Excitable Tissues and Neurons Overview

Excitable Tissues

• Nerves and muscles are excitable tissues, responding to environmental changes (stimuli) by altering electrical properties and generating impulses. These impulses transmit signals.

Nerve Cells (Neurons)

• Neurons are the structural and functional units of the nervous system.
• They have four main parts:
  • Cell Body
  • Dendrites (receive signals)
  • Axon (transmits signals)
  • Axonal Terminals (transmit signals to other cells)
• Myelin sheath covers many nerve fibers, insulating fibers and increasing impulse speed.

Physiology of Cell Membrane

• Extracellular fluid (ECF) and intracellular fluid (ICF) have different compositions.
• Cell membranes are lipid bilayers with proteins, acting as barriers for water and water-soluble substances.
• Membrane potential is created by ion concentration differences across the membrane, due to ion diffusion from high concentration areas to low concentration areas.

Ion Channels

• Plasma membranes have various ion channels, made from membrane proteins:
  • Leakage channels (passive): always open, permitting ion/molecule movement
  • Gated channels (active): open/close in response to signals
    • Chemically-gated channels (ligand): open when a neurotransmitter binds
    • Voltage-gated channels: respond to changes in electrical potential across the membrane

Membrane Potentials

• Polarized state (resting membrane potential): difference in potential between the outer and inner surfaces of the membrane in a resting state.
• Depolarized state: the reduction of membrane potential negativity
• Hyperpolarized state: increase in membrane potential negativity.

Factors Determining RMP

• Selective permeability of the membrane to potassium ions plays a significant role. Potassium leak channels are more permeable to K+ than Na+.

Active Transport

• Sodium-Potassium Pump (Na+-K+ pump): actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell, maintaining the negative potential inside the membrane. ATP provides the energy.

Action Potential

• Definition: a transient reversal in the membrane polarity of an excitable cell.
• Phases:
  • Resting phase: membrane is polarized (e.g., -70 mV).
  • Local excitatory state: depolarization from -70 mV to -55 mV.
  • Depolarization phase: membrane potential changes from -55 mV to +35 mV.
  • Repolarization phase: return to resting potential.
  • Hyperpolarization: temporary overshooting of resting potential.
• Excitability changes:
  • Absolute refractory period (ARP): nerve has no response to stimulus.
  • Relative refractory period (RRP): stronger stimulus required to trigger response.

Local Potential

• A partial, non-propagated change in membrane potential, a response to sub-threshold stimuli.
• Has characteristics differing from action potentials:
  • Smaller amplitude
  • Graded/proportional to stimulus strength
  • Non-propagated
  • Summation possible

All or None Law

• A stimulus must reach a threshold to trigger an action potential—no change in amplitude, with stimuli exceeding threshold causing the same response magnitude.

Propagation

• Conduction in unmyelinated fibers: continuous depolarization, slow.
• Conduction in myelinated fibers: jumps between nodes of Ranvier, fast.

Factors Affecting Conduction Velocity

• Nerve fiber type: type A (fastest), B, and C (slowest).
• Fiber diameter: thicker fibers, faster conduction.
• Myelin sheath: thicker sheath, faster conduction.
• Temperature: higher temperatures, faster conduction.

Muscular Tissue

• Types of muscle tissue: skeletal, cardiac, and smooth muscles.
• Characteristics of each:
  • Skeletal: striated, voluntary, multinucleated.
  • Cardiac: striated, involuntary, uninucleated.
  • Smooth: unstriated, involuntary, uninucleated

Myofibril

• Contains thick (myosin) and thin (actin) filaments. Ordered arrangement of filaments produces striations.
• Myosin heads bind to actin, causing the thin filaments to slide over the thick filaments.

Myofibril Structure

• Thick filaments (myosin): tail and head regions, with heads containing binding sites for actin and ATP.
• Thin filaments (actin), tropomyosin, troponin

Sarcomere

• The functional unit of muscle contraction, consisting of overlapping thick and thin filaments.

Transverse T-tubules (T-tubules)

• Invaginations of the sarcolemma (muscle cell membrane) carrying the action potential into the muscle fiber interior.

Sarcoplasmic Reticulum (SR)

• A network of tubules and cisternae within muscle cells, storing calcium ions, and releasing them upon stimulation to initiate contraction.

Neuromuscular Junction

• The connection between a motor neuron and a muscle fiber.
• Structures involved: axon terminal, motor end plate, synaptic cleft
• Neurotransmitter: Acetylcholine (ACh).

Steps of Neuromuscular Transmission

• Action potential travels to axon terminal.
• Calcium channels open, releasing ACh into the synaptic cleft.
• ACh binds to receptors triggering muscle action potential.
• Acetylcholinesterase breaks down ACh.
• Calcium ions are actively pumped back into the SR.
• Myosin heads detach from actin filaments.

Properties of Neuromuscular Transmission

• Unidirectional
• Time delay
• Fatigue: exhaustion of Ach vesicles during repeated stimulation.
• Ion effect: excess Mg+ ions decrease Ach release, while Ca++ increase it.

Drugs Affecting Neuromuscular Junction

• Some drugs inhibit transmission (e.g., botulinum toxin, curare).
• Others enhance transmission (e.g., nicotine, cholinesterase inhibitors).

Cross Bridge Cycle

• A cyclical series of steps involved in muscle contraction, where myosin heads bind to actin, rotate, and detach causing the sliding movement of filaments.

Factors Affecting Muscle Fiber Length

• Initial length of sarcomere affects the amount of tension a muscle can generate. Maximum tension occurs when there's optimal overlap between actin and myosin filaments.

Types of Muscle Fibers and Mechanical Changes

• Fast and slow twitch muscle fibers differ in their ATP source (oxidative or glycolytic), contraction time, mitochondria quantity, and fatigue resistance.

• Isotonic contractions: muscle shortens, doing work.

• Iso-metric contractions: no muscle shortening, maintaining tension.