OCR UNIT F215 COORDINATED MOVEMENT

Specification Statements

Describe the role of the brain and nervous system in the co-ordination of muscular movement

Describe how co-ordinated movement requires the action of skeletal muscles about joints, with reference to the movement of the elbow joint

Explain, with the aid of diagrams and photographs, the sliding filament model of muscular contraction

Outline the role of ATP in muscular contraction, and how the supply of ATP is maintained in muscles

Compare and contrast the action of synapses and neuromuscular junctions

Outline the structural and functional differences between voluntary, involuntary and cardiac muscle

State that responses to environmental stimuli in mammals are co-ordinated by nervous and endocrine systems

Explain how, in mammals, the fight or flight response to environmental stimuli is co-ordinated by the nervous and endocrine systems

Comparison of Three Types of Muscle

The three muscle types are:

1. Cardiac muscle that makes up the muscle tissue of the heart

2. Involuntary muscle, also called smooth muscle that makes up the muscle tissue in blood vessels and the tubes of the digestive system

3. Voluntary muscle, also called striated or skeletal muscle, that is attached to bones of the skeleton

Similarities of the Muscle Types

Muscle tissue is composed of elongated cells called fibres

Fibres can contract and relax

The force of contraction is due to the protein filaments in the fibres called actin and myosin

The plasma membrane of muscle fibres is called the sarcolemma

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The cytoplasm of the fibres is called sarcoplasm

The sarcoplasm contains many mitochondria to produce ATP for muscle contraction

The rough endoplasmic reticulum is called the sarcoplasmic reticulum

Differences between the Muscle Types

There are distinct differences between the three muscle types in terms of structure and function that are detailed below

Cardiac Muscle

The muscle tissue of the heart is called cardiac muscle.

There are three types of cardiac muscle:

1. Atrial muscle

2. Ventricular muscle

3. Excitatory and conductive muscle fibres

Contraction of atrial and ventricular muscle is similar to that of skeletal muscle but is of a longer duration.

The excitatory and conductive fibres contract feebly. Their main function is to conduct electrical impulses and control the cardiac cycle.

The sinoatrial node is the excitatory and conductive tissue, in the wall of the right atrium. This tissue is myogenic. It is self-exciting and able to contract on its own, without nervous impulses from the CNS.

However, the rate of excitation of the SAN can be changed by autonomic nervous impulses, from the medulla oblongata. The sympathetic system cause an increased rate of depolarisation at the SAN and the parasympathetic system causes a decrease in rate of depolarisation at the SAN

Impulses from the SAN spread throughout the atrial muscle causing atrial systole.

A layer of non-conductive fibres separates the atria from the ventricle, so that the waves of depolarisation/excitation reach the ventricle walls via the AV node and purkyne fibres only.

Cardiac Muscle Structure

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The fibres are branched

The branched fibres create spaces between them that contain blood and lymphatic capillaries and tissue fluid

Each cardiac muscle fibre has one nucleus

The sarcolemma between muscle fibres is a specialised membrane called an intercalated disc. Intercalated discs have gap junctions allowing free diffusion of ions between fibres. Intercalated discs ensure very rapid transmission of the waves of excitation from one fibre to the next

Cardiac muscle tissue appears striated/striped under a light microscope

Each fibre has many mitochondria. These fibres are adapted for aerobic respiration since cardiac muscle relies on ATP production from aerobic respiration for its powerful and continuous muscle contraction.

Cardiac muscle does not fatigue and is not adapted for anaerobic respiration (fatigue is caused by the accumulation of lactate in muscle fibres, from anaerobic respiration)

Involuntary (Smooth) Muscle

Smooth muscle contractions are not under voluntary control. Smooth muscle is therefore innervated by motor neurones of the autonomic nervous system.

Smooth muscle fibres contract more slowly than other muscle fibre types but also tire very slowly.

