Lecture XXIII. Brain Pathways: Sensation & Movement
Bio 3411 Monday
November 20, 2006
Lecture XXIII. Brain Pathways: Sensation & Movement
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Neuroscience; The Brain AtlasPage(s) Feature189 Touch309 Pain184-185 Dorsal Column/Medial Leminscus – Touch & Position186-187 Spinothalmic Tract – Crude touch, pain & temperature229 Eye259 Central Visual Pathways41, 45 Brain stem showing optic pathway192-193 Visual Pathways283 Auditory Function196-197 Auditory Pathways315 Vestibular Function198-199 Vestibular Pathways342 Olfactory receptors194-195 Olfactory Pathways359 Taste buds & receptors190-191 Taste Pathways393 Overall organization of movement200-201 Direct Corticospinal tract202-203 Rubrospinal and Tectospinal tracts204-205 Reticulospinal Pathways
Readings (background only)
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OverviewSensation
Sensory TransductionReceptive FieldsAdaptationFeature DetectionMaps
MovementCST: Activation, MapForce & DirectionMotor Pathways to Spinal CordDescending Control of Movement CST: Effects of Lesions
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Charles Scott Sherrington
1857 – 1957
(Prix Nobel 1932)
Edgar Douglas Adrian
1889 – 1977
(Prix Nobel 1932)
Sensation
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The Five Senses
• Touch: e.g., fine, muscle/position, pain
• Smell: e.g., odorants, “taste”, opposite sex
• Taste: bitter, sweet, sour, salt, ?(glutamate/umami)
• Hearing/Balance: e.g., frequency & amplitude; linear & angular acceleration
• Sight: light/dark, color (chromatic)
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Domains
• Exteroception vs Interoception
• Distance vs Direct
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Sensory Transduction
• Single fiber recording (E. Adrian, Prix Nobel 1932)
• Transduction is the conversion of a relevant physical
stimulus into altered membrane potential, the
currency of the nervous system.
• Stimuli:
– radiant – light and thermal
– mechanical – pressure and sound
– chemical – molecules and ions
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General Scheme for Sensory Transduction
Receptor Potential
Conductance Change
Neural Activation
StimulusInteraction with Cell
Na+Na+ Na+Na+
Na+ Na+
Na+Na+
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Transducers
• Direct: by neurons
• Mediated: by extensions, cell filters, receptor cells, complex organs
• Code: onset, duration, intensity, change
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Touch “Receptors” in Skin
There are many
different kinds of
sensory endings in
the skin. They are
relatively more
sensitive to
movement (amplified
by the lever of a hair
(B)), vibration, light
pressure, pain,
temperature etc.
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Photoreceptors in Eye - Sight
The sensors in the eye
contain “visual pigments” that
change chemically when
exposed to light of different
colors and intensities.
Photoreceptors sensitive to
red, blue and green are
called cones (C) while those
sensitive to low light levels
are rod like (R).
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Hair Cells in Ear
The sensors in the ear are modified “touch” receptors. Sound causes the membrane on which these “hair cells” (because they have cell protrusions that look like hairs) rest to move and this causes the hairs to bend. When the hairs bend the hair cells depolarize and release transmitter to activate the sensory nerve endings.
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With respect to neurons:• Threshold (the magnitude of a stimulus sufficient to
depolarize the sensory neuron)
• Adequate Stimulus (the form of energy to which a
particular sensory cell is most sensitive - light, touch, sound,
etc.)
• Law of specific nerve energies (depolarization of neurons
in a pathway is interpreted as a particular form of stimulation
- pressure to the eyes or direct electrical activation of the
visual cortex are both interpreted as a change in light)
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Thresholds by Location
The threshold to pressure
differs over the body. The
lips and the ends of the
fingers are most sensitive.
In part, this reflects different
innervation densities (higher
in the fingers and lips).
Similar differences
innervation density
associated with high acuity
vision and speech sounds
are found in the eye and the
ear respectively.
