Chapter
1
Neuroscience
Benjamin R. Canida, D.D.S.,
Kyle M. Cheatham. O.D., F.A.A.O.,
1
2
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CHAPTER 1. NEUROSCIENCE
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SECTION 1.1
Neurophysiology, Cellular Neuroscience, and
Electrophysiology
Nerve cells communicate primarily through intercellular electrical signaling mediated by short range chemical messengers (in most cases). It is important to
understand the basic cellular mechanisms giving rise to the complex multicellular behavior in the nervous system. We now cover some fundamentals of
cellular neuroscience (11, ch. 2-6), (8, ch. 4-16), (7, ch. 2).
Cellular Electrophysiology
Resting Potential: Neurons have a resting membrane potential of approximately -60 to -70 mV, meaning that the internal cellular environment is more
negatively charged than its exterior counterpart. Such a potential is maintained by the balance of an electrochemical gradient and the active transport
of ions (most importantly Na+ and K+ ) across the cell membrane.
Conductance: For our purposes, conductance serves as a measure of the
ability of ions to flow across the cellular membrane. For example, a high K+
conductance indicates that potassium ions readily traverse the cellular membrane.
Resting state: When a neuron is in its resting state, K+ conductance is
high; thus, K+ ions are the dominant force in determining the resting potential
of the cell. The Na+ / K+ ATPase pump, which exports Na+ and imports K+ ,
also plays a role in setting this resting potential. The table below lists some
typical (approximate) ion concentrations in mammalian neurons (11, pp. 50).
Note that different sources provide slightly different values.
Ion
K+
Na+
Cl−
Ca2+
Extracellular
Concentration (mM)
5
145
110
1-2
Intracellular
Concentration (mM)
140
10
4-30
10−4
The inside of the cell is negatively charged relative to the exterior
environment. In addition, K+ is more abundant inside the cell.
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1.1. NEUROPHYSIOLOGY, CELLULAR NEUROSCIENCE, AND
ELECTROPHYSIOLOGY
Action Potentials
Action potentials, often referred to as neural spikes, result from an active process which generates a traveling electrical impulse. The process is often termed
an “all or none” process, meaning that if the membrane potential crosses some
threshold, there will be an action potential. For potentials below threshold,
no spike will occur. With some notable exceptions, many cells in the nervous
system communicate primarily via action potentials.
Stages of a Neuron Action Potential:
1. Na+ conductance increases in response to a local depolarization of the
membrane, leading to an inward current of Na+ .
2. The membrane potential rises steeply (depolarizes), resulting in a selfreinforcing cascade whereby more Na+ channels open.
3. Vm , the membrane potential, peaks at approximately 40 mV (although
this varies across cell types).
4. At this stage, K+ conductance increases, meaning that K+ readily exits
the cell. This results in the inactivation of Na+ channels.
5. Membrane potential now falls quickly and briefly overshoots the original.
That is, the membrane becomes more negative than at its resting state;
this is known as hyperpolarization.
6. Immediately following an action potential, there is a period of time during
which another action potential cannot be initiated; this is termed the
refractory period.
• Note that during a relative refractory period, a larger than usual
stimulus can still produce an action potential; during an absolute
refractory period, no stimulus can produce an action potential. This
becomes important, especially in the heart.
The ion channels in the cellular membrane open and close as a result of various stimuli (another neuron, a physical stimulus, etc).
As a result, ions flow in/out of the membrane according to the
electrochemical driving force. This active process leads to a propagating wave of electrical activity along the neuron, an event known
as an action potential or voltage spike.
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Nerve Conduction: Many nerves are covered by a myelin sheath produced
by glial cells.
• Oligodendrocytes form myelin around axons in the CNS.
• Schwann cells form myelin around axons in the PNS.
Every 0.2 - 2 mm there is a break in the myelin sheath called a Node of
Ranvier. The myelin prevents movement of Na+ and K+ through the membrane, forcing the action potential to depolarize the membrane at the Nodes
of Ranvier. While an action potential travels down an unmyelinated nerve
rather slowly (1.0 m/sec), it can progress rapidly along a myelinated nerve
as the action potential jumps from node to node. This is termed saltatory
conduction. Saltatory conduction is not only faster, it also consumes less
energy.
Synapses
Synapses are the physical meeting points between cells that facilitate multineuron communication. Synapses fall into two main categories:
1. Chemical synapse: Communication across a chemical synapse occurs via
a chemical messenger known as a neurotransmitter. The transmission is
limited by factors including the diffusion rate of the neurotransmitter and
the binding of the neurotransmitter to the subsequent (postsynaptic) cell
membrane.
