Muscle Contraction Flashcards
Describe basic histological terms: Trabacula
Trabecula, pl. trabeculae - a supporting or anchoring strand of connective tissue, usually extending from a capsule into the substance of the enclosed organ
What are muscles bundles of?
A muscle is a bundle of fasicles. Fasicles are in turn a bundle of fibres. Fibres are a bundle of myofibrils. Myofibrils are repeating functional units called sacromeres.
- Muscle
- Fasicles
- Fibres
- Myofibrils
- Sacromeres
Describe the structure of a smooth muscle cell
- Smooth muscle is found in the walls of hollow organs like your intestines and stomach and reproductive organs. They work using the ANS.
- Cytoplasm of smooth muscle cells - contains tightly packed contractile protein filaments (CP).
- Single, centrally located, nucleus
- Have Perinuclear lucent zone (PLZ) main area of mitochondria and other non contractile organelles, suurrounding nuscleus
- Smooth muscle cells surrounded by External lamina (EL).
- Caveoli membrane invaginations (V) increase surface area control Ca2+ entry into the cell.
- Terminal sac of sarcoplasmic reticulum (SR) adjacent to the membrane. - calcium store
Histological Terms: Serosa
Serosa - a serous membrane, comprised of a mesothelium and underlying connective tissue, lining the serous cavities of the body.
Histological Terms: Septum
Septum, pl. septa -the dividing wall or partition usually between lobes of an organ.
Histological Terms: Stroma
Stroma - the supporting tissue or scaffolding of an organ, as distinguished from its functional element, or parenchyma.
Histological Terms: Parenchyma
Parenchyma - the essential elements of an organ; a general term used to designate the functional elements of an organ as distinguished from its framework, or stroma.
Histological Terms: Mucosa
Mucosa - a mucous membrane, comprised of several layers (an epithelium, lamina propria, and often a muscularis mucosae).
Histological Terms: Medulla
Medulla - the inner portion of an organ, usually in the center.
Histological Terms: Lumen
Lumen - the cavity or channel within a tube or hollow organ. (Abluminal=on the side away from the lumen; as “the basal lamina is on the abluminal side of the endothelium”. adluminal- toward the lumen; as “the microvilli are on the adluminal side of the gut absorptive epithelium.”)
Histological Terms: Hilus/ Hilum
Hilus or Hilum - a depression or pit at that part of an organ where the vessels and nerves enter.
Histological Terms: Cortex
Cortex - the outer portion of an organ, distinguished from its inner, medullary portion.
Histological Terms: Capsule
Capsule - a structure enclosing an organ, usually composed of dense connective tissue.
Histological Terms: Afferent
Afferent - conveying toward an organ, as an afferent lymphatic vessel, or afferent nerve
Histological Terms: Efferent
Efferent -conveying away from an organ, as efferent lymphatic vessel or efferent nerve. These terms are used with reference to an understood organ.
Sarcolemma?
Muscle cell membrane
◦(Sarco (from Sarx) from Greek; Flesh, and Lemma from Greek; sheath.)
What is the difference between skeletal and smooth muscle?
Skeletal: Single very long cells with obvious striations (muscle tissue that features repeating functional units called sarcomeres). Have multiple nuclei.
Smooth Muscle: Uni-nuceus, no striations from sacromeres, so no sacromeres, contracts differently to cardiac and skeletal muscle.
Describe cardiac muscle
Cardiac: Chains of cells that are branched, uni-nucleus. Have intercalated discs (cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected byintercalated discs to work as a single functional organ, by allowing synchronized muscle contraction).
Describe the structure of a sacromere?
- The sides of the sacromere are called the z disc. It attatches neighbouring sacromeres. The z-line is made of desmin.
- Actin is attatched to the z disc using alpha actinin
- Myosin is attatched to the z disc using titin protein
- The A-band is where actin and myosin overlap
- The I-band is that part of the sarcomere that contains thin filaments (actin only).
- The M- band is the line in the middle of the sacromere formed of cross-connecting elements of the cytoskeleton. The tails of the myosin are attatched to the m-line using myomesin.
- The H-zone is where there is only myosin/ thick filaments.
What happens to the sacromere bands during contraction?
During contraction the muscle shortens.
- The I band (where there is only thin actin filaments and titin protein) pulls on z-disk to make it shorter.
- The A- band stays the same length. This is the area of overlap between myosin and actin filaments.
- The H-zone shortens and dissappears.
Structure of actin
- Is a globuler protein
- Two actin strands form into a double alpha helix
- Tropomyosin is a rope like protein (held by tropinin) that wraps around actin covering the myosin binding sites and blocks myosin from grabbing actin, so the muscle is not contracted all the time.
Structure of Myosin
- Has a head and tail
- Made of two light chains and two heavy chains
Steps in Neuro-muscular Junction Transmission?
- Electrical impuse reaches the end of the motor neurone, and causes an increases in calcium ion permeability in the synaptic knob (the action potential is converted into a chemical message).
