Psych 001 - Chapter 3 Pt I Nervous System

Fundamentals of the Nervous System

• Definition of the Nervous System: A complex network comprised of billions of cells located in the brain and throughout the entire body. It is the biological system responsible for every aspect of human experience, including feeling, thinking, and acting.

• Three Core Functions of the Nervous System:

  • Receiving Sensory Input: The system gathers data from the external world through the five primary senses: vision, hearing, touch, taste, and smell.

  • Processing Information: The brain handles the incoming data by paying attention to specific stimuli, perceiving what they are, and storing them in memory.

  • Responding to Information: The system translates processed information into physical actions or physiological changes.

Cellular Components: Neurons and Glia

• Neurons: These are the discrete cells that serve as the fundamental building blocks of the nervous system. Experience itself is considered a product of the activity of these cells.

  • Population: It is currently estimated that the human nervous system contains nearly $100$ billion neurons.

  • Basic Anatomy of a Neuron:

    • Dendrites: These are branchlike extensions of the neuron. They contain receptors designed to detect chemicals (neurotransmitters) transmitted from neighboring neurons.

    • Cell Body (Soma): The central portion of the neuron where information received from thousands of other neurons is collected and integrated.

    • Axon: A long, narrow outgrowth from the cell body that enables the neuron to transmit information over distances to other neurons.

• Glial Cells (Glia):

  • Support Role: These cells do not transmit information in the same way neurons do but provide critical support functions, including insulation for axons and the removal of waste products and foreign bodies.

  • Size and Statistics: Glia are approximately $1/10\text{th}$ the size of neurons, but they are about $10$ times more numerous within the nervous system.

Axonal Function and Neural Plasticity

• Axon Functionality: The primary purpose of the axon is to send messages to the next cell in the chain. Most axons transmit this information to the dendrites or cell bodies of adjacent neurons.

• Myelin: In vertebrates (animals with backbones), many axons are covered in a coating called myelin.

  • Purpose: Myelin serves to significantly speed up the transmission of electrical signals.

  • Source: Myelin is generated by glial cells.

• Neural Plasticity and Growth:

  • Dynamic Anatomy: Neurons do not possess a fixed structure; they are constantly growing new branches or losing existing ones (both dendrites and axons).

  • Learning and Experience: The growth of neural branches is closely linked to new experiences and the process of learning.

  • Neurogenesis: While neurons can be generated later in life, this occurs only on a very limited scope.

  • Stem Cells: These are undifferentiated cells capable of developing into various types of specialized cells. They appear to be stimulated following certain types of brain damage, suggesting a compensatory or reparative purpose.

The Reflex Arc and Neuron Types

• Reflex Mechanics: A reflex involves a specific pathway of three types of neurons:

  • Sensory Neurons (Afferent): These carry sensory information from the sense organs directly to the central nervous system (CNS).

  • Inter-Neurons: Located within the CNS, these neurons process and interpret the incoming sensory information and then relay commands to other parts of the body.

  • Motor Neurons (Efferent): These carry outgoing commands from the CNS to the muscles, glands, and organs to produce an action.

The Physiology of the Action Potential

• Action Potential Definition: An impulse or electrochemical message that travels the entire length of an axon to convey information.

• Signal Types:

  • Excitatory Messages: These increase the mathematical probability that the next cell in the chain will "fire" (transmit its own message).

  • Inhibitory Messages: These decrease the likelihood of the next cell firing. An example is the brain sending signals to inhibit the sensation of pain in an injured limb.

• Properties of the Action Potential:

  • Constant Strength: The impulse maintains the same strength regardless of how far it travels along the axon.

  • All-or-None Law: An action potential is a binary process; it either happens completely or not at all. There are no partial or weak action potentials.

Resting Potential and Electrochemical Gradients

• Resting Potential: When an axon is unstimulated, it exists in a state of electrical polarization across its membrane.

  • Charge: The inside of the polarized axon carries a negative charge of $-70\text{ millivolts}$ ($-70\text{ mV}$) relative to the outside.

