Neural Integration
The sensory pathways
Chapter 15
Afferent Division of the Nervous System
 Receptors
 Sensory neurons
 Sensory pathways
2
Afferent Division – location in CNS
1. Somatic Sensory info
- Sensory cortex of cerebrum
- Cerebellum
2. Visceral Sensory info
- Reflex centers in brainstem
- Reflex centers in diencephalon
3
The somatic sensory system
 Sensory stimuli that reach the conscious level of
perception
 Specialized cells that monitor specific conditions in the
body or external environment
 General Senses:
 Temp, pain, touch, pressure, vibration, proprioception
 Simple receptors located anywhere on body
 Special Senses:
 Are located in sense organs such as the eye or ear
 Olfaction, vision, gustation, hearing, equilibrium
 Complex receptors located in specialized sense organs
4
General Properties: Sensory Division
Table 10-1 (1 of 2)
From Sensation to Perception
Sensory Pathways – from sensation to perception
 Stimulus as physical energy  sensory receptor
 Receptor acts as a transducer
 Intracellular signal  usually change in membrane
potential
 Stimulus  threshold  action potential to CNS
 Integration in CNS  cerebral cortex or acted on
subconsciously
Sensory Receptors
 Transduction – conversion of environmental stimulus into
action potential by sensory receptor
 Receptors specific for particular type of stimulus
 Specificity is due to structure of receptor
8
From Sensation to Perception
 A stimulus is a change in the environment that is
detected by a receptor
 Sensation: the awareness of changes in the internal
and external environment
 Perception: the conscious interpretation of those
stimuli
Classification by Location
1. Exteroceptors
 Respond to stimuli arising outside the body
 Receptors in the skin for touch, pressure, pain, and
temperature
 Most special sense organs
2. Interoceptors (visceroceptors)
 Respond to stimuli arising in internal viscera and blood
vessels
 Sensitive to chemical changes, tissue stretch, and
temperature changes
Classification by Location
3. Proprioceptors
 Respond to stretch in skeletal muscles, tendons,
joints, ligaments, and connective tissue coverings
of bones and muscles
 Inform the brain of one’s movements
Four types of General Sensory Receptors





Pain: nociceptor
Temperature: thermoreceptor
Physical: mechanoreceptor
Chemicals: chemoreceptors
All can be found in both somatic (exteroceptors) and
visceral (interoceptors) locations except:
 Proprioceptors (a mechanoreceptor) are somatic only
 report the positions of skeletal muscles and joints
12
Pain Receptors: Nociceptors

(noci = harm) sensitive to pain-causing stimuli (e.g.
extreme heat or cold, excessive pressure, inflammatory
chemicals)
 Free nerve ending
 Mode of Action:
 Injured cells release arachidonic acid
 Arachidonic acid is converted into prostaglandins by the
interstitial enzyme cyclo-oxygenase
 Prostaglandins activate nociceptors
- Many pain medications like aspirin function to inhibit
cyclo-oxygenase
- Pain levels are modulated by endorphins which inhibit
CNS function
13
Thermoreceptors
 Detect temperature
 Found in skin, skeletal muscle, liver, and
hypothalamus
 Consist of free nerve endings
 Phasic receptors that adapt easily


