Fundamentals of the Nervous System and Special Senses: Structured Study Notes

Fundamentals of the Nervous System

Organization of the Nervous System

The nervous system (NS) is divided into central and peripheral components, each with distinct functions and structures.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Consists of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

• Components of Each Division: Each division contains specialized cells and structures for signal transmission and processing.

NS Histology

Histology of the nervous system focuses on the cellular and tissue-level organization.

• Neuroglia: Supportive cells in the CNS and PNS, each with specific types and functions.

• Neurons:

  • Function: Transmit electrical impulses.

  • Characteristics: Excitability, conductivity, and longevity.

  • Parts:

    • Cell body (soma): Contains nucleus and organelles.

    • Dendrites: Receive signals.

    • Axon: Transmits impulses away from cell body.

  • Myelination:

    • Function: Increases speed of impulse conduction.

    • Cells: Oligodendrocytes (CNS), Schwann cells (PNS).

    • Method of Conduction: Saltatory conduction in myelinated axons.

  • Neuron Classification:

    • Structural: Multipolar, bipolar, unipolar.

    • Functional: Sensory, motor, interneurons.

• Nerves:

  • Connective Tissue Coverings:

    • Endoneurium: Surrounds individual axons.

    • Perineurium: Surrounds fascicles.

    • Epineurium: Surrounds entire nerve.

  • Types of Nerves: Sensory, motor, mixed.

Physiology of a Nerve Impulse

Nerve impulses are electrical signals that travel along neurons, enabling communication throughout the body.

• Terminology:

  • Voltage (potential): Difference in electrical charge across the membrane.

  • Resting Membrane Potential: Typically -70 mV in neurons.

• Channels:

  • Types: Voltage-gated, ligand-gated, mechanically-gated.

  • Depolarization: Membrane potential becomes less negative.

  • Hyperpolarization: Membrane potential becomes more negative.

  • Threshold: Minimum stimulus required to trigger an action potential.

• Ionic Composition:

  • Specific ions (Na+, K+, Cl-) concentrated in different areas.

  • Plasma membrane permeability regulates ion movement.

• Action Potentials:

  • Initiation: Triggered by stimuli that reach threshold.

  • All-or-None Principle: Action potentials either occur fully or not at all.

  • Propagation:

    • Speed affected by axon diameter and myelination.

    • Two conduction methods: continuous (unmyelinated) and saltatory (myelinated).

  • Refractory Period: Time during which a neuron cannot fire another action potential.

• Synapses:

  • Components: Presynaptic neuron, synaptic cleft, postsynaptic neuron.

  • Types: Chemical (most common), electrical.

• Postsynaptic Potentials:

  • Excitatory (EPSP) and inhibitory (IPSP).

  • Summation: Temporal and spatial.

• Neurotransmitters:

  • Various types with specific effects (e.g., acetylcholine, dopamine).

Central Nervous System (CNS)

CNS Terminology

• Nerve/tract: Bundles of axons in PNS/CNS.

• Ganglia/nucleus: Clusters of neuron cell bodies in PNS/CNS.

• Gray/white matter: Gray = cell bodies, white = myelinated axons.

• Ventricles: Fluid-filled cavities in the brain.

• Gyri/sulci: Ridges and grooves on the brain surface.

Diencephalon

• Thalamus: Relay station for sensory information.

• Epithalamus: Contains pineal gland, secretes melatonin.

• Hypothalamus: Regulates homeostasis, includes mammillary bodies and infundibulum.

Brain Stem

• Midbrain: Visual and auditory reflexes.

• Pons: Relays signals, regulates breathing.

• Medulla Oblongata: Controls vital functions (e.g., heart rate, respiration).

Cerebellum

• Location: Posterior to brainstem.

• Function: Coordinates movement and balance.

Cerebrum

• Corpus Callosum: Connects left and right hemispheres.

• Neocortex: Higher-order brain functions.

Protection of the Brain

• Meninges: Three layers—dura mater, arachnoid mater, pia mater.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the brain.

• Choroid Plexus: Produces CSF.

• Blood Brain Barrier: Protects brain from harmful substances.

Spinal Cord

• Function: Transmits signals between brain and body.

• Reflexes: Automatic responses to stimuli.

Special Senses

Sensory Receptors

Sensory receptors detect changes in the environment and initiate nerve impulses.

• Classification by Stimulus:

  • Mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, nociceptors.

• Classification by Location:

  • Exteroceptors, interoceptors, proprioceptors.

Vision

• Structures of the Eye:

  • Three layers: fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), inner (retina).

  • Chambers: anterior, posterior.

• Photoreceptors:

  • Rods (dim light, peripheral vision), cones (color, sharp vision).

• Phototransduction:

  • Conversion of light into electrical signals.

  • Pathway: Photoreceptors → bipolar cells → ganglion cells → optic nerve → brain.

• Light/Dark Adaptation:

  • Adjustment of eyes to changes in light intensity.

Hearing and Equilibrium

• Major Areas of the Ear:

  • Outer ear, middle ear, inner ear.

• Physiology of Hearing:

  • Sound waves cause vibration of tympanic membrane, transmitted via ossicles to cochlea.

  • Terminology: frequency, wavelength, amplitude, loudness.

  • Pathway: Sound → external ear → middle ear → inner ear → cochlear nerve → brain.

• Equilibrium/Orientation:

  • Structures: semicircular canals (rotational), vestibule (linear acceleration).

Chemical Senses

• Taste: Five basic sensations—sweet, sour, salty, bitter, umami.

• Smell: Olfactory receptors detect airborne chemicals.

Additional info:

• For equations related to membrane potential: $V_m = V_{inside} - V_{outside}$

• For action potential: $I = g(V_m - E)$ where $I$ is current, $g$ is conductance, $V_m$ is membrane potential, $E$ is equilibrium potential.

• For summation: $EPSP_{total} = EPSP_1 + EPSP_2 + ... + EPSP_n$