Ch. 14 - Brain and Cranial Nerves

Rostral Vs. Caudal

rostral - toward the frontal brain

caudal - towards the brain stem

Gray Matter

• unmyelinated

• “thinking matter”

• have neurosomas, dendrites, and synapses

• Dull color due to little myelin

• Forms surface layer (cortex) over cerebrum and cerebellum

• Forms nuclei deep within brain

White Matter

• highly myelinated

• “transmission matter”

• bundles of axons

• Lies below the cortical gray matter

• Pearly white color from myelin around nerve fibers

• made of bundles of axons that connect one part of the brain to another, and to the spinal cord

Ventricles

4 internal chambers within brain:

• two lateral ventricles

• third ventricle

• fourth ventricle

there is a high concentration of ependymal cells in the ventricles, specifically the lateral ventricles

• CSF is pressurized in the lateral ventricles 

• also circulates through subarachnoid space

Interventricular Foramen

a tiny pore that connects to the third ventricle

Choroid Plexus

a spongy mass of blood capillaries on the floor of each ventricle

• covered by ependymal cells which produce CSF

Cerebrospinal Fluid (CSF)

clear, colorless liquid that fills the ventricles and canals of CNS and bathes their surfaces

• basically blood plasma without the proteins

• more lactate than glucose will be floating around in it compared to blood

• most CSF is produced in the meninges

• Brain produces and absorbs 500 mL/day

• Production of CSF begins with filtration of blood plasma through capillaries of the brain, then ependymal cells modify it

Functions of CSF:

1. Buoyancy

Allows brain to gain mass without being impaired by its own weight. If it rested heavily on the floor of the cranium, the pressure would kill the nervous tissue.

2. Protection - shock absorber

Protects the brain from striking the cranium when the head is jolted.

3. Chemical stability

Flow of CSF rinses away metabolic wastes from nervous tissue and regulates its chemical environment.

The Flow of Cerebral Spinal Fluid

1. CSF gets produced in the ventricles

2. CSF then goes through the ventricles to the central canal

3. Then it fills the subarachnoid space and bathes the outer surfaces of the spinal cord and brain

4. there are holes in the dura mater for excess CSF, called arachnoid villi

Blood Supply to the Brain

The brain is only 2% of adult body weight, but receives 15% of our blood

• Neurons have a high demand for ATP so a constant supply of blood is required

• 10s interruption of blood flow may cause loss of consciousness

• 1 to 2 min interruption can cause significant impairment of neural function

• 4 minutes without blood causes irreversible brain damage

Brain Barrier System

regulates what substances can get from the bloodstream into the tissue fluid of the brain

• Although blood is crucial, it can also contain harmful agents

• Two points of entry must be guarded

  • Blood capillaries throughout the brain tissue

  • Capillaries of the choroid plexus

• transcytosis is one of the only ways to get to the tissues of the CNS

Blood-Brain Barrier

protects blood capillaries throughout brain tissue

• Astrocytes build the blood brain barrier by forming tight junctions with the capillaries

• Anything leaving the blood must pass through the cells, and not between them

• Endothelial cells in the capillary walls can exclude harmful substances from passing to the brain tissue while allowing necessary ones to pass

Blood-CSF Barrier

protects brain at the choroid plexus

• Forms tight junctions between the ependymal cells

  • Tight junctions are absent from ependymal cells elsewhere

• Important to allow exchange between brain tissue and CSF

• Brain barrier system is highly permeable to water, glucose, and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, and anesthetics (narcotics)

• Slightly permeable to sodium, potassium, chloride, and the waste products urea and creatinine

What are some issues with the brain barrier system?

1. The brain barrier system (BBS) can be an obstacle for delivering medications such as antibiotics and cancer drugs

2. Trauma and inflammation can damage the BBS and allow pathogens to enter brain tissue

   • Ex) Circumventricular organs

Circumventricular Organs (CVOs)

places in the third and fourth ventricles where the barrier is absent

• Blood has direct access to the brain

• allows the brain to monitor and respond to fluctuations in blood glucose, pH, osmolarity, and other variables

• CVOs allow for invasion of HIV

• any intracellular pathogen can do this (like viruses)

Reticular Formation

A loose web of gray matter that runs up through all levels of the brainstem

• Occupies space between white fiber tracts and brainstem nuclei

• Has connections with many areas of the cerebrum

• Has more than 100 small neural networks without distinct boundaries

• Includes:

