neural integration the sensory pathways chapter 15

Neural IntegrationThe sensory pathways

Chapter 15

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Afferent Division of the Nervous System

• Receptors• Sensory neurons• Sensory pathways

3

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

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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

Table 10-1 (1 of 2)

General Properties: Sensory Division

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

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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

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

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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

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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

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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

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Mechanoreceptors

• Detect membrane distortion – Three receptor types:

• Tactile Receptors• Proprioceptors• Baroreceptors

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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)

Receptor type Structure Location Function

Meissner’s corpuscle/tactile corpuscle

Few spiral terminals surrounded by CT capsule

Between dermal papillae in hairless skin

Touch, pressure

Pacinian corpuscle/lamellated corpuscle

Single dendrite surrounded by capsule with up to 60 layers of collagen fibers

Skin, interosseous membrane, viscera

Deep pressure. Respond only when the pressure is first applied (on/off pressure stimulation)

Ruffini’s corpuscle

Receptor endings enclosed by flatten capsule

All skin, joint capsule

Stretching of skin – continuous pressure

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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 muscle-tendon

junction• Monitor skeletal muscle tension

– Joint capsule receptors• - Monitor pressure, tension and movement in the

joint

Receptor type Structure Location Function

Muscle spindles

Spindle-shape proprioceptors. Modified skeletal muscle fibers enclosed in CT capsule

Perimysium of skeletal muscles

Detect muscle stretch and initiate reflex that resist stretch

Golgi tendon organs

Proprioceptors. Consist of bundle of collagen fibers enclosed in CT capsule with sensory endings coiling between and around the fibers

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

Joint receptors

Proprioceptors (combination of several receptors types – Pacinian, Raffini, free ending and Golgi tendon)

Joints’ CT capsule Monitor stretch in in the articular capsule and provide information on the position and motion of the joint (conscious)

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Mechanoreceptors - Baroreceptors

– Detect pressure changes– Found in elastic tissue of blood vessels and organs of

digestive, reproductive and urinary tracts

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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

Table 10-2

Sensory Receptors

Processing of the sensory information

• Levels of neural integration in sensory systems:

1. Receptor level — the sensor receptors2. Circuit level — ascending pathways in the CNS3. Perceptual level — neuronal circuits in the

cerebral cortex

Figure 13.2

1

2

3

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus

Processing at the Receptor Level

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

Figure 10-3a

One signal goes to the brain.

Compass with pointsseparated by 20 mm

Primarysensoryneurons

Skin surface

Secondarysensoryneurons

(a)

Sensory Neurons: Two-Point Discrimination

• convergence Two-point discrimination

Two signals go to the brain.

Compass with pointsseparated by 20 mm


Skin surface

Secondarysensoryneurons

(b)

Figure 10-3b

Sensory Neurons: Two-Point Discrimination - overlapping

Figure 10-2

The receptive fields of three primary sensory neuronsoverlap to form one large secondary receptive field.

Primary sensoryneurons

Secondarysensoryneuron

SECTION THROUGH SPINAL CORD

Information from thesecondary receptive

field goes to the brain.

The primary sensory neuronsconverge on one secondarysensory neuron.

Receptive Fields of Sensory Neurons - overlapping

Properties of Stimulus: Location

• Lateral inhibition enhances contrast and makes a stimulus easier to perceive

Figure 10-6

Stimulus Stimulus

Primary neuronresponse is proportional

to stimulus strength.

Pathway closest tothe stimulus inhibits

neighbors.

Inhibition of lateralneurons enhances

perception of stimulus.

Tonic level

A B C

A B C

Tonic level

Skin

Pin


Secondaryneurons

Tertiaryneurons

A B C

Freq

uenc

y of

act

ion

pote

ntia

lsFr

eque

ncy

of a

ctio

n po

tent

ials

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

Figure 10-8a

Always active Signal at different rate when stimulated Monitor background levels

Phasic Receptors

Figure 10-8b

Activated by stimulus Become active for a short time whenever a change

occurs Monitor intensity and rate of change of stimulus

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)

Figure 13.2

1

2

3



Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle



Medulla


Motorcortex

Somatosensorycortex

Thalamus

Processing at the circuit Level

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

Pathway Sensation 1st order 2nd order 3rd order Final destinationSpinothalamic pathwayLateral spinothalamic

Pain and temperature

Dorsal root ganglion

Posterior horn Thalamus Primary sensory cortex (opposite side)

Anterior spinothalamic

Crude touch and pressure


Posterior horn Thalamus Primary sensory cortex (opposite side)

Posterior column pathwayFasciculus gracilis

Proprioception, fine touch and pressure from inferior half of the body


Medulla oblongata

Thalamus Primary sensory cortex (opposite side)

Fasciculus cuneatus

Proprioception, fine touch and pressure from superior half of the body


Medulla oblongata

Thalamus Primary sensory cortex (opposite side)

Spinocerebellar pathwayAnterior and posterior

Proprioception Dorsal root ganglion

Posterior horn Not present Cerebellar cortex

Fine touch,proprioception,

vibration

Nociception,temperature,coarse touch

SPINAL CORD

MEDULLA

THALAMUS

Pain, temperature, andcoarse touch cross themidline in the spinal cord.

Fine touch, vibration,and proprioceptionpathways cross themidline in the medulla.

Sensations are perceivedin the primary somaticsensory cortex.

Sensory pathwayssynapse in the thalamus.

Primary sensory neuron

Secondary sensory neuron

Tertiary neuron

KEY1 1

2 2

3 3

4 4

Somatic Senses Pathways

Figure 10-9, steps 1–4

Figure 13.2

1

2

3



Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle



Medulla


Motorcortex

Somatosensorycortex

Thalamus

Processing at the Perceptual Level

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 sub-modalities, & 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)

Rapidly transferred to CNS by small myelinated fibers (within 0.1 seconds after stimulus applied)

Slow pain – more diffused pain (associated with tissue destruction) 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)

http://www.youtube.com/watch?v=IlCstuhpteo

http://www.youtube.com/watch?v=IlCstuhpteo

The Gate-Control Theory of Pain

Figure 10-12a

Pain can be suppressed in the dorsal horn level. Normally, tonically active inhibitory interneuron

inhibit ascending pathways for pain

The Gate Control Theory of Pain Modulation

Figure 10-12b

Fibers from nociceptors synapse on the inhibition interneuron

When activated, the fibers send message to block the interneurons and pain travels to the brain

The Gate Control Theory of Pain Modulation

Figure 10-12c

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

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

Figure 10-13b

Skin(usual stimulus)

Kidney(uncommon stimulus)

Primary sensoryneurons

Secondarysensoryneuron

Ascending sensorypath to somatosensorycortex of brain(b)

Sensory Pathways

Figure 10-4

3

21

1

2

3

Olfactory pathways fromthe nose project throughthe olfactory bulb to theolfactory cortex.

Equilibrium pathways projectprimarily to the cerebellum.

Most sensory pathways projectto the thalamus. The thalamusmodifies and relays informationto cortical centers.

Eye

Nose

Tongue

Equilibrium

Sound

Brainstem

Visualcortex

Auditorycortex

Gustatory cortex Primary somaticsensory cortex

Olfactory cortex

Olfactory bulb

Cerebellum

Thalamus

Somaticsenses

neural integration the sensory pathways chapter 15

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