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Lecture XXIII. Brain Pathways: Sensation & Movement: 

Lecture XXIII. Brain Pathways: Sensation & Movement Bio 3411 Monday November 20, 2006


Neuroscience; The Brain Atlas Page(s) Feature 189 Touch 309 Pain 184-185 Dorsal Column/Medial Leminscus – Touch & Position 186-187 Spinothalmic Tract – Crude touch, pain & temperature 229 Eye 259 Central Visual Pathways 41, 45 Brain stem showing optic pathway 192-193 Visual Pathways 283 Auditory Function 196-197 Auditory Pathways 315 Vestibular Function 198-199 Vestibular Pathways 342 Olfactory receptors 194-195 Olfactory Pathways 359 Taste buds & receptors 190-191 Taste Pathways 393 Overall organization of movement 200-201 Direct Corticospinal tract 202-203 Rubrospinal and Tectospinal tracts 204-205 Reticulospinal Pathways Readings (background only)


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



The Five Senses: 

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)


Domains Exteroception vs Interoception Distance vs Direct

Sensory Transduction: 

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


General Scheme for Sensory Transduction Receptor Potential Conductance Change Neural Activation Stimulus Interaction with Cell


Transducers Direct: by neurons Mediated: by extensions, cell filters, receptor cells, complex organs Code: onset, duration, intensity, change


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.


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).


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.

With respect to neurons:: 

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)


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.


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).

Receptive Fields: 

Receptive Fields Mainly about change Tuning and fidelity Organization - orientation, direction


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).


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.


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.)


General Scheme for Neuron “Adaptation” Sensory Neuron Rapidly Adapting Rapidly/ Slowly Adapting Slowly Adapting


Maps Somatotopic, Visuotopic, Tonotopic, etc. All Levels Distortions ≈ innervation density

Dorsal Column – Medial Lemniscus Pathway : 

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 Foot Hand Face Body


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.


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 : 

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


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).


The map of the visual world (right) onto the visual cortex of the monkey (yellow area in the box to the left).


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.




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


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


THE BRAIN ATLAS, 2nd ed, p. 143 Pyramidal Tracts Cross Section Through Human Medulla


THE BRAIN ATLAS 2nd ed, p. 201


Normal Pyramid Electrical stimulation of different points in motor cortex with small currents (thresholds) causes different 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).


The left hemisphere of the monkey brain - Motor (Ms) and Somatosensory (Sm) Maps


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).


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.


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)


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.


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.


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.


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.


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.


THE BRAIN ATLAS 2nd ed, pp. 36, 43 Corticospial Tract (Pyramid) at Medulla


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.


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.


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.


Relative Size of Different Brain Parts In Phylogeny - The forebrain becomes relatively larger as new pathways (functions) are added.


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”?



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