THE BRAIN
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Develops from the embryonic neural tube
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The anterior end expands and constrictions appear forming the primary brain vesicles
namely:
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Prosencephalon (fore brain)
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Mesencephalon (midbrain)
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Rhombencephalon (hindbrain)
The primary brain vesicles differentiate into secondary brain vesicles
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Procencephalon
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Telencephalon (cerebrum)
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Diencephalon (thalamus, hypothalamus and Epithalamus)
Mesencephalon
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Brain stem: midbrain
Rhombencephalon
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Metencephalon
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Brain stem: Pons and cerebellum
Myelencephalon
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Brain stem: medulla oblongata (Fig 12.2, 12.3 & 12.5)
Regional Organization
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Cerebral Hemispheres
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Has three basic regions; Outer gray matter, internal white matter and the basal
nuclei
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Conscious thoughts, intellect, memory, sensations and complex movements
originate from here regional organization
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Marked by elevated tissue folds called gyri (increase surface area of the cortex)
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Gyri are separated by shallow grooves called sulci
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Deeper grooves, called fissures separate larger brain regions
Longitudinal fissure
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Transverse fissure
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Separates the cerebral hemispheres from the cerebellum
Some sulci divide each hemisphere into lobes,
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Separates the two cerebral hemispheres
Frontal, parietal, temporal, occipital and insula
Central Sulcus
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Lies in the frontal plane
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Separates the frontal lobe from the parietal lobe
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It is bordered anteriorly by the precentral gyrus (primary motor area) and
posteriorly by the postcentral gyrus (primary somatosensory area)
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The parieto-occipital sulcus
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Located on the medial surface of each hemisphere
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Separates the occipital lobe from the parietal lobe
The lateral sulcus
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Outlines the temporal lobe and separates it from the parietal and frontal lobes
The Insula
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Lies deep to the temporal lobe and forms part of its floor
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Covered by portions of the frontal, parietal and temporal lobes (Fig 12.6)
Generalizations about brain regions
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The cortex contains 3 kinds of functional areas: motor, sensory and association
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Each hemisphere is chiefly concerned with motor and sensory functions from the
opposite side of the body
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Two hemispheres are not entirely equal in function (Lateralization) specialization
of cortical functions exist
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No functional area of the cortex acts alone
Regional Organization
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Motor areas
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Primary (somatic) motor cortex
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Conscious control of precise or skilled voluntary movements of the skeletal
muscles
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Premotor cortex
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Lies anterior to the precentral gyrus
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Controls learned motor skills of a repetitious or patterned nature such as typing
or playing a musical instrument (Fig 12.8)
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Motor areas
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Broca’s area
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Lies anterior of the premotor area on one hemisphere (usually left) only
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Controls muscles involved in speech production
Frontal eye field
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Located partially in and anterior to the premotor cortex and superior to the
broca’s area
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Controls voluntary movement of the eyes
Sensory areas
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Primary somatosensory area
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Receives information from general somatic receptors in the skin and
proprioceptors in skeletal muscles
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Spatial discrimination, the ability to identify the regions being stimulated occurs
here (Fig 12.9)
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Somatosensory association cortex
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Lies posterior to the somatosensory cortex
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Integrates temperature, pressure etc, relayed from the primary somatosensory
cortex to produce an understanding of any object being felt
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Visual area
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Lies on the extreme posterior tip of the occipital lobe
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Receives visual information from the retina of the eye
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The visual association area communicates with the visual area to interpret visual
stimuli (color, form, movement)
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Auditory areas
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Located in the temporal lobe
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Converts impulses from the ear and relate them to pitch, rhythm, and loudness
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The auditory association area permits the perception of sound stimulus which
we hear as music, scream, noise etc
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Olfactory cortex
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Found in the frontal and temporal lobes
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Enables us to be aware of changes in odors
Gustatory cortex
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Located in the parietal lobe
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Involved in the reception of taste
Association Areas
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Prefrontal cortex
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Anterior portion of frontal lobe
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Involved with intellect, complex learning abilities (cognition), recall and