Cold response are more superficial and receptors that
respond to heat – deeper
Temperature out of the range of the thermoreceptors will
activate nociceptors
14
Mechanoreceptors
 Detect membrane distortion
 Three receptor types:
 Tactile Receptors
 Proprioceptors
 Baroreceptors
15
Mechanoreceptors - Tactile Receptors
 Detect touch, pressure and vibration on skin
 Detect hair movement
 Detect fine touch
 Detect deep pressure
 respond to itch (respond among other to histamine) and
light touch (detect changes in shape like bending)
16
Receptor type Structure
Location
Function
Meissner’s
Few spiral
corpuscle/tacti terminals
le corpuscle
surrounded by
CT capsule
Between dermal
papillae in
hairless skin
Touch, pressure
Pacinian
corpuscle/lame
llated
corpuscle
Skin,
interosseous
membrane,
viscera
Deep pressure. Respond
only when the pressure
is first applied (on/off
pressure stimulation)
All skin, joint
capsule
Stretching of skin –
continuous pressure
Ruffini’s
corpuscle
Single dendrite
surrounded by
capsule with up
to 60 layers of
collagen fibers
Receptor endings
enclosed by
flatten capsule
Mechanoreceptors - Proprioceptors
 Detect positions of joints and muscles
 Muscle spindles
 Modified skeletal muscle cell
 Monitor skeletal muscle length
 Golgi tendon organs
 Dendrites around collagen fibers at the muscletendon junction
 Monitor skeletal muscle tension
 Joint capsule receptors
 - Monitor pressure, tension and movement in
the joint
18
Receptor type Structure
Location
Function
Muscle
spindles
Perimysium of
skeletal muscles
Detect muscle stretch and
initiate reflex that resist
stretch
In tendons close to
skeletal muscle
insertion
When tendon fibers are
stretched by muscle
contraction the nerve
endings are activated by
compression. When
activated, the contraction of
the muscle is inhibited
which causes relaxation
Monitor stretch in in the
articular capsule and
provide information on the
position and motion of the
joint (conscious)
Golgi tendon
organs
Joint
receptors
Spindle-shape
proprioceptors.
Modified skeletal
muscle fibers enclosed
in CT capsule
Proprioceptors.
Consist of bundle of
collagen fibers
enclosed in CT capsule
with sensory endings
coiling between and
around the fibers
Proprioceptors
Joints’ CT capsule
(combination of
several receptors types
– Pacinian, Raffini,
free ending and Golgi
tendon)
Mechanoreceptors - Baroreceptors
 Detect pressure changes
 Found in elastic tissue of blood vessels and organs of
digestive, reproductive and urinary tracts
20
21
Chemoreceptors
 Detect change in concentration of specific chemicals
or compounds
 pH, CO2, sodium etc.
 Found in respiratory centers of the brain and in
large arteries
22
Sensory Receptors
Table 10-2
Processing of the sensory information

Levels of neural integration in sensory systems:
1. Receptor level — the sensor receptors
2. Circuit level — ascending pathways in the CNS
3. Perceptual level — neuronal circuits in the cerebral
cortex
Processing at the Receptor Level
Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
Figure 13.2
Processing at the Receptor Level
 The receptor must have specificity for the stimulus energy
(as previously discussed)
 The receptor’s receptive field must be stimulated
 The stimulus need to be converted to a nerve impulse
 Receptors have different levels of adaptation
 Information is encoded in the frequency of the stimuli –
the greater the frequency, the stronger is the stimulus.
The stimulation of the receptive field affects the
discharge of the sensory neurons

The receptive field is the a specific physical area that, when
stimulated, affect the discharge of the stimulus.

Most receptive fields activation will result in message sending –
excitatory receptive field

Sensory receptors in the CNS can have inhibitory receptive field
(example: vision fields to determine borders).

Sensory neurons of neighboring receptive field may exhibit


Convergence many sub-threshold stimuli to sum in the
postsynaptic neuron

Overlapping with another receptor’s receptive field – sending 2
sensations from the same area (pressure and pain)
The smaller the receptive field the greater the ability of the brain to
localize the site
Sensory Neurons: Two-Point Discrimination
 convergence Twopoint discrimination
(a)
Compass with points
separated by 20 mm
Skin surface
Primary
sensory
neurons
Secondary
sensory
neurons
One signal goes to the brain.
Figure 10-3a
Sensory Neurons: Two-Point Discrimination overlapping
(b)
Compass with points
separated by 20 mm
Skin surface
Primary
sensory
neurons
Secondary
sensory
neurons
Two signals go to the brain.
Figure 10-3b
Receptive Fields of Sensory Neurons - overlapping
Primary sensory
neurons
The primary sensory neurons
converge on one secondary
sensory neuron.
Information from the
secondary receptive
field goes to the brain.
Secondary
sensory
neuron
The receptive fields of three primary sensory neurons
overlap to form one large secondary receptive field.
SECTION THROUGH SPINAL CORD
Figure 10-2
Properties of Stimulus: Location
 Lateral inhibition enhances contrast and makes a
stimulus easier to perceive
Stimulus
Pin
Skin
Primary neuron
response is proportional
to stimulus strength.
Primary
sensory
neurons
Frequency of action potentials
Stimulus
A
B
C
Tonic level
Inhibition of lateral
neurons enhances
perception of stimulus.
Tertiary
neurons
A
B
C
Frequency of action potentials
Pathway closest to
the stimulus inhibits
neighbors.
Secondary
neurons
A
B
C
Tonic level
Figure 10-6





