BIO 448 - Mechanosensory tracks

transduction mechanisms/pathways

molecular part of the sensory system

light touch

depolarize pressure-sensitive sensory neurons

acid

depolarizes pain-sensitive sensory neurons

vibrations

depolarize hair cells in the cochlea

photons

hyperpolarize photoreceptors

sensory neurons

cell body sits in dorsal root ganglia

axons enter dorsal spnal cord in area called dorsal hoirn

motor neurons

cell body sits in ventral horn

axons exit through ventral spinal cord

dorsal horn

sensory info enters the ___

ventral horn

motor info exit through the __

dorsal column

sometimes sensory info goes up the ___ which is an axon tract going up spinal cord to the brain, or they stay in the ventral horn

pseudounipolar

sensory neurons are ___ and are afferent neurons aka they bring info TO CNS

carry information from the skin/muscles/special senses to central neural circuits

trigeminal ganglia

sensory receptors for the face/neck up sensory neurons

location of cell bodies for somatic sensory neurons delivering info from head and neck travel through the trigeminal nerve

dorsal root ganglia

location of cell bodies for somatic sensory neurons delivering info from trunk, arms and legs, travel through the spinal nerves

mechanosensory neurons

carrying light touch info

central process synapses with neurons in the brainstem (medulla)

pain and temperature neurons

central process synapses with neurons in the spinal cord

bypass brain

dermatome

specific territory innervated by spinal nerve and specific dorsal root ganglion

deep encapsuled receptor

in the dermis/subcutaneous layer

• Pacinian corpuscle

• Ruffini corpuscle

superficial encapsuled receptors

still in dermis

• Meissner corpuscle

  • Merkel cell

pain and temperature

free nerve endings in the epidermis relate info about __

mechanoreceptors

nerve endings encapsuled by specialized receptor cells helping tune the nerve ending to a particular feature of stimulation

• Meissner corpuscle

• Merkel-cell

• Ruffini corpuscle

• Pacinian corpuscle

deformation

mechanoreceptors work by ___ of the membrane, the stretching open the sodium channels and depolarizes the nerve ending encapsuled in specialized cells.

receptive fields

area of the skin that, when stimulated produces or changes responses in the neuron

__ of one neuron can overlap with the __ of another neuron

area monitored by a single receptor cell

within

each neuron has different sensitivities at different locations __ the receptive field

middle, borders

neuron activated the most in the ___ of the receptive field, and activated the least at the ___ of the receptive field

larger

the __ the receptive field, the more difficult to localize a stimulus

2 point discrimination

minimum distance required to perceive 2 simultaneously applied stimuli as distinct

depends on:

• receptive fields sizes

• nerve ending density

high, small

highest acuity in the skin will be with __ innervation density and __ receptive field sizes

adaptation

when neural response decreases despite stimulus remaining constant

slowly adapting

sustained firing

frequency correlated to size and intensity of stimulus

convey info about spatial attributes of stimulus like size and shape

rapidly adapting

fires quickly and then falls silent even in the presence of continued stimulation

convey info about change in ongoing stimulation

ex: socks in the morning vs middle of the day

smaller

superficial encapsuled receptors Meissner and Merkel cell have __ receptive fields compared to deep receptors

Meissner corpuscle

superficial encapsuled receptor that is rapidly adapting

Merkel-cell

superficial encapsuled receptor that is slowly adapting

Pacinian corpuscle

deep encapsuled receptor that is rapidly adapting

Ruffini corpuscle

deep encapsuled receptor that is slowly adapting

concentrated

superficial receptors hav smaller fields but are __ at the fingertips

evenly

deep receptors have bigger fields but are __ distributed

proprioreceptors

provide information about the position of body parts and movement of those body parts

2 classes of proprioceptors

• muscle spindles

• Golgi tendon organ

Muscle spindles

proprioceptors specialized for signaling changes in muscle length (how stretched muscle is)

Golgi tendon organ

proprioceptors specialized for signaling changes in muscle tension (how contracted muscles are)

intrafusal muscle fibers

deep inside muscle fibers surrounded by sensory neurons

gamma motor neuron

motor neuron connected to intrafusal muscle fibers making sure they are tight

Group Ia

afferent axons that are RAPIDLY adapting, aka responds to changes in stimulus, useful for limb dynamics

speed and direction of movement

Group II

SLOWLY adapting

produce sustained response to constant muscle lengths

static position of limbs

Gamma motor neuron

controls contraction of intrafusal muscle fibers

changes in tension can have a large impact on sensitivity of spindle afferent

Tension

Sensory neuron has nerve ending \s that co-mingles with collagen fibrils in tendon which help detect changes in ___

extrafusal

EACH Golgi tendon organ is attached to a subset of ___ muscle fibers

population of Golgi tendon organs give an accurate sample of the tension in the muscles

Fastest axon types

Ia & II

proprioception muscle spindles

thick and myelinated

second fastest axon type

A-Beta

Touch aka coming from skin (Merkel-cells, Meissner, Pacinian, Ruffini)

thick and myelinated

3rd fastest axon type

A-Gamma

Pain and temperature

free nerve endings

thin and myelinated

slowest axon type

C-fibers

pain, temperature, itch

thin and unmyelinated

first order sensory neuron

cell soma in dorsal root ganglia

second order sensory neuron

cell soma in spinal cord or brainstem

third order sensory neuron

cell soma in thalamus

decussation

second order neurons always cross-over meaning that the information travels on the opposite side of the body

aka project across midline

Gracile tract

main mechanosensory pathway from the lower limbs/lower body

synapse on gracile nucleus in caudal medulla

2nd order neuron crosses over

Cuneate tract

main mechanosensory tract from upper body

synapse on cuneate nucleus in caudal medulla

2nd order neuron crosses over

dorsal column

touch information travel up the ____

lower body info travel in MEDIAN ___

upper body info travel in LATERAL-MEDIAN ___

posterior

the dorsal column is in the ___ side of spinal cord

medial lemniscus tract

sensory information traveling up the brainstem

synapse on thalamus

ventral posterior lateral nucleus

VPL part of thalamus

all sensory info from neck DOWN

third order neuron leaves there and synapses in primary somatosensory cortex

ventral posterior medial nucleus

VPM part of thalamus

all sensory info from neck UP

third order neuron leaves there and synapses in primary somatosensory cortex

sensory neck UP info track

aka info from the face

• 1st order neuron has its soma in trigeminal ganglion and synapses on principal nucleus of trigeminal complex

• 2nd order neuron crosses over and go along trigeminal lemniscus tract to VPM in thalamus

• 3rd order neuron has its soma in VPM in thalamus and synapses in primary somatosensory cortex in parietal lobe

separate

Neurons with responses to rapidly and slowly adapting mechanoreceptors were clustered into ___ zones

somatotopic maps

represent the proportions of sensory circuitry

humans have more nerve endings in hands, lips, and face.

functional re-mapping

cortical regions of remaining active zones take over cortical territory that has lost input in order to compensate