Nervous System and Neuron Function

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Functions:
- Sensory Input: Collects information from sensory organs and internal conditions.
- Includes blood pressure, blood CO2 levels, muscle tension, five senses, etc.
- Integration: Central processing of sensory information by interneurons.
- Motor Output: Transmission of signals to muscles for movement.
Types of Nervous Systems:
- Central Nervous System (CNS): Comprises the brain and spinal cord in vertebrates.
- Peripheral Nervous System (PNS): Contains neurons that transmit information to/from the CNS.

Neurons are specialized cells with:
- Dendrites: Branched structures that receive signals from other neurons.
- Cell Body: Contains the nucleus and organelles.
- Axon: A single long structure that transmits signals (generated at the axon hillock).
Types of Neurons:
- Presynaptic Cell: Neuron sending the signal.
- Postsynaptic Cell: Neuron receiving the signal.
Glial Cells (Glia): Support neurons in various functions; there are approximately 86 billion neurons in the human brain, with 10 times more glial cells.

Resting Potential: Generally between -60 ext{ to } -80 ext{ mV}.
Ion Concentrations:
- [ ext{Na}^+] = 145 ext{ mM} (outside), [ ext{K}^+] = 5 ext{ mM} (inside).
- Resting conditions: Na+-K+ pump pushes 3 Na+ out for every 2 K+ in; contributes minimally to voltage.
- Potassium (K+) leaves the cell through leak channels, while sodium (Na+) channels are less permeable, creating a charge imbalance.

Function: Open/close in response to stimuli and are crucial for action potentials.
Types:
- Na+ Gated Channels: Allow Na+ influx, depolarizing the membrane potential (more positive).
- K+ Gated Channels: Allow K+ efflux, repolarizing and hyperpolarizing the membrane potential (more negative).

Graded Potentials: Small changes in membrane potential; magnitude depends on stimulus strength.
Action Potentials: Transmission of impulses along an axon. Initiated when depolarization reaches a threshold (-55 mV), causing voltage-gated Na+ channels to open.
- More Na+ channels lead to further depolarization (positive feedback).
Peak of action potential is fixed due to the closing of Na+ channels and opening of K+ channels.
- K+ exits, causing a decrease in membrane potential, resulting in an undershoot (potential below resting level).

Following depolarization, Na+ channels close during the undershoot, preventing immediate re-firing of action potentials; this is known as the refractory period.

Conduction speed varies with axon diameter; larger axons conduct faster due to reduced resistance.
- Invertebrates: Speeds from several cm/sec to 30 m/sec in squid giant axons.
- Vertebrate axons have myelin sheaths that enable faster saltatory conduction, surpassing invertebrate speeds.