Muscle and Motility

Movement

The change is position or location of an organism or body part relative to its surroundings, it can be voluntary or involuntary.

Motile organisms

Organisms that have the ability to move from one place to another.

Sessile organisms

Organisms fixed to one place

Examples of motile organisms:

1. Bacteria rotating their flagella to propel them forward.

2. Squids and Octopi using jet propulsion to move through water: expelling water through a muscular tube which propels the organism the opposite direction (the direction is controlled).

Example of sessile species

Mimosa pudica folding its leaves and drooping its stem, a response thought to deter herbivores and insects from eating the plant.

What are sacromeres?

Repeating units that compose myofibrils within muscle cells (myofibrils).

Myosin

Thick protein filaments

Actin

Thin protein filaments

Z discs

Define the boundaries of sarcomere, organise and anchor actin.

Sarcomere length

Distance between Z discs.

M line

Organises and anchors myosin filaments

I bands

regions of the sarcomere where only actin filaments are present, located on either side of the Z disc.

A band

Region where myosin is present and contains an area of overlap between actin and myosin.

H band

region in the middle of the sarcomere containing only myosin.

Muscle contraction

The force achieved during muscle contraction is a result of the simultaneous contraction of all the sarcomeres in that muscle.

Sliding filament theory:

Explains the contraction of a sarcomere.

• When a muscle is stimulated to contract, actin filaments slide over the myosin filaments towards the centre of the sarcomere

  • Z discs are pulled closer, shortening the sarcomere and resulting in the overall shortening of the muscle fibre

  • H bands and I bands decrease in length as actin is pulled inwards, overlapping more myosin and reducing the area where only myosin or actin is present

Tropomyosin

• Thin and fibrous protein subunit which blocks myosin binding sites on actin filaments when there is no electrical stimulation.

• It prevents the contraction of the muscle by preventing the binding of myosin to actin.

Troponin

• A muscle is stimulated by a motor neuron

• Calcium ions are released from the sarcoplasmic reticulum (a specialized endoplasmic reticulum found sarcoplasm).

• Calcium ions bind to troponin, causing it to undergo confrontational change

• Troponin as a result of this moves tropomyosin away from the myosin binding sites

ATP’s requirement in muscle contraction

• ATP plays a crucial role in muscle contraction by binding to the myosin head.

• This binding causes the myosin to detach from the actin filament, allowing the muscle fibers to relax and prepare for the next contraction cycle.

• When ATP is hydrolyzed, it provides the energy needed for the myosin head to reattach to a new binding site on the actin filament, facilitating muscle contraction.

Titin

• An immense (giant) protein found inside the sarcomeres of striated muscles.

• Connecting the Z discs and M lines and providing support and stability.

Function of Titin

• Acts as a molecular spring

  • A sarcomere is stretched, so is titin

  • Potential energy is stored that is released when stretching force is released

  • This release of energy helps to return the sarcomere to its original length and resume normal function.

• Acts as an elastic band

  • Providing passive resistance to prevent the overstretching of relaxed muscle,

  • helps maintain the structural integrity of the muscle

  • Prevents it from becoming damaged due to excessive strain or stretching

Antagonistic pairs of muscle

• When one muscle contracts, the other relaxes

• This is because a muscle can only contract and generate force in one direction

Examples of antagonistic muscle pair

• internal and external intercostal muscles: work together to raise and lower the ribs to allow ventilation of the lungs.

• quadriceps and hamstrings: work together to produce movement at the knee joint

• pectoralis major and latissimus dorsi, work together to produce movement at the shoulder joint.

A skeletal muscle unit (aka motor unit)

A functional unit composed of a motor neuron and the muscle fibres that it innervates.

Muscle fibres

long cylindrical structures that make up skeletal muscles .

Composition of muscle fibres:

• many myofibrils that contain sarcomeres

• specialized cytoplasm (sarcoplasm)

• specialized endoplasmic reticulum (sarcoplasmic reticulum) which contains calcium ions.