Transduction allows sensory receptors to respond to
stimuli – converting sensation into a nerve impulse
Sensory transduction – the process that enables a sensory
receptor to respond to a stimulus.
The sensory transduction induces a receptor potential in
the peripheral terminal of the sensory neuron
A receptor potential is a depolarization event that if brings
the membrane to a threshold, will become a nerve impulse
(AP)
The conversion from receptor potential to AP happens in
the trigger zone that can be in the first node of Ranvier.
In some cases, the peripheral terminal is a separate sensory
cell (ex. Photo receptors). In this case there is an
involvement of a synapse and NT
Receptors adaptation




The duration of a stimulus is coded by duration of action
potentials.
A longer stimulus generates longer series of APs.
If a stimulus persists, some receptors adapt or stop responding
There are 2 classes of receptors according to how they adapt:
 Tonic receptors – slowly adapting – they fire rapidly when
first activated, than they slow and maintain firing as long as the
stimulus is present (baroreceptors, proprioceptors)
 Phasic receptors – rapidly adapting receptors – rapidly firing
when first activated but stop firing if the strength of stimulus
remains constant
 This type of reaction allows the body to ignore information
that was evaluated and found not to be a threat to
homeostasis (smell)
Tonic Receptors
 Always active
 Signal at different rate when stimulated
 Monitor background levels
Figure 10-8a
Phasic Receptors
 Activated by stimulus
 Become active for a short time whenever a change
occurs
 Monitor intensity and rate of change of stimulus
Figure 10-8b
Receptors adaptation
 The mechanisms for receptors’ adaptation depends on
the receptors:
 Potassium channels in the receptor’s membrane
open causing the membrane repolarization
 Sodium
channels
inactivated
stopping
depolarization
 Accessory structure may contribute to decrease
sensitivity (muscle in the ear contract and limit the
movement of the auditory oscicles)
Processing at the circuit Level
Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
Figure 13.2
Processing at the circuit Level






