Fundamentals of the Nervous System and Nervous Tissue (Chapter 11) – Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Learning Objectives

• Identify the subdivisions of the nervous system.

• Distinguish between the two main types of cells in nervous tissue.

• Identify the main parts of a neuron.

• Discuss the classification of neurons, including major connective tissues.

• Describe the sequence of events in an action potential.

• Explain the structure and function of both chemical and electrical synapses.

Subdivisions of the Nervous System

Overview and Functions

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to rapidly and specifically coordinate immediate responses.

• Sensory input: Information gathered by sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input.

• Motor output: Activation of effector organs (muscles and glands) to produce a response.

Principal Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and control center, interpreting sensory input and dictating motor output.

• Peripheral Nervous System (PNS): The portion of the nervous system outside the CNS. It consists mainly of nerves that extend from the brain and spinal cord (spinal nerves and cranial nerves).

Functional Divisions of the PNS

• Sensory (afferent) division: Conveys impulses from sensory receptors to the CNS.

  • Somatic sensory fibers: Carry impulses from skin, skeletal muscles, and joints.

  • Visceral sensory fibers: Carry impulses from visceral organs.

• Motor (efferent) division: Transmits impulses from CNS to effector organs (muscles and glands).

  • Somatic nervous system: Conducts impulses to skeletal muscle (voluntary).

  • Autonomic nervous system (ANS): Regulates smooth muscle, cardiac muscle, and glands (involuntary).

    • Sympathetic division

    • Parasympathetic division

Classification of Neurons and Major Connective Tissues

Cell Types in Nervous Tissue

Nervous tissue is highly cellular, consisting of two principal cell types:

• Neuroglia (glial cells): Small cells that surround and wrap delicate neurons, providing support and protection.

• Neurons (nerve cells): Excitable cells that transmit electrical signals.

Types of Neuroglia in the CNS

• Astrocytes: Most abundant; support and brace neurons.

• Microglial cells: Migrate toward injured neurons.

• Ependymal cells: Line central cavities of brain and spinal cord; form a permeable barrier between cerebrospinal fluid (CSF) and tissue fluid.

• Oligodendrocytes: Wrap CNS nerve fibers, forming insulating myelin sheaths.

Structural Classification of Neurons

• Multipolar: Three or more processes (one axon, others dendrites); most common in CNS.

• Bipolar: Two processes (one axon, one dendrite); rare, found in retina and olfactory mucosa.

• Unipolar: One process; associated with sensory receptors.

Functional Classification of Neurons

• Sensory (afferent) neurons: Transmit impulses from sensory receptors to CNS.

• Motor (efferent) neurons: Carry impulses from CNS to effector organs.

• Interneurons (association neurons): Shuttle signals through CNS pathways; most are within CNS.

Main Parts of a Neuron

General Structure

Neurons are large, highly specialized cells that conduct impulses. They have extreme longevity, are mostly amitotic, and have a high metabolic rate requiring continuous oxygen and glucose.

• Cell body (soma): Contains nucleus and cytoplasm; receptive region for signals.

• Processes: Extensions from the cell body; two types:

  • Dendrites: Short, tapering, branched processes; receptive (input) regions; convey graded potentials toward cell body.

  • Axon: Each neuron has one axon, starting at the axon hillock; conducting region; transmits impulses away from cell body; ends in axon terminals (secretory region).

Nuclei: Clusters of neuron cell bodies in CNS. Ganglia: Clusters of neuron cell bodies in PNS.

Myelin Sheath

• White, fatty substance that coats many axons, especially long or large axons.

• Protects and electrically insulates axon; increases speed of impulse transmission.

• Myelinated axons: Segmented sheath surrounds most long or large-diameter axons.

• Nonmyelinated axons: Do not contain sheath; conduct impulses more slowly.

• In the PNS, myelination is formed by Schwann cells; gaps between adjacent Schwann cells are called nodes of Ranvier.

Sequence of Events in an Action Potential

Electrochemical Gradients

• Concentration gradient: Ions move from higher to lower concentration.

• Electrical gradient: Ions move toward area of opposite electrical charge.

