lecture xxiii. brain pathways: sensation & movement bio 3411 monday november 20, 2006

Lecture XXIII. Brain Pathways: Sensation & Movement

Bio 3411 Monday

November 20, 2006


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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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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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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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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”?

lecture xxiii. brain pathways: sensation & movement bio 3411 monday november 20, 2006

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