1. Cardiac Muscle

Function

• The heart must beat continually on a timescale of about 1 beat per second
  • Does not need to sustain a contraction
  • Cannot be prone to Tetanus
• Must be a delay between Atrial and Ventricular Contractions
• Involuntary control
• Not to hang between bones

Anatomy

Auto Rhythmicity

• Contractions are involuntary
• The muscle fibres are electrically coupled through Gap Junctions within each compartment, but electrically separated between compartment
• Basic rhythm and coordination of the activation of different compartments is established by specialised cells, which, themselves don‘t contract.
  • Pacemaker - Sinoatrial Node (SAN)

Muscle Attachment

• Muscle fibres are arranged in a circular spiral order
• Embedded in loose connective tissue, Endomycium.
  • Innervated by Autonomic Nervous System - Modulatory Effects
  • Has many capillaries
• Blood supplied by Coronary Arteries
  • Under tight Metabolic Autoregulation
• Attached to Fibrous Skeleton

Cardiac Muscle Fibre

• Intercalated Discs and Desmosomes
  • Large contact areas where cells are tightly connected
• Gap Junctions
  • Provide Electrical Continuity
    • AP pass unhindered from cell to cell
  • The whole Atrial and Ventricular Muscle each build a functional Syncytium
    • Electrically isolated from each other
    • Syncytium - one stimulus will fire the whole of the compartment
• Muscle Fibres are cylindrical, uni-nucleated, branched and striated
• T-tubule system has a greater diameter and surface area
• SR is less dense and Cisternae are less prominent
  • Cisternae are arborisations of SR which store $Ca^{2+}$
• Cardiac Muscle has more and larger mitochondria than Skeletal Muscles
• Much more Mb - almost exclusive reliance on Aerobic Metabolism
  • use Glucose and Fatty Acid as its main sources of ATP by Oxidative Phosphorylation

2. Cardiac Muscle Contraction

• Some Cardiac Cells are self-excitable and spontaneously trigger their own depolarisation
  • Influence by hormones
• Similar contraction process as for Skeletal Muscle
  • Cardiac Muscle differs in the sequence of events that cause $Ca^{2+}$ release from SR
• In Cardiac Muscle: Calcium Induced Calcium Release CICR
  • AP travels down T-tubules opening VG $Ca^{2+}$ Channels
    • $Ca^{2+}$ diffuse from extracellular fluid into cells causing a small increase in cytosolic $Ca^{2+}$ conc. in the region of the T-tubules and adjacent to SR
  • Small increase in $Ca^{2+}$ induce the binding of $Ca^{2+}$ to receptors on SR membrane
    • These receptors contain intrinsic $Ca^{2+}$ Channels
  • Activation of Receptors opens $Ca^{2+}$ Channels allowing a large net diffusion of $Ca^{2+}$ release from SR
• Thus the movement of $Ca^{2+}$ during AP acts as a signal for contraction
  • Contraction ends when Cytosolic $Ca^{2+}$ conc. is restored to its original extremely low level by active transport of $Ca^{2+}$ back to SR
  • The small amount that entered during excitation is actively transported out into extracellular space
• In Cardiac Muscle the release of $Ca^{2+}$ is not sufficient to saturate all Troponin sites
  • Therefore, increased release of $Ca^{2+}$ from SR will achieve a greater strength of contraction

3. Skeletal Muscle vs. Cardiac Muscle

Skeletal Muscle

1. Latent/Refractory Period while the short (1-2ms) AP spreads over the Sarcolemma
2. Tension$\uparrow$ while the SR releases $Ca^{2+}$ and the Actin-myosin activity starts
3. Tension$\downarrow$ as $Ca^{2+}$ is pumped back into SR
4. Twitch lasts from 10 to 200ms depending on Muscle Fibre Type
5. Essentially no Refractory Period

Cardiac Muscle

4. Smooth Muscle

Function

• Smooth Muscle is found in walls of hollow internal structures
• Functions include:
• Need to function well when being stretched
• Smooth muscle forms Internal Sphincters: