Nervous System Notes

Summary of Nervous System

• Sensory input: Receiving information from the environment.
• Integration: Processing and interpreting the sensory input.
• Motor output: Responding to the processed information.

Organization of the Nervous System

• 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.

Cells of Nervous Systems: Neurons and Glia

• 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.

Glial Cells: Astrocytes and Blood-Brain Barrier

• 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.

Glymphatic System

• 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.

Myelination

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

Neural Stem Cells and Neuron Regeneration

• 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.

Neuron Structure

• 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.

Neuron Communication and Electric Potential

• 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.

Electric Signals and Ion Movement

• 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 and Channels

• 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

• 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.

Gated Ion Channels

• Some ion channels are