Chapter 12: The Nervous System and Nervous Tissue – Structured Study Notes

The Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is a complex network responsible for communication and control throughout the body. It is divided into two major regions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• CNS: Composed of the brain and spinal cord; responsible for processing and integrating information.

• PNS: Includes all nervous tissue outside the CNS; connects the CNS to limbs and organs.

Cell Types in Nervous Tissue

Nervous tissue contains two basic types of cells:

• Neurons: The communicative cells that transmit electrical signals.

• Glial cells: Provide structural and metabolic support for neurons.

Anatomy of Neurons

Neurons are specialized for the transmission of electrical signals. Their structure includes:

• Cell body (soma): Contains the nucleus and most organelles.

• Dendrites: Extensions that receive signals from other neurons.

• Axon: A single, long process that transmits signals to target cells; may branch to communicate with multiple targets.

• Axon terminal: Ends in synaptic end bulbs, forming synapses with target cells.

• Myelin sheath: Insulating layer produced by glial cells, facilitating rapid signal transmission.

• Nodes of Ranvier: Gaps in the myelin sheath important for saltatory conduction.

Organization of Nervous Tissue

• Nucleus (CNS): Localized collection of neuron cell bodies in the CNS.

• Ganglion (PNS): Localized collection of neuron cell bodies in the PNS.

• Tract (CNS): Bundle of axons in the CNS.

• Nerve (PNS): Bundle of axons in the PNS.

Functional Divisions of the Nervous System

The nervous system performs three basic functions:

• Sensation: Detects changes (stimuli) within the body or environment via sensory structures.

• Response: Produces responses based on perceived stimuli; can be voluntary (somatic nervous system) or involuntary (autonomic nervous system).

• Integration: Processes and integrates sensory information in the CNS, leading to specific responses.

Somatic, Autonomic, and Enteric Nervous Systems

• Somatic Nervous System (SNS): Controls voluntary motor responses (skeletal muscle contraction).

• Autonomic Nervous System (ANS): Controls involuntary functions (smooth muscle, cardiac muscle, glands); maintains homeostasis.

• Enteric Nervous System (ENS): Controls smooth muscle and glandular tissue in the digestive system; sometimes considered part of the ANS.

Types of Neurons

Neurons are classified based on their structure:

• Pseudo-unipolar neurons: One process emerging from the cell body, which splits into two branches; sensory neurons in humans.

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

• Multipolar neurons: One axon and two or more dendrites; most common type.

Glial Cells (Neuroglia)

Glial cells support neurons and are classified by location:

• CNS Glial Cells:

  • Astrocytes: Regulate ion concentrations, form blood-brain barrier, remove excess neurotransmitters.

  • Oligodendrocytes: Myelinate axons in the CNS.

  • Ependymal cells: Produce cerebrospinal fluid (CSF).

  • Microglial cells: Immune defense in the CNS.

• PNS Glial Cells:

  • Satellite cells: Support neurons in ganglia, similar to astrocytes.

  • Schwann cells: Myelinate axons in the PNS; each wraps around one axon segment.

Membrane Potential and Ion Channels

The cell membrane separates intracellular and extracellular environments, maintaining a difference in charge known as the membrane potential.

• Ion channels: Allow ions to move across the membrane; types include ligand-gated, mechanically gated, and voltage-gated channels.

• Resting membrane potential: Typically, -70 mV; maintained by sodium-potassium pumps and ion leakage channels.

Types of Ion Channels

• Ligand-gated: Open in response to binding of signaling molecules (ligands) such as neurotransmitters.

• Mechanically gated: Open in response to physical distortion (e.g., pressure).

• Voltage-gated: Open in response to changes in membrane potential.

Action Potential

An action potential is a rapid change in membrane potential that allows neurons to communicate.

• Depolarization: Sodium ions enter the cell, making the inside less negative.

• Threshold: If membrane potential reaches -55 mV, voltage-gated sodium channels open.

• Peak: Membrane potential reaches +30 mV.

• Repolarization: Potassium ions exit the cell, restoring negative charge.

• Hyperpolarization: Temporary overshoot to -90 mV before returning to resting potential. `

• All-or-none principle: Action potentials occur only if threshold is reached; all action potentials peak at the same voltage.

Key Equations

• Resting membrane potential: $V_{m} \approx -70\ \text{mV}$

• Threshold potential: $V_{\text{threshold}} \approx -55\ \text{mV}$

• Peak action potential: $V_{\text{peak}} \approx +30\ \text{mV}$

Graded Potentials

• Local changes in membrane potential caused by opening of ligand-gated or mechanically gated channels.

• A large enough graded potential can trigger an action potential.

Summary Table: Nervous System Components and Functions

Component	Location	Function
Brain (CNS)	Head	Processing sensory stimuli, motor responses, homeostasis
Spinal cord (CNS)	Vertebral column	Reflexes, sensory/motor pathways
Nerves (PNS)	Throughout body	Transmit sensory and motor signals
Ganglia (PNS)	Near spinal cord/limbs	Sensory reception, relay motor responses
Digestive tract (ENS)	Digestive system	Autonomous control of digestion

Additional info:

• Action potentials are fundamental for neural communication and are the basis for all nervous system functions.

• Glial cells are essential for maintaining the health and function of neurons, including myelination and immune defense.