The Structure and Ultrastructure of Muscle Tissues PDF

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The Structure and Ultrastructure of Muscle Tissues...

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The Structure and Ultrastructure of Muscle Tissues Key Terms   

Cytoplasm = sarcoplasm Cell surface membrane = sarcolemma Smooth ER = sarcoplasmic reticulum

LO1: Summarise the structural and functional features that distinguish muscles from other basic tissue types Muscle is the only body tissue with all of the following 4 properties:    

Excitability: Ability to receive and respond to a stimulus via generation of an action potential Contractility – Ability to contract in response to a stimulus. Extensibility – Ability to be stretched or extended in response to a force (other tissues usually tear) Elasticity – Ability of a muscle fibre to recoil back to its resting length when stretch force is removed.

LO2: Outline the distinguishing structural and functional features of the 3 types of muscle tissue (including relative orientation of cytoskeletal fibres)

Skeletal muscle  Skeletal Muscle cell = Myocyte = Muscle fibre (skeletal)  Striated due to light & dark bands (actin and myosin)  Long, cylindrical multinucleated fibres - formed from myoblast fusion  Nuclei are flattened, elongated and found at the periphery which allows for high fibre density  No cell-cell junctions  Myosatellite cells (multipotent cells) found at basement membrane: o Provide additional myonuclei which are used in the repair processes o They can also proliferate to form myoblasts to replace damaged muscle cells (due to wear and tear)

Connective tissue of skeletal muscle  Endomysium: o Thin layer of connective around each individual myocyte. o Composed of basal lamina, small blood capillaries & axons  Perimysium: o Surrounds fascicles (bundles of myocytes) o Contains nerves, larger BVs & lymph vessels  Epimysium: o Dense CT surrounding the entire muscle o Continuous with tendon which binds muscle to its bone

The Effect of Aerobic Exercise on Muscles  Regular aerobic exercise on our skeletal muscles results in an increased blood flow to muscle, and increase in energy stored at the muscle (fat and glycogen) and an increase in the number of mitochondria at muscles.  This increases the muscles ability to undertake aerobic exercise more efficiently, but the muscle does not usually increase in strength or size. The Effect of Anaerobic Exercise on Muscles  After a workout, the Myosatellite cells repairs or replaces damaged muscle fibres.  These repaired myofibrils increase in thickness (fibre hypertrophy) and number (hyperplasia) to create muscle hypertrophy.

Types of skeletal muscle fibres Typical colour Fast/slow twitch Fibre diameter Myoglobin content Mitochondria Metabolism

Type I/S Red (Dark) Slow Small high Numerous Oxidative phosphorylation

Contractions Fatigue Rate Example

Slow and continuous slow fatigue rate Postural back muscles

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Type IIa/FR Red (Dark) Intermediate Medium high Numerous Oxidative phosphorylation and anaerobic glycolysis Fast Medium fatigue rate Major leg muscles

Type IIb/FF White Fast Large low Sparse Aerobic glycolysis

Fast Fast fatigue rate Extraocular muscles

In terms of excitability, slow twitch fibres have the lowest activation threshold so are recruited first. All the Type 1 muscle fibres will be connected to the same motor neurone etc. The motor neurone determine the properties of the muscle fibres.

Motor Unit Recruitment  The activation of one motor neuron will result in a weak but distributed muscle contraction as the myocytes are distributed thought the muscle.  The higher the recruitment (of motor neurones) the stronger the muscle contraction will be.  Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons, and thus slow to fast twitch). This means both fine motor actions and large muscular actions can be performed.

Cardiac muscle  Cardiomyocytes = cardiac muscle cell  Striated due to light & dark bands (actin and myosin)  Cardiomyocytes are surrounded by endomysium which contains a rich capillary network  Cardiac muscle is myogenic (the signal contraction arises within the heart tissue itself). ANS only controls rate & strength of contraction.  Cardiomyocytes are branched so that each cell is in contact with three of four other cardiomyocytes. This allows for the spreading of an action potential from cell to cell allows for fast contraction in a wave-like pattern to effectively pump blood throughout the body.  Cardiomyocytes are connected by intercalated discs – occur at the Z line of the sarcomere. They are made up of three types of cell junctions o Fascia Adherens: are anchoring sites for actin filaments o Desmosomes: stop separation during contraction by joining the cells together. o Gap junctions: allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarisation of the heart muscle.  There are lipid droplets closely associated with mitochondria which store triglycerides. These are rapidly broken down for oxidative phosphorylation to produce energy for contraction.  Purkinje fibres are specialised cardiomyocytes. They are larger with fewer myofibrils and large numbers of mitochondria  conduct cardiac AP faster and more efficiently. They ensure contraction is from apex of heart upwards  allows for more efficient emptying. Fatigue  Cardiac muscle resists fatigue so well because it’s got more mitochondria than skeletal muscle. It also has a rich capillary network which provides enough oxygen to the muscles.  So cardiac muscle continually produces its ATP requirement purely by aerobic respiration.  So as result cardiac muscle does not form Anaerobic processes hence lactic acid is never formed enabling heart muscles to perform their function without fatigue.