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Thresholds by Fiber Type
The thresholds to
mechanical force (mbars)
differ for endings
associated with different
fiber sizes. Smaller forces
activate myelinated faster
conducting fibers (A - )
while greater forces are
required to activate
unmyelinated C and thin
myelinated slower
conducting fibers (A -).
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Receptive Fields
• Mainly about change
• Tuning and fidelity
• Organization - orientation, direction
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Receptive Fields - Endings
Single sensory neurons innervating
the hand have different receptive
fields depending on the kind of
ending they are associated with.
These different endings (here
named for famous guys in Italy or
Germany) respond to very
localized stimulation (Meissner &
Merkel) or to more widely placed
stimuli (Pacini and Ruffini).
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Receptive Fields - Retina (Center/Surround)
The neurons projecting from the eye to
the rest of the brain (ganglion cells)
respond stimuli in the center of their
receptive fields by increasing
depolarization (which will increase
firing) while stimuli in the periphery of
the receptive field will hyperpolarize
them (which will make the cell less
likely to fire). The cell fires best when
the stimulus covers only the central
excitatory part of the receptive field as
shown in the histogram at the bottom.
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Receptive Fields - Visual Cortex (Orientation & Length)
A neuron in the visual
cortex that responds best
to stimuli of a particular
lengths, in a particular
orientation, moving in a
particular direction at a
prefered speed. (The bar
in A is the right length.
The one in B is too long
and the cell fires less.)
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General Scheme for Neuron “Adaptation”
Sensory Neuron
Rapidly Adapting
Rapidly/Slowly Adapting
Slowly Adapting
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Maps
• Somatotopic, Visuotopic, Tonotopic, etc.
• All Levels
• Distortions ≈ innervation density
Dorsal Column – Medial Lemniscus Pathway
This pathway carries fine discriminative and active touch, body and joint position, and vibration sense.
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THE BRAIN ATLAS, 2nd ed, p. 185
Foot
Hand
Face
Body
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This is a sketch of the
left cerebral hemisphere
of a monkey brain. The
body parts to which
neurons in the cerebral
cortex of the monkey
best respond are
organized in 2
systematic maps (Sm I
and Sm II) in the parietal
lobe.
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The whiskers on the
mouse’s face are
innervated by sensory
neurons that
ultimately project to
the somatosensory
cortex. In sections
parallel to the surface
of the brain, simple
stains show a
“visible” map of the
whiskers and easily
identify groups of
cells which fire when
the homologous
whisker is touched.
Visual Pathways
These pathways convey visual information for recognizing scenes and objects, directing gaze, controlling light levels on the retina, and modulating body function with changes in the length of the day.
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THE BRAIN ATLAS, 2nd ed, p. 192 - 193
Hypothalamus
Pretectum
Superior Colliculus (Optic Tectum)
Eye
Optic nerve
Optic chiasm
Optic Tract
Lateral geniculatenucleus
Optic Radiation
Visual Cortex
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Innervation of Visual Cortex from One Eye (via LGN)
The axons to the visual cortex of
monkeys that represent one eye
are separate from those from the
other eye. A technique was used
that labeled axons from one eye.
The image above cuts through the
thickness of the visual cortex
showing patches; the one below
was reconstructed from sections
cut in the plane of the cortex
showing that the patches above
are actually stripes (ocular
dominance stripes).
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The map of the
visual world
(right) onto the
visual cortex of
the monkey
(yellow area in
the box to the
left).
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Orientation Map in the Monkey Visual Cortex (Optical Imaging)
The different
colors represent
areas responding
to bars of light in
different parts of
the visual field in
different
orientations as
indicated in the
key on the left of
the figure.
Movement
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OverviewCorticospinal Tract: Activation & Somatotopy
Activity of Motor Cortex Neurons Directs Movement:
Force & Direction
Four Other Motor Pathways to Spinal Cord
Role(s) of Descending Pathways in Movement Control
Effects of Corticospinal Tract Lesion
Corticospinal (Pyramidal) Pathway.
This is the direct connection from the cerebral cortex for control of fine movements in the face and distal extremities, e.g., buttoning a jacket or playing at trumpet.
