Central Nervous System

I. Embryonic Development

• Ectodermal tissue fold/wrap creating a neural plate and neural fold which create a neural tube
• Organs of the CNS are derived from the neural fold tissue (anterior= brain, posterior= spinal cord)
• As soon as the neural tube is formed, the anterior portion expands quicker than posterior, forming the primary brain vesicles: Prosencephalon (forebrain), Mesencephalon (midbrain), and Rhombencephalon (hind brain).
• The primary brain vesicles differentiate into secondary brain vesicles which further develop into functional adult structures: Diagram

II. Regions and Organization of the Brain

A. Ventricles of Brain Diagram Diagram

• Hollow chambers filled with CSF and lined with ependymal cells
• Four ventricles:
  • (2) lateral, (1) third ventricle, and (1) fourth ventricle
• Laterals separated by a thin membrane called septum pellucidum
• Each lateral ventricle communicates with the third ventricle via a channel called an interventricular foramen (foramen of Monro)
• Third ventricle is continuous with the fourth ventricle via the cerebral aquaduct

B. Cerebral Hemispheres Diagram

• Superior part of the brain
• Covered entirely by deep grooves (gyri), separated by shallow grooves (sulci) and deep shallow grooves (fissures)
• Anatomical landmarks:
  • Longitudinal fissure
  • Frontal, parietal, occipital, and temporal lobes
    • Deep to temporal, parietal, and frontal lobes is a fifth lobe called the insula
  • Central sulcus
  • Pre/post central gyri
  • Lateral sulcus

C. Cerebral Cortex

• All qualities associated with "consciousness"
• Cerebral cortex contains three functional areas:
  • Motor areas - control voluntary motor function
  • Sensory areas - provide for conscious awareness of sensation
  • Association areas - integrate all other information
• Each hemisphere is concerned with the sensory and motor functions of the opposite side of the body.
• Though symmetrical in structure, two hemispheres are not equal in function; there is lateralization (specialization of cortical functions).
• No functional area acts alone and conscious behavior involves the entire cortex in one way or another.

D. Functional Areas Diagram

i. Motor Areas (posterior part of the frontal lobes)

• Primary motor cortex - precentral gyrus in the frontal lobe
  • Large neurons (pyramidal cells) allow conscious control of movement of skeletal muscles
  • The pyramidal cells' long axons from voluntary motor tracts called pyramidal (corticospinal) tracts
  • Motor areas have been spatially mapped = somatotropy.
• Premotor cortex - anterior to the precentral gyrus in the frontal lobe
  • Regions controls learned motor skills that are repeated or patterned
  • Also coordinates the movements of muscles simultaneously and\or sequentially by sending activating
        impulses to the primary motor cortex
• Broca's area - anterior to the premotor area
  • Involved in directing motor speech.
• Frontal eye field - anterior to the premotor cortex and superior to Broca's area  
  • Controls voluntary movement of eyes.

ii. SensoryAreas (parietal, temporal, and occipital lobes)

• Primary somatosensory cortex - postcentral gyrus of parietal lobe (immediately behind primary motor cortex)
  • Neurons receive info (from sensory receptors, skin, and muscles) and identifies body region being stimulated
  • Endows spatial discrimination.
• Somatosensory association area - lies posterior to the primary somatosensory cortex
  • Integrate and analyze somatic sensory inputs (e.g. temperature and pressure) into comprehensive evaluation.
• Visual areas - occipital lobes contain primary visual cortex (receive information from retina) and visual association area (interprets information from retina).
• Auditory areas - temporal lobes contain primary auditory cortex (receives impulses from inner ear) and auditory association area (interprets sound).
• Olfactory cortex - temporal lobe in region called the uncus; enables conscious awareness of odors.
• Gustatory cortex - parietal lobe deep to temporal lobe; involved in perception of taste.

iii. AssociationAreas

• Somatosensory cortex - posterior to the primary somatosensory cortex
  • Somatosensory cortex and each special sensory area have nearby association areas with which they communicate
  • The association areas, in turn, communicate with the motor cortex and with other sensory association areas to analyze, recognize, and act on sensory inputs.
• Prefrontal cortex - anterior portions of frontal lobe
  • Involved with intellect and complex learning (cognition) and personality
  • Tumors may lead to personality disorders - prefrontal lobotomy are performed in severe cases of mental illness.
• Gnostic area - undefined area in temporal, occipital, and parietal lobes
  • Only one hemisphere
  • Receives input from all sensory association areas and stores complex memory patterns associated with sensation
  • Sends assessment of sensations to prefrontal cortex which adds emotional overtones
  • Injury to gnostic area causes one to become an imbecile - interpretation to various sensations/stimuli lost.
• Language areas - found in Wernick's area of temporal lobe of one hemisphere (usually left)
  • Involved in interpretation of language.

