Organization of the Nervous System

Central and Peripheral Nervous System

The nervous system is divided into two main parts, each with distinct structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and coordinates activity throughout the body.

• Peripheral Nervous System (PNS): Composed of all neural tissue outside the CNS, including nerves and ganglia. It connects the CNS to limbs and organs.

Sensory and Motor Divisions

The PNS is further divided based on the direction of information flow:

• Sensory (Afferent) Division: Carries sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits commands from the CNS to effectors (muscles and glands).

Somatic vs. Visceral (Autonomic) Nervous System

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic (Visceral) Nervous System: Regulates involuntary functions (e.g., heart rate, digestion). Subdivided into:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Identifying the Cells of the Nervous System

Nervous Tissue Cell Types

Nervous tissue contains two main types of cells:

• Neurons: The primary signaling cells, responsible for transmitting electrical impulses.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

Neuroglial Cells: Location and Function

• In the CNS:

  • Astrocytes: Form the blood-brain barrier and regulate neurotransmitters.

  • Microglial Cells: Act as immune cells, removing debris and pathogens.

  • Ependymal Cells: Line cavities such as the central canal and ventricles; involved in cerebrospinal fluid production.

  • Oligodendrocytes: Produce myelin sheaths around CNS axons, increasing signal speed.

• In the PNS:

  • Schwann Cells: Produce myelin sheaths around PNS axons.

  • Satellite Cells: Support neuron cell bodies in ganglia.

Neuron Structure Identification

Key structures to identify on a neuron model:

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

• Nucleus: Contains genetic material.

• Mitochondrion: Provides energy for cellular processes.

• Nissl Bodies: Rough endoplasmic reticulum involved in protein synthesis.

• Axon Hillock: Region where the axon originates from the soma; site of action potential initiation.

• Initial Segment: First part of the axon, important for action potential generation.

• Neurofibrils: Cytoskeletal elements that support neuron shape.

• Dendrites: Receive incoming signals from other neurons.

• Schwann Cells: Myelinate axons in the PNS.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal conduction.

• Myelin Sheath: Insulating layer around axons.

• Collateral Branch (Telodendria): Branches of the axon that can form synapses with multiple targets.

Synaptic Terminal Structures

Structures associated with the synaptic terminal:

• Synaptic Vesicles: Store neurotransmitters for release into the synaptic cleft.

• Presynaptic Membrane: The membrane of the neuron sending the signal.

• Endoneurium: Connective tissue surrounding individual axons (see Anatomy of a Nerve).

• Axon: The long process that transmits signals away from the soma.

Neuron Classification

Structural Classification

Neurons are classified by the number and arrangement of their processes:

• Unipolar Neuron: Single process; typical of sensory neurons in the PNS.

• Multipolar Neuron: One axon and multiple dendrites; most common type, typical of motor neurons.

• Bipolar Neuron: One axon and one dendrite; found in special sensory organs (e.g., retina).

• Interneuron: Connects neurons within the CNS; often multipolar.

Functional Classification

Neurons are also classified by their function:

• Afferent (Sensory) Neurons: Transmit sensory information to the CNS; can be somatic or visceral.

• Efferent (Motor) Neurons: Carry commands from the CNS to effectors; can be somatic or visceral.

• Interneurons: Integrate information within the CNS.

Anatomy of a Nerve

Structure of a Nerve

A nerve is a bundle of axons in the PNS, organized by connective tissue layers:

• Epineurium: Outer connective tissue sheath surrounding the entire nerve.

• Fascicle: Sub-bundle of axons within a nerve.

• Perineurium: Connective tissue surrounding each fascicle.

• Endoneurium: Connective tissue surrounding each individual axon.

Additional Definitions

• Tract: Collection of axons within the CNS.

• Ganglion: Collection of neuron cell bodies within the PNS.

• Nucleus: Collection of neuron cell bodies within the CNS.

• White Matter: Regions of the CNS rich in myelinated axons; appears white due to myelin.

• Gray Matter: Regions of the CNS containing neuron cell bodies, dendrites, and unmyelinated axons.

Spinal Cord Model (Reflex Arc) Demonstration

Key components to identify on a spinal cord model, especially in the context of a reflex arc:

1. Receptor: Detects stimulus.

2. Sensory Neuron: Transmits afferent impulses to the CNS.

3. Ganglion: Contains sensory neuron cell bodies.

4. Interneuron: Processes information within the CNS.

5. Motor Neuron: Transmits efferent impulses from the CNS to effectors.

6. Effector: Muscle or gland that responds to the motor neuron signal.

7. Gray and White Matter: Identify regions in the spinal cord cross-section.

8. Neuroglial Cells: Locate different types within the CNS and PNS.

Summary Table: Neuroglial Cells

Key Equations and Concepts

• Resting Membrane Potential: The difference in electric charge across the neuron's membrane at rest, typically about .

• Action Potential: Rapid change in membrane potential that travels along the axon. Initiated at the axon hillock when threshold is reached.

Example: In a reflex arc, a sensory neuron detects a stimulus (e.g., touching a hot object), transmits the signal to the spinal cord, where an interneuron processes it and sends a command via a motor neuron to withdraw the hand.