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THE BRAIN ATLAS, pp. 34, 42
Corticospial Tract (Pyramid) at Medulla
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THE BRAIN ATLAS, 2nd ed, p. 143
Pyramidal Tracts
Cross Section Through Human Medulla
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THE BRAIN ATLAS 2nd ed, p. 201
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Normal Pyramid
Electrical stimulation of different points in motor cortex with small currents (thresholds)causesdifferent movements
Cartoons of movements evoked by direct cortical stimulation. The shading indicates the joint(s) moved.
Currents required to just provoke the above movements (threshold).
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The left hemisphere of the monkey brain - Motor (Ms) and Somatosensory (Sm) Maps
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A neuron in the motor cortex of of an awake behaving monkey fires when the wrist is extended (red arrow in diagram above). It fires more when more force is required (flexors loaded) and not at all if no contraction is needed to extend the rest (extensors loaded).
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A neuron in the motor cortex of of an awake behaving monkey fires in relation to the direction of the movement (see “tuning” curve - left).
Each small vertical line marks an action potential of the neuron.
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Sources of Descending Pathways for Movement Control
4.
3.
2.
1.
4. Medulla (Reticular Formation and Vestibular Nuclei)
3. Pons (Reticular Formation)
2. Midbrain (Red Nucleus & Superior Colliculus)
1. Forebrain (Cortex)
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THE BRAIN ATLAS, 2nd ed, p. 203
Rubrospinal Pathway.
This pathway (from the red nucleus) mediates voluntary control of movements, excepting the fine movements of the fingers, toes and mouth.
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THE BRAIN ATLAS, 2nd ed, p. 203
Tectospinal Pathway.
This pathway (from the superior colliculus) mediates head and body orientation in response to localized visual, auditory and tactile stimuli, often from the same source.
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THE BRAIN ATLAS, pp. 209, 215
Vestibulospinal Pathways.
These pathways (from the vestibular nuclei) mediates head and body orientation in response to changes in head linear and angular velocity and with respect to gravity .
Reticulospinal Pathways.
These pathways carry information from the brain stem reticular formation to the spinal cord to stabilize movement on uneven surfaces.
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NEUROSCIENCE
Descending systems from the brain influence cells in the spinal cord to create movements. The cerebellum and the basal ganglia indirectly influence movements as indicated schematically here.
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NEUROSCIENCE (1st ed), p. 319, Fig 16.8
Other cortical areas influence the initiation of movements to achieve particular goals through specific sequences, as in playing a scale on the piano. These areas are also activated when a person is instructed to think about performing the sequence without actually moving.
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THE BRAIN ATLAS 2nd ed, pp. 36, 43
Corticospial Tract (Pyramid) at Medulla
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After the pyramid was cut
(lesioned) the
opposite hand (the
right hand) was used
to try to get food
from a well but all
fingers were used.
The monkey
could not get food from the smallest
well.
The hand opposite the normal pyramid (the left hand) was used to get food from the small well by opposing the thumb and fore finger. The monkey got the food from the smallest well.
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Cut Pyramid Normal Pyramid
Electrical
stimulation of
different points
in motor cortex
with small
currents
(thresholds)
causes
different
movements
After the pyramid
was cut the
movements were
coarser and the
currents required
to produce them
were larger.
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The corticospinal (pyramidal) tract controls fine
movements particularly of the lips, fingers and toes.
When it is cut, other descending pathways such as the
rubrospinal pathway can be used for grasping
movements. These lack the precision of those activated
by the corticospinal pathway and the monkey cannot
pickup its food.
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Relative Size of
Different Brain
Parts In
Phylogeny -
The forebrain
becomes
relatively larger
as new
pathways
(functions) are
added.
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S. Ramón y Cajal, (1911) Histology of the Nervous System, Volume II.
(English translation by N. & L. Swanson, Oxford: New York pp 309-310,
1995).
Ramón y Cajal suggested
that brain pathways are
crossed to preserve the
appropriate relationships
after optical inversion by
the lens as indicated
schematically by the
arrows in the uncrossed
(left) and the crossed
(right) visual pathways.
Why are brain pathways “crossed”?
END