Ch2 Nervous System

Nervous System (overview)

• There are billions of neurons.
• Neurons regulate almost all physiological variables.
• Neurons sense and respond to the environment.

Organization of the Nervous System

Central Nervous System (CNS)

• Brain
• Spinal cord

Peripheral Nervous System (PNS)

• Afferent divisions (sensing)
• Efferent divisions (motor and control)

Basic Information Flow

A typical pathway:

Sensory receptors → afferent neurons → interneurons → efferent neurons → motor cells

Neuron Structure

A neuron can include:

• Dendrites (receive information)
• Cell body (soma)
• Axon
• Axon terminal
• Axon hillock
• Axon collateral (sometimes)

Key idea:

• One-way flow of signal (functionally, from input regions to output regions)

Convergence and divergence

• Convergence pathway
• Divergence pathway

Electrical Principles

Ohm’s Law

• Current (I) is the product of conductance (G) ((G = 1/R)) and voltage (V).

Ion Equilibrium Potentials

Nernst equation (ion equilibrium)

Typical concentrations

• \([Na^+]_{in} \approx 15,\text{mM}\)
• \([Na^+]_{out} \approx 145,\text{mM}\)
• \([K^+]_{in} \approx 150,\text{mM}\)
• \([K^+]_{out} \approx 5,\text{mM}\)

Typical equilibrium and membrane potentials

Resting Potential (conductance-weighted)

Goldman-Hodgkin-Katz

Channels and Transporters

Na⁺/K⁺ ATPase

• pumps 3 Na⁺ out and 2 K⁺ in

Leak channels

• more K⁺ leak channels than Na⁺ leak channels
• “always open” channels

Voltage-Gated Channels

Voltage-gated Na⁺ channel

• opening tends to increase membrane potential (depolarize)

Chain-ball model

Three states that circulate:

• Closed: channel can open; no ions flow
• Open: activated; “ball” quickly inactivates the channel
• Inactive: no ions pass; can return to closed only after repolarization

Voltage-gated K⁺ channel

• opening tends to decrease membrane potential (repolarize / hyperpolarize)

Membrane Potential Phases

• Depolarization: (V) goes up
• Repolarization: (V) goes down
• Hyperpolarization: (V) goes below resting potential (RP)

Action Potential (AP)

• all-or-none
• fires if reaching threshold potential; does not fire if not reaching threshold
• unitary (can only fire one at once)

Refractory periods

• Absolute refractory period: Na⁺ channels are still inactive and cannot fire
• Relative refractory period: can fire but requires more energy

Directional propagation note

• The absolute refractory period supports one-direction propagation by preventing immediate re-excitation behind the traveling AP.
• The elevated membrane potential can make AP propagate in either direction if conditions permit.

Synapses

Chemical vs electrical

• Chemical synapses: via neurotransmitters (NT)
• Electrical synapses: via gap junctions

Chemical synapses are generally slower than electrical synapses.

Steps in chemical synaptic transmission

1. AP arrives
2. voltage-gated Ca²⁺ channels open
3. vesicles fuse with the cell membrane and release NT
4. NT binds postsynaptic receptors
5. NT removed from the synaptic cleft

Receptor types

• Ionotropic: ion channels
• Metabotropic: via signaling cascades (e.g., GPCR)

Excitatory vs Inhibitory Synapses

Excitatory synapse (EPSP)

• makes the postsynaptic neuron more likely to fire
• \(E_{channel}\) is greater than threshold
• example: AMPA

Inhibitory synapse (IPSP)

• opposite functional effect of excitatory synapses

Example: GABA\(_A\)

• GABA via GABA\(_A\) receptors (Cl⁻ permeable)
• \(E_{Cl}\) is typically around \(-80\) to \(-60\text{mV}\)

Effect:

• can decrease \(V_m\), or
• prevent \(E_m\) from rising above threshold (“clamp” / shunt), reducing spike probability

Three types of IPSP (as listed)

• V decrease IPSP
• No change V IPSP
• V increase IPSP

Note:

• When \(E_{Cl}\) is greater than \(-70\text{mV}\), it can still prohibit AP; thus it is still an IPSP.

Knee Jerk Reflex

• 1A muscle spindle stretch receptor
• if the channel is stretched, it is pulled apart and both Na⁺ and K⁺ can flow through
• generates an AP in the 1a afferent neuron if reaching the necessary level
• receptor is similar to the AMPA receptor

Pathway summary:

• AP propagates to the spinal cord
• AP will inhibit flexor muscle and excite extensor muscle

Summary Notes

• Excitatory synapses: \(E_{channel}\) greater than threshold → higher firing rate and firing chance
• Inhibitory synapses: reduce chance and frequency of neuron firing
• For GABA receptors: \(E_{Cl}\) may be greater than, lower than, or the same as resting potential; they anchor membrane potential and reduce firing likelihood