Anatomy and Physiology of the Nervous System: CNS and PNS Study Guide

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Anatomy of the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals between different parts. It is divided into two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS): Consists of the brain and spinal cord; acts as the main control center.
Peripheral Nervous System (PNS): Composed of cranial nerves, spinal nerves, and sensory organs; connects the CNS to limbs and organs.

Diagram of CNS and PNS divisions in the human body Flowchart of nervous system divisions and functions Anatomical diagram showing CNS and PNS structures

Functions and Divisions of the Nervous System

Functional Organization

The nervous system is organized into sensory (afferent) and motor (efferent) divisions, each with specialized roles:

Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.
Motor (Efferent) Division: Carries commands from the CNS to effectors (muscles and glands).
Somatic Nervous System: Controls voluntary movements of skeletal muscles.
Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion).
Sympathetic Division: Mobilizes body systems during activity (fight or flight).
Parasympathetic Division: Conserves energy and promotes rest (rest and digest).

Flowchart of CNS and PNS with somatic and autonomic divisions

Histology of Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is highly cellular, with two principal cell types:

Neuroglia (Glial Cells): Supporting cells that surround and wrap neurons. Types include:
- Astrocytes (CNS): Support and regulate the environment around neurons.
- Microglial Cells (CNS): Immune defense in the CNS.
- Ependymal Cells (CNS): Line ventricles and produce cerebrospinal fluid (CSF).
- Oligodendrocytes (CNS): Form myelin sheaths in CNS.
- Satellite Cells (PNS): Support neurons in PNS.
- Schwann Cells (PNS): Form myelin sheaths in PNS.
Neurons: Excitable cells that transmit electrical signals; structural units of the nervous system.

Structural and Functional Classification of Neurons

Neurons are classified based on structure and function:

Structural Classification: Based on the number of processes extending from the cell body (e.g., multipolar, bipolar, unipolar).
Functional Classification: Based on the direction of impulse transmission:
- Sensory (Afferent) Neurons: Carry impulses toward CNS.
- Motor (Efferent) Neurons: Carry impulses away from CNS to effectors.
- Interneurons: Connect sensory and motor neurons within CNS.

Table comparing structural classes of neurons Diagram of neurons classified by function

Gray Matter vs. White Matter, Nucleus vs. Ganglion

Key Terminology

Understanding the terminology is essential for distinguishing CNS and PNS structures:

Term	Definition
Nucleus	A collection of neuron cell bodies in the CNS
Ganglion	A collection of neuron cell bodies in the PNS
Tract	A bundle of axons in the CNS
Nerve	A bundle of axons in the PNS

Diagram showing gray and white matter distribution in CNS

Central Nervous System: Brain Regions and Organization

Major Regions of the Brain

The adult brain is divided into four main regions:

Cerebral Hemispheres
Diencephalon (thalamus, hypothalamus, epithalamus)
Brain Stem (midbrain, pons, medulla oblongata)
Cerebellum

Diagram of brain regions: cerebral hemisphere, diencephalon, cerebellum, brain stem

Pattern of Gray and White Matter

The CNS exhibits a basic pattern: a central cavity surrounded by gray matter, with white matter external to gray matter. The brain stem contains additional gray matter nuclei within white matter, and the cerebrum and cerebellum have an outer layer of gray matter called the cortex.

Pattern of gray and white matter in CNS Gray and white matter in brain and spinal cord

Ventricles of the Brain

Structure and Function

The brain contains four ventricles, which are continuous with each other and the central canal of the spinal cord. They are filled with cerebrospinal fluid (CSF) and lined by ependymal cells.

Lateral Ventricles: Paired, separated by septum pellucidum.
Third Ventricle: Located in the diencephalon.
Fourth Ventricle: Located dorsal to the pons; continuous with the central canal.
Apertures: Connect the fourth ventricle to the subarachnoid space.

Diagram of brain ventricles

Cerebral Hemispheres

Surface Markings and Lobes

The surface of the cerebral hemispheres is marked by gyri (ridges), sulci (shallow grooves), and fissures (deep grooves). Sulci divide each hemisphere into five lobes: frontal, parietal, occipital, temporal, and insular.