  • somatic motor control

  • cardiovascular control

  • pain modulation

  • sleep and consciousness

  • habituation

Role of Reticular Formation in Somatic (Voluntary) Motor Control

Adjusts muscle tension to maintain tone, balance, and posture, especially during body movements

• Relays signals from eyes and ears to the cerebellum

• Integrates visual, auditory, balance and motion stimuli into motor coordination

• includes gaze centers and central pattern generators

Gaze Centers

allow eyes to track and fixate on objects

Central Pattern Generators

neural pools that produce rhythmic signals to the muscles of breathing and swallowing

Role of Reticular Formation in Cardiovascular Control

monitors contraction strength and frequency of the heart

Role of Reticular Formation in Pain Modulation

Some pain signals ascend through the reticular formation

• reticular formation downplays pain

• ex) not remembering how you got a bruise

• Some descending analgesic pathways begin in the reticular formation

  • They end in the spinal cord where they block transmission of pain signals

Role of Reticular Formation in Sleep and Consciousness

Reticular formation plays a central role in consciousness, alertness and sleep

• Injury here can result in irreversible coma

Role of Reticular Formation in Habituation

the reticular activating system changs activity in the cerebral cortex so that it ignores repetitive, inconsequential stimuli

• training your central pattern generators

  • ex) wearing glasses and learning to ignore that they are there

Cerebellum

the little brain

• mostly gray matter

• little branching white matter is the arbor vitae

• contains most of our neurons in our brain

• largest part of the hindbrain and second largest part of the brain as a whole

• has right and left cerebellar hemispheres

• important for motor coordination and movement

Nonmotor Functions of the Cerebellum

a lot this is subconscious work

• Comparing textures of objects

• Perceiving space

• Recognizing objects from different views

• Keeping time and maintaining tapping rhythm

• Helping direct eye movements that compensate for head movements (so that gaze stays on a fixed object)

• Judging the pitch of tones and distinguishing between similar spoken words

• Helping in verbal association tasks

• Planning, scheduling, and emotion control

The Forebrain

consists of two parts

• Diencephalon

• Telencephalon (the cerebrum)

Diencephalon

• Encloses the third ventricle

• Most rostral part of the brainstem

• has 3 parts

  • thalamus

  • hypothalamus

  • epithalamus

Thalamus

an oval shaped structure/mass on each side of the brain on the top of the brainstem

• is like the mail room/ receptionist

• makes up 4/5 of the diencephalon

• Two thalami are joined in the middle by a narrow intermediate mass

• Made of 23 nuclei within five major functional groups

Functions of the Thalamus

takes info from brain stem and determines what part of the cerebrum will process this info

• “Gateway to the cerebral cortex”: the thalamus filters nearly all info to the cerebrum as it passes through synapses in the thalamus

• key role in motor control by relaying signals from cerebellum to cerebrum and providing feedback loops between the cerebral cortex and the basal nuclei

• involved in the memory and emotional functions of the limbic system

Hypothalamus

forms part of the walls and floor of the third ventricle

• Extends anteriorly to optic chiasm and posteriorly to mammillary bodies

• Each mammillary body has 3 or 4 mammillary nuclei

  • Relay signals from the limbic system to the thalamus

• Performs regulatory functions

• bypasses the blood-brain barrier

• control center of autonomic nervous system and endocrine system

• maintains homeostasis across the body

Functions of Hypothalamus

1. Thermoregulation

   • is like a thermostat and monitors body temperature

2. Food and water intake

   • Regulates hunger and satiety

   • responds to hunger, energy expenditure, and long-term control of body mass

   • Thirst center monitors osmolarity of blood and can stimulate production of antidiuretic hormone

3. Sleep and circadian rhythms

   • Suprachiasmatic nucleus sits above optic chiasm

4. Memory

   • Mammillary nuclei receive signals from hippocampus

   • short term to long term memory during sleep in the hypothalamus

5. Emotional behavior and sexual response

   • Anger, aggression, fear, pleasure, contentment, sexual drive - aspects of limbic system

Functions of the Nuclei of the Hypothalamus

1. Hormone secretion

   • Controls pituitary gland so it regulates growth, metabolism, reproduction, and stress responses

   • Produces pituitary hormones for labor contractions, lactation, and water conservation

2. Autonomic effects

   • Major integrating center for autonomic nervous system

   • Influences/regulates heart rate, blood pressure, gastrointestinal secretions, motility, etc

Infundibulum

the stalk attaching the pituitary gland to the hypothalamus

Epithalamus

very small mass of tissue composed of:

