Nervous System Structure and Neural Signaling

Nervous System Anatomy

• Functions:

  • Controls perception (sight, touch, hearing, taste).
  • Maintains homeostasis (e.g., blood pressure, body temperature).

• Divisions:

  • Central Nervous System (CNS): Brain & spinal cord (processes information and sends responses).
  • Peripheral Nervous System (PNS): Motor & sensory neurons connecting the CNS to the rest of the body.

• PNS Subdivisions:

  • Somatic Nervous System: Controls voluntary movement (e.g., moving hands).
  • Autonomic Nervous System: Controls involuntary functions (e.g., heart rate).
    • Sympathetic Division (Fight or Flight): Increases alertness, heart rate, etc.
    • Parasympathetic Division (Rest & Digest): Slows heart rate, promotes digestion, etc.

• Structural Differences (Autonomic):

  • Parasympathetic: Long preganglionic neuron, short postganglionic neuron.
  • Sympathetic: Short preganglionic neuron, long postganglionic neuron.

• Neurotransmitters (Autonomic):

  • Both systems use acetylcholine (ACh) in preganglionic neurons.
  • Parasympathetic: Uses acetylcholine (ACh) in postganglionic neurons.
  • Sympathetic: Uses norepinephrine in postganglionic neurons.

• Cells of the Nervous System:

  • Neurons: Transmit electrical and chemical signals.
  • Glial Cells: Support and protect neurons.
    • Examples: Astrocytes, oligodendrocytes, Schwann cells (form myelin), ependymal cells.

Resting Membrane Potential (RMP)

• Definition: The electrical charge difference across the membrane of a resting neuron (approximately -70mV, negative inside).

• Establishment and Maintenance:

  • K⁺ Leak Channels: Allow potassium ions (K⁺) to exit the cell down their concentration gradient, making the inside more negative.
  • Na⁺ Leak Channels: Allow some sodium ions (Na⁺) to enter, but fewer than K⁺ exiting.
  • Na⁺/K⁺ Pump: Actively transports 3 Na⁺ ions out for every 2 K⁺ ions in, using ATP. This maintains the concentration gradients crucial for the RMP.

• Types of Ion Channels:

  • Leak Channels: Always open; responsible for passive ion movement contributing to RMP.
  • Mechanically Gated Channels: Open in response to physical deformation (e.g., pressure, touch).
  • Chemically Gated (Ligand-Gated) Channels: Open when a specific chemical (e.g., neurotransmitter) binds.
  • Voltage-Gated Channels: Open or close in response to changes in membrane potential.

Electrical Signals: Graded vs. Action Potentials

Graded Potentials (GPs)

• Location: Occur primarily in dendrites and the cell body.
• Trigger: Initiated by the opening of mechanically gated or chemically gated channels.

• Characteristics:

  • Can be depolarizing (membrane potential becomes less negative, e.g., -70mV → -60mV) or hyperpolarizing (membrane potential becomes more negative, e.g., -70mV → -80mV).
  • Signal strength varies with stimulus strength.
  • Travel short distances; signal weakens with distance (decremental conduction).
  • Summation: Multiple GPs can combine their effects.
  • No refractory period.

Action Potentials (APs)

• Location: Propagated along the axon, starting at the trigger zone (axon hillock) and moving towards the axon terminal.
• Trigger: Initiated when the sum of graded potentials depolarizes the membrane to a threshold level (typically around -55mV).

• Phases (Ion Movement via Voltage-Gated Channels):

  • Depolarization: Voltage-gated Na⁺ channels open rapidly; Na⁺ influx causes the membrane potential to become positive (e.g., +30mV).
  • Repolarization: Voltage-gated Na⁺ channels inactivate; voltage-gated K⁺ channels open; K⁺ efflux restores the negative membrane potential.
  • Hyperpolarization (Afterpotential): Voltage-gated K⁺ channels close slowly, causing a brief period where the membrane potential is more negative than the RMP.