Simplified idea: A simplified cascade occurs roughly as follows:
• An action potential occurs in cell A.
• Ca2+ permeability of the membrane increases and calcium flows into
cell A.
• Cell A releases small vesicles filled with neurotransmitter.
• The neurotransmitter diffuses across the synaptic cleft, which is
the space between cells.
• The neurotransmitter binds to receptors on the surface of cell B.
• This binding results in a postsynaptic current (called an EPSP or
IPSP, depending on whether the current is excitatory or inhibitory,
respectively) in cell B. Such a current will lead to a change in Vm
in cell B.
• The effects from many PSPs at different locations and times are
added together into an aggregate response in cell B. As a result, a
new local membrane potential is reached.
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1.1. NEUROPHYSIOLOGY, CELLULAR NEUROSCIENCE, AND
ELECTROPHYSIOLOGY
• If this new Vm exceeds a certain threshold, an action potential will
then be generated in cell B. Hence the signal can be passed on to
another cell, and the process begins anew.
Important Neurotransmitters: There are several important neurotransmitters to know. We review some of the major players (11, pp. 127,
ch. 6).
GABA: GABA is the most widely distributed inhibitory neurotransmitter.
Glycine: Glycine is an inhibitory neurotransmitter found in the brainstem, spinal cord, and retina.
Acetylcholine: Binds to two types of cholinergic receptors:
a) Nicotinic: Found in skeletal muscle; blocked by curare.
b) Muscarinic: Found in smooth muscle and cardiac muscle.
Adrenergic: Adrenergic neurotransmitters are often found in the sympathetic nervous system. Important examples are norepinephrine,
epinephrine, and serotonin (in the brainstem).
Acetylcholine is the neurotransmitter found at the neuromuscular junction and the parasympathetic nervous system. Note that
nicotinic Ach receptors are targeted by α - bungarotoxin (snake
venom).
Re-uptake: The action of a neurotransmitter is often terminated by
the re-uptake of the chemical messenger at nerve terminals.
SSRIs and other drugs have an effect by manipulating the rate of
neurotransmitter re-uptake from the synaptic cleft.
2. Electrical synapse: At an electrical synapse, communication occurs via
direct electrical contact between cells. This contact is known as a gap
junction. The transmission is much faster than transmission through a
chemical synapse.
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Local Anesthetic
Local anesthetics work by blocking the voltage gated Na+ channels in a nerve.
When these channels cannot open, an action potential cannot be generated and
the painful stimulus is not communicated to the CNS. The first fibers to be
affected by the anesthetic are small myelinated fibers that communicate pain
and temperature. Fibers communicating touch, proprioception, and skeletal
muscle tone are affected next. Emergence from anesthesia happens in the
reverse order.
Note that during local anesthesia, the conductance is only altered
for sodium but not for potassium, calcium or chloride.
SECTION 1.2
Neuroanatomy
Neuroanatomy is a diverse and complex field. We give only a brief summary
of some fundamentals. Please refer to Gross Anatomy chapters for additional
information.
Peripheral Nervous System
The PNS is organized into a sensory portion and a motor portion, both of which
we briefly describe (11, pp. 11-12), (12, ch. 6,7). The sensory portion includes
a host of sensory neurons and structures, while the motor portion is further
divided into somatic and autonomic divisions. The PNS cellular structures are
organized into ganglia and nerves.
• Ganglia: Local collections of nerve cell bodies (soma). An example is the
dorsal root ganglia.
• Nerves: Collections of bundled axons.
Sensory Division
Ganglia with a sensory function lie near the spinal cord (dorsal root ganglia)
or brainstem (cranial nerve ganglia).
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1.2. NEUROANATOMY
Somatic Sensory Receptors: Free nerve endings and encapsulated nerve
endings receive input signals that they transmit to the CNS. Some of the more
prominent ones are listed below.
• Free Nerve Endings
– Nociceptors - located in most body tissues and sense temperature
change, pain, itch, tickle, and stretch.
– Merkel Disks - sense light pressure and discriminative touch.
– Root hair plexi - sense hair movement.
• Encapsulated Nerve Endings
– Meissner’s corpuscle - senses light pressure, touch, and vibration of
the skin.
– Krause’s corpuscle - senses touch, vibration, and cold on a mucous
membrane.
– Ruffini’s corpuscle - senses heat, and crude and persistent touch on
the skin.
– Pacinian corpuscle - senses deep pressure, high frequency vibration,
and stretch of the skin and joint capsules.