Resting Membrane Potential

The resting membrane potential of a neuron is approximately $-70\,\text{mV}$ (polarized). It is generated by:

• Differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF).

• Differences in plasma membrane permeability.

Depends on differences in K+ and Na+ concentrations inside and outside cells, and permeability of the plasma membrane to these ions.

Changing the Resting Membrane Potential

• Occurs when concentrations of ions across membrane change or membrane permeability to ions changes.

• Produces two types of signals:

  • Graded potentials: Incoming signals operating over short distances.

  • Action potentials: Long-distance signals of axons.

Terms Describing Membrane Potential Changes

• Depolarization: Decrease in membrane potential (moves toward zero and above); inside of membrane becomes less negative.

• Hyperpolarization: Increase in membrane potential (away from zero); inside of membrane becomes more negative.

Graded Potentials

• Short-lived, localized changes in membrane potential.

• The stronger the stimulus, the more voltage changes and the farther current flows.

• Triggered by stimulus that opens gated ion channels.

• Results in depolarization or sometimes hyperpolarization.

Action Potentials (AP)

• Brief reversal of membrane potential with a change in voltage of approximately 100 mV.

• From $-70\,\text{mV}$ to $+30\,\text{mV}$.

• Neurons send signals over long distances by generating and propagating APs.

• Occur only in excitable membranes (neurons and muscle cells).

• APs do not decay over distance as graded potentials do.

• Typically generated in axons of neurons; involves opening of specific voltage-gated channels.

Main Stages of Action Potential

1. Resting state: All voltage-gated Na+ and K+ channels are closed; only leakage channels are open.

2. Depolarization: Voltage-gated Na+ channels open; Na+ rushes into cell, making inside less negative.

3. Repolarization: Na+ channels are inactivating, and voltage-gated K+ channels open; membrane returns to resting membrane potential.

4. Hyperpolarization: Some K+ channels remain open; inside of membrane becomes more negative than resting state.

Threshold and All-or-None Phenomenon

• Not all depolarization events produce an AP; depolarization must reach threshold voltage to trigger AP.

• All-or-none phenomenon: AP either happens completely or does not happen at all.

Clinical Implications

• Multiple sclerosis (MS): An autoimmune disease affecting young adults; myelin sheaths in CNS are destroyed, turning myelin into hardened lesions called scleroses. Impulse conduction slows and eventually ceases.

Structure & Function of Chemical and Electrical Synapses

Synapses Overview

Neurons are functionally connected by synapses, junctions that mediate information transfer.

• Presynaptic neuron: Conducts impulses toward synapse.

• Postsynaptic neuron: Transmits electrical signal away from synapse.

Electrical Synapses

• Less common than chemical synapses.

• Neurons are electrically coupled by gap junctions connecting cytoplasm of adjacent neurons.

• Communication is very rapid; may be unidirectional or bidirectional.

• Found in some brain regions responsible for eye movements and in the hippocampus (emotions and memory).

Chemical Synapses

• Most common type of synapse.

• Specialized for release and reception of chemical messengers (neurotransmitters).

• Composed of:

  • Axon terminal of presynaptic neuron: contains synaptic vesicles filled with neurotransmitter.

  • Receptor region on postsynaptic neuron's membrane: receives neurotransmitter.

  • Separated by fluid-filled synaptic cleft.

• Electrical impulse changed to chemical across synapse, then back into electrical.

• Transmission across synaptic cleft prevents nerve impulses from directly passing from one neuron to next; ensures unidirectional communication.

Neuron Type	Structure	Location	Function
Multipolar	3+ processes (1 axon, many dendrites)	Most CNS neurons	Motor and interneurons
Bipolar	2 processes (1 axon, 1 dendrite)	Retina, olfactory mucosa	Sensory (special senses)
Unipolar	1 process	Sensory ganglia in PNS	Sensory (general senses)

Example: The neuromuscular junction is a chemical synapse where a motor neuron communicates with a muscle fiber to initiate contraction.

Additional info: The above notes expand on the original slides by providing definitions, context, and examples for key terms and processes, ensuring a comprehensive and self-contained study guide for exam preparation.