• Pineal gland - endocrine gland

  • produces melatonin

• Habenula

Habenula

• functions as a relay/connection from the limbic system to the midbrain

• Thin roof over the third ventricle

Cerebrum

largest, most conspicuous part of human brain

• processes sensory perception, memory, thought, judgment, and voluntary motor actions

• the two hemispheres are connected by the corpus callosum

• has gryi and sulci that increase surface area to increase processing

• Made mostly of white matter

5 Lobes of the Cerebrum

1. Frontal

2. Parietal

3. Temporal

4. Occipital

5. Insula

there is an overlap in function of the lobes

3 Types of Tracts in the Cerebrum

1. Projection tracts

2. Association tracts

3. Commissural tracts

Projection Tracts

Extend vertically between higher and lower brain and spinal cord centers

Example: corticospinal tracts

Association Tracts

Connect different regions within the same cerebral hemisphere

• Long fibers connect different lobes

• short fibers connect gyri within a lobe

• ex) connect one lobe to another lobe or one gyri to another gyri

Commissural Tracts

Cross from one cerebral hemisphere to the other, allowing communication between two sides of the cerebrum

Largest example: corpus callusum

Other crossing tracts: anterior and posterior commissures

Where is neural integration carried out?

in the gray matter of the cerebrum

• Cerebral gray matter found in 3 places

  • Cerebral cortex

  • Limbic system

  • Basal nuclei

Cerebral Cortex

gray matter that covers surface of the cerebral hemispheres

• Only 2 to 3 mm thick

• has a high surface area and has lots of neurons and connections

• 90% of human cerebral cortex is neocortex—six-layered tissue

• Contains primarily 2 types of neurons:

  • stellate cells

  • pyramidal cells

Stellate Cells

Have sphere-shaped neurosomas with dendrites pointing in all directions

• Receives sensory input and processes information on a local level

Pyramidal Cells

Tall and cone-shaped, with apex toward the brain surface

• has a thick dendrite with many branches with small knobs

• Includes the output neurons of the cerebrum

• are the only neurons that leave the cortex and connect with other parts of the CNS

Limbic System

important center of emotion and learning

• components:

  • Cingulate gyrus: arches over corpus callosum in frontal and parietal lobes

  • Hippocampus

  • Amygdala: immediately rostral to hippocampus (emotion functions)

• There is a limbic system in each cerebral hemisphere

• Limbic system components are connected through a loop of fiber tracts

  • allows for circular patterns of feedback

• Limbic system structures have centers for both gratification and aversion

Hippocampus

in medial temporal lobe (memory functions)

• performs short term to long term memory

• is completely enclosed by the limbic system

Basal Nuclei

masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus

• Receive input from the substantia nigra of the midbrain and the motor areas of the cortex

• Send signals back to both of these locations

• Involved in motor control*

• Formed by 3 different brain centers (are collectively called the corpus striatum)

  • Caudate nucleus

  • Putamen

  • Globus pallidus

Lentiform nucleus

putamen and globus pallidus together

Higher Brain Functions:

sleep, memory, cognition, emotion, sensation, motor control, and language

• Involve interactions between the cerebral cortex and basal nuclei, brainstem, and cerebellum

• Functions of the brain do not have easily defined anatomical boundaries

Primary Sensory Cortex

sites where sensory input is first received and you becomes conscious of the stimulus

• Association areas near the primary sensory areas process and interpret that sensory information

  • ex) primary visual cortex, multimodal association areas

• located on post-central gyrus

• process things like heat, temperature, pressure, not the special senses

Primary Visual Cortex

makes cognitive sense of visual stimuli

• is bordered by visual association areas

Multimodal Association Areas

receives input from multiple senses and integrates this into an overall perception of our surroundings

Special Senses

senses limited to the head and employ complex sense organs

• vision

• hearing

• equilibrium

• taste and smell

Vision

• Visual primary cortex is in the posterior of the occipital lobe

• Visual association area: takes up the rest of the occipital lobe

• Much of the inferior temporal lobe deals with recognizing faces and familiar objects

Hearing

• Primary auditory cortex is in the superior region of the temporal lobe and insula

• Auditory association : temporal lobe deep and inferior to the primary auditory cortex

• Recognizes spoken words, a familiar piece of music, or a voice on the phone

Equilibrium

• Signals for balance and sense of motion are mainly sent to the cerebellum and several brainstem nuclei involved in head and eye movements and visceral functions

