Download The Brain

Lecture 10
The Brain
Overview of the Brain
 Major landmarks
o Cerebrum – largest and most superior part of the brain, divided into two
hemispheres separated by a longitudinal fissure
o Cerebellum – second largest part of the brain, inferior to the cerebrum
o Brainstem – all of the brain except the cerebrum and cerebellum
 Major components include the medulla oblongata, pons, midbrain,
and diencephalon
 Grey and White Matter
o Grey matter lacks myelin and forms a surface layer called a cortex over
the cerebrum and cerebellum
o Grey matter also forms deeper masses called nuclei surrounded by white
matter
o White matter contains myelin and lies deep to the cortical gray matter in
most of the brain (opposite the pattern of grey and white matter in the
spinal cord)
 Meninges
o Dura mater – different from the dura mater in the spinal cord in that there
are two layers of dura mater
 The periosteal layer adheres to the inside of the cranium
 The meningeal layer lies within and continues into the vertebral
canal
 There is no epidural space in the cranium
o Arachnoid – transparent membrane over the brain surface between the
dura mater and pia mater
o Pia mater – thin delicate membrane that closely follows all the contours of
the brain surface, even dipping into the grooves (sulci)
 Ventricles and Cerebrospinal Fluid
o The brain has 4 fluid-filled chambers
 There are two lateral ventricles, each of which forms an arc in a
cerebral hemisphere
 There is a third ventricle near the center of the cerebrum
 There is a fourth ventricle anterior to the cerebellum
o On the floor or wall of each ventricle there is a choroid plexus
 The choroid plexus is a spongy mass of blood capillaries
 The choroids plexus produces some cerebrospinal fluid; the rest of
the fluid comes from the lining of the ventricles or from the
subarachnoid space
o Cerebrospinal fluid is a clear, colorless liquid that fills the ventricles and
canals of the CNS and bathes its external surface
 It is formed by filtration of blood plasma

Ependymal cells chemically modify the filtrate as it passes
through them into the ventricles and subarachnoid space
 Functions:
 Buoyancy – because the brain and CSF are similar in
density, the brain neither sinks nor floats
 Protection – CSF protects the brain from striking the
cranium when the head is jolted
 Chemical stability – the flow of CSF rinses metabolic
wastes from nervous tissue and regulates its chemical
environment
 Blood supply and the brain barrier system
o Blood is of critical importance to the brain, but blood is also a source of
bacterial toxins and other agents that can harm brain tissue
o The blood-brain barrier strictly regulates which substances get from the
bloodstream into the tissue fluid of the brain
o Anything passing from the blood into the tissue fluid has to pass through
the endothelial cells themselves, which are more selective than gaps
between cells
The hindbrain and the midbrain
 The medulla oblongata
o The most caudal part of the brainstem, immediately superior to the
foramen magnum of the skull
o It connects the spinal cord to the rest of the brain
o It regulates the rate and force of the heartbeat
o It regulates blood pressure and flow
o It regulates the rate and depth of breathing
 The pons
o The section of the brainstem between the midbrain and medulla oblongata
o It is the source of most nerve fibers carrying signals from the brainstem to
the cerebellum
o Nerves from the pons control eye movements, facial expression, chewing,
and swallowing, and they receive sensory signals including taste, hearing,
equilibrium, touch, and pain
 The cerebellum
o Large portion of the brain dorsal to the brainstem and inferior to the
cerebrum
o Two hemispheres are connected by a narrow bridge called the vermis
o In sagittal section, the inner white matter, called the arbor vitae, looks like
a branching fern
o The cerebellum smooths muscle contractions, maintains muscle tone and
posture, coordinates the motions of different joints with each other,
coordinates eye and body movements, and serves in learning and storing
motor skills
 The midbrain
o Short section of the brainstem that connects the hindbrain and forebrain
o Contains the corpora quadrigemina (2 superior and 2 inferior colliculi)

