Nervous System Overview

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

• Role of the Brain:

  • Responsible for correlating and communicating with important systems.

  • Drives the release of hormones, muscle movement, and digestion.

• CNS:

  • Central Nervous System includes the brain and spinal cord.

• PNS:

  • Peripheral Nervous System carries impulses to and from the central nervous system.

  • Subdivisions of PNS:

  • Afferent System:

    • Responsible for sensory input; sends impulses from sensory receptors (e.g., from spider on leg).

    • Pathway:

    • Sensory neurons travel the impulse to the spinal cord for interpretation by the brain.

  • Efferent System:

    • Sends commands from the brain to the muscles or glands.

    • Mnemonic: E for exit (exits the brain).

    • Subdivided into:

    • Somatic Nervous System: Controls voluntary movements.

    • Autonomic Nervous System: Controls involuntary functions (e.g., heart rate, digestion).

      • Further divided into:

      • Sympathetic Nervous System: Prepares the body for 'fight or flight.'

      • Parasympathetic Nervous System: Brings the body back to rest.

Neuron Structure and Function

• Neurons:

  • Basic unit of the nervous system that transmits impulses.

• Neuroglial Cells (Glia):

  • Accessory cells that support neuron function.

  • Types include:

  • Astrocytes:

    • Support connections to the blood-brain barrier.

    • Supply neurons with blood and nutrients.

  • Oligodendrocytes:

    • Produce the myelin sheath in the CNS (around axons).

  • Schwann Cells:

    • Produce myelin sheath in the PNS.

• Myelin Sheath:

  • Protective covering around axons, increases the speed of impulse conduction.

  • Analogy: Similar to rubber coating on electrical cords, preventing impulse leakage.

Neuron Communication

• Dendrites:

  • Receive impulses from other neurons.

• Axon:

  • Transmits impulses away from the neuron.

• Action Potential:

  • A rapid rise and fall in voltage across the membrane of a neuron.

  • Starts at resting membrane potential of around
    -70 ext{ mV}.

  • Triggered when a stimulus causes depolarization, opening voltage-gated sodium channels, leading to a threshold of around
    -55 ext{ mV}.

  • The influx of sodium ions makes the inside of the neuron positive (up to about
    +40 ext{ mV}).

  • Following depolarization, repolarization occurs as potassium channels open and potassium ions leave the cell, returning the membrane potential to a negative state.

  • During hyperpolarization, the potential can drop below resting state (to around
    -75 ext{ mV}).

• Refractory Period:

  • After an action potential, a brief period during which the neuron cannot fire again.

Reflex Arcs and Reactions

• Reflex Arcs:

  • Simple neural pathways that mediate reflex actions (e.g., knee-jerk reaction).

  • Do not require conscious thought and are faster than voluntary actions.

• Examples of Reflex Actions:

  • Babinski reflex in infants.

  • Coughing as a reflex to stimuli.

Nervous System Structure

• Gray Matter vs. White Matter:

  • Gray Matter: Comprised mainly of neuronal cell bodies and unmyelinated axons.

  • White Matter: Comprised of myelinated axons, forms nerve tracts in the CNS.

• Spinal Cord Structure:

  • Continuation of the medulla oblongata, part of the brain stem.

  • Contains 31 segments, each with a pair of spinal nerves.

  • Protected by meninges (membranes surrounding the CNS).

• Spinal Nerve Distribution:

  • 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral nerves.

The Brain

• Main Parts of the Brain:

  • Cerebrum: Responsible for higher brain functions such as thought, memory, and sensory perception.

  • Cerebellum: Regulates motor control and coordination.

  • Brain Stem: Controls autonomic functions (breathing, heart rate).

  • Diencephalon: Contains structures like the thalamus and hypothalamus.

• Cerebrospinal Fluid:

  • Protects the brain and spinal cord from shock.

• Hypothalamus:

  • Regulates autonomic functions and influences mood.

Sensory Systems

• Special Senses:

  • Include smell, vision, and hearing.

• Eye Anatomy:

  • Sclera: The white part of the eye.

  • Cornea: Provides initial light refraction.

  • Iris: Controls light entering the eye.

  • Retina: Contains rods and cones for light interpretation.

• Ear Anatomy:

  • Tympanic Membrane: The eardrum, vibrates to interpret sound.

  • Ossicles: Tiny bones in the ear that transmit sound vibrations.

  • Semicircular Canals: Contribute to balance and spatial orientation.