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