Types of Muscular Tissue and Contraction

Study Notes

Types of Muscular Tissue

Smooth Muscle:
- Unstriated
- Involuntary (controlled by autonomic nervous system)
- Found in walls of visceral organs
- Small, elongated cells with a centrally located nucleus
Cardiac Muscle:
- Striated
- Involuntary
- Located in the heart
- Single nucleus, cells connected by special junctions to conduct electrical impulses
Skeletal Muscle:
- Striated
- Voluntary (controlled by somatic nerves)
- Attached to the skeleton
- Multinucleated

Skeletal Muscle Contraction

Myofilaments:
- Thick filaments: composed of myosin
- Thin filaments: composed of actin and tropomyosin
Contraction-Relaxation Cycle:
- Filaments slide along each other during contraction and relaxation
Sarcomere:
- Functional unit of striated muscle
ATP:
- Used for muscle contraction
- Derived from aerobic and anaerobic glycolysis

Types of Muscle Contraction

Isotonic Contraction:
- Muscle shortens as it contracts
- Generates tension equal to or greater than opposing forces (loads)
Isometric Contraction:
- Muscle generates tension but does not shorten
- Occurs when load is greater than the force generated by the muscle

Nervous Tissue

Neuron (Nerve Cell):
- Basic structural unit of nervous tissue
- Composed of:
  - Cell body
  - Processes:
    - Long (axons)
    - Short (dendrites)

Types of Neurons

Morphological Classification:
- Unipolar
- Bipolar
- Multipolar
Golgi Type Classification:
- Golgi Type I: large with long axons
- Golgi Type II: short axon

Neuroglia

Supporting Cells for Neurons:
- Astrocytes: (fibrous, protoplasmatic) - insulation or barriers
- Oligodendrocytes: – myelin formation in the central nervous system (CNS)
- Microglia: – residual macrophages
- Ependyma: – lining of the cavities of the brain
- Schwann cells: – myelin formation in the peripheral nervous system (PNS)

Nerve Fibers

Myelinated:
- Surrounded by a myelin sheath formed by supporting cells:
  - Central: oligodendrocytes
  - Peripheral: Schwann cells
Nonmyelinated:
- Smaller axons

Neuron Reaction to Injury

Severe Injury:
- Causes cell death
- Neurons cannot be replaced because they are "postmitotic" cells (fully differentiated and do not divide)
Response to Injury:
- Chromatolysis: Disorganization of ribosomes and cellular swelling in the cell body and axon proximal to the injury site
- Wallerian Degeneration: Disintegration of the distal axon and all synaptic endings
- Schwann Cell Activity: Exhibit mitotic activity and produce trophic substances
- Regeneration: Many neurons can regenerate a new axon if lost due to injury

Signal Conductivity in Neurons

Electrical Potential Difference:
- Exists across the plasma membranes of most animal cells
- Cytoplasm is usually electrically negative relative to extracellular fluid
Resting Membrane Potential:
- Electrical potential difference across the plasma membrane of a cell at rest
- Plays a central role in the excitability of nerve and muscle cells
- Due to unequal distribution of ions between the inside and outside of the cell membrane

Action Potential (AP)

- **Excitability:**
    - Shown only by specialized cells like nerves and muscles
    - Involves transient changes in ion conductance and potential of their membranes
- **Action Potential:**
    - Signal transmitted along nerve cells or causing muscle contraction
    - Generated when a stimulus reaches a critical strength (threshold potential)
- **Phases of AP:**
    - **Depolarization:** Stimulus reduces resting membrane potential to a less negative value
    - **Na+ Channel Activation:** At threshold potential, Na+ channels open leading to Na+ influx into the cell
    - **Repolarization:** Na+ conductivity decreases and K+ conductance increases, returning the membrane to resting potential

Action Potential Properties

All-or-None Response:
- Once threshold potential is reached, the cell responds with full depolarization
Refractory Period:
- Absolute: Nerve cannot be excited even with a strong stimulus
- Relative: Nerve can be excited with a stronger stimulus

Propagation of the Action Potential in Nerve Fibers

Types of Propagation:
- Serial:
  - Slow - in non-myelinated nerves
  - Conduction rate - about 1 m/s
- Saltatory:
  - Much faster - in myelinated nerves
  - Conduction rate up to 120 m/s

Synaptic Potentials

Transmitter Release:
- AP at the axon terminal releases a neurotransmitter substance
Postsynaptic Effects:
- Transmitter can depolarize (excitation) or hyperpolarize (inhibition) the postsynaptic membrane
- Excitatory Transmitters: Acetylcholine, Substance P, Glutamate - evoke excitatory post synaptic potentials (EPSP)
- Inhibitory Transmitters: GABA, Glycine - evoke inhibitory postsynaptic potentials (IPSP).
EPSP Summation:
- Single EPSP is usually insufficient to generate AP
- Multiple simultaneous EPSPs can depolarize the cell to the threshold potential (spatial and temporal summation)