2. Smooth and Cardiac muscle Flashcards
What are the three main types of muscle in the body?
Skeletal
Cardiac
Smooth
Describe skeletal muscle
Voluntary muscle
Anchored to bone by tendons
Multinucleate - these are peripherally located
Clear cross striations of actin and myosin
Important in locomotion and breathing
No cellular junctions
Neuromuscular junctions
Describe cardiac muscle
Involuntary muscle
Fibres have centrally located nuclei
Cardiac myocytes are organised in a branched meshwork of fibres which run in various directions
Intrinsic myogenic activity - responsible for beating of the heart
Contractile elements are similar to skeletal muscle - actin and myosin comprised of a sarcomere
Intercalated disks have gap junctions
No neuromuscular junctions
Describe smooth muscle
Involuntary
Non-striated
Spindle-shaped fibres (wide in the middle and tapered at both ends)
Single nucleus
Provides mechanical control of organ systems
Gap junctions
No neuromuscular junctions
Describe the composition of cardiac muscle in detail
Have myofibrils that are comprised of sarcomeres - run from Z disk to Z disk containing actin (thin) and myosin (thick)
Packed full of mitochondria - requires ATP for contraction
Contains sarcolemma - specialised plasma membrane in muscle cells - arranged so it runs into the fibres and action potentials can get right to where they’re needed
Contains gap junctions - connect the cells to each other - allow ions and very small molecules to pass through
Intercalated disks join the fibres together - these are sites of thickening of the sarcolemma where the cardiac myocytes are joined together
Describe cardiac myocytes and how they work with one another
These form ‘electrical syncytium’ or ‘functional syncytium’
The electrical impulses can propagate freely between cells in every direction
Cells contract in synchronous fashion - this is important for the pumping action of the heart
Wave of depolarisation propagates to the adjacent cells via the gap junctions in the intercalated disks - this allows rapid, synchronous depolarisation of the myocardium
Describe the composition of cardiac muscle in detail
Have myofibrils that are comprised of sarcomeres - run from Z disk to Z disk containing actin (thin) and myosin (thick)
Packed full of mitochondria - requires ATP for contraction
Contains sarcolemma - specialised plasma membrane in muscle cells - arranged so it runs into the fibres and action potentials can get right to where they’re needed
Contains gap junctions - connect the cells to each other - allow ions and very small molecules to pass through
Intercalated disks join the fibres together - these are sites of thickening of the sarcolemma where the cardiac myocytes are joined together
These disks also contain desmosomes and gap junctions - gap junctions link the cells electrically
Describe cardiac myocytes
These form ‘electrical syncytium’ or ‘functional syncytium’
The electrical impulse
Describe the sequence of action potentials in cardiac muscle
In muscle cells, the action potential is the first step in the chain of events which lead to the contraction
Many action potentials in the heart - properties of these vary with different locations of cardiac cells in the heart e.e. SA node, AV node, bundle of His, atrium, ventricle
What are the two different tissue types/types of cells in the heart?
Pacemaker tissue
Ventricular cells
Describe pacemaker tissue of the heart
SAN and AVN
Spontaneously depolarise
Have automaticity SO ability to initiate their own beat
Contain different ion channels - allows for initiation of different action potentials
Don’t have a steady, flat baseline - have a leaky baseline that causes the depolarisation
How is the cardiac muscle modulated by the autonomic nervous system
The autonomic nervous system can have both sympathetic and parasympathetic effects on the cardiac muscle/cardiac myocytes
Give the mechanisms of calcium signalling during cardiac contraction
- Depolarisation of the sarcolemma due to influx of Na+ causes opening of Ca2+ VGCs
- Cardiac contraction is initiated by the influx of the Ca2+ through the L type VGCs (LTCC) in the sarcolemma
- The rise in intracellular Ca2+ triggers further Ca2+ release from the sarcoplasmic reticulum (store of calcium) by the RYANODINE RECEPTOR (RyR)
- The calcium associates with troponin C in the sarcomere and this stimulates sarcoplasmic reticulum (systole)
- Release of Ca2+ from the sarcomere causes the relaxation (diastole) and it’s re-uptake into the sarcoplasmic reticulum by SERCA
List common areas where smooth muscle is found
(Provides mechanical control of organ systems)
Bladder
Uterus
Upper respiratory tract
Briefly describe smooth muscle
Involuntary
Non-striated muscle
Heterogenous group of muscle
Wide range of physiological properties
List common areas where smooth muscle is found
(Provides mechanical control of organ systems) Bladder Uterus Upper respiratory tract Gut (oesophagus, stomach, intestines) Blood vessels Bronchi Urethra Erector pili in the skin
Describe in detail the structure of smooth muscle
Loose lattice of actin and myosin - these run obliquely across the muscle (SO not striated)
Cytoskeletal intermediate filaments - these aid the transmission of force within muscle contraction
Dense bodies - serve as attachments for the actin and myosin (equate to Z lines in striated muscle)
Two types of attachment:
-Mechanical attachment between cells
-Gap junctions provide provide a pathway for electrical signals
List common areas where smooth muscle is found
(Provides mechanical control of organ systems)
Bladder
Uterus
Upper respiratory tract
Describe in detail the structure of smooth muscle
There
What are the two different types of smooth muscle in terms of the way it is innervated?
