Chapter 08
CNS
Part 2
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III. Diencephalon
Introduction
1. Part of the forebrain that includes the epithalamus,
thalamus, hypothalamus, part of the pituitary
gland, and the third ventricle
2. Surrounded by the cerebral hemispheres
Diencephalon
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Intermediate mass
Corpus callosum
Choroid plexus of third ventricle
Septum pellucidum
Genu of corpus
callosum
Splenium of corpus callosum
Thalamus
Pineal body
Anterior commissure
Corpora quadrigemina
Hypothalamus
Cortex of cerebellum
Optic chiasma
Infundibulum
Arbor vitae of cerebellum
Pituitary gland
Medulla oblongata
Mammillary body
(a)
Pons
Telencephalon
Forebrain
Diencephalon
Midbrain
Hindbrain
(b)
b: © The McGraw-Hill Company, Inc./Karl Rubin, Photographer
Thalamus and Epithalamus
Thalamus
a. Paired masses of gray matter
b. Relay center through which all sensory
information, except smell, is passed to the
cerebrum
c. Intralaminar nuclei promote a state of arousal
from sleep and alertness
Epithalamus


Contains the choroid plexus over the third ventricle
where cerebrospinal fluid is produced
Also contains the pineal gland, which secretes the
hormone melatonin that helps regulate circadian
rhythms
Hypothalamus

Very important for maintaining homeostasis and
regulating the autonomic system.
 *Contains centers for:
1)
2)
3)
4)
5)
6)
7)
Hunger/satiety and thirst
Regulation of body temperature
Regulation of sleep and wakefulness
Sexual arousal and performance
Emotions of fear, anger, pain, and pleasure
Control of the endocrine system
Controls hormone secretion from the pituitary
gland
Regions of the Hypothalamus & Functions
1) Lateral region: hunger
2) Medial region: satiety
3) Preoptic-anterior: shivering, hyperventilation,
vasodilation, sweating
4) Supraoptic: produces antidiuretic hormone, which
helps control urine formation
5) Paraventricular: produces the hormone oxytocin,
which stimulates uterine contraction and milk
ejection.
Regions of the Hypothalamus
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Paraventricular nucleus
Dorsomedial nucleus
Posterior nucleus
Anterior nucleus
Ventromedial nucleus
Preoptic area
Mammillary body
Suprachiasmatic nucleus
Supraoptic nucleus
Median eminence
Optic chiasma
Anterior pituitary
(adenohypophysis)
Posterior pituitary
(neurohypophysis)
Pituitary gland
Regulation of the Pituitary Gland
1) ADH and oxytocin are transported along the
hypothalamo-hypophyseal tract to the
posterior pituitary gland, where they are stored
until needed.
2) The hypothalamus also produces releasing
hormones and inhibiting hormones that are
transported along the adenohypophysis to the
anterior pituitary to regulate the secretion of
pituitary hormones.
Regulation of Circadian Rhythms
1) Suprachiasmatic nuclei (SCN): contain about
20,000 “clock cells” with activity that oscillates
every 24 hours – main control of circadian rhythms
2) Receive information about day length from retinal
ganglion cells via retinohypothalamic tracts
3) Controls the secretion of melatonin from the pineal
gland which is the major regulator of circadian
rhythms; secreted mainly at night
4) Circadian clock genes are found in cells of the
SCN, other brain areas, heart, liver, kidneys,
skeletal muscle, adipose tissue, and other organs
IV. Midbrain and Hindbrain
Midbrain

Also called the mesencephalon. Includes:
a. Corpora quadrigemina
1) Superior colliculi: visual reflexes
2) Inferior colliculi: auditory reflexes
b. Cerebral peduncles: ascending and
descending tracts
c. Red nucleus: connects the cerebrum and
cerebellum; involved in motor coordination;
vestigial in adults.
Midbrain

