Study Notes on the Nervous System

Introduction to the Nervous System

• Definition: The nervous system is the control and command center for the body.

• 3 Main Functions:

  • Sensory Input: Receives information from both external and internal environments.

  • Integration of Input: Processes this information to make decisions or responses.

  • Motor Output: Sends commands to effector organs to enact responses.

Divisions of the Nervous System

• Anatomical Divisions:

  • Central Nervous System (CNS): Consists of the brain and spinal cord.

  • Peripheral Nervous System (PNS): Composed of nervous tissue that lies outside the brain and spinal cord.

Neural Tissue in the Nervous System

• Components of Neural Tissue: The nervous system comprises all neural tissue in the body, which contains two types of cells:

  1. Neurons: Cells that transmit signals throughout the body.

  2. Neuroglia (Glial Cells): Support and protect neurons.

• Organs of the Nervous System: Include the brain, spinal cord, sensory receptors (e.g., eyes, ears), and nerves that connect the nervous system to other body systems.

Detection and Response of Sensory Information

• Receptors:

  • Function: Detect changes or respond to stimuli.

  • Types: Neurons, specialized cells, and complex sensory organs (such as eyes and ears).

• Integration:

  • Process: The CNS receives input from receptors, processes the information, and sends out responses through an efferent pathway.

• Effectors:

  • Definition: Cells and organs that respond to the efferent signals.

Functions of the Central Nervous System

• Primary Functions:

  • Receives sensory data from the body’s internal and external environments.

  • Integrates and processes the received data.

  • Issues commands to regulate effectors.

  • Higher functions, such as intelligence, memory, learning, and emotions, are coordinated in the brain.

• Nuclei: Collections of neuron cell bodies in the CNS.

The Peripheral Nervous System (PNS)

• Definition: All neural tissue situated outside the CNS.

• Divisions: It consists of two functional divisions:

  • Afferent Division (sensory): Carries impulses towards the CNS.

  • Efferent Division (motor): Carries impulses away from the CNS to effectors.

• Anatomical Components:

  • Ganglia: Collections of neuron cell bodies located outside the CNS.

  • Nerves: Collections of axons that extend from the brain and spinal cord.

• Cranial Nerves: 12 pairs connected to/from the brain.

• Spinal Nerves: 31 pairs connected to/from the spinal cord.

• Nerve Structure: A nerve is a collection of axons wrapped in a connective tissue sheath.

Overview of Nervous System

• Information Processing and Integration: The nervous system integrates and coordinates sensory input with motor commands.

• Components of the PNS:

  • Somatic Nervous System (SNS): Controls voluntary movements.

  • Autonomic Nervous System (ANS): Controls involuntary functions, which includes:

  • Parasympathetic Division: Responsible for rest and digest functions.

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

• Receptors: Special sensory receptors monitor smell, taste, vision, balance, and hearing.

• Effectors: Include skeletal muscle, smooth muscle, cardiac muscle, glands, and adipose tissue.

Histology of Nervous Tissue

• Principal Cell Types:

  1. Neurons: Excitable cells that transmit electrical signals.

  2. Neuroglia (Glial Cells): Supportive cells that include:

  • Astrocytes (CNS): Provide structural support and maintain the blood-brain barrier.

  • Microglia (CNS): Act as immune cells in the CNS.

  • Ependymal Cells (CNS): Line cavities and produce cerebrospinal fluid (CSF).

  • Oligodendrocytes (CNS): Myelinate axons in the CNS.

  • Satellite Cells (PNS): Support neurons in ganglia.

  • Schwann Cells (PNS): Myelinate axons in the PNS.

Characteristics of Neurons (Nerve Cells)

• Special Features:

  • Long lifespan (up to 100 years or more).

  • Amitotic, with very few exceptions.

  • High metabolic rate; dependent on a continuous supply of oxygen and glucose.

  • Plasma membrane plays an important role in electrical signaling and cell interactions during development.

Neuron Cell Body Components

• Components Include:

  • Nucleus and Nucleolus: Contains genetic material.

  • Perikaryon: The cytoplasm of the cell body.

  • Mitochondria: Produce energy for cellular processes.

  • Rough Endoplasmic Reticulum (RER) and Ribosomes: Produce neurotransmitters.

  • Nissl Bodies: Dense areas of RER and ribosomes giving gray color (gray matter).

  • Cytoskeleton: Contains neurofilaments and neurotubules:

    • Neurofibrils: Bundles of neurofilaments providing structural support for dendrites and axon.

Dendrites

• Structure:

  • Highly branched with many fine processes.

  • Dendritic spines are prominent.

  • Function: Receive information from other neurons, accounting for 80-90% of the neuron’s surface area.

The Axon

• Structure:

  • Long structure carrying electrical signals (action potentials) to target cells.

  • Axoplasm: The cytoplasm of the axon, contains neurofibrils, neurotubules, enzymes, and organelles.

  • Axolemma: The specialized cell membrane enveloping the axoplasm.

• Axon Hillock: Tapered area at the beginning of the axon, connecting it to the cell body.

• Initial Segment: Forms the attachment of the axon to the axon hillock.

Structures of the Axon

• Collaterals: Branches that extend from the axon.

• Telodendria: Fine extensions of the distal end of the axon leading to synaptic terminals.

• Synaptic Terminals: Bulbous tips of telodendria involved in neurotransmitter release to communicate with other cells.

Axon Transport

• Definition: Movement of materials from the neuron cell body through the axon, known as axoplasmic transport.

• Flow Directions:

  • Anterograde Flow: Movement from cell body outward (to the axon terminal).

  • Retrograde Flow: Movement from axon terminal back to the cell body.

• Flow Speeds:

  • Fast Stream: Carries neurotransmitters and important materials quickly.

  • Slow Stream: Involves repair and regeneration materials.

• Example of Retrograde Flow: Rabies virus utilizes this transport method.

Structural Classification of Neurons

• Types of Neurons:

  1. Anaxonic Neurons: Found in the brain and sense organs, with similar processes.

  2. Bipolar Neurons: Small neurons with one dendrite and one axon; found in special sensory organs controlling sight, smell, and hearing.

  3. Unipolar Neurons: Also referred to as pseudounipolar; have long axons and fused dendrites/axon, typically found in sensory neurons of the PNS.

  4. Multipolar Neurons: These have multiple dendrites and one axon; are the most common type found in the CNS, including all skeletal muscle motor neurons.

Visualization of Neurons

• Figure of Neuron Types:

  • Multipolar Neuron: Characterized by many dendrites.

  • Unipolar Neuron: Distinguished with a single process bifurcating into axon and dendrites.

  • Bipolar Neuron: Features one dendrite and one axon.

  • Anaxonic Neuron: Appears with all processes looking alike.

Functional Classification of Neurons

• 1. Sensory Neurons: Also known as afferent neurons, located primarily in the PNS (with few exceptions).

• 2. Interneurons: Also known as association neurons; facilitate communication between sensory and motor neurons.

• 3. Motor Neurons: Known as efferent neurons found mainly in the PNS (with some exceptions).