– Muscle spindles - sense mechanical stretch of the skeletal muscle
length.
– Golgi tendon receptors - sense muscle tension within the tendons.
Somatic Motor Division
The somatic motor division includes neurons that innervate skeletal muscles.
This division is responsible for most voluntary motor behavior.
Autonomic Pathways
Remember that the autonomic nervous system (ANS) is composed of neurons
within the central and peripheral nervous systems that control input to the
visceral organs, secretory glands, and smooth muscle of the cardiovascular, digestive, excretory, and thermoregulatory systems of the body. Input
from the ANS is involuntary and helps to maintain homeostasis (2).
The ANS is composed of a sequence of two neurons between the
CNS and the target tissue. The first (pre-ganglionic) neuron is
located within the brainstem or spinal cord. The second (postganglionic) neuron is located in the autonomic ganglia in the periphery (outside the CNS).
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The autonomic nervous system is separated into two divisions: the sympathetic nervous system and the parasympathetic nervous system (2).
Sympathetic nervous system: Responsible for the“fight or flight”response.
It increases heart rate and blood pressure, dilates the bronchioles, causes
vasodilation within skeletal muscles, increases blood glucose levels, and
decreases GI motility and blood flow.
• Pre-ganglionic neurons are located in the thoracic and lumbar sections of the spinal cord in the lateral horn of the grey matter. Their
axons ascend the spinal cord to enter the sympathetic chain of ganglia located along the vertebral column.
• Fibers that carry information to the head and thorax regions synapse
within the ganglia of the sympathetic chain. Post-ganglionic fibers
then continue to travel up the spinal cord to their target tissue.
• Fibers carrying information to the pelvic and abdominal viscera pass
through the sympathetic chain WITHOUT synapsing. They travel
to the autonomic plexi that surround the large branches of the abdominal aorta, where they eventually synapse. Post-ganglionic fibers
then travel a short distance from the autonomic ganglia to the target
tissue.
Autonomic ganglia include the celiac, superior mesenteric, and inferior mesenteric ganglia.
• Pre-ganglionic sympathetic fibers release acetylcholine. Post-ganglionic
sympathetic fibers release norepinephrine.
The adrenal gland is the ONLY gland that is innervated directly
by pre-ganglionic sympathetic fibers (2).
Parasympathetic nervous system: Responsible for the “rest and digest”
response. It decreases heart rate, constricts the bronchioles, increases
salivary and lacrimal gland secretions, increases GI motility, and causes
pupil constriction and accommodation (2).
• Pre-ganglionic neurons are located within the cranial nerve nuclei of
the brainstem, or in the 2nd-4th sacral segments of the spinal cord.
The brainstem parasympathetic fibers innervate structures of the
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1.2. NEUROANATOMY
head, thorax, and abdomen. The sacral spinal cord parasympathetic
fibers innervate pelvic viscera.
• Post-ganglionic neurons are located within ganglia that are very
close or adjacent to their target tissue.
• Pre- AND post-ganglionic parasympathetic fibers release acetylcholine.
Central Nervous System
The CNS is organized into several main divisions (11, ch. 1), (12, ch. 5) (see
also Figure 1.1). The cellular structures are organized into nuclei and cortex.
Nuclei: Collections of neurons with similar structure and function. They are
the CNS analog of ganglia.
Cortex: Refers to sheet-like layers of cells. Cortical cells are typically responsible for high level cognitive, sensory, and motor processing. The cortex
can be divided into the following lobes:
• Frontal Lobe - contains the premotor cortex for motor activity (planning and execution of motor tasks). The frontal lobe also contributes
significantly to general personality of the patient (reasoning, planning).
For example, a patient with a frontal lobe tumor can suddenly start making comments not at all characteristic of past behavior. The frontal lobe
Parietal Lobe
Frontal Lobe
Occipital Lobe
Calcarine Fissure
Temporal
Lobe
Midbrain
Pons
Medulla
Cerebellum
Spinal Cord
Figure 1.1: Brain and brainstem: Note in particular, the location of the occipital lobe. As the location of V1, this region serves a very important role in
visual processing. Drawing modified from SA Kinkel original.
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CHAPTER 1. NEUROSCIENCE
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contains Broca’s area, which is responsible for speech production (recall
that the frontal lobe is for motor actions).
A lesion in Broca’s area is very frustrating to the patient because
they cannot produce the words that they want to say (termed aphasia) - Broca’s lesions causes “broken speech”.
