Download Why are brain pathways

Lecture XXIII. Brain Pathways:
Sensation & Movement
Bio 3411
Monday
November 20, 2006
Readings (background only)
Neuroscience; The Brain Atlas
Page(s)
189
309
184-185
186-187
229
259
41, 45
192-193
283
196-197
315
198-199
342
194-195
359
190-191
393
200-201
202-203
204-205
Feature
Touch
Pain
Dorsal Column/Medial Leminscus – Touch & Position
Spinothalmic Tract – Crude touch,
pain & temperature
Eye
Central Visual Pathways
Brain stem showing optic pathway
Visual Pathways
Auditory Function
Auditory Pathways
Vestibular Function
Vestibular Pathways
Olfactory receptors
Olfactory Pathways
Taste buds & receptors
Taste Pathways
Overall organization of movement
Direct Corticospinal tract
Rubrospinal and Tectospinal tracts
Reticulospinal Pathways
Lecture XXIII. Brain Pathways:
Sensation & Movement
2
Overview
Sensation
Sensory Transduction
Receptive Fields
Adaptation
Feature Detection
Maps
Movement
CST: Activation, Map
Force & Direction
Motor Pathways to Spinal Cord
Descending Control of Movement
CST: Effects of Lesions
Lecture XXIII. Brain Pathways:
Sensation & Movement
3
Charles Scott Sherrington
Edgar Douglas Adrian
1857 – 1957
1889 – 1977
(Prix Nobel 1932)
(Prix Nobel 1932)
Lecture XXIII. Brain Pathways:
Sensation & Movement
4
Sensation
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)
Lecture XXIII. Brain Pathways:
Sensation & Movement
6
Domains
• Exteroception vs Interoception
• Distance vs Direct
Lecture XXIII. Brain Pathways:
Sensation & Movement
7
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
Lecture XXIII. Brain Pathways:
Sensation & Movement
8
General Scheme for Sensory Transduction
Interaction
with Cell
Conductance
Change
Stimulus
Na+ Na+Na+ +
Na
Na+
Na+
Na+
Na+
Receptor
Potential
Neural
Activation
Lecture XXIII. Brain Pathways:
Sensation & Movement
9
Transducers
• Direct: by neurons
• Mediated: by extensions, cell filters,
receptor cells, complex organs
• Code: onset, duration, intensity, change
Lecture XXIII. Brain Pathways:
Sensation & Movement
10
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
11
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).
Lecture XXIII. Brain Pathways:
Sensation & Movement
12
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
13
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)
Lecture XXIII. Brain Pathways:
Sensation & Movement
14
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
15
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 - b)
while greater forces are
required to activate
unmyelinated C and thin
myelinated slower
conducting fibers (A -d).
Lecture XXIII. Brain Pathways:
Sensation & Movement
16
Receptive Fields
• Mainly about change
• Tuning and fidelity
• Organization - orientation, direction
Lecture XXIII. Brain Pathways:
Sensation & Movement
17
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).
Lecture XXIII. Brain Pathways:
Sensation & Movement
18
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
19
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.)
Lecture XXIII. Brain Pathways:
Sensation & Movement
20
General Scheme for Neuron “Adaptation”
Sensory
Neuron
Rapidly
Adapting
Rapidly/
Slowly
Adapting
Slowly
Adapting
Lecture XXIII. Brain Pathways:
Sensation & Movement
21
Maps
• Somatotopic, Visuotopic, Tonotopic, etc.
• All Levels
• Distortions ≈ innervation density
Lecture XXIII. Brain Pathways:
Sensation & Movement
22
Dorsal Column – Medial Lemniscus
Pathway
This pathway carries fine discriminative and
active touch, body and joint position, and
vibration sense.
THE BRAIN ATLAS, 2nd ed, p. 185
Face
Hand
Body
Foot
Lecture XXIII. Brain Pathways:
Sensation & Movement
24
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
25
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
26
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.
