Flashcards in Muscular System Deck (12):
What are the 3 main muscle types?
- Skeletal (striated)
- Cardiac (striated)
- Smooth (non-striated)
Describe smooth muscle cells.
- Length = 20-200 micrometers (up to 0.5 millimetres in uterus)
- Fusiform (spindle-shaped) with a central nucleus
- Connected via connective tissue, gap and desomosome-type junctions
- Form sheets, bundles or layers containing 1000s of cells
Where is smooth muscle found?
- Forms contractile walls of passageways or cavities (modify volume), e.g. Of vascular structures, or in gut, respiratory tract and genitourinary system
Describe how smooth muscle cells contract.
- Not striated: no sarcomeres or T tubules, but contraction still relies on actin-myosin interactions. Thick and thin filaments are arranged diagonally within the cell, spiralling down the long axis - contracts in a twisting way.
- Contraction is involuntary but can respond to stimuli in form of nerve signals, hormones, drugs or local concentrations of blood gases. Most cells are innervate by autonomic NS fibres that release their NTs from varicosities into a wide synaptic cleft.
- Contraction is slower, more sustained and requires less ATP - may remain contracted for hours or days.
Give examples of disorders in which the involuntary nature of smooth muscle contraction is of clinical significance.
- High blood pressure (i.e. Primary hypertension)
- Abnormal gut mobility, e.g. IBS
In which form can modified smooth muscle cells occur singly?
- Myoepithelial cells: stellate cells forming a basketwork around the secretory units of some exocrine glands (e.g. Sweat, salivary and mammary glands). Contraction assists secretion of sweat, saliva or milk into secretory ducts. Also occur in the ocular iris to contract/dilate the pupil.
- Myofibroblasts: produce collagenous matrix and contract (abundant actin and myosin) at sites of would healing.
Describe cardiac muscle cells.
- Length: 50-100 micrometers, diameter: 10-20 micrometers
- Cells form short, branched cylinders with a single central nucleus and striations.
- Distinct myofibrils are absent - actin and myosin filaments form continuous masses in cytoplasm.
- Intercalated discs join cells end to end (substitute for Z bands). Have gap junctions for electrical coupling and adherents-type junctions to anchor cells and provide anchorage for actin filaments.
- T tubules lie in register with Z bands and are closely associated with sarcopasmic reticulum at diads.
Describe the contraction of cardiac muscle cells.
- Intrinsic (lifelong variable) rhythm, involuntary autonomic modulation.
- Spontaneous contractions occur due to APs generated in the sinoatrial node, passing on to atrioventricular nodes and from there to the ventricles via Purkinje fibres.
What are Purkinje fibres?
- Large modified myocytes with: abundant glycogen, sparse myofilaments and extensive gap junction sites.
- Conduct APs rapidly, enabling ventricles to contract in a synchronous manner.
What are natriuretic peptides?
- Peptide hormones synthesised by the heart, brain and other organs.
What are the roles of natriuretic peptides in the heart?
- Main action is to reduce arterial pressure by decreasing blood volume and systemic vascular resistance (i.e. Promote vasodilation). Serve as a counter-regulatory system for the renin-angiotensin-aldosterone system.
- Release is stimulated by atrial and ventricular distension, usually in response to heart failure.
- Normal hearts secrete extremely small amounts of ANP, but elevated levels are found in patients with left ventricular hypertrophy and mitral valve disease.