• Parietal Lobe - sensory activity and recognition. For example, a patient
with a parietal lobe lesion would be able to tell you that a pen is used
for writing, but they would be unable to call the object a pen.
• Occipital Lobe - visual processing.
• Temporal Lobe - perception, sensory recognition (auditory stimuli,
speech), and memory. The temporal lobe contains the hippocampus,
which is responsible for short term memory and spatial orientation. The
hippocampus allows the association of smell to past memories. The
temporal lobe houses Wernicke’s area, which is responsible for speech
recognition (not production).
A patient with a lesion in Wernicke’s area has perfectly sounding
speech (because Broca’s area is normal), but the words do not make
any sense - Wernicke’s lesions cause “wordy speech”.
SECTION 1.3
Divisions of the Central Nervous System
1. Spinal cord
2. Medulla
3. Pons
4. Midbrain
5. Diencephalon
6. Cerebral Hemispheres
7. Cerebellum
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1.3. DIVISIONS OF THE CENTRAL NERVOUS SYSTEM
Spinal Cord
The spinal cord consists of gray matter and white matter.
• Gray matter: Makes up the butterfly-shaped region of the spinal cord
(as seen in slices). It consists of cell bodies and unmyelinated axons.
There are both dorsal root neurons (sensory) and ventral root neurons
(motor).
• White matter: Consists of bundles of myelinated axons (called fasciculi or tracts). The white matter is sectioned into three fiber divisions:
1. Posterior funiculus
2. Lateral funiculus
3. Anterior funiculus
These tracts contain a host of both ascending and descending pathways
(see below for further discussion of specific ascending and descending
pathways).
• Spinal Nerves: There are 31 pairs of spinal nerves that innervate most
of the body. Some of these nerves are sensory, while others have motor
function.
Region
Cervical
Thoracic
Lumbar
Sacral
Coccygeal
Nerves
C1-C8
T1-T12
L1-L5
S1-S5
Innervation
1-4: neck, 5-8: upper extremities
T1-12: upper extremities
1-4: thigh, 4-5: thigh, leg,foot
1-3: thigh, leg, foot, 2-4 pelvis
1 Coccygeal nerve
The brainstem consists of the medulla, pons, and the midbrain. The medulla controls autonomic functions (heart rate,
digestion, breathing), the pons coordinates movement-related information transfer between the cerebral hemisphere and the cerebellum, and the midbrain controls an array of sensory and motor
functions, including the coordination of eye movements and visual
reflexes.
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Medulla
• The upper medulla contains the pyramids (ventral descending tracts)
and the medial lemniscus (ascending dorsal tracts). The fourth ventricle
becomes apparent at this level.
• The lower/middle medulla denotes the location of the vestibular nuclei
and the olivary nuclei, which are associated with learning and memory in
cerebellar function. The large motor tracts of the pyramids are also seen
here.
Pons
The pons relays information between the midbrain and the medulla. The pons
contains the pontine nuclei, which serve as relay stations for motion-related
information transferred between the cortex and the cerebellum. The pons is
also involved in control of respiration and sleep, and serves as the location of
cranial nerves V-VIII.
Midbrain
• The upper midbrain is the location of the superior colliculus (colliculus
= small mound), which contains motor neurons that control orientation
of the head/eyes. The oculomotor nuclei and the red nucleus (controls
movement of the arms) are also located in the midbrain. The EdingerWestphal nuclei within cranial nerve III contribute to the parasympathetic innervation of the iris.
• The lower midbrain contains the inferior colliculus, which is responsible
for reflex response (head / neck) to auditory stimuli, as well as the cranial
nerve IV nucleus, which provides innervation to the contralateral eye.
In addition, at this location we can start to make out the cerebellar
peduncles, which are tracts leading to the cerebellum.
Recall that the neural tube consists of three general areas: the forebrain, midbrain, and hindbrain. The forebrain differentiates into
two additional regions, the telencephalon and diencephalon, which
are separated by the optic chiasm in the adult brain. The telencephalon gives rise to the cerebral hemispheres. Thus, it could
be said that the forebrain gives rise to the diencephalon
and the cerebral hemispheres.
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1.3. DIVISIONS OF THE CENTRAL NERVOUS SYSTEM
Diencephalon
The diencephalon consists of the epithalamus, thalamus, subthalamus, and
hypothalamus.
Epithalamus: Contains the pineal gland, which secretes melatonin.
Thalamus: Relays sensory input to the cortex and contains nuclei for voluntary motor movements. The thalamus is a distribution center that controls
activity.
