Transcript Lecture 5 Sensory and Motor Systems

Sensory and Motor Systems
Sensory System Principles
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Specialized receptors
Limited receptive fields
Localization of stimuli
Coding/Preprocessing
Complex neural pathways
Multiple representation in cortex
Sensory System Principles
• Specialized receptors
– Neurotransmitter receptors are replaced with
chemical- or mechanical motion- sensitive ion
gates which cause neuron depolarization, and in
most cases, action potentials.
– Two general types
• Slow adapting
– Sense stimulus for a longer period.
• Fast adapting
– Sense stimulus changes for a short period.
Sensory System Principles
• Limited receptive fields
– Each sensor responds to only part of the world.
• Localization of stimuli
– Localization by comparison of different views.
– The wider the sensor separation, the better the
localization.
Sensory System Principles
• Coding/Preprocessing
– Very little sensory information is used by the
nervous system exactly as it is transduced.
– High compression: many sensors, few neurons.
– Preprocessing and feature extraction.
– Allows the cortex to concentrate on
recognition, planning and response.
Sensory System Principles
• Complex neural pathways
– Most neuromuscular pathways are 1-2 neurons
– Most sensory pathways are 3-4 neurons
• More responses on different levels.
• Sensory systems can drive multiple centers.
• Multiple representations in cortex
– The same sense can be represented in multiple
ways in the cortex.
– Often related to the sensory feature extraction.
– Separate cortical areas for each sense.
The Senses
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Vision (sight)
Olfaction (smell)
Gustation (taste)
Audition (hearing) / Vestibular (orientation)
Touch
Proprioception (body position)
Nociception (pain)
Chemosensors (BP, glucose, acidity, etc.)
Vision
• Probably one of the most important senses.
– Vision occupies a greater percentage of the
human cerebral cortex than any other sense.
• Almost all organisms are light sensitive and
can react to light intensity and/or direction.
• Ancient sense, possibly derived from a
symbiotic bacterial inclusion.
Vision
• Limited spectral range
– Humans see electromagnetic radiation in the
380-760 nm range,
which corresponds to
peaks of solar radiation.
– Insects can see UV.
– Snakes can see IR (heat).
– Polarized vision in some
species.
Vision
• Eye anatomy
– Protective layers
• Cornea & sclera
– Deformable lens
– Fluid filled
– Photosensitive
retina, optic nerve
and blood supply
in rear
Vision
• Retina
– Rods
• Hi-sensitivity BW vision
– Cones
• Low-sensitivity color vision
– Blood supply
– Several layers of processing
neurons
• Light-sensitive cells involved in
circadian rhythms, not vision
– Tapetum in some species
Vision
• Rods
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Night vision
Slowly adapting
120 million
Not in fovea
Cylindrical outer
segment with stack of
disks w/receptors
– Only one opsin
• Rhodopsin 498 nm (3)
Vision
• Cones
– Color vision
– Fast adapting
– 6 million, mostly in
fovea
– Conical outer segment
of stacked disks
– Three opsins
• Cyanolabe 437 nm (7)
• Chlorolabe 533 nm (X)
• Erythrolabe 564 nm (X)
Vision
• Vision receptors
– 7 TM receptor with
retinal-opsin sensory
complex.
– Retinal is derived from
Vitamin A.
– Opsin is frequency
(color) sensitive.
– Light stimulation
changes retinal
conformation & 2nd
messenger system.
– Releases glutamate
•2nd messenger transduction
•Single photon can be detected!
•Rods and cells produce graded potentials, no APs!
• Presence of cGMP
opens Na+ channels
• Dark
– cGMP keeps Na+
channels open
– Cell is depolarized
• Light
– 2nd messenger
system turns cGMP
into GMP
– No cGMP, Na+
channels close
– Cell hyperpolarizes
Vision
Vision
• Circadian sensor cells
• Newly discovered light-sensitive cells in the
inner ganglion layer of the retina project
directly to the suprachiasmic nucleus
(SCN), site of the human circadian clock.
• Sensitive pigment is melanopsin - a newly
discovered opsin (chromophore, derived
from vitamin B2).
