Neurotransmition,
Neuroanatomy and Functional Localization
I. EARLY HISTORY
1.
Phrenology:
Assumed that the brain is made of functionally discrete modules
2.
Early
psychosurgery: leucotomy, frontal lobotomy.
Moniz. Render aggravated, aggressive,
anxious patients calm, quiet. Also
deadened, flat, lifeless. 1940s – 20,000+ lobotomies
3.
First effective
psychotropic drugs in the 1960s – psychosurgery largely abandoned. Suspicion
over brain surgery as a treatment for mental illness
4.
Increasing
knowledge of both mental illness and normal experience
a.
For example, no
single pain center.
b.
Pain comes from
activation of a constellation of areas associated with attention and
emotion. More stress, more pain. Religious experience of spiritual transcendence,
mystical presence, reproduced by stimulating certain parts of the temporal
lobe.
II.
NEUROTRANSMITION
Brain Cells: [see
image of neuron]
There are two main types
of brain cells: neurons and neuroglia.
1.
Neurons are
responsible for the transmission and analysis of all electrochemical
communication within the brain and other parts of the nervous system. Each
neuron is composed of a cell body called a soma, a major fiber called an axon,
and a system of branches called dendrites. Axons, also called nerve fibers,
convey electrical signals away from the soma and can be up to 1 m (3.3 ft) in
length.
2.
Most axons are
covered with a protective sheath of myelin, a
substance made of fats and protein, which insulates the axon. Myelinated axons
conduct neuronal signals faster than do unmyelinated axons. Dendrites convey
electrical signals toward the soma, are shorter than axons, and are usually
multiple and branching.
3.
Neuroglial cells are twice
as numerous as neurons and account for half of the brain's weight. Neuroglia
(from glia, Greek for “glue”) provide structural support to the neurons.
Neuroglial cells also form myelin, they guide developing neurons, take up
chemicals involved in cell-to-cell communication, and contribute to the
maintenance of the environment around neurons.
Synapse: [see image of synapse]
1.
Synaptic
Sequence: Five Step Process
a.
A
message-carrying impulse travels from one end of a nerve cell to the other by
means of an electrical impulse.
b.
When it reaches
the terminal end of a nerve cell, the impulse triggers tiny sacs called
presynaptic vessicles to release their contents, chemical messengers called
neurotransmitters.
c.
The
neurotransmitters float across the synapse, or gap between adjacent nerve
cells.
d.
When they reach
the neighboring nerve cell, the neurotransmitters fit into specialized receptor
sites much as a key fits into a lock, causing that nerve cell to “fire,” or
generate an electric message-carrying impulse.
e.
As the message
continues through the nervous system, the presynaptic cell absorbs the excess
neurotransmitters, and repackages them in presynaptic vessicles in a process
called neurotransmitter reuptake.
III. NEUROANATOMY
“The human brain is as big as
a coconut, the shape of a walnut, the color of uncooked liver and the
consistency of chilled butter.” 3 lbs.,
100 billion neurons, glial cells
Cerebrum: [see image of ]
Most high-level brain
functions take place in the cerebrum. Its two large hemispheres make up
approximately 85 percent of the brain's weight.
1.
The exterior surface of the cerebrum, the cerebral cortex,
is a convoluted, or folded, grayish layer of cell bodies known as the gray
matter. The gray matter covers an underlying mass of fibers called the white
matter.
2.
The convolutions are made up of ridge-like bulges, known as gyri,
separated by small grooves called sulci and
larger grooves called fissures. Approximately
two-thirds of the cortical surface is hidden in the folds of the sulci. The
extensive convolutions enable a very large surface area of brain cortex—about
16 ft2 in an adult—to fit within the cranium. The pattern of these
convolutions is similar, although not identical, in all humans.
The two cerebral hemispheres are partially separated from each
other by a deep fold known as the longitudinal
fissure.
3.
