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Nervous System Overview

Oct 17, 2025

Overview

This lecture introduces the nervous system, focusing on its functions, organization, neuron structure and classification, the distinction between neurons and nerves, types of synapses, glial cell types, and the importance of myelination. It also covers the structural and functional divisions of the nervous system, neuron microanatomy, and clinical conditions related to nervous tissue.

Functions and Organization of the Nervous System

  • The nervous system is the body’s main communication and control system, responsible for collecting, interpreting, and responding to information from both external and internal environments.
  • It detects stimuli, processes and evaluates information, and initiates appropriate responses through effectors such as muscles or glands.
  • Structural organization:
    • Central Nervous System (CNS): Consists of the brain and spinal cord. The brain is the main control center, responsible for interpreting sensory information, higher-order functions (personality, reasoning, logic), and integrating motor output. The spinal cord relays information to and from the brain and the rest of the body.
    • Peripheral Nervous System (PNS): Composed of all nerves (bundles of axons) and ganglia (clusters of neuron cell bodies) outside the CNS. Nerves extend from the brain (cranial nerves) or spinal cord (spinal nerves) to peripheral parts of the body. Ganglia serve as relay points, containing collections of neuron cell bodies along nerves.
  • Functional organization:
    • Sensory (Afferent) Division: Brings sensory information from receptors to the CNS.
      • Somatic Sensory: Detects consciously perceived stimuli (e.g., touch, sound, light).
      • Visceral Sensory: Detects stimuli not consciously perceived (e.g., internal organ signals, blood pressure, changes in blood glucose).
    • Motor (Efferent) Division: Sends commands from the CNS to effectors.
      • Somatic Motor: Voluntary control of skeletal muscles.
      • Visceral (Autonomic) Motor: Involuntary control of cardiac muscle, smooth muscle, and glands. Subdivided into sympathetic (fight or flight) and parasympathetic (rest and digest) branches.
  • The nervous system’s three main functions are:
    • Sensory input: Detecting stimuli from the environment or within the body.
    • Integration: Processing and interpreting sensory input to determine an appropriate response.
    • Motor output: Initiating responses by sending signals to effectors (muscles or glands).
  • Sensory and motor divisions each have somatic (conscious/voluntary) and visceral (unconscious/involuntary) components, allowing the nervous system to manage both voluntary actions and automatic body functions.

Structure and Classification of Neurons

  • Neurons are the functional cells of nervous tissue, specialized for transmitting electrical impulses and forming the basis of communication within the nervous system.
  • Key characteristics:
    • Excitability: Ability to respond to stimuli by changing membrane potential.
    • Conductivity: Ability to propagate electrical signals (action potentials) along their membranes.
    • Secretion: Release of neurotransmitters at axon terminals in response to electrical activity.
    • Longevity: Neurons are long-lived, often lasting a lifetime.
    • Amitotic: Most neurons do not divide after fetal development; if lost, they are not replaced.
  • Major parts of a neuron:
    • Cell body (Soma): Contains the nucleus and Nissl bodies (chromatophilic substance with ribosomes for protein synthesis). The cell body is the metabolic and biosynthetic center of the neuron.
    • Dendrites: Short, branched extensions that receive input and transmit it to the cell body. Dendrites increase the surface area for receiving signals from other neurons.
    • Axon: Long process that conducts impulses away from the cell body. The axon originates at the axon hillock, contains axoplasm, may have axon collaterals (branches), and ends in axon terminals (with synaptic vesicles containing neurotransmitters).
    • Myelin sheath: Insulating layer around some axons, formed by glial cells, which increases conduction speed and efficiency of electrical impulses.
    • Nodes of Ranvier (Neurofibril nodes): Gaps in the myelin sheath that facilitate rapid signal transmission by allowing the action potential to "jump" from node to node.
  • Axonal transport: Movement of materials along the axon, essential for neuron function and survival.
    • Anterograde transport: Moves materials from the cell body to the axon terminal (e.g., neurotransmitters, enzymes).
    • Retrograde transport: Moves materials from the axon terminal back to the cell body (e.g., recycled materials, signaling molecules).
    • Fast axonal transport: ATP-dependent, uses the cytoskeleton (microtubules) as tracks for rapid movement of organelles and vesicles. Can be anterograde or retrograde.
    • Slow axonal transport: Relies on diffusion and the flow of axoplasm, moving materials more slowly, mainly in the anterograde direction.
  • Structural classification of neurons:
    • Multipolar: Many dendrites, one axon (most common type; found throughout the CNS and in motor pathways).
    • Bipolar: One dendrite and one axon extending from the soma (found in special sensory organs, such as the retina of the eye).
    • Unipolar (Pseudounipolar): Single process splits into two branches (peripheral and central processes); common in sensory neurons of the PNS.
    • Anaxonic: Only dendrites, no axon; rare, mainly found in the CNS, involved in local processing.
  • Functional classification of neurons:
    • Sensory (Afferent) Neurons: Transmit sensory input from receptors to the CNS; mostly unipolar, some bipolar (e.g., in the eye).
    • Motor (Efferent) Neurons: Carry commands from the CNS to effectors (muscles/glands); all are multipolar.
    • Interneurons (Association Neurons): Integrate and process information within the CNS; most are multipolar and make up the majority of neurons. They connect sensory and motor pathways and are essential for reflexes and higher functions.
  • Cytoskeleton: Neurons have a cytoskeleton made of microfilaments, intermediate filaments, and microtubules, providing structural support and serving as tracks for axonal transport.