Substantia nigra: important part of the motor
circuit; part of the dopaminergic nigrostriatal
system
 Ventral tegmental area (VTA): Part of the
dopaminergic mesolimbic system that sends
neurons to the limbic system and nucleus
accumbens in the forebrain
 Involved in the behavioral reward system
and has been implicated in addiction and
psychiatric disturbances
Dopaminergic Pathways
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Caudate
nucleus (tail)
Corpus callosum
Putamen
Ventral
tegmental area
Substantia nigra
Caudate
nucleus
(head)
Locus ceruleus
Fourth
ventricle
Nucleus
accumbens
Corpus striatum
Mesolimbic
dopamine system
Nigrostriatal
dopamine system
Prefrontal
cortex
Cerebellum
Medial
forebrain
bundle
Pons
Hindbrain
1. Introduction
a. Also called the rhombencephalon
b. Composed of the metencephalon and
myelencephalon
Metencephalon


Composed of the pons and cerebellum
The pons houses sensory and motor tracts
heading from/to the spinal cord.
1) The trigeminal, abducens, facial, and
vestibulocochlear nerves arise from the pons
2) Two respiratory control centers are found here:
a) Apneustic – controls intensity of breathing
b) Pneumotaxic – regulates amount of air with each
breath
3) Contain fibers that connect the cerebrum to and
from the cerebellum
Cerebellum
1) Second largest brain structure; gray matter
outside, white matter inside
2) Receives input from proprioceptors in joints,
tendons, and muscles
3) Works with the basal nuclei and motor cortex to:
a) Contribute to coordination, precision, and
accurate timing of motor movements
b) Fine tune motor activities
Cerebellum
4) The cerebellum is needed for motor learning and
the proper timing and force required to move limbs
in a specific task.
5) The cerebellum influences motor coordination
through inhibition on the motor cortex from
Purkinje cells.
6) May have roles in acquisition of sensory data,
memory, emotion, and other higher functions
Myelencephalon


Made up of the medulla oblongata
All ascending and descending tracts between
the brain and spinal cord pass through the
medulla.
1) Motor tracts cross to opposite sides in the
pyramidal decussation.
2) Contains nuclei of cranial nerves VIII, IX, X,
XI, and XII.
Medulla Oblongata

Contains nuclei required for regulation of
breathing and cardiovascular response = vital
centers
1) Vasomotor center controls blood vessel
diameter.
2) Cardiac control center controls heart
rate.
3) Respiratory center works with areas in
the pons to control breathing.
Respiratory Control Centers in the Brain Stem
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Midbrain
Pons
Brain stem
respiratory
centers
Pneumotaxic area
Apneustic area
Rhythmicity area
Reticular formation
Medulla oblongata
Reticular Activating System (RAS)



Includes the pons and reticular formation of the
midbrain.
When activated, it maintains wakefulness and
alertness
When inhibited, it tunes out sensory stimuli to
help us fall asleep.
Structures of the RAS
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Thalamus
Hypothalamus
Pons
Medulla
Cerebellum
Brainstem
Reticular Activating System (RAS)

Arousal from sleep and alertness:
a. Neurons from the pons release ACh on the
thalamus. This enhances passing of sensory
information to the cerebral cortex.
b. Neurons from the hypothalamus and basal
forebrain release monoamines (histamine) onto
the cerebrum, further enhancing alertness.
c. Neurons from the lateral hypothalamic area
release arousing polypeptide hormones, orexin
and hypocretin-1
1) Loss of these neurons leads to narcolepsy.
Reticular Activating System (RAS)

Sleep
a. Neurons from the ventrolateral preoptic nucleus
(VLPO) of the hypothalamus release GABA into
other areas of the RAS.
b. This inhibits the RAS and allows sleep.
c. This activity is increased with depth of sleep.
 Many drugs act on the RAS to promote either
sleep or wakefulness
V. Spinal Cord Tracts
Introduction

The spinal cord is composed of white matter
surrounding a gray matter core
a. The gray matter is arranged with a left and right
dorsal horn and a left and right ventral horn.
Introduction

The white matter is composed of ascending and
descending fiber tracts.
a. Arranged into six columns called funiculi
b. Ascending tracts carry sensory impulses and
are given the prefix spino- with a suffix that
indicates the brain region it synapses on; ex –
lateral spinothalamic tract
c. Descending tracts carry motor impulses and
are given the suffix -spinal, and the prefix
indicates the brain region they come from; ex –
anterior corticospinal tract
Ascending Tracts