Smooth muscle  The smooth muscle cells are anchored to the surrounding connective tissue (endomysium) by a basal lamina.  Non-striated  Smooth muscle produces its own endomysium.  Smooth muscle is myogenic (the signal contraction arises within the muscle tissue itself). ANS only controls rate & strength of contraction.  Axonal swellings (varicosities) lie in close contact with the sarcolemma providing the neurotransmitter to the smooth muscle cells. There are two types of smooth muscles: 

Visceral (single unit) smooth muscle - Visceral smooth muscle has gap junctions to

allow for rapid spread of depolarisation. This allows for simultaneous contraction of cells - all work together at the same time as one unit. Found in the uterus, gastro-intestinal tract, and the bladder. 

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Multi-unit smooth muscle - cells can act and function independently of each other. This allows for finer motor control. Multiunit smooth muscle is found in airways to the lungs and large arteries

Peristaltic Waves Peristalsis is the involuntary wave like contraction of the longitudinal and circular muscles, primarily in the digestive tract. In the esophagus, peristaltic waves begin at the upper portion of the tube and travel the whole length, pushing food ahead of the wave into the stomach. Particles of food left behind in the oesophagus initiate secondary peristaltic waves that remove leftover substances. Peristaltic waves start as weak contractions at the beginning of the stomach and progressively become stronger as they near the distal stomach regions. The waves help to mix the stomach contents and propel food to the small intestine.

Summary

Striations No. of nuclei Position of nucleus Shape of nucleus Branching Shape of fibres Fibre length Gap Junctions T tubules SR Contraction Protein filaments present Innervation Function Location

Skeletal muscle Cardiac muscle Present Present Multinucleate 1 Peripheral, adjacent to Central sarcolemma Elongated Oval Absent Present Cylindrical Cylindrical

Smooth muscle Absent 1 Central

Longest Absent Centre of triads at A-I junction Well developed

shortest Present In diads at z discs

Elongated Absent Fusiform (spindleshaped) middle Present Absent

SR less well developed

Not well developed

Same Actin and myosin

Same Actin and myosin

Different/same Actin and myosin

Somatic Skeletal movement + posture Everywhere

Autonomic Cardiac contraction

Autonomic Slow, steady contraction GIT, BV walls, respiratory tracts, bladder, uterus

Heart

LO3: Understand the role of sarcoplasmic reticulum, t-tubules and NMJ in the normal functioning of striated muscles (Skeletal and cardiac muscle both contract via sliding filament model)  

The NMJ is where the motor neurone depolarises the sarcolemma T-tubules are deep folds of sarcolemma which allow depolarisation to reach deep into muscle so uniform contraction occurs.  Sarcoplasmic Reticulum stores Ca2+ ions, releases them on reception of AP which causes contraction, so low conc. Ca2+ in sarcoplasm at rest  Triad: a structure formed by the 2 Sarcoplasmic Reticulum on either side of t-tubule (typical at A-I junction - junction between A and I bands) (for full explanation on contraction go to notes)

Ultrastructure of Skeletal muscle 1. Myofibrils have alternating light and dark bands:

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Light bands (also known as I-bands) — these areas appear light as they are the region where only actin is present. Dark bands (also known as A-bands) — these areas appear dark as the thick myosin filament is overlapped with actin. Z-line — found at the centre of each light band. This is where actin is anchored. The distance between adjacent Z-lines is called a sarcomere. When a muscle contracts the sarcomere shortens. H-zone — this is a lighter coloured region found in the centre of each dark band. Only myosin filaments are present at this point. When the muscle contracts the H-zone decreases. M-Line – where myosin is anchored.

