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Structure-Function II
Brainstem and Cerebellum
Reading:
BCP Chapter 7/7A
Major Divisions of the Brain
Cerebrum
Brainstem
Divisions are based on developmental origins; they do not sub-serve discrete
functions.
Diencephalon 1
The diencephalon consists of a
set of paired structures at the
base of the forebrain and lateral
to the third ventricle: thalamus,
hypothalamus, epithalamus
(pineal gland) and subthalamus.
Epithalamus
Subthalamus
The thalamus is the largest
structure, with the hypothalamus
located below its anterior aspect.
Both lobes of the thalamus, in
the human, are about the size
and shape of a walnut (3 cm in
length, 2 cm in width and height).
Each part of the hypothalamus is
the size of an almond.
Lateral
ventricle
Thalamus
Third
ventricle
Hypothalamus
Diencephalon 2: Thalamus
The thalamus comprises
many distinct nuclei,
separated, in part, by
lamellae (fibers; white
areas).
The thalamus is richly and
reciprocally connected to the
cortex; contributes fibers to
the internal capsule
Functions:
• Process/relay somatic
nervous system info to
cerebral cortex
• Sleep/wake states
• Consciousness
Thalamus
Diencephalon 3: Hypothalamus
The hypothalamus, like the
thalamus, is comprised of many
distinct (and some not-so-distinct)
nuclei.
The hypothalamus performs many
“primitive” functions. In particular,
in response to the needs of the
organism, it:
•
•
•
motivates the search for food,
drink, sleep, temperature, mates
controls activities of the autonomic
nervous system
links the nervous system to the
endocrine system by synthesizing
and secreting hormones (some into
systemic circulation, others to
stimulate/inhibit secretion of
hormones from the pituitary)
Mesencephalon 1
The mesencephalon or midbrain
consists of a set of paired
structures that surround the
cerebral aqueduct.
Two divisions:
• the tectum (roof) is the dorsal
surface
• the tegmentum is ventral to the
tectum
The tegmentum (also called the
cerebral peduncles) initiates the
division of the forebrain into two
separate hemispheres.
Mesencephalon 2: Tectum
In mammals, the tectum is composed of two pairs of bumps, called the colliculi
(little hills). The top pair, called the superior colliculi, have a visual-motor function,
specifically to direct the body’s orientation towards or away from a visual stimulus.
The lower pair, called the inferior colliculi, are part of the auditory system (will
send output to MGB of thalamus).
Mesencephalon 3: Tegmentum
The tegmentum includes three
“colorful” nuclei:
•
•
•
periaqueductal gray (cell bodies)
– pain modulation
– defensive behavior
red nucleus (iron)
– motor coordination
substantia nigra (melanin)
– movement selection (part of
basal ganglia)
and two major fiber tracts
•
•
Medial lemniscus – somatosensory
fibers ascending to VPN of
thalamus
Pyramidal tract – motor axons from
primary motor cortex descending
towards spinal cord
Pyramidal
tract fibers
Ventral
tegmental
area
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Metencephalon 1
The metencephalon is the
higher/rostral half of the
hindbrain and contains two
major structures that border
the fourth ventricle:
• the pons
• the cerebellum
The pons is part of the brain
stem, and in humans lies
between the medulla
oblongata (below) and the
midbrain (above) and in front
of the cerebellum.
Spinal cord
Mid brain
Metencephalon 2: Pons
The pons can be broadly
divided into two parts: the
dorsal and ventral pons.
The dorsal pons contains
the nuclei for four cranial
nerves (V-VIII), which serve
both sensory and motor
functions.
The ventral pons contains
pontine nuclei scattered
among, and receiving input
from, the descending fibers
of the pyramidal tract. The
pontine nuclei then project
their axons into the
cerebellum via the middle
cerebellar peduncle.
Metencephalon: The Cerebellum
The cerebellum (Latin for “little brain”)
consists of a single, tightly-folded layer
of cortex, with several deep nuclei
embedded in the white matter below.
The cerebellar peduncles connect the
cerebellum to the brain stem.
•
•
•
Superior – primary output (VL thalamus;
red nucleus)
Middle – input from the contralateral
motor cortex via pontine nuclei
Inferior – input from ipsilateral inferior
olive (which in turn receives somatosensory information from the spinal cord
via the dorsal column nuclei)
The cerebellum plays an important role
in movement coordination, precision
and timing. Newer findings suggest
roles related to attention, language,
and emotional responses.
Myelencephalon 1
The myelencephalon (medulla
oblongata) is located in the
hindbrain between the spinal
cord and the pons. The ventral
medulla surrounds the central
canal; the dorsal medulla is
anterior to the fourth ventricle.
The medulla connects the higher
levels of the brain to the spinal
cord, and is responsible for
regulating a variety of basic
functions of the autonomous
nervous system (e.g., heart rate).
It is evolutionarily the oldest
portion of the brain.
Spinal cord
Myelencephalon 2: Nuclei
The medulla is responsible for many functions. For example, it contains cardiac,
respiratory and vasomotor centers which regulate basic autonomic (involuntary)
functions such as heart rate, respiration and blood pressure. In addition, the medulla
contains the dorsal column nuclei (cuneatus and gracilis; touch), the solitary nucleus
(taste), the cochlear nucleus and the ventral part of the superior olive (hearing), and
the inferior olivary nucleus (motor coordination).
Superior
olive
Cochlear
nucleus
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Myelencephalon 3: Tracts
Dorsal
column
nuclei
The medulla is a complex region
of the brain containing many
fiber tracts and nuclei.
Spinal
canal
Two major fiber tracts include:
Medial lemniscus
Ascending (somato)sensory fibers in
the dorsal aspect of the spinal cord
synapse in the dorsal column nuclei.
Axons from these nuclei cross the
brain stem (decussate) and ascend
to the thalamus via the medial
lemniscus.
Medullary Pyramids
Descending motor axons from
primary motor cortex. Most will
decussate and send fibers down the
dorsolateral zone in the spinal cord.
Medial
lemniscus
Medullary
pyramids
Ascending
tracts
Nuclei
Descending
tracts
The Reticular Formation
The reticular formation is not a single structure but a complex network of about
100 tiny nuclei that span the lower brainstem. It is involved in a variety of
behaviors including sleep-wake transitions and arousal/attention (the reticular
activating system; RAS), voluntary motor control, reward and addiction (ventral
tegmental area; VTA), and mood (locus coeruleus, Raphe nuclei).
RAS
Ventral tegmental area
Locus coeruleus
Raphe nuclei
motor
StructureFunction: Brainstem/Cerebellum