chapter 15 neural integration i: sensory pathways and the somatic nervous system

Chapter 15

Neural Integration I:Sensory Pathways and the Somatic Nervous System

fig. 15-1

Sensory Motor

General (15)

Special (17)

Somatic (15)

Autonomic (16)

General senses

temperaturepaintouchpressurevibrationproprioception

most associated with the skin

Special senses

smellsighttastehearing

special “sense”organs

General senses

receptors distributed throughout the body

relatively simple

General senses

receptors send info to CNS

arriving info is called sensation

our awareness of it is perception

Sensory receptors

interface between environment

and the

body

translate stimulus into an AP

transduction

Sensory receptors

receptors have selective sensivity

chemicalphysical touchlightheat transfer

receptors may or may not have accessory structures associated with them

Sensory receptors

receptive field

area monitored by a receptor

fig. 15-2

size of receptive field

70 mm1 mm

specificity

Sensory receptors

receptor

stimulus

stimulus changesmembrane potential

receptor potential (+ or -)

greater stimulus means larger receptor potentialif stimulus is large enough to get to threshold is is called generator potential (

generates an AP)transduction

Sensory receptors

receptor

stimulus

action potential

CNSfor processing and

interpretation(cortical areas)

receptor A

cortex

receptor 2receptor Blabeled line

interpretation is based on which “line” information travels on

a “line” carries the same “type” (modality) of information

receptor A

cortex

receptor 2

labeled line

receptor Boptic nerve

shut eyes and rub them gently

When CNS receives info…

which “line” type of stimuluswhere “line” ends perception

all other attributes (strength, duration, variation) are determined by the frequency and pattern of AP’s

receptor types:

tonic: always “on”

phasic: only on with stimulus

some receptors combine the two

greater stimulus higher freq.lesser stimulus lower freq.

adaptation

reduction in sensitivity in the presence of a constant stimulus

peripheral

centralchange in receptor activity

inhibition of nuclei in pathway

peripheral adaptation

phasic receptors(aka fast-adapting receptors)

example: thermoreceptors

you usually don’t notice roomtemperature unless it changes

central adaptation

example: smell

you walk into a room and notice a new smell…

…but not for long

adaptation reduces the amount of information

reaching the cerebral cortex

about 1% of sensory information coming in reaches our awareness

100 Keys (pg 498)

“Stimulation of a receptor produces action potentials along the axon of a sensory neuron. The frequency or pattern of action potentials contains information about the strength, duration, and variation of the stimulus. Your perception of the nature of that stimulus depends on the path it takes inside the CNS.”

General senses (from chapter 12)

exteroceptors

proprioceptors

interoceptors

outside

position

inside

General senses

classificationbased on nature of stimulus

nociceptorsthermoreceptorsmechanoreceptorschemoreceptors

pain

heat flow

physical distortion

chemical concentration

General senses

nociceptorscommon in:

skinjoint capsulescoverings of bonesaround blood vessel walls

free nerve endingslarge receptive fields

nociceptors

sensitive to:extreme temperature mechanical damagedissolved chemicals

(like those release by damaged cells)

stimulation causes depolarization

two fiber types convey infotype A

fast pain (cut, etc.,)easy to localize

type Cslow pain (“burning, aching”)difficult to localize

nociceptors

tonic receptorsno significant peripheral adaptation

as long as the stimulus is present, it will hurt

but central adaptation can occur(perception of pain may decrease)

nociceptors

sensory neurons bringing in pain info use glutamate and/or substance P as their neurotransmitter

nociceptors

these nts can cause facilitation (?)

pain may be disproportional(feels worse than it should)

pain can be reduced by endorphins and enkephalins (inhibit activity in pathway) [neuromodulators chpt. 12]

nociceptors

endorphins

pain centers use substance Pas nt.

endorphins bind to presynaptic membrane and inhibit substance P release, reducing perception of pain

