Nervous System

NERVOUS SYSTEM

I. Organization and Functions of the Nervous System

  • Overview of the nervous system's components and their functions.

II. Structure of Nervous System Components

  • Neuron: Basic structural and functional unit of the nervous system; capable of transmitting nerve impulses.

    • Components:

    • Cell Membrane: Encloses cell contents.

    • Dendrites: Extensions that receive signals from other neurons.

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

    • Axon: Long projection that transmits impulses away from the cell body.

  • Glia: Supporting cells in the nervous system critical for maintaining neuron health.

    • Functions include protection, nourishment, insulation, and myelin formation.

  • Ganglion: Collection of neuron cell bodies located outside the Central Nervous System (CNS).

  • Nerve: Bundle of axons traveling together in the Peripheral Nervous System (PNS).

    • Types of nerves:

    • Cranial Nerves: 12 pairs originating from the brain.

    • Spinal Nerves: 31 pairs emanating from the spinal cord.

  • Gray Matter: Region containing neuron cell bodies and dendrites.

    • Located in cortex, brain nuclei, and spinal horns.

  • White Matter: Contains myelinated axon bundles (tracts) in the CNS.

III. Main Areas of the Human Brain

  • Cerebrum: Largest part; responsible for higher brain functions (thought, memory, and voluntary movement).

  • Cerebral Cortex: Outer layer; controls sensory processing and voluntary actions.

  • Basal Nuclei: Involved in motor control and movement regulation.

  • Thalamus: Major relay center for sensory input to the cerebrum.

  • Hypothalamus: Regulates autonomic functions, hormonal activity, and maintains homeostasis.

  • Diencephalon: Architectural part of the brain that includes the thalamus and hypothalamus.

  • Epithalamus: Contains the pineal gland which regulates circadian rhythms.

  • Midbrain: Controls visual and auditory responses as well as reflexes.

  • Cerebellum: Coordinates balance and movement.

  • Brain Stem: Composed of midbrain, pons, and medulla; responsible for autonomic functions (breathing, heart rate).

IV. Structure and Functions of the Spinal Cord

  • Serves as the main communication pathway between the brain and peripheral nervous system.

  • Functions include carrying sensory impulses to the brain (via posterior horn) and motor impulses from the brain (via anterior horn).

  • Also involved in spinal reflexes, providing quick responses to stimuli without direct involvement of the brain.

V. Components and Function of a Reflex Arc

  • Reflex Arc: Neural pathway that mediates reflexes.

    1. Sensory Receptor: Detects a stimulus, producing a generator/receptor potential.

    2. Sensory Neuron: Conducts impulses from receptor to integrative center (CNS).

    3. Integrating Center: Processes information and relays impulses to motor neurons.

    4. Motor Neuron: Conducts impulses from integrating center to effector.

    5. Effector: Muscle or gland that responds to motor neuron impulses.

  • Example: Withdrawal Reflex - A rapid response to a painful stimulus.

VI. Autonomic Nervous System (ANS)

  • Divisions:

    • Parasympathetic Division: Generally promotes rest-and-digest functions, conserves energy, and assists in involuntary actions.

    • Sympathetic Division: Activates fight-or-flight responses, prepares body for emergencies, increases heart rate, and dilates airways.

Comparative Functions:

  • Parasympathetic: Decreases heart rate, increases digestive activity.

  • Sympathetic: Increases heart rate, inhibits digestive activity.

VII. Resting Membrane Potential and Action Potential Generation

  • Resting Membrane Potential:

    • Defined as the electrical potential difference across a neuron's plasma membrane at rest: approximately -70mV.

    • Maintained by differences in ion concentrations: high K+ concentration inside and high Na+ concentration outside the neuron.

Maintaining resting potential involves:

  1. Open K+ channels facilitating K+ diffusion out and limited Na+ diffusion in, resulting in a negative internal charge.

  2. Negatively charged proteins inside the cell also add to this negativity.

  3. Sodium-potassium pumps actively transport 3 Na+ out and 2 K+ in using ATP, preserving concentration gradients.

VIII. Neuronal Action Potential

  • Generation of Action Potential: A momentary reversal of the membrane potential that allows for communication along the axon.

    • Characteristics: all-or-none response, constant magnitude.

  • Stages:

    1. Resting state: Na+ and K+ channels are closed.

    2. Depolarization: Na+ channels open, Na+ enters the neuron, making it more positive.

    3. Threshold is reached, resulting in complete opening of Na+ channels.

    4. Repolarization: Na+ channels close, and K+ channels open; K+ exits, restoring negative charge.

    5. Hyperpolarization: Brief period during which K+ permeability is higher than at rest, potential overshoots and then stabilizes back to resting.

X. Neurotransmission and Neurotransmitters

  • Neurotransmission: Process by which signals are transmitted across a synapse from one neuron to another via neurotransmitters.

  • Types of Neurotransmitters:

    • Excitatory: E.g., Glutamate - Main neurotransmitter for excitatory signals in the brain.

    • Inhibitory: E.g., GABA - Main inhibitory neurotransmitter, helps regulate neuronal excitability.

    • Biogenic Amines: E.g., Dopamine and Norepinephrine - involved in mood regulation and responses to stress.

    • Neuropeptides: Such as Met-Enkephalin and Beta-Endorphin - modulate pain response and provide analgesia.