A sensory pathway is a set of neurons arranged in series.
The circuit level role is to deliver the impulses to the appropriate
region in the cerebral cortex.
The ascending tract typically consists of 3 neurons
First order neurons
 cell bodies in a ganglion (dorsal or cranial)
 Impulses from skin and proprioceptors to spinal cord or brain stem
to a 2nd order neuron
Second order neuron
 In the dorsal horn of the spinal cord or in the medulary nuclei
 Transmit impulses to thalamus or cerebellum
Third order neurons
 Cell bodies in the thalamus (no 3rd-order neurons in the
cerebellum)
 Transmit signals to the somatosensory cortex of the cerebrum
Pathways for somatic perception
 Receptors for the somatic sensations are found both in
the skin and viscera
 Receptor activation triggers AP in the 1st order neuron
 In the spinal cord, sensory neurons synapse with
interneurons – 2nd order neurons
 All 2nd order neurons cross over at some point
(sensations are being integrated in the opposite side)
 The synapse between the 2nd and the 3rd happens in the
thalamus
 The axons of the 3rd order neurons project to the
appropriate somatosensory area in the cerebral cortex
Processing at the circuit Level
 Impulses ascend in :
 Non specific pathway that in general transmit pain,
temperature and touch
 Give branches to reticular formation and
thalamus on the way up
 Sends general information that is also involved in
emotional aspects of perception
 Specific ascending pathways involve in more precise
aspect of sensation
Thalamic Function
 The thalamus is the “gateway to the cerebral cortex”
 Major relay station for most sensory impulses that
arrive to the primary sensory areas in the cerebral cortex:
 taste, smell, hearing, equilibrium, vision, touch, pain,
pressure, temperature
 Contributes to motor functions by transmitting
information from the cerebellum and basal ganglia to the
cerebral primary motor area
 Connects areas of the cerebrum
 Impulses of similar function are sorted out, edited, and
relayed as a group
3 major somatosensory pathways –1) spinothalamic
pathway
 Conscious sensation of poorly localized sensations
 Anterior spinothalamic tracts – crude touch and
pressure
 Lateral spinothalamic tracts – pain and temperature
 1st order neurons synapse with the 2nd in the posterior
gray horn at the level of entrance
 The 2nd cross before ascending to the thalamus
 3rd order synapse at the level of the primary
somatosensory cortex
http://webanatomy.net/anatomy/spinothalamic.jpg
3 major somatosensory pathways - 2) Posterior column pathway
 Sensation of precise touch, vibration and
proprioception
 Includes
 Left and right fasciculus gracilis (inferior part of
the body)
 Left and right fasciculus cuneatus (superior part
of the body)
 First order neurons enter the CNS at the dorsal roots
and the sensory roots of cranial nerves.
 Synapse with 2nd order in the medulla
 2nd order neurons cross over in the brain stem
 3rd order in the thalamus where the stimuli are sorted by
the nature of stimulus and the region of body involved
http://webanatomy.net/anatomy/gracilis_cuneatus.jpg
3 major somatosensory pathways – 3) The spinocerebellar
pathway
 Information about muscle, tendon and joint position
from the spine to the cerebellum
 This information is subconscious
 1st order neurons synapse in the dorsal horn
 2nd order neurons ascend via anterior and posterior
spinocerebellar tracts to the cerebellar cortex
 Used to coordinate movements
 In this pathway there is no 3rd order neuron
http://webanatomy.net/anatomy/spinocerebellar.jpg
1st order
2nd order
3rd order
Pathway
Sensation
Spinothalamic pathway
Lateral
Pain and
spinothalamic temperature
Dorsal root
ganglion
Posterior horn
Thalamus
Anterior
Crude touch and
spinothalamic pressure
Dorsal root
ganglion
Posterior horn
Thalamus
Medulla
oblongata
Thalamus
Primary sensory
cortex (opposite
side)
Medulla
oblongata
Thalamus
Primary sensory
cortex (opposite
side)
Posterior horn
Not present Cerebellar cortex
Posterior column pathway
Fasciculus
Proprioception, fine Dorsal root
gracilis
touch and pressure ganglion
from inferior half of
the body
Fasciculus
cuneatus
Proprioception, fine Dorsal root
touch and pressure ganglion
from superior half of
the body
Spinocerebellar pathway
Anterior and Proprioception
posterior
Dorsal root
ganglion
Final destination
Primary sensory
cortex (opposite
side)
Primary sensory
cortex (opposite
side)
Somatic Senses Pathways
4 Sensations are perceived
in the primary somatic
sensory cortex.
4
3 Sensory pathways
synapse in the thalamus.
3
THALAMUS
MEDULLA
2
2
Fine touch, vibration,
and proprioception
pathways cross the
midline in the medulla.
Fine touch,
proprioception,
vibration
Nociception,
temperature,
coarse touch
1
1
Pain, temperature, and
coarse touch cross the
midline in the spinal cord.
KEY
Primary sensory neuron
Secondary sensory neuron
Tertiary neuron
SPINAL CORD
Figure 10-9, steps 1–4
Processing at the Perceptual Level
Perceptual level (processing in
cortical sensory centers)
3
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Pons
2 Circuit level
(processing in
Spinal
ascending pathways) cord
Cerebellum
Medulla
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle
Receptor level
(sensory reception Joint
and transmission
kinesthetic
to CNS)
receptor
1
Figure 13.2
Processing at the Perceptual Level
 Interpretation of sensory input occurs in the cerebral
cortex
 The ability to identify the sensation depends on the
specific location of the target neurons in the sensory
cortex not on the nature of the message (all messages
are action potentials)
The CNS integrate sensory information
 Most of the somatic sensory information enters the
spinal cord and travels via ascending pathways to the
brain
 Some information goes directly to the brain through
the cranial nerves
 Autonomic sensory information does not arrive
conscious perception
Main Aspects of Sensory Perception
 Perceptual detection – detecting that a stimulus has
occurred and requires summation
 Magnitude estimation – the ability to detect how intense
the stimulus is
 Spatial discrimination – identifying the site or pattern of
the stimulus
 Feature abstraction – used to identify a substance that has
specific texture or shape
 Quality discrimination – the ability to identify
submodalities of a sensation (e.g., sweet or sour tastes)
 Pattern recognition – ability to recognize patterns in stimuli
(e.g., melody, familiar face)
Somatosensation perception
 The specific sensation depends on the 2nd and 3rd
neurons
 The ability to localize the specific location of a
stimulus depends on the stimulation of a specific area
in the primary somatosensory cortex
 A sensory “homunculus” (little human) is a functional
map of the primary somatosensory cortex
Somatosensory Association Cortex
 Located posterior to the primary somatosensory
cortex and has connection with it
 Integrates sensory information like temperature and
pressure coming from the primary somatosensory
cortex.
 Forms understanding of the stimulus like size, texture,
and relationship of parts
 Ex.: putting the hand in the pocket and feeling
something. The center integrate previous information
to identify objects without seeing them
The main Sensory Areas in the cerebral cortex
Figure 12.8a
Properties of the sensory system - summary