E. Matter and Nuclei

i. Cerebral White Matter Diagram

• Deep to grey matter, provides communication between cerebral areas and between the cerebral cortex and lower CNS centers
• Consists of myelinated fibers bundled into large tracts; classified according to the direction they run:
  • Commissures - composed of commissure fibers connect two areas of hemisphere (corpus callosum and anterior commissure)
  • Association fibers - transmit impulses within a single hemisphere
  • Projection fibers - fibers entering the cerebral hemispheres from lower brain or cord centers, and fibers leaving the cortex to travel to lower areas.
• Upper brain stem fibers form compact band (internal capsule) then radiate outward (corona radiata)

ii. Basal Nuclei Diagram

• Receive extensive inputs from the entire cerebral cortex and project messages (via relays) to the premotor and prefrontal cortices and through the thalamus influence muscle movements directed by premotor cortex.
• Structures:
  • Corpus Striatum, composed of
    • Caudate nucleus
    • Lentiform nucleus, composed of
      • Putamen
      • Globus pallidus)

F. Diencephalon (thalamus, hypothalamus, and epithalamus) Diagram

• Thalamus - superiolateral walls of third ventricle
  • Composed of masses of grey matter held together by a midline commissure (intermediate mass)
  • Contains many nuclei, each projecting fibers to and receives from a specific region of the cerebral cortex.
• Hypothalamus - inferolateral walls of third ventricle
  • Walls of hypothalamus (tissue) meet and extend, forming infundibulum, connecting the pituitary to base of
        hypothalamus
  • The hypothalamus is the main visceral control center of the body
  • Homeostatic roles:
    1. Autonomic control center - regulates involuntary nervous system activity (influences BP, HR, GI motility, Respiration rate and depth, and pupil size).
    2. Center for emotional response and behavior - numerous connections with cortical association areas (initiates physical expressions of emotion).
    3. Body temperature regulation - hypothalamus neurons monitor blood temperature flowing through hypothalamus (initiates sweating, shivering, etc.).
    4. Regulation of food intake - responds to hormones and blood levels of nutrients.
    5. Regulation of water balance and thirst - osmoreceptors are activated, triggers ADH release, causing kidneys to retain water; thirst centers stimulated, cause us to drink.
    6. Sleep-wake cycle regulation - set timing of our sleep cycle in response to daylight-darkness cues.
    7. Control of endocrine system - hypothalamus produces releasing hormones that control the secretion of anterior pituitary hormones.
• Epithalamus - dorsal portion of diencephalon and forms the roof of the third ventricle
  • Contains two structures, the hormone-secreting pineal body (secrets melatonin to regulate the sleep/wake cycle) and CSF-forming choroid plexus.

G. Brain Stem (midbrain, pons, medulla oblongata ) 

NOTE: brainstem contains grey matter surrounded by white matter; centers of brain stem regulate autonomic behaviors (head and eye movements, respiration, heart contraction, etc...)

• Midbrain - inferior to third ventricle/thalamus  Diagram
  • Structures:
    • Cerebral peduncles - pyramidal motor tracts descending toward spinal cord
    • Cerebral aqueduct - connect 3rd and 4th ventricle, enclosed by nuclei
    • Nuclei - corpora quadragemina (two pair)
      • Superior colliculi (visual reflex, head/eye movement)
      • Inferior colliculi (auditory relay, startle reflex)
  • Also embedded in white matter of midbrain are two pigmented nuclei substantia nigra (contains melanin and regulates subconscious muscle control) and red nucleus (contains iron &hemoglobin and coordinates muscular movements)
• Pons - between midbrain and medulla oblongata Diagram Diagram
  • Conduction pathway between higher and lower brain centers
  • Middle cerebellar peduncles connect pons with the cerebellum
  • Some pons nuclei (pneumotaxic and apneustic) are respiratory centers that help maintain normal rhythm of breathing.
• Medulla Oblongata - most inferior part of brain stem, blends into spinal cord at the level of the foramen magnum Diagram
  • Structures:
    • Pyramids - pyramidal tracts descending from motor cortex
    • Decussation of pyramids - fibers of pyramids cross over at one point; supporting that each hemisphere controls voluntary muscles
    • Olives - lateral to pyramids, relay sensory information on the state of stretch of our muscles and joints to the cerebellum
  • Medulla oblongata contains visceral motor nuclei that control:
    • Force and rate of heart contraction
    • Regulate BP (by regulating smooth muscle respiratory contraction)
    • Rate and depth of breathing, therefore maintain respiratory rhythm
    • Vomiting, hiccuping, swallowing, coughing, and sneezing

H. Cerebellum Diagram Diagram

• Processes inputs from the cerebral motor cortex, brainstem nuclei, and sensory receptors to provide precise timing and patterns of skeletal muscle contraction.
• Has bilateral symmetry - two hemispheres connected by vermis, each hemisphere has many transverse gyri called folia
• Contains lobes - posterior and anterior lobes regulate subconscious skeletal movement and flocculonodular lobe controlling movement that regulates equilibrium
• Matter - superficial layer (cerebellar cortex) made up of gray matter and deep layer made up of white matter tracts called arbor vitae
• Cerebellar Processing:
  • Initiates voluntary muscle contraction
  • Determines body orientation (body parts in space)
  • Coordinates forces, direction, extent of muscle contraction
  • Dispatches "blueprint" for coordination to brain stem nuclei then to motor neurons of spinal cord.