Vision
• Retinal processing
– Receptors
• Graded potentials, gap junctions, hyperpolarize in light
– Horizontal cells
• L-type (luminosity) hyperpolarize in all light
• C-type (color) hyperpolarizes only for certain colors
• Only inhibitory
– Bipolar cells
• Concentric oppositional surround response
• 1:1 bypass for foveal receptors only
Vision
• Retinal processing
– Amacrine cells
• Seem to be responsive to changes
• Gets input from midbrain in some species
– Ganglion cells
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First to generate action potentials
800,000 outputs (150:1 for rods, 8:1 for cones)
X (slow moving objects in fovea)
Y (fast moving objects in periphery)
W (orientation specific, vestigal in humans)
M (moving object detection, transient)
P (form, color, and fine detail, sustained)
- - + - - + + + - - + - -
Vision
• Anopsias
– Partial field of view
– Hemianopsia = ½ field
• Lesions before LGN
– Semi-hemianopsia =
¼ of field
• Lesions after LGN
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4, 5
Vision
• Lateral Geniculate
Nucleus (LGN) in
thalamus:
– 80% of optic nerve
– Left hemi-fields of both
eyes project to right LGN
of the thalamus and then
onto the V1 cortex.
Both the LGN and Superior Colliculus are mapped retinotopically
Vision
• Superior Colliculi
– 20% of optic nerves project to 3 (of 7) most
superficial layers of the superior colliculi.
– Other 4 layers receive projections from
auditory and somatosensory systems.
– Topographically mapped – each column
corresponds to a spatial direction.
– Outputs project to ocular muscles, tectospinal
area (head and neck movements) and pulvinar
area of the thalamus (attention).
Vision
• Cortical cell types
– Simple
• Responsive to linear edges that fall on receptive area
– Complex
• Responsive to linear edges of preferred orientation
anywhere in the visual field
– Hypercomplex
• Additionally responsive to edge length, angles,
corners, and discontinuities
Vision
• Visual cortex
– V1, Brodmann area 17,
Striate cortex, orientation
– V2, Brodmann 18
– V3, Brodmann 19 shape
– V4, color
– V5, middle temporal
(MT) and inferotemporal
(IT) cortex, motion
– V6, depth
V1
All of LGN output.
Mostly simple cells.
Ocular dominance.
Orientation sensitive.
Foveal area
Exaggerated.
V1
Orientation
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Vision
• V2, Brodmann’s area 18
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Only covers central 50% of visual field.
Mostly complex and simple cells.
Larger receptive fields than V1.
80% of cells respond to ocular disparity (i.e.
distance sensitive).
– Electrical stimulation produces fully formed
and recognizable hallucinations (people,
animals, familiar objects, etc.).
Vision
• V3 (Shape, What?)
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Very large receptive field, almost entire FOV
Binocular
Prefers static and slow moving objects
Primary shape discrimination area
• Complex and hypercomplex cells decode shapes
from edges, angles, corners, discontinuities and
lengths.
• V4 (Color)
– Color and texture processing
Vision
• V5 (Motion)
– Middle temporal cortex (MT in monkeys)
• Insensitive to color
• Prefers motion of 5-100 deg/sec either across the
field or towards/away from the eye.
• Some nerves respond to particular speeds and others
to disparities between near and far objects.
– Inferotemporal cortex
• Some respond to very specific shapes (i.e. faces) and
respond less if any component is missing (ex. nose).
• V6 (Depth)
Olfactory Neurons
• Specialized for the
detection of
airborne aromas.
• Two sets of
sensors in animals
– Smell for foods
– Smell for sex
(VNO is
degenerate in
human adults)
VNO
Olfactory Neurons
Olfactory Neurons
• Transduction
– Sensitive to over
2000 odorants, over
10% of genome!
– By 2nd messenger
– Odorant binds to Gprotein receptor
– G-protein activates
adenylyl cyclase
producing cAMP
– cAMP opens Na+
channels
• Most senses project
contralaterally.
• Gustatory and olfactory
neurons project primarily
ipsilaterally.
• Some neurons are
diverted to control centers
in the medulla which
control swallowing,
salivation, gagging,
vomiting, digestion, etc.
Somatosenses
Merkel
Ruffini
Meisner
Pacinian
Pressure
Pressure
Vibration
Vibration
Small
Large
Small
Large
Somatosenses
• Somatosensory pathway
– Cell bodies in dorsal root ganglia
– Spinothalamic tract (lateral)
• Pain and temperature
• Cross cord at entry level and ascend contralaterally
– Dorsal columns
• Precise touch
• Ascend ipsilaterally until medulla, then crosses
– Synapse in medial lemniscus of thalamus
– Somatotopic primary somatosensory cortex on
postcentral gyrus (contralateral to sensors).