THE BRAIN ATLAS, 2nd ed, p. 192 - 193
Eye
Optic nerve
Hypothalamus
Optic chiasm
Optic Tract
Pretectum
Lateral geniculate
nucleus
Optic Radiation
Superior Colliculus
(Optic Tectum)
Visual Cortex
Lecture XXIII. Brain Pathways:
Sensation & Movement
28
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).
Lecture XXIII. Brain Pathways:
Sensation & Movement
29
The map of the
visual world
(right) onto the
visual cortex of
the monkey
(yellow area in
the box to the
left).
Lecture XXIII. Brain Pathways:
Sensation & Movement
30
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
31
Movement
Overview
Corticospinal 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
Lecture XXIII. Brain Pathways:
Sensation & Movement
33
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.
THE BRAIN ATLAS, pp. 34, 42
Corticospial Tract
(Pyramid) at Medulla
Lecture XXIII. Brain Pathways:
Sensation & Movement
35
THE BRAIN ATLAS, 2nd ed, p. 143
Cross Section
Through Human
Medulla
Pyramidal Tracts
Lecture XXIII. Brain Pathways:
Sensation & Movement
36
THE BRAIN ATLAS 2nd ed, p. 201
Lecture XXIII. Brain Pathways:
Sensation & Movement
37
Normal Pyramid
Cartoons of
movements evoked
by direct cortical
stimulation. The
shading indicates
the joint(s) moved.
Electrical
stimulation of
different points
in motor cortex
with small
currents
(thresholds)
causes
different
movements
Currents required to
just provoke the
above movements
(threshold).
Lecture XXIII. Brain Pathways:
Sensation & Movement
38
The left
hemisphere of
the monkey
brain - Motor
(Ms) and
Somatosensory
(Sm) Maps
Lecture XXIII. Brain Pathways:
Sensation & Movement
39
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).
Lecture XXIII. Brain Pathways:
Sensation & Movement
40
Each small vertical
line marks an action
potential of the
neuron.
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).
Lecture XXIII. Brain Pathways:
Sensation & Movement
41
Sources of
Descending
Pathways for
Movement Control
1.
1. Forebrain
(Cortex)
2. Midbrain (Red
Nucleus & Superior
Colliculus)
2.
3.
3. Pons (Reticular
Formation)
4. Medulla
(Reticular
Formation and
Vestibular Nuclei)
4.
Lecture XXIII. Brain Pathways:
Sensation & Movement
42
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
43
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
44
THE BRAIN ATLAS, pp. 209, 215
Vestibulospinal
Pathways.
Reticulospinal
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
.
These pathways carry
information from the
brain stem reticular
formation to the spinal
cord to stabilize
movement on uneven
surfaces.
Lecture XXIII. Brain Pathways:
Sensation & Movement
45
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
46
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.
Lecture XXIII. Brain Pathways:
47
Sensation & Movement
THE BRAIN ATLAS 2nd ed, pp. 36, 43
Corticospial Tract
(Pyramid) at Medulla
Lecture XXIII. Brain Pathways:
Sensation & Movement
48
Lecture XXIII. Brain Pathways:
Sensation & Movement
49
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
50
Cut Pyramid
Normal Pyramid
Electrical
After the pyramid
stimulation of
was cut the
different points
movements were
in motor cortex
with small
coarser and the
currents
currents required
(thresholds)
causes
to produce them
different
were larger.
movements
Lecture XXIII. Brain Pathways:
Sensation & Movement
51
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.
Lecture XXIII. Brain Pathways:
Sensation & Movement
52
Relative Size of
Different Brain
Parts In
Phylogeny The forebrain
becomes
relatively larger
as new
pathways
(functions) are
added.
Lecture XXIII. Brain Pathways:
Sensation & Movement
53
Why are brain pathways “crossed”?
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.
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).
Lecture XXIII. Brain Pathways:
Sensation & Movement
54
END