Subthalamus: Communicates with the basal ganglia to help control muscle movement.
Hypothalamus: Regulates body temperature and eating and sleeping
behavior.
A reduction in core temperature stimulates the hypothalamus and
produces shivering.
Cerebral Hemispheres
The cerebral hemispheres are responsible for high level processing related to
sensory interpretation, motor control, intelligence, and emotion. The dominant
hemisphere is more in control of understanding and processing language, intermediate and long term memory, word retrieval, and emotional stability. The
non-dominant side is more responsible for recognizing facial expressions and
vocal intonation, music, and visual learning.
Cerebellum
The cerebellum is involved in fine motor movements, posture, and balance.
While the architecture is well organized and therefore largely understood, the
number of cells making up the cerebellum is positively immense.
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CHAPTER 1. NEUROSCIENCE
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Major Neural Pathways
We now introduce several major neural pathways, paying particular attention
to the points of midline crossover. It is important to remember the basic
anatomy of ascending and descending pathways:
• Ascending pathways carry sensory information from the periphery of the
body to the brain. The generic ascending pathway consists of three neurons:
1. 1st order neuron: Soma located in the DRG
2. 2nd order neuron: Connects 1 and 3
3. 3rd order neuron: Cell body in thalamus; projects to the cortex
• Descending pathways carry motor impulses from the brain to the muscles.
Pyramidal Motor Pathway
The pyramidal motor pathway (PMP) (11, pp. 377) (8, pp. 346) begins in the
motor cortex (located in the precentral gyrus) and plays a large role in complicated voluntary movements.
• Pyramidal motor cell axons come together, forming the internal capsule
in the forebrain. These fibers then travel through the cerebral peduncles,
pons, and medulla and form the medulla pyramids.
– Note that fibers that innervate cranial nerves break away from this
path at certain regions of the middle pons and middle medulla; this
“break away” tract is called the corticobulbar tract.
• The major pathway continues until it reaches the pyramidal decussation in the caudal medulla, where most (85-90%) of the fibers cross to the
opposite side of the spinal column and become the lateral corticospinal
tract, which controls the proximal musculature (9).
• The remaining fibers make up the anterior corticospinal tract and
eventually decussate at the level of the spinal cord. These fibers control
the distal musculature (9).
A lesion above the medulla will lead to problems with motor
control on the contralateral side.
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1.3. DIVISIONS OF THE CENTRAL NERVOUS SYSTEM
Auditory and Vestibular Pathways
The cochlear and vestibular nerves combine to form the vestibulocochlear
nerve (CN VIII), which carries information to the primary auditory cortex,
the cerebellum, and the spinal cord for hearing and balance (4) (5).
Cochlear nerve: Composed of fibers that originate from the spiral ganglion
of the cochlea. These fibers travel through the organ of Corti before
exiting via the internal meatus and ending at their cell bodies located in
the cochlear nuclei of the medulla (4).
• The second order neuron axons ascend on both sides (i.e. crossed
and uncrossed fibers) of the trapezoid body to the superior olivary
complex within the brainstem. This is the first location of bilateral
auditory input.
• Fibers from the superior olivary complex (third order neurons) form
the lemniscus pathway and eventually synapse in the inferior
colliculus of the midbrain and the medial geniculate body in the
thalamus (fourth order neurons) before traveling to the primary auditory cortex.
Vestibular nerve: Composed of axons originating from the vestibular ganglia
at the distal end of the internal auditory meatus. These fibers join the
cochlear nerve of CN VIII and carry sensory information from the semicircular canals and otolith organs of the ear. Most of the fibers synapse
with 4 vestibular nuclei in the medulla and pons. The remaining fibers
directly project to the cerebellum via the inferior cerebellar peduncle to
control movements necessary for balance (5).
• Primary ascending fibers from the superior and lateral vestibular
nuclei carry sensory information to the thalamus, which then sends
fibers to the primary vestibular cortex (exact location in the cerebrum is unknown).
• Ascending fibers from the superior and medial vestibular nuclei
travel through the medial longitudinal fasciculus to the nuclei
of CN 3, 4, and 6 and help to coordinate head and eye movements.
• Ascending fibers from the inferior and medial vestibular nuclei travel
to the cerebellum to help coordinate balance.
• Descending fibers from the lateral vestibular nuclei form the lateral
vestibulospinal pathway that travels along the ipsilateral spinal
cord and helps control movements that allow us to walk upright.
• Descending fibers from the medial vestibular nuclei form the medial
vestibulospinal pathway that travels along either side to the thoracic segments of the spinal cord. This pathway helps to integrate
head movements with eye movements.