• Association cortex: in the roof of the lateral sulcus near the lower end of the central sulcus

• consciousness of our body movements and orientation in space is processed here

Taste and Smell

Gustatory (taste) signals:

received by primary gustatory cortex in inferior end of the postcentral gyrus of the parietal lobe and anterior region of the insula

Olfactory (smell) signals:

received by the primary olfactory cortex in the medial surface of the temporal lobe and inferior surface of the frontal lobe

• sense of smell skips the thalamus, goes straight to cerebrum

General (somesthetic, somatosensory, or somatic) Senses

distributed over entire body and employ simple receptors

• Includes touch, pressure, stretch, movement, heat, cold, and pain

• For the head, cranial nerves carry general sensory information

• For the rest of the body, ascending tracts bring general sensory information to the brain

How are general senses passed along/processed?

• The thalamus processes the input from the contralateral side

• it then selectively relays signals to the postcentral gyrus of the parietal lobe

  • Functionally known as the primary somesthetic cortex

  • Provides awareness of stimulus

• processed on postcentral gyrus

Sensory Homunculus

diagram of the primary somesthetic cortex which resembles an upside-down sensory map of the contralateral side of the body

• Shows receptors in lower limbs projecting to superior and medial parts of the gyrus

• Shows receptors from face projecting to the inferior and lateral parts of the gyrus

Somatotopy

point-to-point correspondence between an area of the body and an area of the CNS

• how we were able to map out the homunculi on the postcentral gyrus

The intention to contract a muscle begins in…

The motor association (premotor) area of the frontal lobes

• this is where we plan our behavior

• also where neurons make a “program” for the degree and sequence of a muscle contraction required for a certain action

Where does the motor “program” get sent to after being created?

the program is transmitted to neurons of the precentral gyrus (primary motor area)

• These neurons send signals to the brainstem and spinal cord leading ultimately to muscle contractions

The Pre-Central Gyrus in Motor Control

exhibits somatotopy

• Neurons for toe movement are deep in the longitudinal fissure of the medial side of the gyrus

• The summit of the gyrus controls the trunk, shoulder, and arm

• The inferolateral region controls the facial muscles

Motor Homunculus

diagram of the motor cortex

• has a distorted look because the amount of cortex devoted to a given body region is proportional to the number of muscles and motor units of that body region (not body region size)

• voluntary movements only

• no abdominal viscera or genitals in this diagram because they are not voluntary

• if more fine motor control is needed, the organ takes more surface area on the homunculus

Pyramidal Cells of the Precentral Gyrus

are called upper motor neurons

• Their fibers project caudally

• lots of their fibers end in nuclei of the brainstem

• their fibers also form the corticospinal tracts

• Most fibers decussate in the lower medulla oblongata

• In the brainstem/spinal cord, the fibers from upper motor neurons connect with lower motor neurons whose axons innervate skeletal muscles

Basal Nuclei in Motor Control

Important motor functions include helping to control:

• start and stop of intentional movements

• Repetitive hip and shoulder movements in walking

• Highly practiced, learned behaviors such as writing, typing, driving a car

- Lie in a feedback circuit from the cerebrum, to the basal nuclei, to the thalamus, and back to the cerebrum

Dyskinesias

movement disorders caused by lesions in the basal nuclei involving abnormal movement initiation

Cerebellum in Motor Control

• Highly important in motor coordination

• Aids in learning motor skills

• Maintains muscle tone and posture

• Smooths muscle contraction

• Coordinates eye and body movements

• Coordinates motions of different joints with each other

• Lesions can cause ataxia

Ataxia

clumsy, awkward gait

• alcoholics can have lesions in their cerebellum, causing ataxia

  • causes them lose ability to regulate motor movements

  • causes constant tremors

Language includes:

reading, writing, speaking, and understanding words

Wernicke’s Area

Posterior to lateral sulcus usually in left hemisphere

• Permits recognition of spoken and written language

• When we want to speak, Wernicke’s area makes phrases and transmits a plan of speech to Broca’s area

• controls comprehension of language

Broca’s Area

Inferior to the prefrontal cortex, usually in left hemisphere

• Generates a motor program for the muscles of the larynx, tongue, cheeks, and lips for speaking and for hands when signing

• Transmits the program to the primary motor cortex for commands to the lower motor neurons that lead to muscles

