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:

1. AP arrives and opens Ca2+ channels at the axon terminal.

2. Ca2+ influx causes neurotransmitter release from synaptic vesicles.

3. Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic neuron, generating graded potentials.

4. Neurotransmitter effects are terminated by diffusion, reuptake, or enzymatic degradation.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Moves membrane voltage toward threshold (due to Na+ influx).

• Inhibitory Postsynaptic Potential (IPSP): Moves membrane voltage away from threshold (due to Cl- influx or K+ efflux).

Summation of Postsynaptic Potentials

• Summation: Additive effects of multiple EPSPs needed to reach threshold.

  • Temporal Summation: Rapid sequence of EPSPs.

  • Spatial Summation: EPSPs from multiple sources occurring close in time.

Overview of Neuron Physiology

• Summary Diagram: Shows the sequence from receiving graded potentials at dendrites to action potential propagation along the axon to neurotransmitter release at the synapse.

Neurotransmitters

• Structural Classification:

  • Acetylcholine (ACh): Used in motor pathways and autonomic nervous system.

  • Biogenic Amines: Includes dopamine, serotonin; relate to mood and arousal.

  • Other types include: Proteins, Amino Acids, ATP.

• Functional Classification:

  • Excitatory: Causes depolarization (EPSP).

  • Inhibitory: Causes hyperpolarization (IPSP).

  • Some neurotransmitters can have both effects depending on receptors.