Nervous System Study Notes

Introduction to The Nervous System

  • Arranged by Dr. Andrew Jensen for the College of Western Idaho.

General Functions of the Nervous System

  • Nervous System: A communication and control system of the body.

  • Functions:

    • Collect Information:

    • Receptors detect stimuli.

    • Sends sensory signals to spinal cord and brain.

    • / Process and Evaluate Information:

    • Brain and spinal cord determine response to sensory input.

    • Initiate Response to Information:

    • Brain and spinal cord send motor output via nerves to effectors (muscles or glands).

Organization of the Nervous System

  • Structural Organization:

    • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS):

    • Comprises peripheral nerves and ganglia.

Functional Organization of the Nervous System

  • Sensory vs. Motor:

    • Sensory Nervous System (Afferent Nervous System):

    • Receives sensory information from receptors and transmits it to CNS.

      • Somatic Sensory System: Detects stimuli that are consciously perceived (e.g., from eyes, ears, skin). ‘

      • Visceral Sensory System: Detects stimuli that are typically not consciously perceived (e.g., signals from heart or kidneys).

    • Motor Nervous System (Efferent Nervous System):

    • Initiates motor output and transmits it from CNS to effectors.

      • Somatic Motor System: Sends voluntary signals to skeletal muscles.

      • Autonomic Motor System (Visceral Motor): Sends involuntary commands to heart, smooth muscle, and glands.

      • Divided into sympathetic and parasympathetic divisions.

Nerves and Ganglia

  • Nerve: A bundle of parallel axons in the PNS.

  • Connective Tissue Wrappings:

    • Epineurium: Encloses entire nerve; thick layer of dense irregular connective tissue.

    • Perineurium: Wraps fascicle (bundle of axons); layer of dense irregular connective tissue.

    • Endoneurium: Wraps an individual axon; delicate layer of areolar connective tissue that separates and electrically insulates each axon.

  • Vascularization:

    • Blood vessels branch through epineurium and perineurium to become capillaries, allowing for exchange between axons and blood.

Structural Classification of Nerves

  • Cranial Nerves: Extend from the brain.

  • Spinal Nerves: Extend from the spinal cord.

  • Functional Classification of Nerves:

    • Sensory Nerves: Contain sensory neurons sending signals to CNS.

    • Motor Nerves: Contain motor neurons sending signals from CNS.

    • Mixed Nerves: Contain both sensory and motor neurons; most named nerves fall into this category.

  • Ganglion: A cluster of neuron cell bodies in the PNS.

General Characteristics of Neurons

  • Excitability: Responsiveness to a stimulus.

    • Stimulus causes change in cell’s membrane potential.

  • Conductivity: Ability to propagate an electrical signal.

    • Voltage-gated channels open sequentially along the membrane, which is a one-way event.

  • Secretion: Release of neurotransmitters in response to conductive activity.

    • Messenger is released from vesicles to influence target cell.

  • Extreme Longevity: Neurons can live throughout a person’s lifetime.

  • Amitotic: After fetal development, most neurons lose mitotic activity.

Neuron Structure

  • Cell Body (Soma):

    • Plasma membrane encloses cytoplasm (perikaryon).

    • Contains nucleus; initiates some graded potentials and receives others from dendrites, conducting these to the axon.

    • Chromatophilic Substance (Nissl Bodies): Made of ribosomes (free and bound).

  • Dendrites:

    • Short, unmyelinated processes that branch off the cell body.

    • Function: Receive input and transfer it to the cell body.

  • Axon:

    • Long process emanating from the cell body; makes contact with other neurons, muscle cells, or glands.

    • Attaches to cell body at axon hillock (triangular region of the soma).

    • Cytoplasm called axoplasm; membrane called axolemma.

    • Splits into branches called axon collaterals and ends in several telodendria (axon terminals).

    • Tips of telodendria are synaptic knobs (terminal boutons) that house synaptic vesicles containing neurotransmitter.

    • Functions to conduct action potentials and release neurotransmitters at synaptic knobs.

Cytoskeleton in Neurons

  • Components:

    • Composed of microfilaments, intermediate filaments, and microtubules.

    • Intermediate filaments are termed neurofilaments which aggregate to form bundles, neurofibrils, providing tensile strength.

Neuron Transport

  • Mechanism: Axons move materials to and from the cell body.

  • Anterograde Transport: Moves materials from the cell body to the synaptic knobs.

    • Transports newly synthesized materials.

  • Retrograde Transport: Moves materials to the cell body.

    • Used materials are sent for breakdown and recycling in the soma.

  • Modes of Transport:

    • Fast Axonal Transport:

    • Occurs at about 400 mm per day; involves movement along microtubules powered by motor proteins splitting ATP.

    • Can be anterograde or retrograde.

      • Anterograde: vesicles, organelles, glycoproteins.

      • Retrograde: used vesicles, potentially harmful agents.

    • Slow Axonal Transport:

    • Occurs at about 0.1 to 3 mm per day; results from the flow of axoplasm.

    • Substances moved from cell body toward knob are enzymes, cytoskeletal components, and new axoplasm.

L Classification of Neurons

  • Structural Classification:

    • Multipolar Neurons: Many dendrites and one axon; most common type.

    • Bipolar Neurons: One dendrite and one axon; limited, e.g., in the retina of the eye.

    • Unipolar Neurons (Pseudounipolar): One process that extends from cell body and splits into two processes (peripheral and central processes).

    • Anaxonic Neurons: Have dendrites but no axons.

  • Functional Classification:

    • Sensory Neurons (Afferent Neurons): Conduct input from somatic and visceral receptors to the CNS; most are unipolar (some bipolar).

    • Motor Neurons (Efferent Neurons): Conduct output from the CNS to somatic and visceral effectors; all are multipolar.

    • Interneurons (Association Neurons): Receive, process, and integrate information from many neurons; located within the CNS and make up 99% of neurons; generally multipolar.

Synapses

  • Definition: A synapse is a place where a neuron connects to another neuron or an effector.

  • Components:

    • Presynaptic Neuron: The axon terminal that produces the signal.

    • Postsynaptic Neuron: Receives the signal, often via its dendrites.

    • Synaptic Cleft: A small fluid-filled gap between the two neurons.

  • Events of Synaptic Communication:

    • Neurotransmitter molecules released from vesicles in the synaptic knob into the cleft.

    • Neurotransmitter diffuses across cleft and binds to postsynaptic receptors.

    • Binding initiates postsynaptic potential (a graded potential).

    • Synaptic Delay: The time taken for these events to occur.