Nerves and Synapses

  • Nerves are bundles of axons in the PNS, often containing both sensory and motor fibers (mixed nerves), but individual axons carry signals in only one direction.
    • Cranial nerves: Originate from the brain (12 pairs), serving the head and neck.
    • Spinal nerves: Originate from the spinal cord (31 pairs), serving the rest of the body.
    • Nerve structure:
      • Epineurium: Outer connective tissue covering the entire nerve, providing protection and support.
      • Perineurium: Surrounds bundles of axons (fascicles), providing a barrier and support for each fascicle.
      • Endoneurium: Encloses individual axons and their myelin sheaths, providing insulation and support.
  • Ganglia: Clusters of neuron cell bodies in the PNS, often associated with nerves. Ganglia serve as relay points for transmitting signals.
  • Synapse: The junction where communication occurs between two neurons or between a neuron and another cell (muscle or gland).
    • Chemical synapses: Use neurotransmitters to transmit signals across a synaptic cleft; most common type, with a slight synaptic delay due to neurotransmitter release, diffusion, and binding.
    • Electrical synapses: Directly transmit electrical signals via gap junctions, allowing rapid communication with no synaptic delay. These are less common than chemical synapses.
    • Presynaptic neuron: The neuron sending the signal (releases neurotransmitter).
    • Postsynaptic neuron/cell: The neuron or cell receiving the signal (responds to neurotransmitter).
  • Synaptic delay: Occurs in chemical synapses due to the time required for neurotransmitter release, diffusion across the synaptic cleft, and binding to receptors on the postsynaptic cell.
  • Nerve vs. neuron: A nerve is a bundle of axons from many neurons, often containing both sensory and motor fibers. Each individual neuron transmits signals in only one direction, but a nerve can carry both types of information because it contains many neurons.

