Chapter 14

The Brain and Cranial Nerves

 

Lecture Outline

INTRODUCTION

•      The brain is the center for registering sensations, correlating them with one another and with stored information, making decisions, and taking action.

•      It is also the center for intellect, emotions, behavior, and memory.

•      It also directs our behavior towards others.

•      In this chapter we will consider the principal parts of the brain, how the brain is protected and nourished, and how it is related to the spinal cord and to the 12 pairs of cranial nerves.

Chapter 14
The Brain and Cranial Nerves

•       Largest organ in the body at almost 3 lb.

•       Brain functions in sensations, memory, emotions, decision making, behavior

 

OVERVIEW OF BRAIN ORGANIZATION AND BLOOD SUPPLY

•      The major parts of the brain are the brain stem, diencephalon, cerebrum, and cerebellum (Figure 14.1).

•      The CNS develops from an ectodermal neural tube

–   Three primary vesicles: prosencephalon, mesencephalon, and rhombencephalon develop from the neural tube. (Figure 14.29)

•      The embryologic development of the CNS is summarized in table 14.1

Principal Parts of the Brain

•       Cerebrum

•       Diencephalon

–    thalamus & hypothalamus

•       Cerebellum

•       Brainstem

–    medulla, pons & midbrain

Blood Supply to Brain

•       Arterial blood supply is branches from circle of Willis on base of brain

•       Vessels on surface of brain----penetrate tissue

•       Uses 20% of our bodies oxygen & glucose needs

–    blood flow to an area increases with activity in that area

–    deprivation of O2 for 4 min does permanent injury

•    at that time, lysosome release enzymes

•       Blood-brain barrier (BBB)

–    protects cells from some toxins and pathogens

•    proteins & antibiotics can not pass but alcohol & anesthetics do

–    tight junctions seal together epithelial cells, continuous basement membrane, astrocyte processes covering capillaries

 

 

 

Blood Flow and the Blood-Brain Barrier

•      An interruption of blood flow for 1 or 2 minutes impairs neuronal function. 

–   A total deprivation of oxygen for 4 minutes causes permanent injury.

•      Because carbohydrate storage in the brain is limited, the supply of glucose to the brain must be continuous.

–   Glucose deficiency may produce mental confusion, dizziness, convulsions, and unconsciousness.

BBB

•      A blood-brain barrier (BBB) protects brain cells from harmful substances and pathogens by serving as a selective barrier to prevent passage of many substances from the blood to the brain.

•      An injury to the brain due to trauma, inflammation, or toxins causes a breakdown of the BBB, permitting the passage of normally restricted substances into brain tissue.

•       The BBB may also prevent entry of drugs that could be used as therapy for brain cancer or other CNS disorders, so research is exploring ways to transport drugs past the BBB.

Protective Covering of the Brain

•      The brain is protected by the cranial bones (Figure 7.4) and the cranial meninges (Figure 14.2).

–   The cranial meninges are continuous with the spinal meninges and are named dura mater, arachnoid, and pia mater.

–   Three extensions of the dura mater separate parts of the brain: the falx cerebri, falx cerebelli, and the tentorium cerebelli.

Protective Coverings of the Brain

•       Bone, meninges & fluid

•       Meninges same as around the spinal cord

–    dura mater

–    arachnoid mater

–    pia mater

•       Dura mater extensions

–    falx cerebri

–    tentorium cerebelli

–    falx cerebelli

CEREBROSPINAL FLUID

•      Cerebrospinal fluid (CSF) is a clear, colorless liquid that protects the brain and spinal cord against chemical and physical injuries.

Cerebrospinal Fluid (CSF)

•      80-150 ml (3-5oz)

•      Clear liquid containing glucose, proteins, & ions

•      Functions

–   mechanical protection

•   floats brain & softens impact with bony walls

–   chemical protection

•   optimal ionic concentrations for action potentials

–   circulation

•   nutrients and waste products to and from bloodstream

Ventricles

•      There are four CSF filled cavities within the brain called ventricles (Figure 14.3).

–   A lateral ventricle is located in each hemisphere of the cerebrum.  The lateral ventricles are separated by the septum pellucidum.

–   The third ventricle is a narrow cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus.

–   The fourth ventricle is between the brain stem and the cerebellum.

Origin of CSF

•       Choroid plexus = capillaries covered  by ependymal cells

–    2 lateral ventricles, one within each cerebral hemisphere

–    roof of 3rd ventricle

–    fourth ventricle

 

Drainage of CSF from Ventricles

•       One median aperture & two lateral apertures allow CSF to exit from the interior of the brain

Flow of Cerebrospinal Fluid

Reabsorption of CSF

•       Reabsorbed through arachnoid villi

–    grapelike clusters of arachnoid penetrate dural venous sinus

•       20 ml/hour reabsorption rate = same as production rate

•    Reabsorption of CSF

–   Reabsorbed through arachnoid villi

»   grapelike clusters of arachnoid penetrate dural venous sinus

–   20 ml/hour reabsorption rate = same as production rate

Hydrocephalus

•      Blockage of drainage of CSF (tumor, inflammation, developmental malformation, meningitis, hemorrhage or injury)

–   Continued production cause an increase in pressure --- hydrocephalus

–   In newborn or fetus, the fontanels allow this internal pressure to cause expansion of the skull and damage to the brain tissue

•      Neurosurgeon implants a drain shunting the CSF to the veins of the neck or the abdomen

THE BRAIN STEM

Medulla Oblongata

•       Continuation of spinal cord

•       Ascending sensory tracts

•       Descending motor tracts

•       Nuclei of 5 cranial nerves

•       Cardiovascular center

–    force & rate of heart beat

–    diameter of blood vessels

•       Respiratory center

–    medullary rhythmicity area sets basic rhythm of breathing

•       Information in & out of cerebellum

•       Reflex centers for coughing, sneezing, swallowing etc.

Ventral Surface of  Medulla Oblongata

•       Ventral surface bulge

–    pyramids

–    large motor tract

–    decussation of most fibers

•    left cortex controls right muscles

•       Olive = olivary nucleus

–    neurons send input to cerebellum

–    proprioceptive signals

–    gives precision to movements

Dorsal Surface of Medulla Oblongata

•       Nucleus gracilis & nucleus cuneatus = sensory neurons

–    relay information to thalamus on opposite side of brain

•       5 cranial nerves arise from medulla -- 8 thru 12

XII = Hypoglossal Nerve

•       Controls muscles of tongue during speech and swallowing

•       Injury deviates tongue to injured side when protruded

•       Mixed, primarily motor

 

XI = Spinal Accessory Nerve

•       Cranial portion

–    arises medulla

–    skeletal mm of throat & soft palate

•       Spinal portion

–    arises cervical spinal cord

–    sternocleidomastoid and trapezius mm.

X = Vagus Nerve

•       Receives sensations from viscera

•       Controls cardiac muscle and smooth muscle of the viscera

•       Controls secretion of digestive fluids

 

IX = Glossopharyngeal Nerve

•       Stylopharyngeus m. (lifts throat during swallowing)

•       Secretions of parotid gland

•       Somatic sensations & taste on posterior 1/3 of tongue

VIII = Vestibulocochlear Nerve

•       Cochlear branch begins in medulla

–    receptors in cochlea

–    hearing

–    if damaged deafness or tinnitus (ringing) is produced

•       Vestibular branch begins in pons

–    receptors in vestibular apparatus

–    sense of balance

–    vertigo (feeling of rotation)

–    ataxia (lack of coordination)

 

Injury to the Medulla

•       Hard blow to the back of the head may be fatal

•       Cranial nerve malfunctions on same side as injury;
loss of sensation or paralysis of throat or tongue;
irregularities in breathing and heart rhythm

Pons

•      The pons is located superior to the medulla. It connects the spinal cord with the brain and links parts of the brain with one another by way of tracts (Figures 14.1, 14.5).

–   relays nerve impulses related to voluntary skeletal movements from the cerebral cortex to the cerebellum.

–   contains the pneumotaxic and apneustic areas, which help control respiration along with the respiratory center in the medulla (Figure 23.24).

–   contains nuclei for cranial nerves V  trigeminal, VI abducens, VII facial, and VIII vestibulocochlear (vestibular branch only).(Figure 14.5).

Pons

•      One inch long

•      White fiber tracts ascend and descend

•      Pneumotaxic & apneustic areas help control breathing

•      Middle cerebellar peduncles carry sensory info to the cerebellum

•      Cranial nerves 5 through 7

 

VII = Facial Nerve

•       Motor portion

–    facial muscles

–    salivary & nasal and oral mucous glands & tears

•       Sensory portion

–    taste buds on anterior 2/3’s of tongue

VI = Abducens Nerve

•       Lateral rectus eye muscle

V = Trigeminal Nerve

•       Motor portion

–    muscles of mastication

•       Sensory portion

–    touch, pain, & temperature receptors of the face

•    ophthalmic branch

•    maxillary branch

•    mandibular branch

Midbrain

•       One inch in length

•       Extends from pons to diencephalon

•       Cerebral aqueduct connects 3rd ventricle above to 4th ventricle below

Midbrain in Section

•       Cerebral peduncles---clusters of motor & sensory fibers

•       Substantia nigra---helps controls subconscious muscle activity

•       Red nucleus-- rich blood supply & iron-containing pigment

–    cortex & cerebellum coordinate muscular movements by sending information here from the cortex and cerebellum

Dorsal Surface of Midbrain

•      Corpora quadrigemina = superior & inferior colliculi

–   coordinate eye movements with visual stimuli

–   coordinate head movements with auditory stimuli

 

IV = Trochlear Nerve

•       Superior oblique eye muscle

III = Oculomotor Nerve

•       Levator palpebrae raises eyelid (ptosis)

•       4 extrinsic eye muscles

•       2 intrinsic eye muscles

–    accomodation for near vision (changing shape of lens during reading)

–    constriction of pupil

Reticular Formation

•      Scattered nuclei in medulla, pons & midbrain

•      Reticular activating system

–   alerts cerebral cortex to sensory signals (sound of alarm, flash light, smoke or intruder) to awaken from sleep

–   maintains consciousness & helps keep you awake with stimuli from ears, eyes, skin and muscles

•      Motor function is involvement with maintaining muscle tone

Cerebellum

•       2 cerebellar hemispheres and vermis (central area)

•       Function

–    correct voluntary muscle contraction and posture based on sensory data from body about actual movements

–    sense of equilibrium

 

Cerebellum

•       Transverse fissure between cerebellum & cerebrum

•       Cerebellar cortex (folia) & central nuclei are grey matter

•       Arbor vitae = tree of life = white matter

 

Cerebellar Peduncles

•      Superior, middle & inferior peduncles attach to brainstem

–   inferior carries sensory information from spinal cord

–   middle carries sensory fibers from cerebral cortex & basal ganglia

–   superior carries motor fibers that extend to motor control areas

THE DIENCEPHALON

Diencephalon Surrounds 3rd Ventricle

•       Surrounds 3rd ventricle

•       Superior part of walls is thalamus

•       Inferior part of walls & floor is hypothalamus

Thalamus

•      The thalamus is located superior to the midbrain and contains nuclei that serve as relay stations for all sensory impulses, except smell, to the cerebral cortex (Figure 14.9).

–   seven major groups of thalamic nuclei on each side (Figure 14.9 c and d). 

–   They are the Anterior nucleus, medial nuclei, lateral group, ventral group, intralaminar nuclei, midline nucleus, and the reticular nucleus.

•      It also registers conscious recognition of pain and temperature and some awareness of light touch and pressure.

•      It plays an essential role in awareness and the acquisition of knowledge (cognition.)

Thalamus

•      1 inch long mass of gray mater in each half of brain (connected across the 3rd ventricle by intermediate mass)

•      Relay station for sensory information on way to cortex

•      Crude perception of some sensations

Thalamic Nuclei

•      Nuclei have different roles

–    relays auditory and visual impulses, taste and somatic sensations

–   receives impulses from cerebellum or basal ganglia

–   anterior nucleus concerned with emotions, memory and acquisition of knowledge (cognition)

Hypothalamus

•      The hypothalamus

–   inferior to the thalamus, has four major regions (mammillary, tuberal, supraoptic, and preoptic)

–   controls many body activities, and is one of the major regulators of homeostasis (Figure 14.10).

•      The hypothalamus has a great number of functions.

–    It controls the ANS.

–    It produces hormones.

–    It functions in regulation of emotional and behavioral  patterns.

–    It regulates eating and drinking through the feeding center, satiety center, and thirst center.

–    It aids in controlling body temperature.

–    It regulates circadian rhythms and states of consciousness.

Hypothalamus

•       Dozen or so nuclei in 4 major regions

–    mammillary bodies are relay station for olfactory reflexes;
infundibulum suspends the pituitary gland

•       Major regulator of homeostasis

–    receives somatic and visceral input, taste, smell & hearing information; monitors osmotic pressure, temperature of blood

Epithalamus

•      The epithalamus lies superior and posterior to the thalamus and contains the pineal gland and the habenular nuclei (Figure 14.7).

–   The pineal gland secretes melatonin to influence diurnal cycles in conjunction with the hypothalamus.

–   The habenular nuclei (Figure 14.7a) are involved in olfaction, especially emotional responses to odors.

Epithalamus

•       Pineal gland

–    endocrine gland the size of small pea

–    secretes melatonin during darkness

–    promotes sleepiness & sets biological clock

•       Habenular nuclei

–    emotional responses to odors

Subthalamus

•      The subthalamus lies immediately inferior to the thalamus  and includes tracts and the paired subthalamic nuclei, which connect to motor areas of the cerebrum.

–   The subthalamic nuclei and red nucleus and substantia nigra of the midbrain work together with the basal ganglia, cerebellum, and cerebrum in control of body movements.

•      Table 14.2 summarizes the functions of the parts of the diencephalon.

Circumventricular Organs

•      Parts of the diencephalon, called circumventricular organs (CVOs), can monitor chemical changes in the blood because they lack a blood-brain barrier.

•      CVOs include

–   part of the hypothalamus,

–   the pineal gland,

–   the pituitary gland, and a few other nearby structures.

•      They function to coordinate homeostatic activities of the endocrine and nervous systems.

•      They are also thought to be the site of entry into the brain of HIV.

THE CEREBRUM

•      The cerebrum is the largest part of the brain .

–   The surface layer, the cerebral cortex, is 2-4 mm thick and is composed of gray matter. The cortex contains billions of neurons.

–   The cortex contains gyri (convolutions), deep grooves called fissures, and shallower sulci. (Figure 14.11a)

•      Beneath the cortex lies the cerebral white matter, tracts that connect parts of the brain with itself and other parts of the nervous system.

•      The cerebrum is nearly separated into right and left halves, called hemispheres, by the longitudinal fissure.

–   Internally it remains connected by the corpus callosum, a bundle of transverse white fibers. Figure 14.12)

Cerebrum
 (Cerebral Hemispheres)

•      Cerebral cortex is gray matter
overlying white matter

–   2-4 mm thick containing billions of cells

–   grew quickly; formed folds
(gyri) and grooves (sulci or fissures)

•      Longitudinal fissure separates left & right cerebral hemispheres

–   Corpus callosum is a commisure (band of white matter) connecting left and right cerebral hemispheres

•      Each hemisphere is subdivided into 4 lobes

Lobes

•      Each cerebral hemisphere is further subdivided into four lobes by sulci or fissures (Figure 14.11 a,b)

–    frontal, parietal, temporal, and occipital.

•      A fifth part of the cerebrum, the insula, lies deep to the parietal, frontal, and temporal lobes and cannot be seen in an external view of the brain.

 

 Lobes and Fissures

•      Longitudinal fissure (green)

•      Frontal lobe

•      Central sulcus (yellow)

–   precentral & postcentral gyrus

•      Parietal lobe

•      Parieto-occipital sulcus

•      Occipital lobe

•      Lateral sulcus (blue)

•      Temporal lobe

•      Insula

Insula within Lateral Fissure

White Matter

•      The white matter is under the cortex and consists of myelinated axons running in three principal directions (Figure 14.12).

–