Chapter 13

ANATOMY OF THE NERVOUS SYSTEM

Chapter 13 Overview

  • The document contains detailed information regarding the structure, development, and organization of the nervous system with a focus on various components.

ANATOMY

  • Key anatomical structures mentioned include:

    • Frontal lobe

    • Prefrontal cortex

    • Septum

    • Parietal lobe

    • Motor area

    • Cingulate gyrus

    • Olfactory bulb

    • Amygdala

    • Hypothalamus

    • Hippocampus

    • Mammillary bodies

    • Occipital lobe

    • Fornix

    • Thalamus

    • Thalamic nuclei

THE NEURAL TUBE

Origins of the Nervous System

  • The formation of the nervous system begins with fertilization, where the sperm and egg cells fuse to become a zygote.

  • The zygote divides, leading to the generation of germ layers approximately 16 days after fertilization.

  • The ectoderm differentiates into a specialized region known as neuroectoderm, which serves as the precursor for nervous system tissue.

Developmental Stages

  • The process of the neural tube formation consists of several key steps:

    1. Neuroectodermal Tissue Formation:

    • The ectoderm thickens to form a neural plate, which separates into two borders identified as the neural crest.

    • Neural crest cells ultimately form components of the peripheral nervous system (PNS).

    1. Neural Tube Closure:

    • The neural plate bends, leading to a deeper groove and eventual joining of the edges to form the neural tube.

    1. Differentiation and Function:

    • The notochord degenerates, persisting only as the nucleus pulposus of intervertebral discs.

    • Mesoderm cells differentiate into somites, precursors for skeletal structures.

TIMELINE OF DEVELOPMENT

  • Day 18 Post-Fertilization:

    • The embryo measures 2-3 mm; cells begin to organize into the neural plate.

  • Day 20:

    • Neural folds start to come together to form the neural tube.

  • Days 22-26:

    • Closure initially occurs at the upper neck area and moves downward and forward.

  • Day 28 (4 weeks):

    • The neural tube completes its closure; failure to do so can cause defects such as anencephaly or spina bifida.

PRIMARY AND SECONDARY VESICLE STAGES

Primary Brain Vesicles

  • The anterior end of the neural tube enlarges to form three primary brain vesicles:

    • Prosencephalon (Forebrain)

    • Mesencephalon (Midbrain)

    • Rhombencephalon (Hindbrain)

Secondary Brain Vesicles

  • At 5 weeks of development, five secondary brain vesicles emerge from the primary ones:

    • Telencephalon (from Prosencephalon)

    • Diencephalon (from Prosencephalon)

    • Mesencephalon remains unchanged

    • Metencephalon (from Rhombencephalon)

    • Myelencephalon (from Rhombencephalon)

SPINAL CORD DEVELOPMENT

  • The spinal cord develops from the posterior neural tube.

  • As it develops, the cells of the neural tube proliferate, differentiating into neurons and glia.

  • The organization of the neural tube ensures dorsal tissues are associated with sensory functions while ventral tissues correspond to motor functions.

HUMAN NEURAXIS

  • The mammalian nervous system is oriented along an anterior-posterior axis, with specific adaptations for bipedal organisms.

  • In humans, there is a bend in the neuraxis between the brainstem and the diencephalon, allowing for forward orientation of the face and eyes.

CEREBRUM ANATOMY

Cerebral Cortex

  • The cerebral cortex appears wrinkled with regions delineated by fissures and sulci.

  • Features significant to the structure:

    • Longitudinal fissure separates hemispheres.

    • Gyri are folds of gray matter; sulci are the grooves separating the gyri.

Lobes of the Cerebral Cortex

  • The lobes of the cerebral cortex include:

    • Frontal lobe

    • Parietal lobe (2)

    • Temporal lobe (2)

    • Occipital lobe

  • Sulci separating these lobes include:

    • Lateral sulcus

    • Central sulcus

    • Parieto-occipital sulcus

BRODMANN’S AREAS

  • The mapping of functionally distinct regions within the cerebral cortex, known as Brodmann mapping, is based on its cellular architecture at a microscopic level.

  • Localization of function indicates specific areas in the cortex are associated with distinct functions.

MOTOR AND SENSORY REGIONS

  • Motor regions include:

    • Primary motor cortex (precentral gyrus)

    • Somatic motor association area (premotor cortex)

    • Prefrontal cortex

  • Sensory regions include:

    • Primary sensory cortex (postcentral gyrus)

    • Somatic sensory association area

    • Language areas such as Broca's area (speech production) and Wernicke's area (comprehension).

IMPACT OF CEREBRAL CORTEX DAMAGE

  1. Premotor Cortex: Responsible for motor planning; damage results in spasticity and tightness.

  2. Primary Motor Cortex: Damage affects execution of movement; impacts depend on extent of damage.

  3. Visual Cortex: Two areas -

    • Primary detects basic visual features, damage can distort sights.

    • Association recognizes complex stimuli, damage leads to inability to recognize familiar images.

  4. Somatosensory Association Area: Injury interferes with awareness of body and spatial motion.

  5. Language Areas (Aphasia):

    • Broca’s Area: Damage causes inability to express words despite knowing them.

    • Wernicke’s Area: Damage leads to fluent speech that lacks comprehension (words appear nonsensical).

