Properties of excitable tissues and types of stimuli
Excitable tissues include muscle, nerve, and secretory tissues. Their basic properties are:
- Excitability is the ability of a cell to generate an action potential in response to stimulation.
- Contractility is the ability to change length and tension.
- Conductivity is the ability to transmit excitation to other cells and tissues.
- Irritability is the ability to respond to a stimulus with changes in metabolism, shape, etc.
- Automaticity is the generation of excitation without an external stimulus.
- Lability is the ability to maintain rhythmic activity.
Stimuli are classified by strength into subthreshold, threshold, and suprathreshold; by application site into contact and distant; by nature into adequate and inadequate; and by type into chemical, mechanical, thermal, and physical.
Ion channels and resting potential
A distinction is made between leak channels (constantly open, poorly sensitive to membrane potential) and gated channels: voltage-gated, ligand-gated, and mechanically sensitive.
Resting potential is the potential difference between the inner and outer surfaces of the membrane at rest (about −70 mV). The conditions for its formation are:
- Ionic asymmetry — the difference in concentrations of K⁺, Na⁺, and Cl⁻ inside and outside the cell.
- High membrane permeability to K⁺ and low permeability to Na⁺ at rest.
Diffusion of K⁺ through leak channels charges the outer surface positively and intracellular anions charge the inner surface negatively. Concentration gradients are maintained by Na⁺/K⁺-ATPase: using ATP energy, it pumps 3 Na⁺ out of the cell and 2 K⁺ in.
Action potential
Action potential occurs when a threshold-strength stimulus is applied and includes several phases:
- Local response (slow depolarization) — partial activation of Na⁺ channels; the membrane depolarizes to the critical level of depolarization (CLD).
- Rapid depolarization — avalanche-like influx of Na⁺, charge inversion (inside +, outside −), peak around +30 to +37 mV.
- Repolarization — inactivation of Na⁺ channels, opening of voltage-gated K⁺ channels, and K⁺ efflux restoring the potential.
- After-potentials — after-depolarization (residual Na⁺ current) and after-hyperpolarization (residual K⁺ efflux).
Laws of stimulation
- Strength-duration law: the duration of stimulation depends on current strength. Rheobase is the minimum strength that causes excitation; useful time is the minimum duration of action of one rheobase; chronaxie is the duration of action of double the rheobase.
- All-or-none law: a subthreshold stimulus produces no response, while a threshold or higher stimulus produces a maximum response (characteristic of the myocardium).
- Gradient law (accommodation): the rate of rise of current strength matters.
- Polar law of direct current stimulation: on closure, the cathode stimulates; on opening, the anode stimulates; the stimulation threshold is lower at the cathode.
- Law of strength relations: for a whole nerve or muscle, the response increases with stimulus strength up to a certain limit; optimum and pessimum of frequency are distinguished.
Laws of excitation conduction
- Anatomical and physiological integrity of the fiber is a prerequisite for conduction.
- Bilateral conduction — excitation spreads in both directions from the point of origin.
- Decrementless conduction — the amplitude of the action potential does not decrease along the length of the fiber.
- Isolated conduction — excitation does not pass to neighboring fibers.
- Saltatory conduction — in myelinated fibers, excitation "jumps" from one node of Ranvier to the next, increasing velocity.
- Continuous conduction — in unmyelinated fibers, excitation spreads along the entire membrane and proceeds more slowly.
Skeletal muscle contraction
Types of contractions:
- Isometric — length does not change, tension changes (holding a load).
- Isotonic — length changes at constant tension (lifting a load).
- Auxotonic — length and tension change simultaneously (jumping).
Types of contractions: concentric (shortening while overcoming resistance) and eccentric (lengthening while controlling resistance).
Contraction modes: single twitch — response to a single stimulus; tonic — posture maintenance; tetanic — sustained tension during a series of stimuli. A distinction is made between unfused (serrated) tetanus (stimuli arrive during the relaxation phase) and fused (smooth) tetanus (stimuli arrive during the shortening phase).
Synapses and features of synaptic transmission
According to the mode of transmission, synapses are divided into chemical (via a neurotransmitter), electrical (via gap junctions), and myoneural (neuromuscular). Based on the neurotransmitter, they are classified as cholinergic (acetylcholine), adrenergic (norepinephrine, epinephrine), serotoninergic, dopaminergic, glutamatergic, GABAergic, etc.
Features of conduction through a chemical synapse:
- Unidirectional conduction — from the presynaptic to the postsynaptic membrane.
- Synaptic delay — the time between the arrival of the action potential and the generation of the postsynaptic potential.
- Low lability and high fatigability during prolonged stimulation.
- Summation of excitations (temporal and spatial).
- Transformation of the rhythm of excitation.
- Synaptic aftereffect — persistence of influence after the cessation of the stimulus.
- High chemical sensitivity and selectivity to neurotransmitters and pharmacological substances.
Key points
- Excitable tissues — nervous, muscle, and secretory — possess excitability, conductivity, contractility, and lability.
- The resting potential is maintained by ionic asymmetry, predominant permeability to K⁺, and the activity of Na⁺/K⁺-ATPase.
- The action potential includes a local response, rapid depolarization with inversion, repolarization, and after-potentials.
- Excitability parameters are described by rheobase, useful time, and chronaxie.
- In myelinated fibers, conduction is saltatory and faster, whereas in unmyelinated fibers it is continuous.
- Muscle contractions are subdivided into isometric, isotonic, and auxotonic; modes include single twitch, tetanus (unfused and smooth), and tonic.
- The chemical synapse provides unidirectional conduction with synaptic delay, summation, and high fatigability.
- The classification of synapses by neurotransmitter determines their pharmacological sensitivity and functional characteristics.