AP 1 - Nervous System and Nervous Tissue 2020

The Nervous System

• Functions of the Nervous System:

  • Sensory Input: Gathering information from receptors throughout the body.

  • Integration: Processing information and making decisions based on sensory input.

  • Motor Output: Sending signals to effectors (muscles or glands) to elicit a response.

Organization of the Nervous System

• Central Nervous System (CNS):

  • Composed of the brain and spinal cord.

  • Functions as integrative and control centers.

• Peripheral Nervous System (PNS):

  • Consists of cranial and spinal nerves.

  • Serves as the communication lines between CNS and the rest of the body.

Divisions of the PNS

• Sensory (Afferent) Division:

  • Neurons that conduct impulses from receptors to the CNS (composed of somatic and visceral sensory nerve fibers).

• Motor (Efferent) Division:

  • Neurons that conduct impulses from the CNS to effectors.

  • Divided into:

    • Somatic Nervous System: Controls voluntary skeletal muscles.

    • Autonomic Nervous System (ANS): Controls involuntary functions involving cardiac and smooth muscles, and glands.

      • Sympathetic Division: Mobilizes body systems during activity (fight or flight).

      • Parasympathetic Division: Conserves energy and promotes housekeeping functions during rest (rest and digest).

Cells of Nervous Tissue

1. Neurons: Functional units that carry nerve impulses.

2. Neuroglia: Supportive cells that assist neurons.

Neuroglia of the CNS

• Astrocytes: Attach neurons to blood vessels and regulate the extracellular environment.

• Microglia: Act as phagocytes to monitor and maintain health.

• Ependymal Cells: Line fluid-filled spaces in the CNS and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS neurons.

Neuroglia of the PNS

• Satellite Cells: Surround and insulate neuron cell bodies.

• Schwann Cells: Form myelin sheaths around PNS neurons.

Characteristics of Neurons

• Longevity: Neurons can last a lifetime.

• Amitotic: Neurons cannot divide once mature.

• High Metabolic Rate: Require large amounts of oxygen and nutrients.

Structure of a Neuron

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

• Dendrites: Short processes that receive signals from other neurons.

• Axon: Long process that transmits impulses away from the soma.

• Terminal Branches: End of the axon that relays signals to other neurons or effectors.

• Axon Terminal: Contains neurotransmitters to transmit signals.

Neuron Organization

• Nucleus: A cluster of neuron cell bodies in the CNS.

• Ganglion: A cluster of neuron cell bodies in the PNS.

• Tract: A bundle of axons in the CNS.

• Nerve: A bundle of axons in the PNS.

Myelin Sheaths

• Function: Insulate nerve fibers and increase the speed of impulse transmission.

• Formation: Created by Schwann cells in the PNS; the part of the Schwann cell that remains is called the neurilemma.

• Nodes of Ranvier: Gaps between myelinated sections of the axon.

Classification of Neurons

Structural Classification:

• Multipolar Neurons: Most common, have many dendrites.

• Bipolar Neurons: Rare, with one dendrite and one axon.

• Unipolar Neurons: Have a single process that bifurcates into dendritic and axonic ends.

Functional Classification:

• Sensory (Afferent) Neurons: Relay impulses to the CNS.

• Motor (Efferent) Neurons: Relay impulses from the CNS to muscles and glands.

• Interneurons: Found within the CNS; connect sensory and motor neurons.

Neurophysiology and Electrical Principles

Basic Terms:

• Voltage (Potential): Potential energy due to charge separation (Na+ and K+).

• Current: Flow of electric charges (movement of Na+ and K+ across membranes).

• Resistance: Hindrance to charge flow (membrane resistance to Na+).

• Ohm’s Law: Current = Voltage / Resistance.

Membrane Ion Channels:

• Ion Channels: Proteins that allow ion diffusion across membranes.

• Types of Channels:

  • Nongated (Leakage) Channels: Always open.

  • Chemically Gated Channels: Open in response to chemicals.

  • Voltage-Gated Channels: Open in response to voltage changes.

Resting Membrane Potential

• Definition: Voltage across a neuron’s membrane, typically -70mV.

• Establishment:

  • More K+ ions diffuse out than Na+ diffuse in, making the inside negative.

  • Sodium-Potassium Pump maintains resting potential by pumping 3 Na+ out for every 2 K+ in.

Changes in Membrane Potential

• Depolarization: Inside of the cell becomes less negative or more positive.

• Hyperpolarization: Inside of the cell becomes more negative.

Graded Potentials

• Definition: Localized changes in membrane potential that dissipate over short distances; may be depolarizations or hyperpolarizations.

• Role: Can lead to action potentials.

Action Potentials

• Definition: Large membrane potential changes that travel over long distances without losing strength; generated primarily in axons.

• Phases of Action Potentials:

  1. Resting State: Membrane is polarized at -70mV, all gated channels closed.

  2. Depolarization: Na+ channels open, Na+ enters, membrane reaches +30mV.

  3. Repolarization: Na+ channels close, K+ channels open, K+ exits.

  4. Hyperpolarization: K+ channels remain open.

  5. Ion Redistribution: Na+/K+ pump restores resting ion concentrations.

Propagation of Action Potential

• Process: Movement of AP along a neuron’s membrane, where each segment undergoes depolarization and repolarization.

• Mechanism: Involves opening of voltage-gated channels downstream.

• In Myelinated Axons: AP propagation occurs at nodes of Ranvier, speeds up signal transmission.

All-or-None Phenomenon

• A threshold voltage (typically -55mV) must be reached for an action potential to occur; if not reached, no action potential occurs.

Refractory Periods

• Refractory Periods: Times when a new AP cannot be generated.

  • Absolute Refractory Period: Na+ and K+ channels are open.

  • Relative Refractory Period: K+ channels remain open; requires a higher threshold to generate an AP.

Graded Potentials vs. Action Potentials

• Graded Potentials: Occur at dendrites; vary in magnitude; decremental.

• Action Potentials: Occur at axons; all-or-none response; non-decremental; propagate fully.

The Synapse

• Definition: Junction between two neurons or between a neuron and an effector.

• Parts:

  • Presynaptic Neuron: Sends information.

  • Postsynaptic Neuron: Receives information (typically at dendrites).

Events at a Chemical Synapse: