Ch. 14 The Brain and Cranial Nerves - Comprehensive Notes

Brain Organization, Protection, and Blood Supply

• The development of the nervous system begins with a thickening of the ectoderm called the neural tube.

Protection

• The brain is protected by:
  • Cranial bones
  • Cranial meninges
    • Pia mater
    • Arachnoid mater
    • Dura mater
      • Composed of 2 layers
  • Cerebrospinal fluid (CSF)

Extensions of the Dura Mater

• Falx cerebri
• Tentorium cerebelli
• Falx cerebelli

Blood Flow to the Brain

• Blood flows to the brain via the vertebral and carotid arteries and flows back to the heart via the jugular veins.
• The brain utilizes about 20% of the body’s oxygen supply.
• Any interruption of the oxygen supply can result in weakening, permanent damage, or death of brain cells.
• Glucose deficiency may produce mental confusion, dizziness, convulsions, and unconsciousness.

Blood-Brain Barrier (BBB)

• The BBB protects brain cells from harmful substances and pathogens by serving as a selective barrier to prevent passage of many substances from the blood into the brain.
• The BBB can prevent the entry of therapeutic drugs.
• Injury to the brain may cause a breakdown of the BBB, permitting the passage of normally restricted substances into the brain tissue.

Cerebrospinal Fluid (CSF)

• CSF is a liquid that protects the brain and spinal cord against chemical and physical injuries.
• CSF carries oxygen, glucose, and other important substances from the blood to nervous tissue cells.

Ventricles and the Choroid Plexus

• The ventricles of the brain contain CSF
• The choroid plexuses make CSF

CSF Flow

• CSF is produced by the choroid plexuses in the lateral ventricles.
• It flows through the interventricular foramina to the third ventricle, where more CSF is added by the choroid plexus of the third ventricle.
• CSF then flows through the aqueduct of the midbrain to the fourth ventricle, where the choroid plexus of the fourth ventricle adds more CSF.
• From the fourth ventricle, CSF flows through the lateral and median apertures into the subarachnoid space.
• In the subarachnoid space, CSF circulates around the brain and spinal cord.
• CSF is then absorbed into the venous blood through the arachnoid granulations of the dural venous sinuses.

Regions of the Brain

• Brainstem
• Cerebellum
• Diencephalon
• Cerebrum

The Brainstem

• Composed of the:
  • Medulla oblongata
  • Pons
  • Midbrain

Medulla Oblongata

• Continuous with the superior aspect of the spinal cord and contains portions of both motor and sensory tracts.
• Cranial nerves:
  • Vestibulocochlear (VIII)
  • Hypoglossal (XII)
• Structural regions:
  • Pyramids
  • Inferior olivary nuclei
• Functional regions:
  • Heart rate
  • Respiratory rate
  • Vasoconstriction
  • Swallowing
  • Coughing
  • Vomiting
  • Sneezing
  • Hiccupping

Pons

• Located superior to the medulla oblongata and it links parts of the brain with one another by way of tracts.
• Cranial nerves:
  • Trigeminal (V)
  • Abducens (VI)
  • Facial (VII)
  • Vestibular branch of vestibulocochlear (VIII)
• Functional regions:
  • Relays nerve impulses related to voluntary skeletal muscle movements from cerebrum to cerebellum
  • Pneumotaxic and apneustic areas (control of respiration)

Midbrain

• Located superior to the medulla oblongata and extends from the pons to the diencephalon.
• Cranial nerves:
  • Oculomotor (III)
  • Trochlear (IV)
• Structural regions:
  • Cerebral peduncles
  • Corpora quadrigemina
  • Substantia nigra
  • Red nuclei
  • Medial lemniscus
• Functional regions:
  • Conveys motor impulses from the cerebrum to the cerebellum and spinal cord.
  • Sends sensory impulses from the spinal cord to the thalamus.
  • Regulates auditory and visual reflexes.

Reticular Formation

• Helps regulate muscle tone
• Alerts the cortex to incoming sensory signals
• Responsible for maintaining consciousness and awakening from sleep

Summary of Functions of Brainstem

• Medulla oblongata
  • Contains sensory (ascending) and motor (descending) tracts.
  • Cardiovascular center regulates heartbeat and blood vessel diameter.
  • Medullary respiratory center (together with pons) regulates breathing.
  • Contains gracile nucleus, cuneate nucleus, gustatory nucleus, cochlear nuclei, and vestibular nuclei (components of sensory pathways to brain).
  • Inferior olivary nucleus provides instructions that cerebellum uses to adjust muscle activity when learning new motor skills.
  • Other nuclei coordinate vomiting, swallowing, sneezing, coughing, and hiccupping.
  • Contains nuclei of origin for vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII) nerves.
  • Reticular formation (also in pons, midbrain, and diencephalon) functions in consciousness and arousal.
• Pons
  • Contains sensory and motor tracts.
  • Pontine nuclei relay nerve impulses from motor areas of cerebral cortex to cerebellum.
  • Contains vestibular nuclei (along with medulla) that are part of equilibrium pathway to brain.
  • Pontine respiratory group (together with the medulla) helps control breathing.
  • Contains nuclei of origin for trigeminal (V), abducens (VI), facial (VII), and vestibulocochlear (VIII) nerves.
• Midbrain
  • Contains sensory and motor tracts.
  • Superior colliculi coordinate movements of head, eyes, and trunk in response to visual stimuli.
  • Inferior colliculi coordinate movements of head, eyes, and trunk in response to auditory stimuli.
  • Substantia nigra and red nucleus contribute to control of movement.
  • Contains nuclei of origin for oculomotor (III) and trochlear (IV) nerves.

The Cerebellum

• Occupies the inferior and posterior aspects of the cranial cavity.
• Consists of two hemispheres and a central vermis.
• Functions in the coordination of skeletal muscle contractions and in the maintenance of normal muscle tone, posture, and balance.

The Diencephalon

• Composed of the:
  • Thalamus
  • Hypothalamus
  • Epithalamus

Thalamus

• Located superior to the midbrain.
• Contains nuclei that serve as relay stations for all sensory impulses (except smell) to the cerebral cortex.

Hypothalamus

• Found inferior to the thalamus.
• Has four major regions.
• Controls many body activities.
• One of the major regulators of homeostasis.

Epithalamus

• Lies superior and posterior to the thalamus and contains the pineal gland.
• The pineal gland secretes melatonin and the habenular nuclei are involved in olfaction.

Circumventricular Organs (CVOs) of the Diencephalon

• Parts of the diencephalon, the CVOs, can monitor chemical changes in the blood because they lack a blood-brain barrier.
• CVOs include a portion of the hypothalamus, the pineal gland, and the pituitary gland.
• CVOs coordinate homeostatic activities of the endocrine and nervous systems.

Summary of Functions of the Diencephalon

• Thalamus: Relays almost all sensory input to cerebral cortex. Contributes to motor functions by transmitting information from cerebellum and corpus striatum to primary motor cortex of cerebral cortex. Plays role in maintenance of consciousness.
• Hypothalamus: Controls and integrates activities of autonomic nervous system. Produces hormones, including releasing hormones, inhibiting hormones, oxytocin, and antidiuretic hormone. Regulates emotional and behavioral patterns (together with limbic system). Contains feeding and satiety centers (regulate eating), thirst center (regulates drinking), and suprachiasmatic nucleus (regulates circadian rhythms). Controls body temperature by serving as body’s thermostat.
• Epithalamus: Consists of pineal gland (secretes melatonin) and habenular nuclei (involved in olfaction).

The Cerebrum

• The cerebral cortex is composed of gray matter which contains billions of neurons.
  • Gyri, fissures, and sulci can be identified on the cortex.
• Deep to the cortex is white matter composed of tracts of neurons that connect parts of the brain to each other and the spinal cord.
  • A bundle of white matter tracts called the corpus callosum connects the right and left hemispheres of the cerebrum.

Lobes of the Cerebrum

• Frontal lobe
• Parietal lobe
• Temporal lobe
• Occipital lobe
• Insula

Basal Nuclei of the Cerebrum

• The basal nuclei are paired masses of gray matter in each cerebral hemisphere.

The Limbic System

• The limbic system is found in the cerebral hemispheres and diencephalon.

Functional Organization of the Cerebral Cortex

• Specific types of sensory, motor, and integrative signals are processed in certain regions of the cerebral cortex.
  • Sensory areas
  • Motor areas
  • Association areas

Summary of Functions of the Cerebrum

• Sensory areas of cerebral cortex are involved in perception of sensory information; motor areas control execution of voluntary movements; association areas deal with more complex integrative functions such as memory, personality traits, and intelligence.
• Corpus striatum helps initiate and terminate movements, suppresses unwanted movements, and regulates muscle tone.
• Limbic system promotes range of emotions, including pleasure, pain, docility, affection, fear, and anger.

Hemispheric Lateralization

• Right Hemisphere Functions
  • Receives somatic sensory signals from and controls muscles on left side of body.
  • Musical and artistic awareness.
  • Space and pattern perception.
  • Recognition of faces and emotional content of facial expressions.
  • Generating emotional content of language.
  • Generating mental images to compare spatial relationships.
  • Identifying and discriminating among odors.
  • Patients with damage in right hemisphere regions that correspond to Broca's and Wernicke's areas in the left hemisphere speak in a monotonous voice, having lost the ability to impart emotional inflection to what they say.
• Left Hemisphere Functions
  • Receives somatic sensory signals from and controls muscles on right side of body.
  • Reasoning.
  • Numerical and scientific skills.
  • Ability to use and understand sign language.
  • Spoken and written language.
  • Persons with damage in the left hemisphere often exhibit aphasia.

Brain Waves

• Brain waves indicate electrical activity of the cerebral cortex.
• They are recorded via electroencephalogram (EEG).
  • Alpha waves occur when people are awake and resting with their eyes closed. They disappear during sleep.
  • Beta waves appear when the nervous system is active; during periods of sensory input and mental activity.
  • Theta waves occur when children and adults experience emotional stress.
  • Delta waves occur during deep sleep in adults and in awake infants.

Cranial Nerves

Cranial Nerve Mnemonic: "Oh Oh Oh To Touch And Feel Very Good Vegetables, Ah!"

• Olfactory (I)
  • Special sensory
  • Olfaction (smell).
• Optic (II)
  • Special sensory
  • Vision (sight).
• Oculomotor (III)
  • Motor (Somatic)
    • Movement of eyeballs and upper eyelid.
  • Motor (autonomic)
    • Adjusts lens for near vision (accommodation).
    • Constriction of pupil
• Trochlear (IV)
  • Motor (Somatic)
    • Somatic Movement of eyeballs.
• Trigeminal (V)
  • Sensory
    • Touch, pain, and thermal sensations from scalp, face, and oral cavity (including teeth and anterior two-thirds of tongue).
  • Motor (branchial)
    • Chewing and controls middle ear muscle.
• Abducens (VI)
  • Motor (somatic)
    • Movement of eyeballs.
• Facial (VII)
  • Sensory
    • Touch, pain, and thermal sensations from scalp, face, and oral cavity (including teeth and anterior two-thirds of tongue).
  • Motor (branchial)
    • Control of muscles of facial expression and middle ear muscle.
  • Motor (autonomic)
    • Secretion of tears and saliva.
• Vestibulocochlear (VIII)
  • Special sensory
    • Hearing and equilibrium.
• Glossopharyngeal (IX)
  • Sensory
    • Taste from posterior one-third of tongue.
    • Proprioception in some swallowing muscles.
    • Monitors blood pressure and oxygen and carbon dioxide levels in blood.
    • Touch, pain, and thermal sensations from skin of external ear and upper pharynx.
  • Motor (branchial)
    • Assists in swallowing.
  • Motor (autonomic)
    • Secretion of saliva.
• Vagus (X)
  • Sensory
    • Taste from epiglottis.
    • Proprioception from throat and voice box muscles.
    • Monitors blood pressure and oxygen and carbon dioxide levels in blood.
    • Touch, pain, and thermal sensations from skin of external ear.
    • Sensations from thoracic and abdominal organs.
  • Motor (branchial)
    • Swallowing, vocalization, and coughing.
  • Motor (autonomic)
    • Motility and secretion of digestive canal organs.
    • Constriction of respiratory passageways.
    • Decreases heart rate
• Accessory (XI)
  • Motor (branchial)
    • Branchial Movement of head and pectoral girdle.
• Hypoglossal (XII)
  • Motor (somatic)
    • Somatic Speech, manipulation of food, and swallowing.

Development of the Nervous System

• During the 3rd week of gestation, the ectoderm thickens, forming the neural plate.
• The neural plate folds inward and forms the longitudinal groove.
  • On either side of the groove, the raised edges are the neural folds.
• Eventually the folds meet to form the neural tube.

Aging and the Nervous System

• Aging can result in:
  • Loss of neurons
  • Diminished capacity for sending nerve impulses to and from the brain
  • Diminished ability to process information
  • Decreased conduction velocity
  • Slowing of voluntary motor movements
  • Increased reflex time
  • Degenerative changes in vision, hearing, sight, taste, smell, touch, and balance

Disorders

• Cerebrovascular accident (CVA) or Stroke
  • Leads to death of brain cells due to lack of oxygen
    • 85% of strokes are ischemic (due to blood clot)
    • 15% of strokes are hemorrhagic (leaky or ruptured vessel)
• Transient ischemic attack (TIA)
  • Temporary cerebral dysfunction caused by impaired blood flow
    • Typically persists for 5-10 minutes
    • Symptoms include dizziness, weakness, numbness, etc.
• Alzheimer’s disease
  • Progressive dementia, characterized by loss of reasoning and ability to care for oneself
    • Hallmarks include: neuronal degeneration, beta- amyloid plaques, neurofibrillary tangles
• Brain Tumor
  • Abnormal growth of brain tissue; malignant or benign
    • Symptoms depend on size, location and rate of growth
• Attention Deficit Hyperactivity Disorder (ADHD)
  • Learning disorder characterized by poor or short attention span, consistent level of hyperactivity and impulsiveness
    • Likely has a genetic component although the causes are not fully understood