During contraction the myosin filaments pull the actin filaments inwards towards the centre of the sarcomere. This results in:  the light band becoming narrower  the H -zone becoming narrower  the Z lines moving closer together, shortening the sarcomere The dark band remains the same width, as the myosin filaments themselves have not shortened, but now overlap more with the actin filaments.

H-Zone

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The simultaneous contraction of lots of sarcomeres means that the myofibrils and muscle fibres contract. This results in enough force to pull on a bone and cause movement. When sarcomeres return to their original length the muscle relaxes.

LO4: Recognise and identify histological specimens of muscles in longitudinal and transverse sections

Longitudinal sections of skeletal muscles (things to look out for):    

Very long multinucleate cells Many peripheral Nuclei Striations No Branching

Transverse sections of skeletal muscles (things to look out for):  Peripherally located nuclei 

Multinucleated

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Massive Cytoplasm

Skeletal muscle micrograph – transverse section

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A: Perimysium B: Endomysium C: Extrafusal Myocyte: muscle fibres that produce detectable force D: Muscle Spindles: Muscle spindles are skeletal muscle sensory receptors within the body of a muscle that primarily detect changes in the length of this muscle contributing to fine motor control and providing axial and limb position information to the central nervous system. E: Intrafusal Myocyte: Produce electrical activity – (see above)

Longitudinal sections of Cardiac muscles (things to look out for):

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A: centrally placed round to oblong nuclei striations branching, intercalated discs (*)

Transverse sections of Cardiac muscles (things to look out for):   

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central nuclei You may notice a high density of blood vessels and a relatively high proportion of cells that have been sectioned through the nucleus. a large ring of surrounding cytoplasm. These features would enable you to distinguish between cardiac and smooth muscle.

Longitudinal sections of Smooth muscles (things to look out for):

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Long, thin central nuclei and an absence of striations.

Relaxed

Contracted (Grainy Nucleus)

Transverse sections of Smooth muscles (things to look out for): 

central nuclei

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small ring of surrounding cytoplasm. It may be evident that relatively few cells have been sectioned through the nucleus. Compare this to cardiac muscle in TS.

NB: Smooth Muscle in structures (this example is the wall of the digestive tube) may show both a longitudinal and transverse cross section. 2 layers:  A longitudinal smooth muscle (outer)  B circular smooth muscle (inner)

LO5: Describe the basic histological response of each category of muscle to disease and trauma and appreciate the functional effects of such damage

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Repair and regeneration can occur in skeletal muscle because of a population of reserve Myosatellite cells that can proliferate, fuse and form new muscle fibres Cardiac muscle lacks satellite cells and has little capacity for regeneration Regeneration is rapid in smooth muscle because the cells/fibres are small and relatively less differentiated, which allow renewed mitotic activity after injury

Clinical correlations Which junctions are present in the connection between cardiac myocytes? Why is the function of each junction type important to normal cardiac function?

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Gap junctions (provide ionic continuity between cells) These serve as “electrical synapses” and allow cells of cardiac muscle to act like a multinucleated syncytium as in skeletal muscle, with contraction signals passing in a wave from cell to cell

Why is the blood supply so important to cardiac cells? What happens if it is lost?  All cells in the body, including cardiac cells, need oxygen and the vital nutrients found in the blood – without these the cells will die  Cardiac cells are specifically important as the heart helps to provide oxygen and nutrients to the body’s tissues and organs by ensuring a rich supply of blood  Blood supply is also important for transporting carbon dioxide and waste products away from cells  Lack of blood supply to cardiac cells → cardiac cells cannot respire → cardiac cells die What are muscle spindles and what is their function? Are they present in all muscle types?  Among the muscle fascicles are stretch detectors known as muscle spindles A muscle spindle is encapsulated by modified perimysium, with concentric (of or denoting circles, share the same centre, the larger often completely surrounding the smaller) layers of flattened cells, containing interstitial fluid and a few thin muscle fibres filled with nuclei called intrafusal fibers  Several sensory nerve axon penetrate each muscle spindle  Changes in length of the surrounding muscle fibres caused by body movements are detected by the muscle spindles and sensory nerves relay this information to the spinal cord  Muscle spindles are found in skeletal muscle 

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If the somatic motor fibre supplying skeletal muscle is lost, the muscle will eventually atrophy (waste away) – this is covered in later sessions. Why is this relationship not the same if the nerves supplying cardiac and smooth muscle are lost? Despite losing body fat, a young person doing heavy exercise in a gym is likely to increase weight....