Multiunit smooth muscle (works more like skeletal)
Single unit smooth muscle (works more like cardiac)
Describe the mechanism of contraction of smooth muscle
Innervation occurs via the autonomic nervous system
Nerves make multiple contacts with the muscle cells called VARICOSITIES - these release neurotransmitter in space surrounding the membrane and the receptors are widely spread across the post-synaptic membrane
Gap junctions also permit electrical coupling between cells
Both are used in cells and the degree of each usage is tissue specific
What are the two different types of smooth muscle?
Multiunit smooth muscle (works more like skeletal)
Single unit smooth muscle (works more like cardiac)
Describe single unit smooth muscle
ANS innervates single cell within a sheet or bundle
Action potential is propagated by gap junctions to neighbouring cells
Contraction occurs as a bundle or sheet as a functional SYNCYTIUM
Generally produces slow, steady contractions - allows substances to move through the body e.g. digestive tract
Where is single unit smooth muscle found?
Found in the walls of all visceral organs (apart from the heart)
Describe single unit smooth muscle
ANS innervates single cell within a sheet or bundle
Action potential is propagated by gap junctions to neighbouring cells
Contraction occurs as a bundle or sheet as a functional SYNCYTIUM
Generally produces slow, steady contractions
Where is single unit smooth muscle found?
Found in the walls of all visceral organs (apart from the heart)
Describe the different action potentials that occur in smooth muscle
There are different types - simple spike, spikes followed by a plateau, spikes on top of slow waves
Action potentials last much longer than in skeletal muscle
Dependent on the opening of Ca2+ VGCs (sometimes Na+)
Ca2+ VGC open more slowly than Na+ and so there is a slower contraction compared to skeletal muscles
Contribution of each ion depends on the type of muscle
Describe how smooth muscle can be regulated
Contraction stimulated by:
Hormones
Neural stimulation
ANS
Local factors
In certain locations, e.g. walls of visceral organs, stretching of muscle stimulates contraction (stretch-relaxation response)
Smooth muscle also expresses wide variety of neurotransmitter and hormone receptors
Give the mechanism of contraction of smooth muscle
- Thin filaments slide past the thick filaments and pull on the dense bodies
- These dense bodies pull on the intermediate filament network throughout the sarcoplasm
- This arrangement causes the entire muscle fibre to contract - ends are pulled towards the centre and the midsection bulges
Describe how smooth muscle can be regulated
Contraction stimulated by:
Hormones
Neural stimulation
ANS
Local factors
In certain locations, e.g. walls of visceral organs, stretching of muscle stimulates contraction (stretch-relaxation response)
Smooth muscle also expresses wide variety of neurotransmitter and hormone receptors
What are the two different types of smooth muscle?
Vascular smooth muscle
Bronchial smooth muscle
Describe vascular smooth muscle
Found within, and composes, majority of the wall of blood vessels
Describe the sympathetic impact on bronchiole smooth muscle
Causes bronchiole dilation
Adrenaline/Noradrenaline - B2 adrenergic receptor
Short acting beta agonists or long acting beta agonists
Describe the parasympathetic impact on bronchiole smooth muscle
Causes bronchiole constriction
Mucus secretion
Actylcholine - muscarinic receptprs (M3)
Short acting or long acting - can be used as treatments
Describe the sympathetic impact on bronchiole smooth muscle
Causes bronchiole dilation
Adrenaline/Noradrenaline - B2 adrenergic receptor
Short acting beta agonists or long acting beta agonists
What is meant by the ‘excitation - contraction’ coupling in smooth muscle?
Excitation of the smooth muscle stimulates and is followed by the contraction of the smooth muscle
These two stages happen consecutively