- Release of transmitter (Ach) at motor end-plate (where a neuron and muscle meet) by exocytosis.
- The ACH binds to nicotinic receptors, causing increased Na+ and K+ conductance in motor end-plate.
- Generation of end-plate potential.
- Generation of action potential in muscle fibres.
Resting potential in muscle cells, neurones and epithelial cells?
- In a typical neuron, its value is −70 mV,
- In a typical skeletal muscle cell, its value is −90 mV
- In a typical epithelial cell, its value is closer to −50 mV
What happens when the muscle receives the action potential?
- The action potential passes along the sacrolemma (muscle membrane) until it reaches the transverse tubles (invaginations of the sacrolemma that go deep into the muscle fibre).
- This causes depolarisation on the t- tubule. Activates voltage sensitive calcium channel. Mechanical coupling with calcium channel on SR causes SR calcium channel to open. Calcium leaves SR.
- The calcium binds to tropinin, causing a conformational change in troponin, and moves away from the troponin binding site. Myosin head free to interact with actin.
What is cross bridge cycling?
- How the sacromere contracts.
- Calcium ions released from sacroplasmic reticulum bind to troponin causing it to change shape, so tropomyosin moves away from the myosin binding sites.
- Myosin head binds to actin and is in its ‘rigor state’
- ATP attatches to the myosin head and is hydrolysed to ADP and inorganic phosphate.
- The energy released from hydrolysing ATP allows the myosin head to move to its ‘cocked state’.
- The now activated myosin head binds to a new place on the actin. Inorganic phosphate is released. Myosin head performs power stroke. Sliding filaments
- ADP released. Cycle repeats.
Smooth Muscle Innervation
- Innervated by the ANS
- Autonomic neurones can make multiple contacts on a smooth muscle cell.
- Autonomic neurones form swellings at contact point called a varicosity - vesicular neurotransmitter release.
- Neurotransmitters: Post ganglionic parasympathetic nerve releases Acetylcholine (Ach). Post ganglionic sympathetic nerve release Norepinephrine
Types of smooth muscle innervation?
Smooth muscle has two distinct organisations:
◦Multiunit:- Each cell receives nervous input, cells are electrically isolated, therefore act independently from one another. No spontaneous contractions. Contractions specifically controlled by ANS input from varicosities. Requires distinct action potentials. Characterised by fast contraction, like in the iris.
◦Unitary:- Few cells receive nervous input, gap junctions allow communication, cells coordinate. Cells can contract spontaneously, without input from the NS. ANS input can increase and decrease the level of contraction from varicosities. Signals communicated through gap junctions- Ca2+ waves. Characterised by slow contraction -eg, blood vessels.
Smooth Muscle and Hormonal Input
Epinephrine / Norepinephrine released from adrenal gland on kidney
Though under control of ANS, because released into blood, classed as hormones
Act on specific receptors on smooth muscle cells
Structure of Contractile Elements
In a smooth muscle cell in it’s relaxed state, actin and myosin filaments are attatched to the membrane using focal densities. They are attatched to other filaments using dense bodies. When the filaments contract (actin moves across myosin, filaments shorten) they pull the cell into a more globular shape. External lamina helps cells contract together
Differences between smooth muscle and skeletal contraction?
Similar to Skeletal muscle but some differences:
- Ca2+ influx in smooth muscle from both intracellular AND extracellular sources (sarcoplasmic reticulum – calcium store)
- Action potential not always required
- Many smooth muscle cells initiate spontaneous electrical activity (unitary)
- Smooth muscle contraction not always dependent on electrical activity – Pharmacomechanical coupling
- Smooth muscle Ca2+ sensor is calmodulin, skeletal muscle is troponin
Calcium entry into smooth muscle cytoplasm?
3 routes of entry:
◦Voltage independent (from sarcoplasmic reticulum or though other calcium channels (eg. ion gated).
◦Voltage dependent (voltage gated channels in response to graded depolarisations or action potentials).
◦Smooth muscle contraction occurs when cytoplasmic Ca2+ increases.
◦Smooth muscle relaxation occurs when cytoplasmic Ca2+ decreases.
Mechanism of contraction in smooth muscle
- A trigger like an action potential causes an increase in calcium ion contraction (from intracellular- sacroplasmic reticulum and extracellular - calcium channels or hormonal)
- The intracellular calcium binds to calmodulin and this activates myosin light chain kinase.
- Myosin light chain kinase phosphorylates light chains on myosin heads and causes increase in ATPase activity (an enzyme that catalyses the hydrolysis or decomposition of ATP into ADP and a free phosphate ion, thereby releasing energy for use in another biochemical reaction).
- Active myosin crossbridges walk along actin and create muscle tension.
Mechanism of relaxation in smooth muscle
1) Ca++ decreases as it is pumped out of the cell or back to the SR.
2) Ca++ unbinds from Calmodulin (CM).
3) Myosin phosphatase removes phosphate from myosin, this decreases myosin ATPase activity.
4) Reduced ATP hydrolysis results in decreased muscle tension.
