Lecture 10
The Brain
Overview of the Brain
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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
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Major components include the medulla oblongata, pons, midbrain, and diencephalon
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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)
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Meninges o Dura mater – different from the dura mater in the spinal cord in that there are two layers of dura mater
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The periosteal layer adheres to the inside of the cranium
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The meningeal layer lies within and continues into the vertebral canal
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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)
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Ventricles and Cerebrospinal Fluid o The brain has 4 fluid-filled chambers
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There are two lateral ventricles, each of which forms an arc in a cerebral hemisphere
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There is a third ventricle near the center of the cerebrum
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There is a fourth ventricle anterior to the cerebellum o On the floor or wall of each ventricle there is a choroid plexus
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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
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It is formed by filtration of blood plasma

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Ependymal cells chemically modify the filtrate as it passes through them into the ventricles and subarachnoid space
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Functions:
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Buoyancy – because the brain and CSF are similar in density, the brain neither sinks nor floats
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Protection – CSF protects the brain from striking the cranium when the head is jolted
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Chemical stability – the flow of CSF rinses metabolic wastes from nervous tissue and regulates its chemical environment
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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
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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
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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
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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
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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)

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Superior colliculi – functions in visual attention, such as turning the eyes and head in response to a visual stimulus
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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
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Center that improves motor performance by suppressing unwanted muscle contractions
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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
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The diencephalon – includes thalamus, hypothalamus, and epithalamus o Thalamus – oval mass of gray matter underlying each cerebral hemisphere
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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
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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
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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
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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:
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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
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Occipital lobe – at the rear of the head, concerned with vision
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Temporal lobe – lateral and horizontal lobe, concerned with hearing, smell, learning, memory, and some aspects of vision and emotion
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Insula – deep lobe (normally covered), plays some roles in taste, hearing, and visceral sensation

The cranial nerves
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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
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Optic o Composition: Sensory o Function: Vision o Origin: Retina o Termination: thalamus o Cranial passage: Optic foramen
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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
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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
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Trigeminal o Opthalmic division
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Composition: Sensory
 Function: touch, temperature, and pain sensation in upper face
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Origin: superior region of face
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Termination: pons
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Cranial passage: superior orbital fissure o Maxillary division
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Composition: sensory
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Function: touch, temperature, and pain sensation in lower face
 Origin: middle region of face
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Termination: pons
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Cranial passage: foramen rotundum o Mandibular division
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Composition: sensory and motor
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Function: touch, temperature, and pain sensation in lower jaw, mastication
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Sensory Origin: inferior region of face
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Sensory Termination: pons

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Motor Origin: Pons
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Motor Termination: muscles of mastication
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Cranial passage: Foramen ovale
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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
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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
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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
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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
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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
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Accessory o Composition: Motor with some proprioceptive fibers o Function: swallowing; head, neck, and shoulder movements

o Origin: medulla oblongata and segments of C1-C5 o Termination: Palate, pharynx, sternocleidomastoid and trapezius o Cranial passage: jugular foramen
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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