Nervous System & Tissue

Nervous System Overview

The nervous system is a complex organ system that plays a crucial role in how organisms perceive and respond to their environment. The study of the nervous system focuses on neural tissue, its structure, and its various functions.

Chapter Objectives

After studying this chapter, students should be able to:

  • Identify Major Divisions: Understand both anatomical and functional divisions of the nervous system.

  • Differentiate Gray and White Matter: Describe the functional and structural differences between gray matter and white matter.

  • Understand Neuron Anatomy: Identify parts of a multipolar neuron in order of polarity.

  • Recognize Glial Cells: List types of glial cells, their locations, and functions in the nervous system.

  • Explain Nervous System Functions: Distinguish the major functions of the nervous system including sensation, integration, and response.

  • Membrane Potential: Describe how resting membrane potential is established and the changes that lead to action potentials.

  • Graded Potentials: Explain differences between types of graded potentials.

  • Neurotransmitter Classification: Categorize major neurotransmitters based on their chemical types and their effects.

Basic Structure and Function of the Nervous System

Organization of the Nervous System

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

  • Peripheral Nervous System (PNS): Includes cranial nerves, spinal nerves, and specialized receptors.

    • Sensory Division: Conducts impulses from sensory receptors to the CNS.

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

Overall, the nervous system can be divided into:

  • CNS Functions: Integrative and control centers.

  • PNS Functions: Communication pathways between the CNS and the rest of the body.

    • Sympathetic Division: Mobilizes body systems during activity; known for "fight or flight" responses.

    • Parasympathetic Division: Conserves energy and promotes "housekeeping" functions at rest.

Characteristics of Neurons

  • Neurons are excitable cells with a high metabolic rate that can transmit signals through electrical impulses. They typically possess a cell body (soma), dendrites for receiving inputs, and an axon that transmits impulses away from the cell body.

Parts of a Neuron

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

  • Dendrites: Receive signals from other neurons.

  • Axon: Transmits impulses to other neurons.

  • Myelin Sheath: Insulates the axon to facilitate faster signal transmission.

Neurophysiology

Action Potentials

  • The action potential is a crucial electrical signal used for neuron communication. It involves a series of steps that includes resting potential, depolarization, propagation, repolarization, and hyperpolarization.

  • Resting Membrane Potential: The membrane maintains a resting voltage of around -70mV due to the uneven distribution of ions across the membrane, primarily sodium (Na+) and potassium (K+).

  • Depolarization: A stimulus causes Na+ channels to open, leading to a rapid influx of Na+ and a shift towards 0mV.

  • Action Potential Propagation: If the membrane reaches threshold (-55mV), an action potential is generated, allowing Na+ channels to open sequentially along the axon.

  • Repolarization: K+ channels open to return the membrane potential to resting state, often overshooting to -90mV (hyperpolarization).

Communication Between Neurons

Synapses

  • Synapses are the sites where communication occurs between neurons. Two main types of synapses are:

    • Chemical Synapses: Utilize neurotransmitters to transmit signals across the synaptic gap.

    • Electrical Synapses: Allow direct communication via gap junctions that facilitate rapid signal transmission.

Neurotransmitters

  • Neurotransmitters are classified based on their effects and systems they operate within, including excitatory and inhibitory neurotransmitters.

    • Examples: Acetylcholine, serotonin, glutamate, GABA, and various neuropeptides.

    • Mechanisms of Action: Neurotransmitters bind to receptors on the postsynaptic neuron, leading to changes in membrane potential and potentially triggering an action potential.