Neural 6.

Organization of the Nervous System

• Comprises two main divisions:

  • Sensory Nervous System (SNS): Carries information from sensory receptors to the Central Nervous System (CNS).

  • Motor Nervous System (MNS): Transmits signals from CNS to effectors (muscles and glands).

  • Effectors: Structures that respond to motor commands.

Cells of Nervous System

• Neurons or Nerve Cells:

  • Receive stimuli and transmit action potentials.

  • Structure:

    • Cell Body (Soma): Contains nucleus and organelles.

    • Dendrites: Receive input signals.

    • Axons: Transmit output signals.

• Neuroglia (Glial Cells): Support and protect neurons.

Electrical Signals in the Nervous System

• Action Potentials: Electrical signals produced by cells for communication.

• Mechanism:

  • Result from ionic concentration differences across the plasma membrane and their permeability.

Resting Potential of the Membrane

• Ion concentrations maintained by:

  • Na/K Pump: Pumps 3 Na+ out for every 2 K+ in, creating a concentration gradient.

  • Membrane Permeability: High Na+ and Cl- outside; high K+ and proteins inside the cell.

• Resting potential ranges from -70 to -90 mV due to steep gradients of Na+ and K+.

Sodium-Potassium Exchange Pump

• Functionality involves:

  • Active Transport: Requires ATP to change carrier shape.

  • Effective exchange of Na+ and K+ across the membrane.

Membrane Permeability

• Protein Dynamics:

  • Negatively charged proteins remain inside as they are too large to exit through the membrane.

• Ion Movement:

  • Cl-: Diffuses out due to repulsion from negatively charged proteins via always-open channels.

  • Gated Channels: Open/close in response to stimuli (e.g., ligand-gated, voltage-gated).

Gated Ion Channels

• Types include:

  • Ligand-Gated Channels: Open in response to a specific binding event (e.g., ACh binding allows Na+ entry).

  • Voltage-Gated Channels: Open in response to membrane voltage changes (e.g., Na+ and K+ channels).

Changes in Membrane Potential

• Resting Membrane Potential: Adjustments in K+ and Na+ concentrations impact potential.

• Depolarization: Membrane becomes less negative when Na+ enters.

• Hyperpolarization: Membrane becomes more negative when K+ exits, increasing potential difference.

Action Potentials

• Triggered by reaching a threshold level from a local potential, characterized by:

  • All-or-None Principle: Once threshold is reached, an action potential occurs.

  • Phases of Action Potential:

    • Depolarization: Membrane potential becomes more positive.

    • Repolarization: Returns to resting potential, may cause afterpotential.

Refractory Periods

• Absolute Refractory Period: Neuron cannot respond again until repolarization is nearly complete.

• Relative Refractory Period: A stronger-than-threshold stimulus can trigger a new action potential.

Action Potential Propagation

• Occurs along axons:

  • Unmyelinated Axons: Action potentials propagate continuously.

  • Myelinated Axons: Faster transmission via saltatory conduction at Nodes of Ranvier.

Structures of the Synapse

• Types of Synapses:

  • Electrical Synapses: Direct current flow between cells via gap junctions.

  • Chemical Synapses: Involve neurotransmitter release from presynaptic to postsynaptic cells.

Neurotransmitter Dynamics

• Removal Mechanisms:

  • Neurotransmitter clearance (e.g., ACh is broken down by acetylcholinesterase).

  • Reuptake mechanisms for others (e.g., norepinephrine).

• Types of Neurotransmitter Effects: Can be excitatory or inhibitory based on receptor binding.

Sensory Processing

• Senses allow brain to receive information:

  • Sight, sound, touch, taste, and smell.

• Steps in Sensation:

  • Detection by receptors, action potential generation, CNS processing, and perception.

Taste and Smell

• Olfaction and Gustation: Involves complex pathways involving receptors in the nasal epithelium and taste buds.

• Taste Perception: Modified by texture and temperature, rapid adaptation, and varied responsive thresholds.