Some examples of smooth muscles are tabulated below:

Location of Smooth Muscle

Arrangement of muscle fibres

Action of Smooth Muscle

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Walls of oesophagus, small and large intestines

Circular and longitudinal fibre bundles

Peristalsis

Iris of the eye Circular and radial fibre bundles

Control light entering the eye through the pupil:

Contraction of radial muscles dilate the pupil

Contraction of circular muscles constrict the pupil

Walls of arteries and arterioles

Circular fibre bundles Bring about vasoconstriction/vasodilation of arterioles important in:

temperature control redistribution of

blood to organs eg more blood to skeletal muscles during exercise

Uterus wall and cervix Circular bundle fibres Contractions during childbirth

Smooth Muscle Structure

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Does not appear striated under the light microscope

Spindle shaped muscle fibres

Each muscle fibre contains a single nucleus

Relaxed fibres are 500 µm long and 5 µm wide

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Voluntary (Skeletal/Striated) Muscle

Contraction of voluntary muscle leads to movement of bones at joints.

Contraction is quick and powerful.

Voluntary muscle fatigues easily because it is adapted to carry out anaerobic respiration. Accumulation of lactate causes muscle fatigue

Voluntary Muscle Fibre Structure

Shows a striated pattern under the light microscope

Relaxed fibres are 100 µm in diameter

Each fibre contains several nuclei

These muscle fibres do not have intercalated discs, are not branched and do not have spaces between the fibres

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Role of the Brain and Nervous System in the Co-ordination of Muscular Movement

Muscles contract when they receive nervous impulses, transmitted along motor neurones, from the brain or spinal cord.

Cardiac and smooth muscles are under the involuntary control of the autonomic nervous system in the medulla oblongata.

Skeletal muscle is under voluntary control, involving the cerebrum and the cerebellum.

The motor neurones are connected to the muscle fibres that they control, at neuromuscular junctions

Skeletal Movement at Joints with Reference to Movement of the Elbow Joint

Voluntary muscles are attached to bones of the skeleton by tendons

Tendons are made of inelastic collagen fibres. They are continuous with the muscle tissue at one end and the connective tissue around the bone (the periosteum) at the other end

The force of muscle contraction moves the bone

Movement of any bone at a joint involves at least two muscles

When muscles work in pairs, they are described as antagonistic. Whilst one muscle contracts to move a bone, the other muscle relaxes, to allow smooth movement

The movement of many bones is more complex than this. It involves a wider range of actions and is under the control of groups of muscles called synergists

Movement at the Elbow Joint

At the elbow joint, the biceps muscle contracts to raise the forearm (radius and ulna bones) whist the triceps relaxes. The biceps is the flexor muscle

Contraction of the triceps muscle lowers the forearm. The triceps is the extensor muscle

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The Elbow Joint – an Example of a Synovial Joint

Structure of the Synovial Joints

A joint is where at least two bones meet

A synovial joint contains synovial fluid (a viscous lubricant containing mucin)

A synovial joint allows a large amount of movement.

The knee, hip, elbow and shoulder joints are examples of synovial joints.

The features of a synovial joint include the following:

1. The ligament capsule (made by the ligaments) that joins bone to bone and encloses the joint. Ligaments contain collagen fibres and are tough, but allow some flexibility. Ligaments prevent dislocation of the joint

2. The synovial membrane. This lies beneath the ligament capsule and is a membrane that secretes synovial fluid

3. Synovial fluid, a viscous lubricant containing mucin that fills the joint capsule. Synovial fluid reduces friction as the two bones in the joint articulate against each other and produces easier movement

4. Smooth articular cartilage. This soft smooth tissue covers the rounded ends of the bones that articulate against each other. It reduces friction and therefore reduces wear/damage of the bones during movement. Cartilage also acts as a shock absorber during movement

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The main features of synovial joints are shown in the figure below. Label cartilage, synovial membrane, synovial fluid, bones and the ligament capsule

Neuromuscular Junctions

Definition:

