Nervous System Overview and Divisions

Nervous System Overview

• The nervous system is the master controlling and communicating system of the body
• Its cells communicate via electrical and chemical signals, enabling rapid and specific responses
• Sensory receptors detect stimuli, transmitting sensory input along afferent pathways
• Integration—processing and interpreting sensory input and deciding on an appropriate response—happens in a control center.
• Motor output, via efferent pathways, triggers a reaction in effectors (muscles or glands)

Nervous System Divisions

• Central Nervous System (CNS): consisting of the brain and spinal cord
• Peripheral Nervous System (PNS): consisting of cranial nerves and spinal nerves

Nervous Tissue

• Highly cellular, with little extracellular space
• Two major cell types:
  • Neuroglia: Supporting cells that surround and wrap delicate neurons
  • Neurons: Excitable nerve cells that transmit electrical signals (structural and functional units of the nervous system)

Neuron Anatomy

• Has a cell body (soma) and processes (dendrites and axons)
• Three Functional Zones
  • Receptive (dendritic zone): Receives input
  • Conduct excitation (axon): Conducts the excitation. Initiates in initial segment
    • Secretory/Transmission zone (telodendritic zone): Transmits output

Myelin Sheath

• Composed of Schwann cells in the PNS and oligodendrocytes in the CNS
• Insulates axons, increasing speed of nerve impulse transmission

Nerve and Tract Difference

• Nerve: A bundle of axons in the PNS
• Tract: Bundle of axons in the CNS

Membrane Potentials

• Resting Membrane Potential (RMP): The voltage across the plasma membrane of a resting neuron, typically ranging from -50 to -90 mV
• RMP is generated by differences in ion concentrations (inside vs. outside) and membrane permeability.

Graded Potentials

• Short-lived, local changes in membrane potential
• Strength decreases with distance
• Can be depolarizing (EPSP) or hyperpolarizing (IPSP)
• They can summate (add together) to reach a threshold

Action Potentials (AP)

• Brief, large depolarization events
• "All-or-none" phenomenon
• Generated by voltage-gated ion channels, resulting in rapid and specific movement of ions
• They propagate along axons and are essential for long-distance communication.
• Contain 4 phases, and steps to an Action Potential (depolarization, repolarization, hyperpolarization)
  • Threshold: membrane depolarization must reach a certain voltage to initiate an AP
  • Activation Gate: voltage-gated Na+ channels open (depolarization)
  • Inactivation Gate: voltage-gated Na+ channels close, K+ channels open. (Repolarization)

Refractory Periods

• Absolute Refractory Period: A period after an AP where no new AP can be elicited, no matter the size of the stimulus
• Relative Refractory Period: A period after the absolute refractory period where a new AP can be elicited, but it will require a larger stimulus than normal

Conduction Velocity Factors

• Larger diameter axons
• Presence of myelin

Saltatory Conduction

• APs jump between myelinated sections (nodes of Ranvier) along the axon, increasing the speed of conduction

Synapses

• Junctions between neurons or between a neuron and another cell type (e.g., muscle)
• Chemical Synapse: Neurotransmitters transmit the signal across the synaptic cleft; most common

Synaptic Transmission Steps

• Action potential arrives at the axon terminal
• Voltage-gated Ca2+ channels open, Ca2+ enters
• Neurotransmitter is released via exocytosis
• Neurotransmitter binds to receptors on the postsynaptic membrane
• Response in the postsynaptic cell (either excitation or Inhibition)
• Neurotransmitter removal

Synaptic Potentials

• EPSPs (Excitatory): Graded depolarizations that increase the likelihood of an AP
• IPSPs (Inhibitory): Graded hyperpolarizations that decrease the likelihood of an AP

Synaptic Summation

• Spatial Summation: Combined effect of multiple simultaneous inputs from different synapses
• Temporal Summation: Combined effect of repeated inputs from the same synapse

Neuron Classification

• Grouped by number of processes (multipolar, bipolar, unipolar).
• Classified by functions (sensory, motor, interneurons)

Neurotransmitters and Receptors

• Chemical messengers that transmit signals across synapses
• Classes include excitatory (e.g., acetylcholine, glutamate) and inhibitory (e.g., GABA, glycine)
• Action at Specific receptors

Neural Integration

• Common patterns of neuronal organization, including diverging, converging, reverberating, and parallel after-discharge circuits.