PREFRONTAL CORTEX

  • The prefrontal cortex governs executive functions including:

    • Working memory

    • Attention and inhibition

    • Personality traits and decision-making behaviors

  • It develops into the mid-twenties; its underdevelopment is linked with impulsive behaviors in adolescents.

SUBCORTICAL STRUCTURES

Key Subcortical Structures

  • Striatum: Comprises Caudate and Putamen, controlling movement.

  • Globus Pallidus: Also involved in the regulation of movement.

  • Hippocampus: Involved in long-term memory formation.

  • Amygdala: Involved in emotional response regulation.

DIENCEPHALON

Thalamus

  • The thalamus acts as a final relay point for sensory information projected to the cerebral cortex, processing and filtering incoming data, excluding olfactory signals.

  • It assists in integrating motor output as well.

Hypothalamus

  • Highly influential in regulating the autonomic nervous system (ANS) and the endocrine system via the pituitary gland.

    • Controls cardiovascular and respiratory functions through the medulla oblongata.

    • Regulates body temperature and circadian rhythms.

    • Plays roles in memory and emotional responses via the limbic system.

BRAIN STEM FUNCTIONS

Components

  • Midbrain:

    • Contains superior and inferior colliculi; manages head reflexes and sound perception.

  • Pons: Links cerebellum to other brain regions.

  • Medulla Oblongata: Involved in critical functions like sleep regulation and controlling cardiovascular and respiratory activities.

CEREBELLUM

Functions

  • Monitors various sensory inputs to coordinate balance and fine motor skills.

  • Dysfunction leads to ataxia, characterized by loss of control and clumsiness.

SPINAL CORD ANATOMY

Key Features

  • Composed of 31 pairs of spinal nerves:

    • Cervical enlargement for upper limbs

    • Lumbar enlargement for lower limbs

    • The conus medullaris stops near L1/L2 vertebra

    • Cauda equina spreads from L2 to S5.

Gray and White Matter Organization

  • Gray Matter:

    • Dorsal (posterior) horn: Somatic/visceral sensory nuclei.

    • Lateral horn: Contains sympathetic motor neurons (only in thoracic/upper lumbar).

    • Anterior (ventral) horn: Contains somatic motor nuclei.

  • White Matter:

    • Ascending (sensory) tracts carry information to the brain; descending (motor) tracts send commands from the brain.

CIRCULATION IN CNS

Arterial System

  • Blood supply commences from the aorta, leading to:

    • Common carotid arteries → Internal carotid arteries

    • Subclavian arteries → Vertebral arteries

  • The anterior spinal artery supplies the anterior spinal cord, merging into the basilar artery for the brain stem and cerebellum supply.

Venous Return

  • The superior sagittal sinus collects blood from deeper brain structures, flowing toward the heart through jugular veins.

STROKE AND PROTECTIVE LAYERS

Stroke Overview

  • A stroke is characterized by disrupted blood supply, leading to neurological deficits.

  • Transient ischemic attacks (TIAs) indicate temporary blockages; can be preventable via proper diet/lifestyle.

Protective Coverings

  • Meninges:

    • Dura Mater: Outermost protective layer.

    • Arachnoid Mater: Middle layer with subarachnoid space containing cerebrospinal fluid.

    • Pia Mater: Innermost layer, impervious to fluid.

Meningitis

  • A serious infection of the meninges, with bacterial forms being more severe but treatable. Symptoms include severe headache and fever.

PERIPHERAL NERVOUS SYSTEM

Nerve Structure

  • Nerves are wrapped in connective sheaths:

    • Epineurium surrounds the whole nerve.

    • Perineurium surrounds individual fascicles.

    • Endoneurium surrounds the axons.

Cranial Nerves

  • 12 pairs serve various sensory and motor functions of the head and neck, classified as sensory, motor, or mixed.

    • Mnemonic to remember the cranial nerve functions:

      • S = Sensory

      • M = Motor

      • B = Both

  • List of Cranial Nerves:

    • Olfactory (CN I)

    • Optic (CN II)

    • Oculomotor (CN III)

    • Trochlear (CN IV)

    • Trigeminal (CN V)

    • Abducens (CN VI)

    • Facial (CN VII)

    • Vestibulocochlear (CN VIII)

    • Glossopharyngeal (CN IX)

    • Vagus (CN X)

    • Spinal Accessory (CN XI)

    • Hypoglossal (CN XII)

SPINAL NERVES

  • 31 spinal nerves consist of sensory (dorsal nerve root) and motor roots (ventral nerve root), creating a mixed function. Nerve plexuses include cervical, brachial, lumbar, and sacral.

  • Dermatomes: Areas of skin associated with specific nerve connections to the spinal cord, crucial for sensory mapping.

  • Myotomes: Groups of muscles innervated by ventral roots of spinal nerves, important for diagnosing muscle function.

DERMATOME TABLE

  • C2: Lower jaw; back of head.

  • C3: Upper neck; back of head.

  • C4: Lower neck; upper shoulders.

  • C5: Collarbone area; upper shoulders; lateral forearm.

  • C6: Shoulders; outside of arm; thumb.

  • C7: Upper back; back of arm; pointer and middle fingers.

  • C8: Upper back; inside the arm; ring and little fingers.

  • T1-T12: Upper chest to lower abdomen.

  • L1 - L5: Lower back to thighs and legs.

  • S1 - S5: Buttocks, genitals, and the lower body.