Stimulus – works on a receptor
 The receptor is a transducer that converts the stimulus into a
change of membrane potential
The message from the receptor will be sent in the form of action
potential to the CNS
Stimuli that will reach the cerebral cortex will be come
conscious
Somatosensory information ascends the spinal column along
several pathways, which synapse at the midbrain &/or thalamus
before reaching the cortex
Sensory processes have different sub-modalities of
somatosensory information
Later stages of processing combine information across the submodalities, & with information from other senses
Pain pathways

Pain is a protective mechanism

Pain is a subjective perception

It is individual and can vary depending on emotional state

Types of pain sensations:

Fast pain – sharp and localized – in superficial parts of the body
(cut, burn)


Slow pain – more diffused pain (associated with tissue
destruction)


Rapidly transferred to CNS by small myelinated fibers
(within 0.1 seconds after stimulus applied)
Carried by small unmyelinated fibers
Often fast pain will follow a slow one
Pain pathways
 Pain from the body – via spinal cord
 Pain from face – via trigeminal (V) that enters the
pons, descend to the medulla where they cross over
and ascend to the thalamus
 The ascending pathway sends branches not only to
thalamus and the cerebral cortex but also to the limbic
system (emotions) and hypothalamus (autonomic
reaction)
 The result is that pain may be accompanied by
emotional distress and autonomic reactions such as
nausea, vomiting or sweating
Pain perception
 Pain can be felt in skeletal muscle when anaerobic
metabolism
 In cardiac muscle, pain is a result of ischemia (lack of
oxygen due to reduced blood flow) during myocardial
infraction (heart attack)
 Visceral pain is poorly localized and called referred
pain
Pain perception – the gate control theory
 Pain perception is subjected to modulation that can
happen in several levels of the nervous system
 Pain can be magnified by past experiences
 Pain can be suppressed when in emergencies when
surviving depends on ignoring the injury
 http://www.youtube.com/watch?v=IlCstuhpteo
 (minute 13.41)
The Gate-Control Theory of Pain
 Pain can be suppressed in the dorsal horn level.
 Normally, tonically active inhibitory interneuron
inhibit ascending pathways for pain
Figure 10-12a
The Gate Control Theory of Pain Modulation
 Fibers from nociceptors synapse on the inhibition
interneuron
 When activated, the fibers send message to block the
interneurons and pain travels to the brain
Figure 10-12b
The Gate Control Theory of Pain Modulation





In the gate control theory of pain modulation fibers carrying sensory
information about mechanical stimuli help block pain transmission
Those fibers synapse on the interneuron and increase its inhibitory
activity
If both pain stimulus and nonpainful stimulus arrive at the same
time, there will be partial inhibition of pain
The sensation of pain will be perceived by the brain as lower
Explains why rubbing a bumped elbow lessens the pain feeling
Figure 10-12c
Visceral sensory pain pathways
 Collected by interoceptors within the closed ventral body
cavities
 The interoceptors include nociceptors, thermoreceptors,
tactile receptors, baroreceptors and chemoreceptors
 The axons of the 1st order neuron usually travel with the
autonomic motor fibers innervating the same visceral
structures
 2nd order neurons within the spinal cord use the spinothalamic
pathway and arrive to the medulla oblongata
 Cranial nerves V, VII, IX and X carry visceral sensory
information also to the medulla
 (all parasympathetic will be discussed with the ANS)
Referred Pain
Skin
(usual stimulus)
Primary sensory
neurons
Kidney
(uncommon stimulus)
(b)
Secondary
sensory
neuron
Ascending sensory
path to somatosensory
cortex of brain
Figure 10-13b
Sensory Pathways
Primary somatic
sensory cortex
Gustatory cortex
Olfactory cortex
Olfactory bulb
1
Auditory
cortex
Visual
cortex
Olfactory pathways from
the nose project through
the olfactory bulb to the
olfactory cortex.
Eye
2
2
3
Most sensory pathways project
to the thalamus. The thalamus
modifies and relays information
to cortical centers.
Equilibrium pathways project
primarily to the cerebellum.
Nose
1
Cerebellum
Thalamus
Sound
Brain
stem
Equilibrium
3
Tongue
Somatic
senses
Figure 10-4