Each half of the cerebrum is divided into four major
lobes: the frontal, parietal,
temporal, and occipital lobes. The
insula, a fifth lobe, lies within the deep folds of the lateral sulcus.
4.
Each
lobe processes its own group of things:
a.
Frontal lobe: the largest of the five and consists of all the
cortex in front of the central sulcus. Broca's
area, a part of the cortex related to speech, is located in the frontal
lobe. Concerned with thinking,
conceptualization, planning (integrated functions). Conscious appreciation of emotion.
b.
Parietal lobe: consists of the cortex behind the central sulcus to a
sulcus near the back of the cerebrum known as the parieto-occipital sulcus.
Parietal functions are connected with movement, calculation and some
recognition.
c.
Occipital lobe: at the back.
Almost entirely visual processing areas.
d.
Temporal lobe: The temporal lobe is to the side of and below the
lateral sulcus. Wernicke's area, a part of the cortex related to the understanding
of language, is located in the temporal lobe.
Temporal lobe deals with sound, speech comprehension (usually on left
only), some aspects of memory.
5.
Many other areas of the
cerebral cortex have also been mapped according to their specific functions, such
as vision, hearing, speech, emotions, language, and other aspects of
perceiving, thinking, and remembering.
a.
Cortical regions
known as associative cortex are responsible
for integrating multiple inputs, processing the information, and carrying out
complex responses.
b.
Communication
between the two hemispheres is through several concentrated bundles of axons,
called commissures, the largest of which is the corpus
callosum. [see image of right hemisphere]
Limbic System: Mammalian Brain [see image of
limbic system]
Beneath the cortex. Contraption of tubes, chambers, lumps and
blobs. Each little unit has its own
function, all are interconnected by criss-crossing axons. The limbic system is a group of brain structures that play a role in
emotion, memory, and motivation.
1.
For example,
electrical stimulation of the amygdala in
laboratory animals can provoke fear, anger, and aggression.
2.
The thalamus and the hypothalamus
lie underneath the cerebrum and connect it to the brain stem.
a.
The thalamus
consists of two rounded masses of gray tissue lying within the middle of the
brain, between the two cerebral hemispheres.
b.
The thalamus is
the main relay station for incoming sensory signals to the cerebral cortex and
for outgoing motor signals from it. All sensory input to the brain, except that
of the sense of smell, connects to individual nuclei of the thalamus.
c.
The hypothalamus lies
beneath the thalamus on the midline at the base of the brain. It regulates or
is involved directly in the control of many of the body's vital drives and
activities, such as eating, drinking, temperature regulation, sleep, emotional
behavior, and sexual activity. It also controls the function of internal body
organs by means of the autonomic nervous system, interacts closely with the
pituitary gland, and helps coordinate activities of the brain stem.
1.
Brain injury can
cause deregulation of limbic functions.
Four Fs
Brain Stem and Cerebellum: Reptilian
Brain [see image of brain stem]
3.
The
brain stem, shown here in cross section,
is the lowest part of the brain. It serves as the path for messages traveling
between the upper brain and spinal cord but is also the seat of basic and vital
functions such as breathing, blood pressure, and heart rate, as well as
reflexes like eye movement and vomiting.
4.
The brain stem
has three main parts: the medulla, pons, and midbrain. A canal runs longitudinally through
these structures carrying cerebrospinal fluid. Also distributed along its
length is a network of cells, referred to as the reticular formation, that
governs the state of alertness.
The Cerebellum is at the back of the main body of the brain, tucked
under its tail, partly fused to it.
The cerebellum coordinates voluntary movements by fine-tuning
commands from the motor cortex in the cerebrum. The cerebellum also maintains
posture and balance by controlling muscle tone and sensing the position of the
limbs. All motor activity depends on the cerebellum.
Neuroscience shining light on human
qualities associated with the soul:
1.
Functional
localization and will: Research by Chris
Frith in