A neuromuscular junction is where the motor neurone meets the muscle fibre. It is a specialised type of synapse

Structure of the Neuromuscular Junction

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Features to note are:

The terminal knob of the motor neurone has many mitochondria

The terminal knob of the motor neurone has vesicles containing acetylcholine (Ach)

The gap between the presynaptic membrane and the sarcolemma of the muscle fibre is called the synaptic cleft

The sarcolemma of the muscle fibre (post-synaptic membrane) is highly folded (providing a large surface area) and contains receptors for acetylcholine

The endings of the motor neurone are also referred to as the motor end plates

Comparison of the Actions of the Synapse and the Neuromuscular Junction

Similarities

1. Action potentials travel down the presynaptic neurone to the presynaptic knob

2. The action potentials stimulate the uptake of calcium ions into the presynaptic knob

3. Calcium ions cause the vesicles in the presynaptic knob, containing neurotransmitter, to fuse with the presynaptic membrane

4. The neurotransmitter diffuses across the synaptic cleft and binds to neurotransmitter receptors in the postsynaptic membrane of the next neurone or of the sarcolemma

5. The binding of neurotransmitter causes sodium voltage gated channels to open and sodium ions to flood into the next neurone or into the muscle fibre

6. In the postsynaptic neurone, a postsynaptic potential is produced as the membrane is depolarised. In the muscle fibre, an end plate potential is produced, as the sarcolemma is depolarised

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7. The sarcolemma and neurone plasma membranes have the same channel proteins and Na+/K+ pumps

8. At both synapses, an impulse is transmitted across a gap

9. The neurotransmitter is broken down by an enzyme, in the synaptic cleft, to avoid overstimulation of the postsynaptic neurone or muscle fibre

Differences

Neurone to Neurone Synapse Neuromuscular Junction

Various types of neurotransmitter released, including acetylcholine

Neurotransmitter is always aceytylcholine

Release of neurotransmitter causes an action potential in the postsynaptic neurone

Release of neurotransmitter is linked to muscle contraction

Skeletal Muscle Structure

A muscle e.g. the biceps muscle, is an organ made up of several different tissues that carry out the function of contraction to move a bone. The tissues include:

1. Muscle made up of muscle fibres (muscle cells)2. Connective 3. Nervous 4. Blood

The figure below shows the arrangement of tissues in the muscle

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(A) Shows a section through a muscle. The muscle contains a bundle of muscle fibres

embedded in connective tissue. The muscle is supplied by an artery, vein and motor nerve

(B) Shows one fibre bundle containing muscle fibres (cells) embedded in connective tissue and surrounded by blood capillaries and nerve fibres/axons

(C) Shows a muscle fibre containing many myofibrils

(D) Shows a more detailed picture of a muscle fibre. Each myofibril has a striped appearance due to the arrangement of actin and myosin microfilaments it contains

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The Detailed Structure of a Skeletal Muscle Fibre

1. The plasma membrane of the muscle fibre is called the sarcolemma

2. The cytoplasm is called the sarcoplasm

3. The fibre contains many mitochondria that provide ATP for muscle contraction. Identify these on the diagram below

4. Each muscle fibre has many nuclei, described as multinucleate

5. There is an extensive system of ER called the sarcoplasmic reticulum, used to store Ca2+ for muscle contraction

6. At intervals, there are infoldings of the sarcolemma called T-tubules (transverse tubules) that penetrate the fibre

7. There are many glycogen granules in each fibre. Why are these present?

It is energy storage of glucose that can be quickly hydrolysed when contracting. This is useful storage as it is insoluble so doesn’t affect water potential. Lots of glucose is necessary because lots of ATP is needed via aerobic and anaerobic respiration to contract the muscle.

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Structure of the myofibril and arrangement of the microfilaments

The myofibril is divided into functional units called sarcomeres

The myofibril has a characteristic banding pattern because of the arrangement of the protein microfilaments, actin and myosin.

Myosin filaments are thicker than actin.