Somatosenses
Spinal Reflexes
– Afferent (sensory) nerve enters cord thru dorsal root.
– Synapses either directly on efferent nerve, or an
intermediary (interneuron) in spinal cord.
– Intermediaries can cross to opposite side.
– Efferent (motor) nerve leaves ventral root.
– Status sent to brain, which can modulate response.
Spinal Reflexes
• Ex: Knee-jerk reflex
(patellar tendon reflex)
– Spinal reflexes normally act to
keep flexors and extensors in
balance (no motion).
– The doctor’s hammer stretches the
quadriceps tendon (as if knee was
flexed).
– Stretch receptors synapse in spinal
cord on quadriceps motor neurons.
– Quadriceps contract and knee
extends to counter the “flexing”
knee.
Sensory
Somatotopic
Mappings
Motor
Neural Control of Movement
• Several parts necessary for movement:
– Sensory nerves (input)
• Proprioceptors
• Touch sensors
• Vestibular sensors
– Something to control the process:
• Spinal (reflex) control of movement (involuntary)
• Brain control of movement (voluntary)
– Muscles (output)
Involuntary Motor Control
• Reflexes
– In spinal cord
– Uni- or bi-lateral
– Can be sensed and
moderated by the cortex
• Balance
– Vestibular sensors
– Vestibular nuceli (VN) of
Medulla
– Ventromedial pathways
Voluntary Motor Control
• Motivation (what?)
– Frontal lobe
• Strategy (how?)
– SMA, PMA, basal ganglia &
posterior parietal cortices
• Tactics (details)
– Pre-motor cortex and
cerebellum
• Execution
– Primary motor cortex,
brainstem, spinal cord and
muscles
Voluntary Motor Control
• Voluntary movements
– Thought originates in frontal cortex.
– Posterior parietal area knows body position.
– Descends to the basal ganglia (caudate nucleus,
putamen and globus pallidus) for focusing.
– Proceeds to premotor cortex for preplanning.
– Cerebellum coordinates multiple muscles and is
responsible for “motor memory.”
– Then onto primary motor cortex on the precentral
gyrus for final movement commands.
– Lateral descending pathways to effector muscles.
Voluntary Motor Control
Dopamine inhibits the basal ganglia from inhibiting movement.
The Cerebellum
• Contains over 50% of CNS neurons!
It must be important!
• Cerebellum coordinates complex sequences of
actions by many muscles.
• Takes an idea of a motion apart, and returns
details of force, direction and timing.
• Lesions result in ataxias. Alcohol-induced
cerebellar inhibition causes same
uncoordinated and inaccurate movements.
The Cerebellum
• Important site of motor learning
• Consider ballistic motions - they are too fast
for sensory information to be incorporated, so
they must be based on estimations.
• These estimations can only be made on the
basis of experience, which must be adjusted
and fine-tuned over time.
• “Practice makes perfect”
Descending Pathways
• Lateral pathway
– Corticospinal and
rubrospinal tracts
– Voluntary movements
controlled by the
cortex
• Ventromedial pathway
– Tectospinal,
vestibulospinal,
reticulospinal tracts
– Involuntary
movements controlled
by the brainstem
– Balance & visual
orientation
Muscles
Muscles
• Made of interleaved
bundles of myosin
and actin.
• Tropomyosin heads
on myosin ratchet
and walk down the
actin fibers,
shortening the
muscle.
Muscles
• One alpha motor neuron and all of its
associated muscle fibers are collectively
known as a motor unit.
• Motor units can only fire as a unit, fiber
contraction is all or nothing.
• Strength of muscle contraction is controlled
by the recruitment of varying numbers of
motor units.
Muscles
• Function similar to neurons.
• All efferent motor neurons all emit acetylcholine
(ACh) from their terminal buttons.
• Nicotinic ACh receptors (Na+) on muscles cause an
EPSP in the muscle unit.
• Muscle depolarization allows influx of Ca++ into
muscle and Ca++ release from sarcolemma.
• Ca++ causes tropomyosin heads to ratchet.
• The two sets of actin fibers surrounding the myosin
are drawn together.