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CHAPTER 1. NEUROSCIENCE
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Cerebrum
VPS
Location of cerebrum cross section
Midbrain
Spinothalamic
Tract
Locations of Sections through Brainstem
Mid-Pons
Midbrain
Mid-Pons
Caudal Medulla
Caudal
Medulla
Synapse in
Substansia
Gelatinosa
Pain/Temp Info from
Upper Body (not face)
Anterolateral
System
Cervical
Spinal
Cord
Lumbar
Spinal
Cord
Pain/Temp Info from
Lower Body
Figure 1.2: Spinothalamic Pathway. Drawing modified from SA Kinkel original.
Spinothalamic Pathway
The spinothalamic pathway (8, pp. 482) (11, pp. 213) carries pain and temperature information from the body. Note that this overall pathway is
sometimes called the anterolateral system.
• Nerve endings in the periphery synapse at the substantia gelatinosa
within the dorsal horn of the spinal cord. Fibers that leave the substantia gelatinosa cross the midline and become the lateral spinothalamic
pathway.
• The fibers remain contralateral until they terminate in the ventral posterior thalamus (VPL) (see Figure 1.2).
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1.3. DIVISIONS OF THE CENTRAL NERVOUS SYSTEM
Cerebrum
Location of cerebrum cross section
Midbrain
Locations of Sections through Brainstem
Mid-Pons
Midbrain
Pain/Temp Info
from face
Mid-Pons
Caudal Medulla
Spinal tract
of Trigeminal
Nerve
Nucleus of spinal
tract of Trigeminal
Nerve
Caudal
Medulla
Figure 1.3: Trigeminothalamic Pathway. Drawing modified from SA Kinkel
original.
Trigeminothalamic Pathway
The trigeminothalamic pathway (TGP) (11, ch. 10) (8, ch. 23,24) carries pain
and temperature information from the face. The pathway originates in
the trigeminal ganglion cells, as well as facial pain and temperature receptors
that extend into the brainstem at the level of the pons.
• These axons descend into the medulla (forming a tract known as the
spinal tract of cranial nerve V), where they synapse onto second
order neurons in one of two sub-regions of the trigeminal complex of the
spinal cord.
• Axons from the neurons within the trigeminal complex then cross the
spinal column in the medulla and ascend contralaterally until they terminate in the thalamus (see Figure 1.3).
A lesion to the trigeminothalamic pathway above the crossover
point will result in a loss of pain or temperature information from
the contralateral side of the face.
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CHAPTER 1. NEUROSCIENCE
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Cerebrum
Location of cerebrum cross section
VPL
Midbrain
Locations of Sections through Brainstem
Mid-Pons
Midbrain
Mid-Pons
Nucleus
Gracilis
Nucleus
Cuneatus
Cuneate Tract
Mechanoreceptors
from upper body (not face)
Medial
Lemniscus
Caudal Medulla
Caudal
Medulla
Gracile Tract
Cervical
Spinal Cord
Lumbar
Spinal Cord
Mechanoreceptors
from lower body
Figure 1.4: Medial Lemniscus Pathway. Drawing modified from SA Kinkel
original.
Medial Lemniscus Pathway
The medial lemniscus pathway (8, pp. 34) (11, pp. 200) carries information
about touch, pressure, and vibration.
• Peripheral information from mechanoreceptors in the upper body travels
along the cuneate tract (located more laterally), while information from
the lower body travels along the gracilis tract (located more medially).
• These tracts enter at the cervical and lumbar regions of the spinal cord,
respectively, and ascend to the cuneatus and gracilis nuclei in the caudal
medulla, respectively.
• Axons from the secondary neurons in this region cross the midline at the
level of the medulla and become the internal arcuate fibers. These
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1.4. SPECIAL SENSORY SYSTEMS
fibers continue to travel contralaterally until terminating in the VPL (see
Figure 1.4).
A lesion in the medial lemniscus pathway below the crossover
point affects the ipsilateral side, while a lesion above the
crossover point affects the contralateral side.
SECTION 1.4
Special Sensory Systems
Vision
The eye is divided into two fluid filled segments. The anterior segment has
two chambers filled with a watery fluid called aqueous humor. The posterior
segment is filled with a thick gelatinous material called vitreous humor.
The eye can be divided into three concentric layers. The outer layer consists
of the sclera and the cornea.
• The sclera (tough connective tissue) makes up the white of the eye and
serves to maintain the size and form of the eyeball.