Actin and myosin filaments are arranged horizontally across the myofibril and are attached to vertical strips of connective tissue – the Z lines and the M line

A sarcomere is a unit bordered by two vertical strips of connective tissue called Z lines.

The thinner actin filaments are connected to these Z lines

The thicker myosin filaments are found in the centre of the sarcomere. They also lie horizontally and are attached to a vertical strip of connective tissue called the M line (the M line is in the middle of the sarcomere)

For parts of their lengths, actin and myosin filaments overlap

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Complete the diagram above as follows:

1. Label 2 more Z lines

2. Label 1 more M line

3. How many complete sarcomeres are shown in this diagram? ...................Apart from the Z and M lines, three regions can be distinguished across the myofibril as follows:

1. I band or light band: this band consists only of the thinner actin filaments and hence is light in shade. The Z line is in the centre of the I band (to remember that the I band is light, remember that I is the second letter in LIGHT)

2. A band or dark band: this region includes the whole width of the myosin filaments across the M line. The outer parts of the A band contain overlapping actin and myosin filaments (to remember that the A band is dark, remember that A is the second letter of DARK)

3. H zone: this is in the very centre of the sarcomere where only thick myosin filaments cross the M line. It is darker than the I band but lighter than the A band

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In two dimensional diagrams, as in the figure above, it appears that each myosin is associated with only two actin filaments, in the region where they over-lap.

In cross/transverse section, it can be seen that each myosin filament is surrounded by six actin filaments as shown above

Another Representation of the Myofibril Structure

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Changes in Banding Pattern During Muscle Contraction

When a muscle contracts, the actin filaments are pulled in towards the myosin filaments, creating a greater degree of overlap between the two filaments. The banding pattern changes as follows:

The I band becomes narrower, as actin slides in between the myosin

The H zone becomes narrower, as the overlap between the two filaments increases

The sarcomere shortens, as the Z lines move closer together

The A band remains the same width as the length of the myosin filaments does not change

Width (mm) Relaxed Sarcomere Contracted SarcomereSarcomere

A band

I band

H zone17

Electron micrograph showing a longitudinal section of contracted striated muscle

Identify T, U and V by labelling the diagram.

Name the structure between positions X and Y

Also identify and label the M line, A band H zone and I band

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The Structure of Myosin and Actin

Diagrams showing the structures of these molecules are shown in the figure below

Myosin filaments are made up of hundreds of molecules of the protein myosin

Each myosin molecule has a rod-shaped fibrous protein tail and two globular protein heads made of ATPase (see above)

The fibrous tails are anchored in the M line

The heads have binding sites for actin

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Actin filaments

The actin filaments are made up of two long chains of the globular protein actin

The two chains are twisted around each other in a helical arrangement

The actin filaments have many binding sites for myosin along their length

Actin filaments are anchored in the Z line

Each actin filament is associated with two other proteins:

Tropomyosin – this forms long thin strands around the actin filament. In the relaxed muscle, tropomyosin blocks the myosin binding sites on the actin molecule

Troponin – is a globular protein. Troponin molecules are attached to the tropomyosin molecule at regular intervals. In the relaxed muscle fibre, troponin molecules keep the tropomyosin in position, blocking the myosin binding sites on the actin filaments.

Mechanism of muscle contraction

The mechanism of muscle fibres is called the sliding filament hypothesis, as the actin and myosin filaments slide between each other to cause the contraction and shorten the muscle fibres.