Superior colliculi – functions in visual attention, such as turning
the eyes and head in response to a visual stimulus
 Inferior colliculi – receives and processes auditory input from
lower levels of the brainstem and relays it to other parts of the
brain
o Contains the substantia nigra
 Center that improves motor performance by suppressing unwanted
muscle contractions
 The reticular formation
o Loosely organized web of gray matter that runs vertically through all
levels of brainstem and to many areas of the cerebrum
o Plays roles in somatic muscle control, cardiovascular control, pain
modulation, consciousness, and habituation
The forebrain
 The diencephalon – includes thalamus, hypothalamus, and epithalamus
o Thalamus – oval mass of gray matter underlying each cerebral hemisphere
 Gateway to the cerebral cortex – signals going to and from the
cerebrum pass through this region
o Hypothalamus – sits below the thalamus and connects to the pituitary
gland
 Involved in:
 hormone secretion,
 autonomic effects
 thermoregulation
 food and water intake
 sleep and circadian rhythms
 emotional responses
 memory
o Epithalamus – consists mostly of the pineal gland, which produces
melatonin which helps regulate our sleep-wake cycle
 The cerebrum – largest and most superior portion of the brain
o Is marked by gyri (thick folds) and sulci (depressed grooves)
o The two hemispheres are separated by a longitudinal fissure
 At the bottom of this fissure, the hemispheres are connected by a
thick “C” shaped bundle of nerve fibers called the corpus callosum
o Lobes:
 Frontal lobe – behind frontal bone, concerned with cognition,
speech, and motor control
 Parietal lobe – under parietal bones, concerned with receiving and
interpreting general senses as well as taste
 Occipital lobe – at the rear of the head, concerned with vision
 Temporal lobe – lateral and horizontal lobe, concerned with
hearing, smell, learning, memory, and some aspects of vision and
emotion
 Insula – deep lobe (normally covered), plays some roles in taste,
hearing, and visceral sensation
The cranial nerves
 Olfactory
o Composition: Sensory
o Function: Smell
o Origin: Olfactory mucosa in nasal cavity
o Termination: olfactory bulb
o Cranial passage: Cribiform plate of the ethmoid bone
 Optic
o Composition: Sensory
o Function: Vision
o Origin: Retina
o Termination: thalamus
o Cranial passage: Optic foramen
 Oculomotor
o Composition: Motor with some proprioceptor fibers
o Function: Eye movement, opening of eyelid, constriction of pupil,
focusing
o Origin: Midbrain
o Termination: superior, medial, and inferior rectus; and inferior oblique eye
muscles, constrictor of iris and ciliary muscles of lens
o Cranial passage: superior orbital fissure
 Trochlear
o Composition: Motor with some proprioceptor fibers
o Function: Eye movements
o Origin: Midbrain
o Termination: Superior oblique eye muscle
o Cranial passage: Superior orbital fissure
 Trigeminal
o Opthalmic division
 Composition: Sensory
 Function: touch, temperature, and pain sensation in upper face
 Origin: superior region of face
 Termination: pons
 Cranial passage: superior orbital fissure
o Maxillary division
 Composition: sensory
 Function: touch, temperature, and pain sensation in lower face
 Origin: middle region of face
 Termination: pons
 Cranial passage: foramen rotundum
o Mandibular division
 Composition: sensory and motor
 Function: touch, temperature, and pain sensation in lower jaw,
mastication
 Sensory Origin: inferior region of face
 Sensory Termination: pons
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Motor Origin: Pons
Motor Termination: muscles of mastication
Cranial passage: Foramen ovale
Abducens
o Composition: Motor with some proprioceptor fibers
o Function: eye movements
o Origin: inferior pons
o Termination: lateral rectus
o Cranial passage: superior orbital fissure
Facial
o Composition: Mixed
o Function: motor nerve of facial expression, control of salivary glands,
sensation of taste on anterior two-thirds of tongue
o Sensory Origin: Taste buds on anterior two-thirds of tongue
o Sensory Termination: thalamus
o Motor Origin: pons
o Motor Termination: muscle of facial expression, salivary glands
o Cranial passage: mastoid foramen
Vestibulocochlear
o Composition: sensory
o Function: hearing and equilibrium
o Origin: inner ear
o Termination: pons and medulla oblongata
o Cranial passage: internal auditory meatus
Glossopharyngeal
o Composition: mixed
o Function: swallowing, regulation of blood pressure and respiration, taste
sensations on the posterior one-third of the tongue
o Sensory Origin: Pharynx, posterior one-third of tongue, internal carotid
arteries
o Sensory Termination: medulla oblongata
o Motor Origin: Medulla oblongata
o Motor Termination: salivary glands, muscles of swallowing
o Cranial passage: jugular foramen
Vagus
o Composition: Mixed
o Function: cardiovascular and gastrointestinal regulation; sensations of
hunger, fullness, and intestinal discomfort
o Sensory Origin: thoracic and abdominal viscera
o Sensory Termination: medulla oblongata
o Motor Origin: medulla oblongata
o Motor Termination: thoracic and abdominal viscera
o Cranial passage: jugular foramen
Accessory
o Composition: Motor with some proprioceptive fibers
o Function: swallowing; head, neck, and shoulder movements
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o Origin: medulla oblongata and segments of C1-C5
o Termination: Palate, pharynx, sternocleidomastoid and trapezius
o Cranial passage: jugular foramen
Hypoglossal
o Composition: Motor with some proprioceptive fibers
o Function: food manipulation, swallowing, speech
o Origin: medulla oblongata
o Termination: muscles of the tongue
o Cranial passage: hypoglossal canal