Nervous System Notes

CNS (Central Nervous System): Brain and spinal cord – the control center.
PNS (Peripheral Nervous System): Peripheral nerves and ganglia, sensory receptors – communication lines.
Afferent: Sensory division carrying information to the CNS.
- Somatic: Conscious senses (e.g., touch, vision).
- Autonomic: Visceral organs (e.g., heart, stomach).
Efferent: Motor division carrying information from the CNS.
- Somatic: Skeletal muscle (voluntary).
- Autonomic: Smooth muscle, glands (involuntary).
  - Sympathetic: "Fight or flight" responses.
  - Parasympathetic: "Rest and digest" functions.

Neurons: Nerve cells that generate and conduct electrical signals.
Glia: Support cells of the nervous system.
- Macroglia: Modulate neuron activity and provide support.
- Microglia: Small phagocytic cells; major immune defense mechanism in the nervous system.
Glia provide physical and biochemical support for neurons and outnumber them by about 10 to 1.
CNS Glia: Oligodendrocytes, microglia, astrocytes, ependymal cells.
PNS Glia: Schwann cells, satellite cells.

Astrocytes contribute to the blood-brain barrier, protecting the brain from toxic substances.
Functions of Astrocytes:
- Take up neurotransmitters from the synapse, controlling communication.
- Store glycogen to supply neurons with fuel.
- Release neurotransmitters to alter neuron activities.
- Aid in repair and regeneration of neurons.
- Signal changes in blood composition by contacting blood vessels and neurons.
- A single astrocyte can contact over 100,000 synapses.
Tripartite Synapse: Includes pre- and postsynaptic neurons and astrocyte connections.

Interstitial fluid enters the brain through perivascular spaces between arteries and astrocytes.
Astrocytes use aquaporins to take up fluid and distribute it.
Fluid leaves through perivascular spaces of the veins, removing metabolic waste products.

Oligodendrocytes (in the brain and spinal cord) wrap around neuron axons to form myelin sheaths.
Schwann cells wrap the axons of other nerves.

Neurons and macroglia originate from neural stem cells in the neural tube during early embryonic development.
Stem cells divide into a stem cell and either a neuroblast (neuron progenitor) or a gliablast (glia progenitor).
Committed neurons do NOT regenerate (divide) after differentiation.

Cell Body (Soma): Contains the nucleus and organelles.
Dendrites: Bring information to the cell body.
Axon: Carries information away from the cell body.
Axon Terminals: Located at the tip of the axon.
Neuron form reflects function: the number of dendrites reflects the amount of incoming information; axon length varies based on communication distance.

Neurons communicate through changes in electric potential across their membranes.
Action Potentials (APs): Rapidly reversed changes in membrane potential.
Sodium-potassium pumps create gradients of $Na^+$ and $K^+$ across the cell membrane.
The inside of the cell is usually negative due to K+ leak channels.
Membrane Potential: Electrical charge difference across a cell membrane due to the balance between K+ diffusion and electrical potential ( $E_K$ ).
Resting Membrane Potential: Steady-state membrane potential of a neuron.

Voltage (Electric Potential Difference): Force causing charged particles to move between two points.
Electric current is carried by ions.
Major Ions in Neurons: Sodium ( $Na^+$ ), potassium ( $K^+$ ), calcium ( $Ca^{2+}$ ), chloride ( $Cl^−$ ).
Resting potential of an axon: -60 to -70 mV (millivolts).
An action potential is a sudden reversal in voltage across the cell membrane.

Ion transporters (e.g., sodium-potassium pump) move ions against their concentration gradients, requiring energy.
Ion channels allow ions to pass through the membrane, selective for each type of ion.
Net ion movement depends on concentration gradient and voltage difference (electrochemical gradient).
Potassium channels are open in the resting membrane, allowing K+ to diffuse out.
The Nernst equation calculates the value of $E<em>K$ from the concentrations of $K^+$ on both sides of the membrane: $E = 2.3 \frac{RT}{zF} \text{log} \frac{[ion]</em>o}{[ion]_i}$

Patch clamping is a technique to study ion channels by placing an electrode/pipette against the membrane to isolate a patch with intact ion channels.
Ion movement and channel opening/closing are recorded as electric currents.