Initiation of Contraction (numbers refer to the diagram on page 21)

Action potentials pass down the motor neurone (1) to the presynaptic membrane and stimulate calcium ions to enter the synaptic bulb (2)

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The calcium ions cause the movement of vesicles, containing acetylcholine, towards the presynaptic membrane (3). The vesicles fuse with the presynaptic membrane and release the acetylcholine into the synaptic cleft by exocytosis

Acetyl choline diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma of the muscle fibre (4)

The binding of ACh causes Na+ voltage gated channels to open in the sarcolemma and Na+ ions flood into the muscle fibre

The sarcolemma is depolarised (its membrane permeability has been changed) and if this depolarisation is above a threshold level, action potentials are transmitted along the sarcolemma (5) and down into the transverse system tubules (the T- tubules) (6)

The action potential travelling down the T-tubules causes the opening of calcium channels in the sarcoplasmic reticulum membrane and the release of calcium ions into the sarcoplasm (7)

The calcium ions bind to the troponin molecules causing troponin to change shape

This troponin shape change results in the tropomyosin filaments moving away from the actin filaments, exposing the myosin-binding sites on the actin filaments

(A) in the diagram below shows the structure of the actin filaments during relaxation. (B) shows actin filaments ready for contraction

Contraction

ATP is a source of energy that can be used directly by muscles during contraction

The myosin heads attach to the exposed binding sites on the actin filaments, forming actin-myosin cross bridges

The myosin heads swivel and change angle causing the actin filaments to slide past the stationary myosin towards the centre of the sarcomere. In this state, the myosin molecules are in their most stable form

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The swivelling of the myosin head to move actin towards the M line is called the power stroke

Energy from ATP is required to break the cross bridge and re-position the myosin head. For this to occur, ATP binds to the myosin head whilst it remains in the actin-myosin cross bridge

The ATP is hydrolysed to ADP and Pi as the myosin head swivels back to its original position. The myosin head can then form another cross bridge to actin, further along the actin filament and bend again to move the actin further towards the middle of the sarcomere

In a contracting muscle, several million cross-bridges are continuously being made and broken, causing the actin filaments to pass over the myosin filaments and shorten the sarcomere

Shortening of all the sarcomeres contracts the muscle

Complete the picture below by adding molecules of ATP or ADP to the myosin head. ATP attachment to the myosin head is required before myosin can detach from actin and ATP hydrolysis is needed to swivel myosin heads back to their original position

Relaxation

Contraction stops when nervous stimulation stops

Calcium ions are pumped out of the myofibrils and back into the sarcoplasmic reticulum, through carrier proteins on the sarcoplasmic reticulum membrane This also requires ATP

The troponin molecules regain their original shape, pushing against the tropomyosin molecules to re- block the myosin-binding sites on the actin filaments

The actin filaments will naturally slide away from the myosin filaments, and this relaxation process is helped by the contraction of the antagonistic muscle (remember that skeletal muscles always work in pairs – one contracts whilst the other relaxes)

Roles of ATP in Muscle Contraction

1. ATP is required for the release of acetylcholine from vesicles in the presynaptic bulb, into the synaptic cleft by exocytosis

2. Attachment of ATP to the myosin head is required and ATP hydrolysis, to detach myosin heads from the cross-bridges and re-position the myosin heads to their original position. The myosin head can then bind to the next myosin binding site on the actin molecule and swivel, to move the actin filament closer to the M line

3. ATP is required for the active transport of calcium ions back into the sarcoplasmic reticulum after muscle contraction

Sources of ATP in Muscle Fibres

1. From aerobic respiration in muscle fibres. There are many mitochondria in these cells but oxygen is required and this may be in limited supply. Muscle fibres do store the

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respiratory pigment myoglobin that binds oxygen. Myoglobin has a high affinity for oxygen, but releases it when the partial pressure of oxygen in the muscle tissue is very low. ATP production in aerobic respiration takes longer than the other two processes described

2. From anaerobic respiration in muscle fibres. Voluntary fibres are adapted for anaerobic respiration but less ATP is formed and lactate causes muscle fatigue

3. From creatinine phosphate. This molecule is stored in muscle fibres. It is a source of phosphate that can combine with ADP to synthesise ATP rapidly.

Creatinine phosphate is used to synthesise ATP during the first few seconds of a sprint race, followed by anaerobic respiration. The race is far too fast to rely on ATP from aerobic respiration

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