• On the front of the eye, the sclera gives way to a clear dome called the
cornea. It allows light to enter the eye and focus on the retina at the
back of the eye. The majority of focusing is accomplished by the cornea,
with fine tuning accomplished by the lens.
The middle layer consists of the choroid, ciliary body, and iris.
• The choroid lies beneath the sclera and contains blood vessels that help
supply the retina.
• The ciliary body contains the ciliary muscle, which alters the shape of
the lens to focus an image on the back of the eye. Zonules from the ciliary
body hold the lens in place.
• The lens focuses light on the retina. Cataracts are cloudiness within the
lens.
• The iris is located in front of the lens and consists of two pigmented
layers of epithelium, loose connective tissue, and smooth muscle. Iris
pigmentation determines eye color.
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• The pupil is not a structure but rather an opening in the iris. The iris
regulates the diameter of the pupil and the amount of light allowed to
enter the eye.
• Miosis - refers to constriction of the pupil. Miosis can be a normal
response to increased light, or secondary to drugs, pathologic conditions,
or parasympathetic stimulation.
• Mydriasis - refers to abnormal dilation of the pupil for a prolonged time.
Mydriasis can be caused by drugs or disease. Normal dilation of the pupil
occurs as a response to decreased light or sympathetic stimulation.
The innermost layer of the eye is the retina, which senses light and sends nerve
signals to the brain.
• The fovea is the center of the retina where images are focused. There is
a high density of cones in this area.
• The optic disc is the portion of the retina where the optic nerve and
blood vessels enter. There are no photoreceptors in the optic disc, making
it a blind spot in the visual field.
• Rods and cones are photoreceptor cells within the retina.
– Rods contain the photopigment rhodopsin (derived from Vitamin
A).
– Rods perceive degrees of brightness, especially in low light situations,
but lack color discrimination. Rods are effective during night vision.
– Rods are concentrated at the periphery of the retina.
– Rods have lower acuity than cones.
– Cones are primarily responsible for color vision due to three different photopigments, each sensitive to a different wavelength (red,
green, and blue).
– Cones are concentrated in the fovea of the retina.
– Cones are the main photoreceptors in bright or daylight situations.
Clinical Defects
• In emmetropia, or normal vision, the eye can focus light from both near
and far images on the retina.
• Near-sightedness, or myopia, occurs when the length of the eye is longer
than normal or the cornea+lens power is stronger than normal, resulting
in a far image that is focused in front of the retina (blurred image). Near
images are seen clearly. Myopia is treated by placing a concave (minus)
lens in front of the eye.
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1.4. SPECIAL SENSORY SYSTEMS
• Far-sightedness, or hyperopia, occurs when light focuses behind the
retina. This is caused by a cornea that is flatter or an eye that is shorter
than normal. Patients with hyperopia have trouble seeing up close, but
may also have some trouble seeing far away. Hyperopia is corrected by
placing a convex (positive) lens in front of the eye.
• Astigmatism occurs when the surface of the lens is irregular and light
gets bent erratically towards the retina. Glasses or contact lenses can
correct for astigmatism.
• Presbyopia is a hardening of the lens that occurs with aging. As the
lens loses flexibility, the eye can no longer focus sharply on near objects.
Presbyopia is often treated with bifocals.
Hearing and Equilibrium
Anatomy of the Ear
The anatomy of the ear can be divided into three parts:
• The external ear functions to gather sound waves and conduct them to
the ear drum. The external ear consists of the following:
– Pinna (Auricle) - the external part - gathers sound waves and directs
them into the ear.
– External auditory meatus - ear canal - contains hair and earwax
(cerumen). It serves as a conduit and resonator for sound waves to
reach the ear drum (tympanic membrane).
• The middle ear or tympanic cavity functions to amplify sound and
transmit soundwaves from air to a fluid. It consists of the following:
– Eustacian tube (auditory tube) - connects the middle ear with the
pharynx and serves to equalize pressure.
– Three ossicles - malleus, incus, stapes - together transmit sound
from the eardrum to the oval window. A 22 fold amplification of
sound is achieved from the eardrum to the oval window.
• The inner ear is composed of a bony labyrinth and a membranous
labyrinth. It consists of the following:
– Cochlea - a spiral shaped organ that contains the receptor (hair)
cells for hearing. Contains two membranes (vestibular and basilar)
between which lies the organ of Corti that has stereocillia (hair cells)
which bend from sound waves converting them into nerve impulses.
– Vestibule (saccule and utricle) - are associated with the sense of
balance.
