Nervous System - Excitable Tissues

Nervous System - Excitable Tissues

• The neuron is the structural unit of the nervous system.
• Reflex action is the functional unit, involving receptor, afferent, center, efferent, and effector organs.
• Anatomical Classification:
  • CNS (Central Nervous System) includes the brain (cerebrum, subcortical centers, cerebellum, brain stem) and spinal cord (31 segments with horns and columns).
  • PNS (Peripheral Nervous System) includes 12 pairs of cranial nerves and 31 pairs of spinal nerves.
• Physiological Classification:
  • Sensory part transmits sensations from receptors to sensory centers (e.g., somatic, visceral, special senses).
  • Motor part transmits orders from motor centers to effector organs (somatic and autonomic).

Nerve Properties

• Excitability:
  • Nerves respond to stimuli (electrical, chemical, mechanical, thermal).
  • Threshold stimulus is the minimum intensity needed for a response.
  • Subthreshold stimuli have intensities less than the threshold.
  • Suprathreshold stimuli have intensities greater than the threshold.
  • Responses involve electrical changes (action potentials), excitability changes, metabolic changes, and thermal changes.
• Conductivity:
  • Nerve impulses are conducted along nerve fibers.
  • Unmyelinated fibers conduct impulses via sweeping conduction.
  • Myelinated fibers conduct impulses via saltatory conduction.

All or None Law

• Nerve fibers either respond maximally or not at all to a threshold stimulus, provided other factors affecting excitability remain constant.
• Applies to: single nerve fiber, single motor unit, cardiac muscle, and unitary smooth muscle.
• Does not apply to: whole skeletal muscle, nerve trunks, and multiunit smooth muscle.

Summation

• Subthreshold stimuli, applied successively within short periods (<0.5 ms), can summate to produce an action potential.

Adaptation (Accommodation)

• Nerve fibers quickly adapt to constant or gradually increasing stimuli.
• The response to a constant stimulus is only at the beginning and end; no response occurs during the actual stimulus.

Infatigability

• Nerve fibers don't become fatigued with repeated stimulation.

Electrical Changes in Nerves

• Resting Membrane Potential (RMP): The potential difference between the outer and inner surfaces of a nerve cell membrane at rest. Typically around -70mV.
• Selective permeability and Na+/K+ pump are crucial for maintaining RMP. The Na+/K+ pump pumps 3 sodium ions out and 2 potassium ions in for each ATP.
• Depolarization: A reduction in the membrane potential (e.g. moving toward 0 mV).
• Repolarization: A return to the resting membrane potential.
• Hyperpolarization: An increase in the membrane potential (e.g. moving further from 0 mV).
• Action Potential: The sequence of depolarization and repolarization that occurs in response to a stimulus strong enough to reach the threshold potential.

Mechanism of Neurotransmission

• Nerve impulse transmission occurs at the neuromuscular junction.
• This involves the release of acetylcholine, which triggers depolarization in the muscle.

Mechanism of Skeletal Muscle Contraction

• Depolarization of the muscle fiber initiates contraction.
• Release of Ca2+ from the sarcoplasmic reticulum allows binding of myosin heads to actin, leading to a sliding filament mechanism.

Propagation of Nerve Impulse

• Absolute Refractory Period: During this period, the nerve cannot respond to further stimuli. Coincides with ascending limb of action potential).
• Relative Refractory Period: The nerve can respond to stimulation but a stronger stimulus is required. Coincides with remaining part of repolarization).
• Supernormal Phase: Excitability is higher than normal.
• Subnormal Phase: Excitability is lower than normal.