Convey sensory information from receptors in the
skin, muscles, joints, and organs
Crossover of tracts (decussation) may occur in
the spinal cord or in the medulla. This means that
the origin of the input and the brain area are
contralateral.
Ascending Tracts
Ascending Tracts
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Postcentral gyrus
Axons of
third-order
neurons
Thalamus
Cerebral
cortex
Medial lemniscal tract
(axons of second-order
neurons)
Medulla oblongata
Fasciculus cuneatus
(axons of first-order
sensory neurons)
Lateral spinothalamic tract
(axons of second-order neurons)
Joint stretch receptor
(proprioceptor)
Pain receptor
Spinal cord
Axons of first-order
neurons (not part of
spinothalamic tract)
Fasciculus gracilis
(axons of first-order
sensory neurons)
Temperature
receptor
(a)
Touch receptor
(b)
Descending Tracts

Two major groups:
a. Corticospinal or pyramidal: descend directly without
synaptic interruption from the cerebral cortex to the
spinal cord
1) Cell bodies of these neurons are located in the
precentral gyrus (Primary motor cortex) and superior
frontal gyrus (Supplementary motor complex).
2) 80−90% cross in the pyramidal decussation and
descend as lateral corticospinal tracts.
3) 10-20% descend as anterior corticospinal tracts
and cross in the spinal cord at the level that the
nerves leave the cord.
Descending Motor Tracts
Descending Pyramidal Tracts
Descending Tracts

Extrapyramidal tracts: originate in the brain
stem and are controlled by the motor circuits of
the corpus striatum, substantia nigra, and
thalamus
 Symptoms of Parkinson disease reveal the
importance of these tracts for initiating body
movements, maintaining posture, and
controlling facial expression.
Extrapyramidal Tracts

Reticulospinal tracts are the major descending
extrapyramidal tracts.
 These originate in the reticular formation of the
brain stem. This area is stimulated or inhibited
by neurons from the cerebellum, basal nuclei,
and cerebrum.
 Vestibulospinal tracts arise from the vestibular
nuclei in the medulla oblongata
 Rubrospinal tracts arise from the red nuclei.
Higher Motor Neuron Control of Skeletal Muscles
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Cerebral cortex
Cerebellum
Red nucleus
Vestibular nucleus
Lower motor neurons
Vestibulospinal
tract
Brain stem
reticular
formation
Rubrospinal
tract
Basal nuclei
Reticulospinal
tract
Pyramidal (corticospinal) tracts
Thalamus
VI. Cranial and Spinal Nerves
Cranial Nerves
1. Part of the PNS
2. Nerves that arise directly from nuclei in the
brainstem
3. Twelve pairs
4. Most are mixed nerves with both sensory and
motor neurons (somatic and parasympathetic)
5. Those associated with vision, olfaction, and
hearing are sensory only and have their cell bodies
in ganglia located near the sensory organ.
Cranial Nerves
Spinal Nerves
1. Part of the PNS
2. Nerves that arise directly from the spinal cord
3. 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral,
1 coccygeal
4. All are mixed nerves that separate near the spinal
cord into a dorsal root carrying sensory fibers and
a ventral root carrying motor fibers.
a. The dorsal root ganglion houses the sensory
neuron cell bodies.
b. Motor neuron cell bodies are in the ventral gray
horns
Distribution of Spinal Nerves
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Cranial nerves
(12 pairs)
Cervical plexus
Brachial plexus
Cervical
(8 pairs)
Thoracic
(12 pairs)
Spinal
nerves
Lumbar plexus
Sacral plexus
Some peripheral
nerves:
Ulnar
Median
Radial
Femoral
Lateral femoral
cutaneous
Sciatic
Lumbar
(5 pairs)
Sacral
(5 pairs)
Coccygeal
(1 pair)
Reflex arc

Unconscious motor response to a sensory
stimulus
 Parts of an arc
a. Sensory receptor
b. Sensory neuron
c. Association neuron in CNS
d. Motor neuron
e. Effector – muscle or gland that responds
Types of arcs


Somatic reflex – effectors are skeletal muscles
Autonomic reflex – effectors are smooth muscle,
cardiac muscle, or glands
Reflex Arch
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Upper motor neuron
(association neuron
in brain)
Dorsal root
ganglion
Cell body
of neuron
Dorsal
root
Sensory
neuron
Somatic
motor neuron
Spinal
nerve
Association neuron
Spinal cord
Ventral
root
Skeletal muscle