I. Functional Brain Systems (limbic and reticular)

NOTE: Functional brain systems are networks of neurons that work together but span large distances within the brain

i. Limbic System - complex group of fiber tracts and grey matter areas Diagram

• Components:
  • septal nuclei
  • cingulate gyrus
  • parahippocampal gyrus
  • hippocampus
  • amygdala
  • hypothalamus
  • anterior thalamic nuclei
  • fornix
• Emotional, affective brain

ii. Reticular Formation - loosely clustered neurons localized into three broad columns along the length of the brain stem

• Raphe nucleus (flanked by) medial and lateral nuclear group
• Reticular formation function
  • Aids in the control of skeletal muscle (and muscle tone)
  • Alerts cortex to incoming sensory signals
  • Maintain alertness and causes arousal from sleep

III. Protection of the Brain

NOTE: Brain is protected by skull, membranes (meninges), cerebrospinal fluid, and blood-brain barrier.

A. Meninges - connective tissue membranes external to CNS Diagram

• Functions
  • Cover and protect CNS
  • Protect blood vessels and enclose the venous sinuses
  • Contains CSF
  • Forms partitions within skull
  • Composed of dura mater, arachnoid mater, and pia mater (each = menix)
• Components: Pia Mater, Arachnoid, and Dura Mater
  • Pia Mater - (internal menix) delicate C.T., invested with blood, "clings" to brain (follows each convolution)
  • Arachnoid Mater - (middle menix) forms loose brain covering, it separated from dura mater by subdural space and separated from pia mater by subarachnoid space; knoblike extensions (arachnoid villi) protrude through dura mater - CSF absorbed into blood throughout villi sinuses.
  • Dura Mater - double-membrane surrounding brain:
    
    • Periosteallayer - attached to skull
    • Meningeallayer - deep to periosteal, outermost brain covering; extends inward to form flat septa that anchor brain to skull: Diagram
      • Falx cerebri (in longitudinal fissure)
      • Falx cerebelli (runs along vermis of cerebellum)
      • Tentoriumcerebelli (in transverse fissure)

B. CerebrospinalFluid - liquid cushion surrounding brain & spinal cord

• Aids in supplying nourishment
• Contains less proteins but more sodium, chloride, magnesium, and hydrogen ions than blood
• Produced by choroid plexuses.
• Circulation Diagram

C. Blood-brain Barrier

• Extracellular concentration of hormones, ions, and amino acids are in constant flux
• Neurons in brain would fire uncontrollably and therefore need protective mechanism:
  • Capillaries have relatively thick basal lamina
  • Capillaries have continuous endothelium with tight junctions
  • The bulbous "feet' of astrocytes cling to capillaries
• Glucose, essential amino acids, and some electrolytes pass through barrier via passive facilitated diffusion
• Wastes, proteins, and toxins impermeable into CSF - K⁺ and nonessential amino acids are actively pumped across the endothelium of capillary.
• The barrier is ineffective against fats, fatty acids, oxygen, and carbon dioxide.
• The blood-brain barrier is not completely uniform: Hypothalamus, root of diencephalon, and medulla contains noncontinuous capillaries (fenestrated).

IV. Spinal Cord Anatomy

A. Protection and Coverings

• Spinal cord located in the vertebral foramen/vertebral canal
• Meninges (cranial and spinal) and spaces: Diagram  Diagram
  
  • Epidural space
  • Dura mater
  • Subdural space
  • Arachnoid
  • Subarachnoid space
  • Denticulate ligaments
  • Pia mater

B. External Anatomy of Spinal Cord Diagram Diagram

• Cervical and lumbar enlargements
• Anterior median fissure and posterior median sulcus
• Conis medularis (L1-L2), cauda equina, and finum terminale
• 31 pairs of nerves each two points of attachment called roots
• Posterior/dorsal root containing sensory nerve fibers -- includes a swelling (ganglion) containing cell bodies of sensory neurons
• Anterior/ventral root containing motor nerve fibers

C. Interior Anatomy of the Spinal Cord Diagram

• Interior gray matter (unmyelinated neurons) surrounded by white matter (myelinated neurons)
• Central canal (continuous with the fourth canal)
• Gray matter divided into horns: anterior, posterior, and lateral
• White matter divided into columns: anterior, posterior, and lateral
• Each white column contains bundles of nerve axons (called tracts) having a common origin or destination
• Two types of tracts: ascending (sensory) and descending (motor)