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thermoreceptors

free nerve endings in the dermisskeletal m.hypothalamusliver

warm receptorsor

cold receptors

thermoreceptors

phasic receptorsactive when temperature is changing, quickly adapting to stable temperature

detect transfer of heatheat loss from skin coolheat gain to skin warm

mechanoreceptors

contain mechanically regulated ion channels (chapter 12)

c. mechanically regulated channels

fig. 12-10c

mechanicalstimulus-

opens

removestimulus-

closed

closed

mechanoreceptors

three classes

tactile receptors

baroreceptors

proprioceptors

touch, pressure, vibration

pressure changes(gut, genitourinary)

position of joints/muscles

mechanoreceptors

tactile receptors

fine touch/pressure

crude touch/pressure

small (narrow) receptive fielddetailed informationsensitive

wide receptive fieldpoor localization

fig. 15-3

tactile receptors

range of complexity

free nerve endingsroot hair plexustactile discstactile corpuscles (Meissner’s)lamellated corpuscles (pacinian)Ruffini corpuscles

tactile receptors

free nerve endings

in epidermis of skincornea of eyesensitive to touch and pressuretonic receptorssmall receptive field

tactile receptors

root hair plexus

around each hair folliclesense movement of hairadapt quickly

tactile receptors

tactile discs

sensitive, tonic receptorsin epidermis

fine touch and pressure

tactile receptors

tactile corpuscles (Meissner’s)

fine touch, pressure , vibrationadapt quicklysurrounded by Schwann cellsin dermis of skineyelids, fingertips (sensitive areas)

tactile receptors

lamellated corpuscles (pacinian)

sensitive to deep pressurehigh-frequency vibrations

adapt quicklynerve ending is encapsulatedby layers of supporting cells

(onion)dermis, pancreas, fingers…

tactile receptors

Ruffini corpuscles

pressure and skin distortionlocated deep in the dermistonic, little if any adaptation

fig. 15-3

sensivitity can be altered

infectiondiseasedamage to pathway

e.g., damage to a spinal nervewould affect an entire dermatome

tickle and itch

closely related to touch and pain

baroreceptors

free nerve endings in the walls of organs that stretch

e.g., blood vessels

when pressure changes they expand or contract

changes activity of receptors

proprioceptors

muscle spindles

Golgi tendon organs

receptors in joint capsules

stretch reflex

monitor tendon tension

free nerve endings in joints

proprioceptors

no adaptation

continuously send info to CNS

most processed at subconscious level

chemoreceptors

respond to chemicals dissolved in the surrounding fluidsrespiratory centers in brain

pH, CO2 levels in blood

carotid bodies and aortic bodiespH, CO2, O2 levels in blood

Pathways in the CNS

spinothalamic tractspine to thalamus=sensory

corticospinal tractcortex to spine=moto

r

Pathways in the CNS

sensory pathways

neurons involved

first order neuron

second order neuron

third order neuron

sensory neuron (DRG)

in CNS (crosses over)

in thalamus

Pathways in the CNS

sensory pathways

carry sensory infofrom skin and muscles ofbody wall, head, neck, limbs

Somatic sensory pathways

Pathways in the CNS

sensory pathways


posterior column pathwayanterolateral column pathwayspinocerebellar pathway

fig. 15-4

The Posterior Column Pathway

fine touchpressurevibrationsproprioception


inferior half of bodyfirst order neuron in DRG

up the fasciculus gracilis to thenucleus gracilis of med. oblong.

superior half of bodyfirst order neuron in DRG

up the fasciculus cuneatus to thenucleus cuneatus of med. oblong.


second order neuron in nucleus ?cross to other side and ascend to

the ventral nucleus of thalamus

third order neuron in thalamusproject to the primary sensory cortex

fig. 15-4

fig. 15-5a

The Anterolateral Pathway

“crude” touchpressurepaintemperature


first order neuron in DRGsynapses on 2nd order neuron

in dorsal horn of spinal cord

second order neuroncross to opposite side and ascend


second order neuroncross to opposite side and ascend

anterior spinothalamic tract

lateral spinothalamic tract

crude touch and pressureto ventral nucleus of thalamus

pain and temperatureto ventral nucleus of thalamus


second order neuron in spinal cordcross to other side and ascend to

the ventral nucleus of thalamus

third order neuron in ventral thalamusproject to the primary sensory cortex

fig. 15-4

fig. 15-5b


phantom pain ?

referred pain?

activity along pathway, even if “limb” is not there

viceral pains sensations may stimulate neurons of AL pathway

fig. 15-6

The Spinocerebellar Pathway

posterior s.c. tracts

anterior s.c. tracts

axons from same side to cerebellum

axons cross over andascend to cerebellum

information goes to Purkinjie cellsin the cerebellum (proprioception)

fig. 15-4

fig. 15-7

100 Keys (pg. 507)

Most somatic sensory informationis relayed to the thalamus for processing. A small fraction of the arriving information is projected to the cerebral cortex and reaches our awareness.

