Notes on the Nervous System: Overview, Cells, and Potentials

Overview of the Nervous System

  • Main Divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS)
    • CNS: Comprises the brain and spinal cord; responsible for information processing
    • PNS: Includes all neural tissue outside the CNS; delivers information to and from the CNS
    • Components of PNS: Afferent (sensory) and Efferent (motor) pathways
    • Afferent: Carries signals from sensory neurons to CNS
    • Efferent: Carries signals from CNS to effectors (muscles/glands)

Nervous System Cell Types

  • Neurons (Functional Cells):

    • Types based on function:
    • Afferent (Sensory Neurons): Relay information to CNS
    • Interneurons: Connect neurons within the CNS
    • Efferent (Motor Neurons): Transmit signals from CNS to muscles or glands
    • Structure: Includes dendrites (receive signals), soma (integrate signals), and axon (transmit signals)

  • Glial Cells (Support Cells):

    • Types of glial cells:
    • Astrocytes: Regulate extracellular fluid, facilitate signaling
    • Microglia: Immune defense in the CNS
    • Oligodendrocytes (CNS) & Schwann Cells (PNS): Insulation of neuronal axons (myelination)

Membrane Potentials

  • Resting Membrane Potential (Vm):

    • Average resting potential of a neuron is approximately -70 mV.
    • Maintained by the Sodium-Potassium Pump (Na+/K+ ATPase), which pumps out 3 Na+ for every 2 K+ it takes in.
    • Equilibrium Potentials:
    • Potassium (EK): -90 mV
    • Sodium (ENa): +60 mV

  • Types of Potentials:

    • Graded Potentials: Localized changes in membrane potential, can be depolarizing (EPSP) or hyperpolarizing (IPSP).
    • Action Potentials (AP):
    • Characteristic spikes in membrane potential, involves phases:
      1. Depolarization: Influx of Na+ ions when thresholds are reached.
      2. Repolarization: Efflux of K+ ions, bringing the membrane back to resting potential.
      3. Hyperpolarization (Undershoot): Potential becomes more negative than resting, before returning to resting.
      4. Recovery: Resting state is reestablished.

Reflexes and Reflex Arc

  • Components of a Reflex Arc:

    1. Receptor: Detects stimulus
    2. Sensory Neuron (Afferent Fiber): Conveys information to CNS
    3. Integrator (CNS): Processes information and coordinates response
    4. Motor Neuron (Efferent Fiber): Conveys signals to effector
    5. Effector: Muscle or gland that responds to signal (e.g., contraction or secretion)

  • Reflex Characteristics:

    • Involuntary: Occurs without conscious control.
    • Requires Adequate Stimulus: Stimulus must exceed a threshold to elicit a response.
    • Specific Receptor Involvement: Receptors respond to specific stimuli.
    • Patterned Response: Consistent response to specific stimuli.
    • Graded Response: The strength of the response is proportional to the strength of the stimulus.

Ion Movement and Influencing Factors

  • Diffusion Influences:
    • Chemical gradients dictate ion movement based on concentration differences.
    • Electrical gradients dictate movement based on charge attraction or repulsion.
  • Equilibrium Potential (Eion):
    • Voltage at which the electrical and chemical gradients for an ion are balanced, creating no net movement.

Summary of Key Concepts

  • The nervous system is crucial for maintaining homeostasis and facilitating interaction with the environment through sensory input and motor output.
  • Understanding the structure and functions of neurons and glial cells is fundamental to grasping the workings of the nervous system and how signals are transmitted throughout the body.
  • Recognizing how membrane potentials function is essential for understanding nerve signal conduction and reflex arc behavior in the nervous system.