• controls movement of mouth for speech

Affective Language Area

usually in right hemisphere

• Lesions produce aprosody

Aprosody

flat emotionless speech

Aphasia

a language deficit from lesions to the hemisphere with Wernicke and Broca areas

Nonfluent (Broca) Aphasia

caused from a lesion in Broca’s area

• Slow speech

• difficulty in choosing words

• using words that only approximate the correct word

Fluent (Wernicke) Aphasia

Caused from a lesion in Wernicke’s area

• Speech is normal and excessive, but uses senseless sentences

• Cannot comprehend written and spoken words

Anomic Aphasia

Can speak normally and understand speech, but cannot identify written words or pictures

Cerebral Lateralization

the difference in the structure and function of the cerebral hemispheres

Left Hemisphere

usually the categorical hemisphere

• Specialized for spoken and written language

• Sequential and analytical reasoning (math and science)

• Breaks information into fragments and analyzes it

Right Hemisphere

usually the representational hemisphere

• Perceives information in a more integrated way

• Involves imagination and insight

• Musical and artistic skill

• Perception of patterns and spatial relationships

• Comparison of sights, sounds, smells, and taste

Differences in lateralization between genders and right vs. left handed:

• Right handed:

  • left hemisphere is typically the categorical one

• Left-handed:

  • left hemisphere is typically the categorical one, but less than righties

  • some lefties have neither hemisphere specialized

• Lateralization differs with age and sex

  • Children are more resilient to lesions on one side

  • Males exhibit more lateralization than females and suffer more functional loss when one hemisphere is damaged

Cranial Nerves

12 pairs of cranial nerves

• arise from the base of the brain

• Exit the cranium through foramina

• Lead to muscles and sense organs located mainly in the head and neck

• allows us to still have coordination even if we have a spinal cord injury

Cranial Nerve Pathways (sensory vs motor)

motor fibers of the cranial nerves begin in the brainstem and lead to glands and muscles

Sensory fibers begin in receptors located mainly in head and neck and lead mainly to the brainstem

• Most cranial nerves carry fibers between brainstem and ipsilateral receptors and effectors

  • so a lesion in the brainstem causes a deficit on the same side

  • Exceptions:

    • optic nerve: half the fibers decussate

    • trochlear nerve: all efferent fibers lead to a muscle of the contralateral eye

Sensory Cranial Nerves:

l, ll, Vlll

Motor Cranial Nerves:

III, IV, VI, XI, and XII

• Stimulate muscle but also contain fibers of proprioception

Mixed Cranial Nerves:

V, VII, IX, X

• Sensory functions may be quite unrelated to their motor function

Olfactory Nerve (l)

sends the sense of smell to the brain

• has rootlets into the nasal cavity

Optic Nerve (ll)

Used to send visual signals to the brain using the optic chiasma

Oculomotor Nerve (lll)

Control the size of the pupil and movement of eye

Controls:

• Superior Rectus

• Inferior Rectus

• Medial Rectus

• Inferior Oblique

Trochlear Nerve (lV)

Controls movement of eye

• controls the superior oblique muscle

Trigeminal Nerve (V)

• largest cranial nerve

• most important sensory nerve of the face

• forks into 3 divisions

  • Ophthalmic Division (V1)

    • sensory

  • Maxillary Division (V2)

    • sensory

  • Mandibular Division (V3)

    • mixed

Abducens Nerve (Vl)

Movement of eyeball

• controls lateral rectus muscle

• allows us to move eyes laterally

Facial Nerve (Vll)

Motor function:

• is the major motor nerve of facial muscles:

  • facial expressions, salivary glands, tear, nasal, and palatine glands

Sensory function:

• controls taste on anterior two-thirds of tongue

Damage:

• Damage produces sagging facial muscles and disturbed sense of taste

• Clinical test:

  • test anterior two-thirds of tongue with sugar, salt, vinegar, and quinine

  • test response of tear glands to ammonia fumes

  • test motor functions by asking subject to close eyes, smile, whistle, etc.

Vestibulocochlear Nerve (Vlll)

Controls sense of equilibrium and sound

Glossopharyngeal Nerve (lX)

Used to control tongue, salivary glands, and swallowing muscles

Vagus Nerve (X)

The wandering nerve

• Most extensive distribution of any cranial nerve

• Major role in the control of cardiac, pulmonary, digestive, and urinary function

• Controls swallowing, speech, and regulation of viscera

• Damage causes hoarseness or loss of voice, impaired swallowing

• fatal if both are cut

Accessory Nerve (Xl)

Movement of sternocleidomastoid and trapezius