Motor neurons

nerve cells that transmit messages from the central nervous system to effector organs such as muscles or glands

The role that motor neurons play in muscle contraction

• Action potential travels down the motor neuron

• Motor neuroron releases a neurotransmitter acetylcholine

• AcH diffuses across the synaptic cleft and binds to its eceptors on the motor end plate of the muscle fibres

• Causes calcium ions to be released from the sarcoplasmic reticulum

• Muscle contraction is triggered

The whole process summary:

1. An electrical message is sent from the brain via a motor neuron

2. The electrical message is released from the axon terminal of the motor neuron

3. Acetylcholine is released from the axon terminal of the motor neuron

4. Acetylcholine diffuses across the synaptic cleft

5. Acetylcholine binds to acetylcholine receptors on the motor end plate of the muscle fibre

6. Calcium ions are released from the sarcoplasmic reticulum

7. Calcium ions bind to troponin, altering its conformation

8. Troponin moves tropomyosin away from the myosin binding sites on the acting filament

9. ATP binds to the myosin head and is hydrolysed, causing the myosin head to detach from its binding site on the actin filaments and move upward

10. Myosin head binds to a new binding site, forming a cross-bridge

11. The release of ADP and Pi causes the myosin head to change position, pulling the actin filament towards the centre of the sarcomere in a process known as power stroke

12. This cycle of cross-bridges forming and a power stroke being executed is repeated when another ATP binds to the myosin head, resulting in the actin filaments being pulled towards the M-line, shortening the sarcomere.

Skeleton

A structural framework composed of bone and other connective tissue that provides support, shape, and protection for the organism’s body

Two types of skeletal systems found in animals:

1. Endoskeletons

2. Exoskeletons

Arthropods

A diverse group of organisms that are characterised by jointed legs, segmented bodies and tough exoskeletons made of complex poly saccharide chitin.

Exoskeletons

• Acts as a type of external armour to protect the soft body parts inside from physical damage and dehydration

• Due to being located outside the body, must periodically be shedand replaced as the animal grows

• Acts as an attachment site to muscles

  • When muscles contract, they pull on the tendons which in turn pull on the exoskeleton and produce movement

Vertebrates

Animals with a backbone and an endoskeleton

Endoskeleton

• An internal structure made of bone and cartilage

• Provides protection and support for the body’s internal organs

• Act as an anchorage site for muscles

• Muscles contract to general the generate the force necessary to move the bone they are attached to and allow movement of the body

Joints

The articulating surfaces between two or more bones

Synovial joints

Joints that are enclosed in a joint capsule, where bones are separated by a fluid-filled cavity, allowing free movement between the bones

Synovial fluid

Acts as a lubricant to reduce friction between bones

Articular cartilage

Covers the end of the bones in a synovial joint, acting as a cushion to absorb shock , the smooth surface of cartilage helps facilitate smooth movement of bones over each other

Ligaments

Strong, flexible bands of connective tissue that provide stability to the joint and prevent excess movement or dislocation of the joint

Tendons

Strong fibrous bands of connective tissue that connect the bone to the muscles

Collagen

Fibrous protein that provides strength and support to various tissues in the body, including bones, tendons, and cartilage. It is rigid relatively resistant to stretching.

Hip joint

Synovial joint that connects the femur bone in the thigh to the pelvis bone

The range of motion (ROM)

Refers to the type and amount of movement that is possible at that joint

How is ROM determined

• The joint’s anatomical structure

• the surrounding muscles

• ligaments

• tendons

• presence of other tissues that can facilitate or limit movement

Factors that affect ROM

• injury

• desease

• ageing

How to measure the amount of movement of a joint

• Goniometer: joint angle is measured at an angle at which a bone can move relative to its resting position using a goniometer

• Computer analysis: tends to be able to measure joint angles faster and in multiple planes of movement, providing a more complete picture of joint function

Intercostal muscles

A group of muscles located between the ribs and the thoracic cavity that are involved in breathing.

Two main types of intercostal muscles

1. External intercostal muscles

2. Internal intercostal muscles

Innermost intercostal muscles

responsible for assisting in forced expiration during heavy breathing

Internal intercostal muscles

• Deeper intercostal muscles

• Run in an upward and forward direction towards the centre of the chest

• When they contract the rib is lifted up and out

External intercostal muscles

• Most superficial (closest to the surface of the body)

• Run downward and forward direction diagonally towards the center of the chest

What happens due to the different orientation of muscle fibres in the internal and external layers of the intercostal muscles

• when one layer contracts, the other layer is stretched

• this stores potential energy in the sarcomere protein titin

• when contraction of the sarcomere ends the release of potential energy stored in the titin helped turn the sarcomere to its original length,

• This allows the sarcomere to return to its original length