personality
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Necessary for the production of abstract ideas, judgment, reasoning, persistence,
long term planning concern for others and conscience
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Language areas
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Surrounds the lateral sulcus in the left hemisphere
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Parts include
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Wernicke’s area (Understanding of written and spoken language and in
sounding unfamiliar words)
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Broca’s area
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Lateral prefrontal cortex (language comprehension and word analysis)
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Lateral and ventral parts of temporal lobe (coordinate auditory and visual
aspects of language)
Regional Organization
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Cerebral white matter
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Lies deep to the cortical gray matter
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Responsible for communication between the cerebral areas, cortex and lower CNS
centers
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Consists mostly of myelinated fibers bundled into large tracts
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Fibers are classified according to the direction in which they run
Commisures (Commisural fibers)
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Connect corresponding gray areas of the two hemispheres enabling them to
function as a whole
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The largest commisure is the corpus callosum
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Anterior and posterior commisures are less important
Association fibers
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Connect different parts of the same hemisphere
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Short fibers connect adjacent gyri
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Long fibers are bundled in tracts that connect different lobes
Projection fibers
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Diencephalon
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Fibers that enter and leave the to lower brain or cord centers (Fig 12.10)
Forms the central core of the forebrain
Thalamus
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Egg-shaped and forms the superolateral walls of the third ventricle and contains
several nuclei
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Its bilateral masses of gray matter are held together by the intermediate mass
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Principal relay station for directing sensory information from the spinal cord,
medulla and cerebellum to the cortex
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Relays and processes taste, visual, auditory and somatosnsory information
Hypothalamus
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Lies below the thalamus, caps the brain stem and forms the inferolateral wall of the
third ventricle
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It contains many functionally important nuclei and is the main visceral control
center of the body
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Processes and relays olfactory information, integrates the ANS, release hormones
(ADH and Oxytocin), control over heart rate, digestive tract activity, rage and
aggression, body temperature regulation, water balance and thirst, hunger and
satiety centers and sleep pattern (Fig 12.13)
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Brain stem
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Made up of the midbrain, pons, and medulla
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Has a deep gray matter surrounded by white matter with nuclei of gray matter
embedded in the white matter
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Produce rigidly programmed automatic behavior necessary for survival
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Provides a pathway for fibers running between higher and lower neural centers
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Is associated with 10 of the 12 cranial nerves
Midbrain
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Located between the diencephalon and the pons
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The ventral surface has cerebral peduncles that consists of fiber tracts descending
toward the spinal cord
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The superior peduncles contain tracts that link the midbrain to the cerebellum
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The cerebral aqueduct runs through the midbrain connecting the third and forth
ventricle
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On the dorsal surface are superior and inferior colliculi (corpora quadrigemina) that
coordinate head and neck movements in response to visual and auditory stimuli
respectively (Fig 12.15 & 12.16)
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Pons
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Brainstem swellings that lie between the mid brain and the medulla oblongata
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Dorsally it forms part of the anterior wall of the fourth ventricle
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Longitudinal fibers connects the medulla with higher brain center
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Transverse fibers connect the cerebral hemispheres within the midbrain
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Respiratory centers of the reticular formation that help to maintain the normal
rhythm of breathing are located here
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Medulla oblongata
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Cone shaped and lies between the pons and the spinal cord
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The foramen magnum is the anatomical landmark used to differentiate brain
structures from spinal cord structures
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The ventral aspect has two longitudinal ridges called pyramids formed by large
pyramidal fibers descending from the motor cortex
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These fibers decussate (pyramidal decussation) before they enter the spinal cord
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On the dorsal surface are the nucleus cuneatus and gracilis that serve as relay nuclei
for tracts that carry sensory information from the spinal cord and relay them to the
thalamus
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These fibers decussate in the medulla
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Has a cardiovascular center (cardiac and vasomotor center), respiratory center (rate
and depth of breathing) and other centers that regulates vomiting, sneezing,
hiccups, swallowing etc
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Cerebellum
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Located dorsal to the pons and medulla and lies under the occipital lobe of the
cerebral hemisphere from which it is separated by the transverse fissure
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Consists of two hemispheres that are connected in the midline by the vermis
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It functions primarily by fine-tuning movement and adjusting body posture to
maintain balance