• Characteristics:

  • All-or-None Principle: An AP either occurs fully if the threshold is reached or not at all.
  • Signal strength is constant (non-decremental).
  • Used for long-distance communication.
  • Refractory Periods:
    • Absolute Refractory Period: No new AP can be initiated, regardless of stimulus strength (due to Na⁺ channel inactivation).
    • Relative Refractory Period: A new AP can be initiated, but requires a stronger-than-usual stimulus (due to ongoing K⁺ efflux).

Synaptic Transmission: Neuron Communication

1. An action potential arrives at the presynaptic axon terminal.
2. Voltage-gated Ca²⁺ channels open, allowing calcium ions (Ca²⁺) to enter the terminal.
3. The influx of Ca²⁺ triggers synaptic vesicles (containing neurotransmitters) to fuse with the presynaptic membrane and release neurotransmitters (e.g., acetylcholine) into the synaptic cleft via exocytosis.
4. Neurotransmitters diffuse across the synaptic cleft and bind to specific receptors (often ligand-gated ion channels) on the postsynaptic membrane.
5. Binding opens ion channels (e.g., Na⁺ channels), causing a flow of ions that changes the postsynaptic membrane potential, creating a postsynaptic potential (a type of graded potential).
6. If this postsynaptic potential reaches the threshold potential at the axon hillock of the postsynaptic neuron, it triggers a new action potential.
7. Neurotransmitter effects are terminated by:
   • Enzymatic degradation in the synaptic cleft (e.g., acetylcholinesterase breaks down ACh).
   • Reuptake into the presynaptic terminal or nearby glial cells.
   • Diffusion away from the synapse.

Neural Signal Coding and Integration

Stimulus Strength Coding

• A weak stimulus typically triggers a low frequency of action potentials, leading to less neurotransmitter release.
• A strong stimulus triggers a high frequency of action potentials, leading to more neurotransmitter release. (The amplitude of individual APs remains constant due to the all-or-none principle).

Summation: Combining Neural Signals

• Temporal Summation: Multiple graded potentials arriving at the same location in rapid succession from a single presynaptic neuron combine their effects.
• Spatial Summation: Graded potentials originating from multiple different presynaptic neurons arriving simultaneously at different locations on the postsynaptic neuron combine their effects.

Excitatory & Inhibitory Signals

• Excitatory Postsynaptic Potentials (EPSPs): Graded potentials that cause depolarization, making the postsynaptic neuron more likely to reach threshold and fire an action potential.
• Inhibitory Postsynaptic Potentials (IPSPs): Graded potentials that cause hyperpolarization or stabilize the resting potential, making the postsynaptic neuron less likely to reach threshold.

Action Potential Conduction and Myelination

• Action potentials travel in one direction along the axon (from cell body to axon terminal) due to the refractory period preventing backward propagation.
• Myelin: An insulating layer formed by glial cells (Schwann cells in PNS, oligodendrocytes in CNS) around many axons.
• Nodes of Ranvier: Gaps in the myelin sheath where voltage-gated channels are concentrated.
• Saltatory Conduction: In myelinated axons, the AP appears to "jump" from one Node of Ranvier to the next. This process is much faster than continuous conduction in unmyelinated axons.
• Myelination significantly increases the speed of action potential conduction by preventing ion leakage across the membrane except at the nodes.
• Loss of myelin (demyelination), as seen in diseases like multiple sclerosis, disrupts and slows down or blocks nerve signal transmission.

Final Thoughts

• The nervous system is a complex network responsible for processing information, integrating signals, and coordinating responses to internal and external stimuli.
• Understanding electrical signals (action potentials, graded potentials), chemical signaling at synapses (neurotransmitter release), and the integration of these signals (summation) is fundamental to comprehending neural function.
• These mechanisms underpin essential processes like perception, voluntary and involuntary motor control, and the maintenance of homeostasis.