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.
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.
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.
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.
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.
