Anatomy & Physiology Study Notes: Nervous System Structure and Function

Nervous System Overview

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It is divided into two principal parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes incoming sensory information and issues instructions based on past experience.

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves, ganglia, and sensory receptors. It connects the CNS to limbs and organs.

The PNS is further subdivided into:

• Sensory Portion: Conducts impulses from sensory receptors toward the CNS.

• Motor Portion: Conducts impulses away from the CNS to effectors (muscles and glands).

• Somatic Division: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): Controls involuntary actions of smooth and cardiac muscles and glands.

Structural Organization of the Brain

Major Brain Regions

The brain is organized into four major regions, each with distinct functions and anatomical features:

• Cerebral Hemispheres: The largest part of the brain, divided into left and right hemispheres. Responsible for higher cognitive functions, sensory perception, and voluntary motor actions.

• Diencephalon: Contains structures such as the thalamus and hypothalamus, which relay sensory information and regulate autonomic functions.

• Brain Stem: Includes the midbrain, pons, and medulla oblongata. Controls basic life functions such as breathing, heart rate, and reflexes.

• Cerebellum: Coordinates movement and balance.

Development of the Human Brain

During embryonic development, the brain forms from the neural tube, which differentiates into primary and secondary brain vesicles. These vesicles give rise to the adult brain structures.

• Primary Brain Vesicles: Prosencephalon (forebrain), mesencephalon (midbrain), rhombencephalon (hindbrain).

• Secondary Brain Vesicles: Telencephalon, diencephalon, metencephalon, myelencephalon.

The adult brain structures derived from these vesicles include the cerebral hemispheres, thalamus, hypothalamus, pons, medulla oblongata, and cerebellum.

Table: Embryonic Development of the Human Brain

Primary Vesicle	Secondary Vesicle	Adult Brain Structure
Forebrain (Prosencephalon)	Telencephalon	Cerebral hemispheres
Forebrain (Prosencephalon)	Diencephalon	Thalamus, hypothalamus
Midbrain (Mesencephalon)	Mesencephalon	Midbrain
Hindbrain (Rhombencephalon)	Metencephalon	Pons, cerebellum
Hindbrain (Rhombencephalon)	Myelencephalon	Medulla oblongata

Brain Anatomy and Terminology

Gyri, Sulci, and Fissures

The surface of the brain is marked by ridges and grooves:

• Gyri: Elevated ridges of tissue on the cerebral cortex.

• Sulci: Shallow grooves separating gyri.

• Fissures: Deeper grooves that separate large regions of the brain.

These structures increase the surface area of the brain, allowing for greater cognitive capacity.

White Matter vs. Gray Matter

The brain and spinal cord are composed of two types of tissue:

• White Matter: Consists mainly of myelinated axons, responsible for transmitting signals between different brain regions.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; involved in processing and integration of information.

Neuronal Function and Action Potentials

Action Potential Generation

An action potential is a rapid change in membrane potential that travels along the neuron, allowing for communication within the nervous system.

• Stimulus: Any event or agent that initiates an action potential in a neuron.

• Threshold: The minimum stimulus required to trigger an action potential.

• Propagation: The movement of the action potential along the axon.

Formula:

$V_m = V_{rest} + ext{change in membrane potential}$

Where $V_m$ is the membrane potential and $V_{rest}$ is the resting membrane potential.

Effect of Local Anesthetics on Action Potentials

Local anesthetics block nerve pain by decreasing the permeability of sodium ions in the plasma membrane of neurons. This prevents the generation of action potentials, as sodium influx is essential for depolarization.

• Mechanism: Sodium channels are blocked, preventing depolarization.

• Result: Nerve impulses cannot be transmitted, leading to loss of sensation.

Formula for Action Potential:

$\text{Action Potential} \propto \text{Na}^+ \text{influx}$

If sodium permeability is decreased, action potential generation is inhibited.

Laboratory Applications

Brain Dissection and Identification

Dissection of human and sheep brains allows for identification of major external and internal structures. Comparative anatomy highlights similarities and differences between species.

• Sheep Brain: Used as a model for human brain anatomy due to structural similarities.

• Lab Materials: Dissectible brain models, preserved specimens, and anatomical charts.

Example: The sheep brain is smaller but contains analogous structures such as the cerebral hemispheres, cerebellum, and brain stem.

Key Terms and Definitions

• Neuron: The basic functional unit of the nervous system, specialized for transmitting electrical impulses.

• Synapse: The junction between two neurons where information is transmitted.

• Ganglion: A cluster of neuron cell bodies in the PNS.

• Ventricle: A chamber within the brain that contains cerebrospinal fluid.

Summary Table: CNS vs. PNS

Feature	Central Nervous System (CNS)	Peripheral Nervous System (PNS)
Main Components	Brain, spinal cord	Cranial nerves, spinal nerves, ganglia
Function	Integration and processing	Transmission of signals to/from CNS
Subdivisions	None	Sensory, motor (somatic, autonomic)

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, formulas, and comparative anatomy context.