side view of dolphin | | | | ▲View from above | - **Streamlining** - marine mammals are shaped to minimize resistance to motion by these features: - shaped to be widest near the front and tapering towards the rear, which causes less drag than other shapes - flippers, flukes and dorsal fin have an elongated teardrop profile in transverse section which reduces drag - body surface is smooth due to even distribution of blubber and absence of hind limbs and ear flaps - skin is hairless, reducing friction. - **Adaptations for locomotion**: - flippers, which are used for steering, instead of front legs - tail flukes, which are lobes to left and right that increase thrust when the tail is moved up and down - dorsal fin to provide stability by preventing rolling - blubber, which provides buoyancy. - **Adaptations for periodic breathing between dives**: - airways can be closed during dives using the nostrils or blowhole - airways are reinforced with rings of cartilage or smooth muscle to ensure ventilation can restart quickly after a dive.