الجهاز العصبي - الأنسجة المثيرة

Nervous System - Excitable Tissues

• The neuron is the basic structural and functional unit of the nervous system.
• Reflex action is a functional unit involving receptor, afferent pathway, integrating center, efferent pathway, and effector organ.
• The nervous system is classified anatomically into central nervous system (CNS) and peripheral nervous system (PNS).
  • CNS includes the brain (cerebrum, subcortical centers, cerebellum, brain stem) and spinal cord (31 segments).
  • PNS includes 12 pairs of cranial nerves and 31 pairs of spinal nerves.
• Physiologically, the nervous system is classified into sensory and motor parts.
  • Sensory part transmits sensations from receptors to sensory centers (somatic, visceral, and special sensations).
  • Motor part transmits orders from motor centers to effector organs, controlling voluntary (somatic) and involuntary (autonomic) movements.
  • Common physiological classification includes CNS (somatic and autonomic) and special senses.

Nerve Properties

• Excitability: Nerves respond to stimuli (electrical, chemical, mechanical, thermal) according to their intensity (threshold, subthreshold, suprathreshold).
• Conductivity: Nerve impulses are conducted along nerve fibers (unmyelinated - sweeping conduction, myelinated - saltatory conduction).

Nerve Function

• All-or-none Law: Nerve responds maximally or not at all to a threshold stimulus.
• Summation: Subthreshold stimuli can summate to reach threshold and produce an action potential if applied successively within a short period.
• Adaptation: The nerve fiber quickly adapts itself when stimulated by a current, or a current with gradually rising intensity.
• Infatigability: Nerve fibers do not tire with repeated stimulation.

Electrical Properties

• Resting Membrane Potential (RMP): The membrane potential difference between the outer and inner surfaces of a nerve cell at rest is approximately -70 mV. Factors contributing to this include selective permeability and the Na+/K+ pump.
• Potential Changes: The nerve potential changes during stimulation, passing through depolarization, repolarization, after-depolarization, and after-hyperpolarization phases. These changes are affected by cathodal (depolarizing) and anodal (hyperpolarizing) currents.

Action Potential Propagation

• Phases of Excitability
  • Absolute Refractory Period: Excitability is completely lost during the ascending limb of action potential.
  • Relative Refractory Period: Excitability is partially recovered (weaker stimulus required) during the repolarization phase.
  • Supernormal Phase: Excitability is higher than normal during the after-depolarization phase.
  • Subnormal Phase: Excitability is lower than normal during the after-hyperpolarization phase.

Neuromuscular Transmission

• Action potentials are transmitted across the neuromuscular junction causing muscle contraction.