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ascending tracts
- always sensory and afferent
- carry info from receptors up to brain and spinal cord
- begin at a sensory receptor that is acted on by a stimulus
- AP travels to spinal cord, up to cerebrum or cerebellum
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descending tracts
- always motor and efferent
- carry info from brain out out effectors
- begin in frontal lobe motor cortex
- AP travels down cord out to muscle or gland
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tracts
- all fibers in a tract have similar origins, destinations, functions
- fibers usually decussate in tract
- if they cross, response is contralateral
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sensory pathways
- every sensory tract contains 3 neurons: FON, SON, TON
- 3 specific pathways: posterior column medial lemniscus/fasciculus gracilis, fasciculus cuneatus
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first order neuron
- begins at sensory receptor, ends at brain stem or spinal cord
- BS: sensations from mouth, teeth, eyes (via cranial nerves)
- SC: sensations from posterior head, neck, everywhere else (via spinal nerves)
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second order neuron
- begins at BS/SC, ends at thalamus
- decussation in medulla pyramids
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third order neuron
begins at thalamus, ends at cerebrum (rarely cerebellum)
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posterior column medial lemniscus pathway
- sensory pathway
- conveys sensations of touch to the cerebrum
- contralateral
- discriminative touch, stereognosis, proprioception, weight discrimination, vibration, kinesthesia
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fasciculus gracilis
- part of posterior column medial lemniscus pathway
- conveys info from lower part of body
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fasciculus cuneatus
- part of posterior column medial lemniscus pathway
- conveys info from upper part of body
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stereognosis
ability to perceive the form of an object by using the sense of touch
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proprioception
awareness of the position of the body without looking
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kinesthesia
awareness of the movement of the body without looking
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anterolateral spinothalamic pathway
- sensory pathway
- anterior: conveys sensations of itch, tickle, pressure, and crude touch
- lateral: conveys sensations of pain and temperature
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anterior posterior spinocerebellar pathway
- sensory pathway
- balance, posture, muscle tone, skilled body movements
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motor pathways
- always begin in frontal lobe of brain and end in muscle or gland
- 2 types: direct and indirect
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direct motor pathways
- aka pyramidal
- 2 neurons: upper motor neuron and lower motor neuron
- produces precise, voluntary contraction of a muscle
- almost all fibers decussate (95%)
- 3 pathways: lateral corticospinal, anterior corticospinal, corticobulbar
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upper motor neuron
- begins in motor cortex, ends in BS/SC
- synapses with lower motor neuron
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lower motor neuron
begins in BS/SC, ends in muscle or gland
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lateral corticospinal pathway
- direct motor pathway
- all fibers decussate
- voluntary movement of the arms, hands, legs, feet
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anterior corticospinal pathway
- direct motor pathway
- 90% of fibers decussate
- voluntary movement of the neck, trunk, axial skeleton
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corticobulbar pathway
- direct motor pathway
- all fibers decussate
- voluntary contraction of the muscles of the head, face, neck
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indirect motor pathways
- aka extrapyramidal
- more complicated than direct (extra neuron)
- begins in motor cortex with association neuron -> UMN -> LMN
- produces involuntary responses
- 5 tracts: tectospinal, rubrospinal, vestibulospinal, lateral reticulospinal, medial reticulospinal
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tectospinal pathway
- indrect motor pathway
- involuntary movement of the eyes and head in response to rapid auditory, visual, or tactile stimulation
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rubrospinal pathway
- indirect motor pathway
- involuntary movement of fingers and toes
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vestibulospinal pathway
- indirect motor pathway
- no fibers cross
- muscle tone, posture, balance
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lateral reticulospinal pathway
- indirect motor pathway
- controls reflexes associated with pain (withdrawal reflex)
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medial reticulospinal pathway
- indirect motor pathway
- inhibits withdrawal reflex
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sensation
a conscious or subconscious awareness of internal or external stimulus
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perception
the way a sensation is interpreted
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sensory modality
- a unique sensations (such as taste, smell, etc)
- a neuron can carry only one modality
- 2 classes: general senses, special senses
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general senses
touch, pressure, temperature
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special senses
vision, hearing, taste, smell
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free nerve endings
- sensory receptors
- exposed dendrites that respond to pain, temp, itch, tickle, light touch
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encapsulated nerve endings
- sensory receptors
- dendrites enclosed in connective tissue capsule (pressure receptors)
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separate cells
- sensory receptors
- hearing and vision receptors
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exteroceptors
receptors on body surface that respond to external stimuli
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interoceptors
- aka visceroceptors
- receptors inside body around blood vessels that respond to stretch, pressure, and pain
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proprioceptors
receptors in muscles, tendons, joints, inner ear that respond to body position, movement, balance
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thermoreceptors
temperature receptors
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photoreceptors
light receptors
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chemoreceptors
chemical receptors
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nociceptors
pain receptors
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chronic pain
long term and constant pain
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phantom pain
pain perceived in a structure that was amputated
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referred pain
deep pain projected to a surface site that shares the same spinal nerve
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sensory adaptations
- long lasting stimulus will cause a sensation to decrease until it is no longer perceived
- 2 receptors: phasic and tonic
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phasic receptors
rapidly adapting receptors (temp, smell, touch)
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tonic receptors
slowly adapting receptors (pain, chemical)
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baro receptors
- heart and blood vessels
- respond to changes in BP
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hair cell receptors
- inner ear
- maintain balance
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muscle spindle cells
- muscles
- prevent muscle damage if muscle has been stretched
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root hair plexus
- hair follicle
- makes hair sensitive to touch
-
meissner's corpuscles
touch receptors in the dermis
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pacinian corpuscles
- pressure receptors in the dermis
- respond to heavy and continuous touch
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