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CHAPTER 1. NEUROSCIENCE
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– Semicircular canals - are concerned with equilibrium.
The vestibulocochlear nerve (CN VIII) transmits hearing and equilibrium signals to the brain.
Humans have the ability to hear sounds ranging in pitch from 20-20,000
Hz. Our greatest sensitivity range is between 1,000 and 4,000 Hz. Pitch is
a measure of the frequency of a sound wave and is measured in hertz (Hz).
Loudness is a measure of the intensity or amplitude of a sound wave and is
measured in decibels (dB). Timbre refers to the quality of the sound.
Taste
Taste is transmitted by way of 10,000 taste buds located primarily on the
tongue and roof of the mouth, but also in the pharynx. Chemicals must be
dissolved in saliva to bind to the taste receptor and be perceived. This is why
a dry mouth diminishes taste perception.
There are four primary tastes:
• Salty - caused by the presence of sodium ions (or other cations).
• Sour - caused by acid in foods.
– Causes an aversive reaction that may serve a protective role.
• Sweet - caused by organics such as sucrose, fructose, maltose, aspartame,
sucralose, etc.
• Bitter - caused by nitrogen containing compounds.
– Our averse reaction to bitter foods may also serve a protective role
as some bitter foods are also toxic.
• Umami is a fifth tasteless category associated with amino acids that enhances other tastes. Glutamate is the most common and is often in the
form of MSG.
Taste perception is a complicated matter as it does not have receptor specificity and also depends on the sense of smell. Cranial nerves VII, IX and X are
all involved in transmitting to the gustatory nucleus of the medulla, then to
the thalamus and on to the gustatory cortex.
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1.5. BLOOD SUPPLY TO BRAIN
Smell
The organ for smell, the olfactory epithelium, is located at the roof of the nasal
cavity.
• The sense of smell has many similarities to the sense of taste. In order
to be smelled, odorants must be dissolved in mucous (generated from
Bowman’s glands), and bind to specific chemoreceptors.
Axons of the olfactory nerve pass through the cribriform plate to the olfactory nerve and travel along the olfactory tract to the olfactory cortex or the
limbic system, which triggers olfactory driven behavior (such as sex). Note
that olfaction is the only sensory system that does not synapse in the thalamus
on its way to the cortex.
SECTION 1.5
Blood Supply to Brain
The blood supplying architecture in the brain is quite complex; we review only
the basic concept. Blood is supplied to the brain primarily through two sets
(left and right) of arteries: internal carotids and vertebrals. The 4 arteries
meet near the pituitary gland.
Vertebrals: Arise from the subclavian arteries and (in concert with the
medullary arteries from the aorta) provide blood to the spinal cord. The right
and left vertebrals come together to form the basilar artery (which supplies the
pons) at the brainstem. The basilar artery then joins the internal carotids at
the Circle of Willis.
Internal carotids: Arise from the common carotid arteries in the neck. The
left common carotid artery branches off of the aortic arch, while the right common carotid artery comes off of the brachiocephalic trunk. They branch into
the anterior and middle cerebral arteries, which supply blood to the forebrain.
Circle of Willis: Serves as the meeting loop for the basilar artery, the internal carotids, and the anterior and posterior communicating arteries, which
are small arteries bridging the basilar and internal carotids. The Circle of Willis
forms an arterial circle beneath the brain and distributes blood to many parts
of the brain.
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CHAPTER 1. NEUROSCIENCE
25
CIRCLE OF WILLIS
Anterior Cerebral
Artery
Anterior Communicating Artery
Optic Nerve
Middle Cerebral
Artery
Internal Carotid Artery
OPTIC CHIASM
Posterior Communicating
Artery
Posterior Cerebral Artery
Superior Cerebellar
Artery
Pontine Arteries
Anterior Cerebellar
Artery
Basilar Artery
Vertebral Artery
Figure 1.5: Circle of Willis
Ascending Pharyngeal
Artery
Superficial Temporal
Artery
Maxillary Artery
Posterior Auricular
Artery
Facial Artery
Occipital Artery
Internal Carotid
Artery
Lingual Artery
Superior Thyroid Artery
External Carotid Artery
Vertebral Artery
Right Common Carotid Artery
BrachiocephalicTrunk
Subclavian Artery
Left Common Carotid Artery
Thyrocervical Trunk
Subclavian Artery
Aortic Arch
Figure 1.6: Aortic Arch Branches
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1.5. BLOOD SUPPLY TO BRAIN
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Copyright 2014 by B & B Dental Educational Services, LLC