Glial Cells (Neuroglia) and Myelination

  • Glial cells are non-excitable support cells in nervous tissue, more numerous than neurons, and capable of mitosis. They protect, nourish, and support neurons, and are essential for normal nervous system function.
  • CNS glial cells:
    • Astrocytes: Star-shaped cells that form the blood-brain barrier by wrapping around blood vessels, regulate ion concentrations (especially potassium), absorb excess potassium, and fill spaces left by dead neurons. They help maintain the environment necessary for action potentials and neuronal health.
    • Ependymal cells: Line the ventricles of the brain and central canal of the spinal cord; form the choroid plexus and produce cerebrospinal fluid (CSF), which cushions and nourishes the CNS.
    • Microglia: Small, phagocytic cells that remove debris, waste, and pathogens from CNS tissue. They act as the immune defense of the CNS.
    • Oligodendrocytes: Form myelin sheaths around CNS axons by wrapping their extensions around multiple axons, enabling faster action potential conduction and insulating axons from each other.
  • PNS glial cells:
    • Satellite cells: Surround and insulate neuron cell bodies in ganglia, regulating their environment and separating cell bodies from each other to prevent electrical interference.
    • Schwann cells (Neurolemmocytes): Myelinate axons in the PNS by wrapping their membrane around the axon, forming the myelin sheath. Each Schwann cell myelinates a single segment of one axon.
  • Myelination:
    • Increases the speed of action potential propagation via saltatory conduction (action potentials "jump" from node to node).
    • Myelin is produced by oligodendrocytes in the CNS and Schwann cells in the PNS.
    • Not all axons are myelinated; unmyelinated axons conduct impulses more slowly and may be partially insulated by glial cells.
    • Myelin sheaths are formed by multiple layers of glial cell membrane wrapping around the axon, with gaps (nodes of Ranvier) between segments. These nodes are critical for rapid signal transmission.
    • In the CNS, oligodendrocytes can myelinate multiple axons, while in the PNS, each Schwann cell myelinates only one segment of a single axon.
    • Unmyelinated axons in the PNS may be partially surrounded by Schwann cells but are not wrapped in multiple layers.
  • Clinical conditions:
    • Multiple sclerosis (MS): Autoimmune disorder causing loss of CNS myelin due to immune attack on oligodendrocytes, leading to scarring, repeated inflammatory events, and progressive loss of function. Symptoms worsen with each flare-up.
    • Guillain-Barre syndrome (GBS): Inflammatory loss of myelin in PNS nerves, often resulting in muscle weakness that can progress from distal to proximal muscles. Function can sometimes recover with treatment.
    • Glial cell tumors (gliomas): Arise from glial cells (not neurons), can be benign or malignant, and may cause problems due to pressure within the skull. Examples include astrocytomas and glioblastomas.

Key Terms & Definitions

  • Neuron: Excitable cell that transmits electrical impulses in the nervous system.
  • Nerve: Bundle of axons in the peripheral nervous system.
  • Ganglion: Cluster of neuron cell bodies in the PNS.
  • Synapse: Junction where communication occurs between two neurons or a neuron and another cell.
  • Glial Cell (Neuroglia): Supporting cell in nervous tissue.
  • Myelination: Process of wrapping axons with a myelin sheath for insulation and faster signal transmission.
  • Afferent: Sensory, carrying signals toward the CNS.
  • Efferent: Motor, carrying signals away from the CNS.
  • Somatic: Related to voluntary/conscious control.
  • Visceral (Autonomic): Related to involuntary/unconscious control.
  • Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission.
  • Action potential: Electrical signal propagated along the membrane of a neuron.
  • Epineurium: Outer connective tissue covering a nerve.
  • Perineurium: Connective tissue surrounding a fascicle (bundle) of axons.
  • Endoneurium: Connective tissue surrounding individual axons.

Action Items / Next Steps

  • Be able to label and draw a neuron, including all major parts (cell body, dendrites, axon, axon hillock, axon terminals, myelin sheath, nodes of Ranvier).
  • Review the four structural and three functional classifications of neurons, and be able to identify examples of each.
  • Read Chapter 12 (Nervous Tissue) in your textbook for more detail and clarification.
  • Prepare to discuss the processes of action potential generation and propagation in the next lecture.
  • Familiarize yourself with the roles of different glial cells and the clinical significance of myelination, including the effects of demyelinating diseases.
  • Review diagrams of neuron and nerve structure, and understand the organization of the nervous system, including the differences between CNS and PNS components.
  • Complete assigned readings on neuron communication and glial cell functions.
  • Prepare for a quiz on nervous system divisions, synapses, neuron types, and the structure and function of glial cells.

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