• Maintenance of Resting Potential:

  • Sodium-Potassium Pump: A mechanism that actively transports $3$ sodium ions ($Na^+$) out of the cell for every $2$ potassium ions ($K^+$) it brings in.

  • Potassium Gates (Channels): These allow $K^+$ to enter or leave based on two specific gradients:

    • Electrostatic Gradient: The principle that positive ions are attracted to negative environments. Since the inside of the axon is negative, the electrostatic gradient pulls $K^+$ into the cell.

    • Concentration Gradient: The principle that ions flow from areas of high concentration to areas of low concentration. This gradient pulls $K^+$ out of the axon because the interior concentration is higher.

The Process of Neural Firing

• Initiation: When a neuron receives sufficient excitatory messages, the electrical difference across the membrane changes. Specifically, sodium gates open where the axon meets the soma (cell body).

• Sodium ($Na^+$) Influx:

  • The electrostatic gradient pulls $Na^+$ into the negative interior of the axon.

  • The concentration gradient pulls $Na^+$ into the axon due to the lower interior concentration of sodium.

  • This influx of $Na^+$ makes the inside of the cell temporarily positive.

• Restoration (Repolarization):

  • The sodium ($Na^+$) gates shut and potassium ($K^+$) gates open.

  • $K^+$ ions leave the cell, carrying away the positive charge and returning the axon to its $-70\text{ mV}$ polarized state.

  • The sodium-potassium pump eventually removes excess sodium and recaptures the exited potassium to restore the original resting balance.

Synaptic Communication

• The Synapse: A specialized junction between two neurons where chemical communication occurs.

• Terminal Bouton: The bulge at the end of an axon that stores neurotransmitters.

• Synaptic Vesicles: Small sacs within the neuron where neurotransmitters are stored.

• The Transmission Process:

  1. An action potential reaches the terminal bouton.

  2. Neurotransmitter molecules are released into the synapse.

  3. The neurotransmitter diffuses across the gap to the surface of the receiving neuron (postsynaptic neuron).

  4. The neurotransmitter binds to specific receptors on the dendrite or cell body of the postsynaptic neuron, causing either excitation or inhibition.

  5. The neurotransmitter detaches from the receptor site to end the message.

• Post-Transmission Fate of Neurotransmitters:

  • Reuptake: The neurotransmitter is reabsorbed by the axon that originally released it.

  • Metabolism: Enzymes break down the chemical, and it is removed from the body as waste.

  • Recycling: The chemical may remain at the synapse and eventually reattach to a receptor.

Neuropharmacology and Parkinson’s Disease

• Parkinson’s Disease:

  • Symptoms: Difficulty executing voluntary movements, tremors, muscle rigidity, and depressed mood.

  • Biological Cause: Linked to the gradual decay of a system of axons that release the neurotransmitter dopamine.

  • Neuroanatomy: Primarily associated with the Substantia nigra.

  • Treatment: Management of mild cases involves the drug L-dopa, which the surviving neurons can synthesize into dopamine.

  • Note: Treating symptoms does not necessarily reveal the original cause of the disease.

• Drug Classifications:

  • Agonists: Drugs that enhance or mimic the actions of neurotransmitters (e.g., Opioids).

  • Antagonists: Drugs that inhibit or block the actions of neurotransmitters (e.g., Neurotoxins that block receptors).

  • SSRIs: Drugs that work by affecting the reuptake process.

  • MAOIs: Drugs that work on the enzymes responsible for breaking down neurotransmitters.

Catalog of Specific Neurotransmitters

• Acetylcholine: Responsible for motor control over muscles, as well as attention, memory, learning, and sleep.

• Epinephrine: Provides energy; formerly known as adrenaline.

• Norepinephrine: Responsible for maintaining arousal and alertness.

• Glutamate: Primary excitatory neurotransmitter; involved in the enhancement of action potentials, learning, and memory.

• Endorphins: Involved in pain reduction and the body's reward system.