Organization of the Nervous System Quiz

Study Notes

The nervous system (NS) ensures sensory detection, interpretation, and motor response.
Functions categorized into sensory (information gathering), integrative (processing and interpretation), and motor (response execution).

Neurons: Functional units responsible for transmitting stimuli, classified as:
- Afferent: Transmit action potentials (APs) to the CNS.
- Interneurons: Relay APs within the CNS.
- Efferent: Convey APs to the PNS.
Neuroglia: Supportive cells categorized into CNS (astrocytes, oligodendrocytes, microglia, ependymal cells) and PNS (neurolemmocytes, satellite cells).

Neurons are electrically excitable due to voltage differences across their membranes.
Two types of electrical signals:
- Graded potentials: Short-distance communication involving local membrane changes.
- Action potentials: Can travel long distances and constitute nerve impulses.

Resting membrane potential is typically around -70mV, influenced by ion channels.
Graded potentials can trigger action potentials by reaching a threshold of -55mV.
Voltage-gated Na+ channels open, allowing Na+ ions to flood into the cell, resulting in depolarization to +30mV.
Following depolarization, voltage-gated K+ channels open, allowing K+ to exit, leading to repolarization.

Depolarization: Initial rising phase as Na+ enters.
Repolarization: K+ exits, returning membrane potential to resting state.
Hyperpolarization: Potential dips below resting state before returning to normal.
The Na+/K+ ATPase pumps restore the resting membrane potential at the end of the action potential cycle.

Leakage (nongated) channels: Always open, with higher permeability to K+.
Gated channels responsive to stimuli:
- Voltage-gated: Respond to membrane potential changes, critical in action potential generation.
- Ligand-gated: Open in response to chemicals such as neurotransmitters.
- Mechanically gated: Open in response to physical stimuli like pressure.

Propagation of action potentials is directional towards the synapse due to the refractory period, preventing backtracking.### Refractory Period of Action Potential
Neurons experience a time frame during which they cannot generate another action potential, known as the refractory period.
Absolute refractory period: No stimulus, regardless of strength, can initiate another action potential due to inactivated Na+ channels needing to return to resting state.
Relative refractory period: A suprathreshold stimulus can initiate an action potential, despite the presence of open K+ channels, as the Na+ inactivation channels have returned to resting state.

Action potentials are propagated along neurons, ensuring effective signal transmission from one region to another.

Signal transmission occurs via electrical and chemical methods, with chemical synapses playing a crucial role.

Ionotropic receptors: Form ion channel pores that open upon activation, allowing ions such as Na+, K+, or Cl- to flow through.
Metabotropic receptors: Function through G-protein coupled mechanisms, affecting cellular processes indirectly.

Effects of neurotransmitters can be excitatory or inhibitory:
- Excitatory Postsynaptic Potential (EPSP): Caused by the opening of ligand-gated Na+ channels, increasing the likelihood of reaching threshold for action potential.
- Inhibitory Postsynaptic Potential (IPSP): Resulting from the opening of ligand-gated Cl- or K+ channels, causing hyperpolarization of the postsynaptic cell.

Neurotransmitters are cleared from the synaptic cleft through several mechanisms:
- Diffusion: Movement down the concentration gradient.
- Enzymatic degradation: Example includes acetylcholinesterase breaking down acetylcholine.
- Uptake by neurons or glial cells: Neurotransmitter transporters facilitate reuptake.

Spatial Summation: Effect results from neurotransmitters released from multiple presynaptic end bulbs onto a single neuron.
Temporal Summation: Effect results from rapid, successive firings of the same presynaptic end bulb on a second neuron.

Small-Molecule Neurotransmitters:
- Acetylcholine (ACh): Released by many neurons in the peripheral nervous system (PNS) and some in the central nervous system (CNS). It acts excitatory at the neuromuscular junction (NMJ) but can be inhibitory in other contexts. Inactivated by acetylcholinesterase.
- Amino Acids: Glutamate functions as an excitatory neurotransmitter in brain neurons and is inactivated via reuptake mechanisms.