Untitled Quiz

Study Notes

Myelinated nerve fibers have a myelin sheath.
- The myelin sheath is interrupted by nodes of Ranvier.
- The myelin sheath is a protein-lipid complex created by Schwann cells.
- It facilitates conduction in axons.
Non-myelinated nerve fibers are not covered by a myelin sheath.

Afferent nerve fibers conduct impulses from receptors to the central nervous system (CNS).
Efferent nerve fibers conduct impulses from the CNS to effector organs (like muscles).

Excitability: The ability of the organism to respond to a stimulus.
Conductivity: The ability of the nerve to transmit an impulse away from the cell body along the axon.
Adaptation: Decreasing impulse discharge in response to constant or continuous stimulation.
Not fatigable: Nerves can continue to transmit impulses without getting tired.

A stimulus is a change in the environment that triggers a response in a living organism.
Electrical stimuli are preferred because intensity, onset, and duration can be controlled.
They do not damage the tissue.
Chemical stimuli include oxygen, carbon dioxide, hydrogen ions, acids, and bases.
Mechanical stimuli include pain and movement.
Physical stimuli include temperature changes (hot and cold).
Electromagnetic stimuli include sunlight and light.

Membrane potential is the electrical potential difference between the inner and outer surfaces of a cell's membrane.
It is present in all body cells.
Types of membrane potentials include resting membrane potential and action potential.

A resting membrane potential occurs due to unequal ion distribution across the cell membrane.
Nerve fibers typically have a resting membrane potential of -70 mV.
Skeletal and cardiac muscle fibers approximately -90 mV.
Causes include selective membrane permeability (higher permeability to K+ than Na+), Na+/K+-ATPase pump (actively pumps 3 Na+ out and 2 K+ in), and intracellular negatively charged proteins.

Cell membranes, including nerve cell membranes, have various ion channels.
Leak channels are always open, allowing ions to passively diffuse across the membrane (permeability to K+ is 100 times higher than Na+ during rest).
Gated channels open or close in response to specific signals (voltage-gated or ligand-gated).

Action potential is a rapid change in membrane potential.
Stages:
- Latent period: time between stimulus and response (membrane potential rises toward zero or -65 mV).
- Depolarization: rapid loss of membrane polarity.
- Repolarization : restoration of normal polarity.
- Hyperpolarization: when membrane potential overshoots (more negative than resting potential).
Action potential typically takes about 40 ms.

As Na+ channels close, inactivation gates close, K+ channels open.
K+ rushes out of the cell, restoring negative membrane potential.
The process may lead to hyperpolarization (more negative than resting potential).

Action potential can propagate in any direction along the nerve.
Propagation mechanisms depend on whether the nerve is myelinated or not.
In unmyelinated nerves, conduction is called "sweeping conduction", where depolarization spreads continuously along the membrane.
In myelinated nerves, conduction is called "saltatory conduction," where depolarization jumps from node to node, making it faster.

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.
The motor neuron has:
- Mitochondria for energy.
- Acetylcholine vesicles.
- Voltage-gated Ca2+ channels.
The synaptic cleft has acetylcholine esterase (AChE).
The motor end plate has receptors for acetylcholine and acetylcholine gated Na+ channels.
The arrival of an action potential at the axon terminal releases acetylcholine.
Acetylcholine binds to receptors on the muscle fiber causing depolarization and initiating the action potential in the muscle fibers leading to muscle contraction.