Gyri: Ridges on the brain surface.
Sulci: Shallow grooves separating gyri.
Fissures: Deep grooves (e.g., longitudinal, transverse cerebral fissure).

Surface markings and lobes of the cerebral hemispheres

Basic Regions of Each Hemisphere

Cerebral Cortex: Superficial gray matter.
White Matter: Internal to the cortex.
Basal Nuclei: Deep within the white matter.

Diagram showing basic regions of the cerebral hemisphere

Cerebral Cortex

Functional Areas

The cerebral cortex is the site of conscious mind, including awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding. It contains three functional areas:

Motor Areas: Control voluntary movements.
Sensory Areas: Conscious awareness of sensation.
Association Areas: Integrate diverse information.

Brodmann areas of the cerebral cortex Motor and sensory regions of the cerebral cortex

Motor Areas

Primary Motor Cortex: Located in the precentral gyrus; controls precise voluntary movements.
Premotor Cortex: Plans and coordinates complex movements.
Broca's Area: Motor speech area, usually in the left hemisphere.
Frontal Eye Field: Controls voluntary eye movements.

Motor and sensory regions of the cerebral cortex Motor homunculus map in precentral gyrus

Sensory Areas

Primary Somatosensory Cortex: Located in postcentral gyrus; receives sensory information from skin and proprioceptors.
Somatosensory Association Cortex: Integrates sensory inputs for interpretation.
Visual Areas: Primary visual cortex and association area in occipital lobe.
Auditory Areas: Primary auditory cortex and association area in temporal lobe.
Vestibular Cortex: Awareness of balance.
Olfactory Cortex: Perception of odors.
Gustatory Cortex: Perception of taste.
Visceral Sensory Area: Perception of visceral sensations.

Sensory regions of the cerebral cortex

Multimodal Association Areas

Integration and Higher Functions

Multimodal association areas receive inputs from multiple sensory areas and send outputs to multiple areas. They are involved in higher cognitive functions, memory, and emotional responses.

Anterior Association Area (Prefrontal Cortex): Intellect, cognition, recall, personality.
Posterior Association Area: Recognition of patterns, faces, and understanding language.
Limbic Association Area: Emotional impact and memory formation.

Lateralization of Cortical Function

Hemispheric Specialization

Lateralization refers to the division of labor between the cerebral hemispheres. The left hemisphere is typically dominant for language, math, and logic, while the right hemisphere is dominant for visual-spatial skills, intuition, and creativity.

Cerebral White Matter

Fiber Types

Commissural Fibers: Connect corresponding areas between hemispheres (e.g., corpus callosum).
Association Fibers: Connect areas within a hemisphere.
Projection Fibers: Connect cortex to lower CNS centers.

Basal Nuclei

Structure and Function

Basal nuclei are deep gray matter structures involved in regulating movement, filtering unnecessary actions, and contributing to cognition and emotion.

Diencephalon

Thalamus, Hypothalamus, Epithalamus

Thalamus: Relay station for sensory and motor signals to the cortex.
Hypothalamus: Main visceral control center; regulates homeostasis, emotions, body temperature, hunger, thirst, sleep, and endocrine functions.
Epithalamus: Contains the pineal gland, which secretes melatonin for sleep regulation.

Brain Stem

Midbrain, Pons, Medulla Oblongata

Midbrain: Contains cerebral peduncles, corpora quadrigemina, substantia nigra, and red nucleus; involved in motor control and reflexes.
Pons: Connects higher brain centers and spinal cord; involved in breathing regulation.
Medulla Oblongata: Autonomic reflex center; regulates cardiovascular, respiratory, and other vital functions.

Cerebellum

Structure and Function

The cerebellum coordinates voluntary movements, balance, and posture. It receives input from the cortex, brain stem, and sensory receptors, and sends output to motor areas for smooth, coordinated movement.

Functional Brain Systems

Limbic System and Reticular Formation

Limbic System: Emotional and memory processing; includes amygdala, hippocampus, and cingulate gyrus.
Reticular Formation: Maintains brain arousal and consciousness; filters sensory input.

Additional info: This study guide covers the main anatomical and functional features of the nervous system, including CNS and PNS divisions, histology, brain regions, and key functional systems, as outlined in the ANP college course chapters.