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Pathways in the CNS

sensory pathways



fig. 15-4

Pathways in the CNS

sensory pathways



Pathways in the CNS

sensory pathways

Somatic sensory pathwaysVisceral sensory pathways

info from interoceptors(internal organs)

Pathways in the CNS


nociceptors, thermoreceptors,tactile receptors, baroreceptors, chemoreceptors

Pathways in the CNS


CN V, VII, IX, X carry info frompharynx, mouth, palate, larynx, trachea and esophagus

project to solitary nucleus(medulla oblongata)

Pathways in the CNS


T1 to L2 abdominal organsS2 to S4 pelvic organs

first order neurons project to interneurons which travel up the anterolateral pathway to sol. nuc.

usually subconscious

Pathways in the CNS

sensory pathwaysmotor pathways

the somatic nervous system (SNS)

autonomic nervous system (ANS)voluntary

involuntary

the somatic nervous system (SNS)

always involve at least two neurons

upper motor neuron

lower motor neuroninside CNS (+ or -)

stimulates a motor unit

motor pathways in the CNS

motor information followsone of three main pathways:

corticospinal pathwaymedial pathwaylateral pathway



corticospinal pathway(aka., pyramidal system)

upper motor neurons arepyramidal cells in primary motor cortex

synapse on lower motor neurons(ventral horn of spinal cord)

also project to other control centers


corticospinal pathway

three pairs of tracts:

corticobulbar tracts

to motor nuclei ofCN III, IV, V, VI, VII, IX, XI, XIIconscious control of eye, jaw and face muscles…


corticospinal pathway

three pairs of tracts:

corticobulbar tractslateral corticospinal tractsanterior corticospinal tracts

fig. 15-9

Pathways in the CNS

motor pathways

motor information followsone of three main pathways:

corticospinal pathwaymedial pathwaylateral pathway

fig. 15-8

Pathways in the CNS

motor pathways

corticospinal pathwaymedial pathway

muscle tonegross movement

necktrunkproximal

limbs

Pathways in the CNS

motor pathways

medial pathwayUMN in:

vestibular nuclei(hind)

superior colliculus(mid)

reticular formation(brain stem)

posture &balance

reflexive head

position

various

Pathways in the CNS

motor pathways

lateral pathway

control of muscle tone

precise movement of distal limbs

UMN in red nucleus (mid)

descend down rubrospinal tract

Basal Nuclei

background patterns of movement(walking, running, etc.)

adjust activities of UMN in cortex

two populations:

ACh stimulatory

GABA inhibitory

normally:

inactive

active

inhibited

Cerebellum

monitors (sensory):

proprioceptionvisualvestibular (balance)

spinocerebellar tract

superior colliculus

vestibular nucleus

output

continually adjusts UMN activity

Several conditionsALS

amyotrophic lateral sclerosis(aka Lou Gerhig’s disease)degeneration of UMN’s and/or LMN’s

atrophy of musclecerebral palsy

affect voluntary muscle performancetrauma, exposure to drugs etc., genetics

cerebrum, cerebellum, basal nuclei, hippocampus, thalamus

abnormal motor skills, posture, speech…

anencephalylack of higher brain development

100 Keys (pg. 513)

“Neurons of the primary motor cortex (UMN) innervate motor neurons in the brain and spinal cord (LMN) responsible for stimulating skeletal muscles. Higher centers in the brain can suppress or facilitate reflex responses; reflexes can complement or increase the complexity of voluntary movements”

chapter 15 neural integration i: sensory pathways and the somatic nervous system

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