1. Neural Integration The sensory pathways Chapter 15

2. Afferent Division of the Nervous System
Receptors
Sensory neurons
Sensory pathways

3. Afferent Division – location in CNS
Somatic Sensory info
Sensory cortex of cerebrum
Cerebellum
Visceral Sensory info
Reflex centers in brainstem
Reflex centers in diencephalon

4. 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

5. General Properties: Sensory Division
Table 10-1 (1 of 2)

6. From Sensation to Perception

7. 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

8. 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

9. 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

10. Classification by Location
Exteroceptors
Respond to stimuli arising outside the body
Receptors in the skin for touch, pressure, pain, and temperature
Most special sense organs
Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes

11. Classification by Location
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

12. 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

13. 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

14. 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

15. Mechanoreceptors Detect membrane distortion Three receptor types:
Tactile Receptors
Proprioceptors
Baroreceptors

16. 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)

17. 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

18. 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

19. 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)

20. Mechanoreceptors - Baroreceptors
Detect pressure changes
Found in elastic tissue of blood vessels and organs of digestive, reproductive and urinary tracts

22. 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

23. Sensory Receptors
Table 10-2

24. Processing of the sensory information
Levels of neural integration in sensory systems:
Receptor level — the sensor receptors
Circuit level — ascending pathways in the CNS
Perceptual level — neuronal circuits in the cerebral cortex

25. Processing at the Receptor Level3
Perceptual level (processing in
cortical sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Cerebellum
Pons
Medulla 2
Circuit level
(processing in
ascending pathways)
Spinal
cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle 1
Receptor level
(sensory reception
and transmission
to CNS)
Joint
kinesthetic
receptor
Figure 13.2

26. 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.

27. 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

28. Sensory Neurons: Two-Point Discrimination
convergence Two-point 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

29. Sensory Neurons: Two-Point Discrimination - overlapping
Two signals go to the brain.
Compass with points separated by 20 mm
Primary sensory neurons
Skin surface
Secondary sensory neurons
(b)
Figure 10-3b

30. 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

31. Properties of Stimulus: Location
Lateral inhibition enhances contrast and makes a stimulus easier to perceive
Stimulus
Stimulus
Pin
Skin A B C
Frequency of action potentials
Tonic level
Primary neuron response is proportional to stimulus strength.
Primary sensory neurons
Pathway closest to the stimulus inhibits neighbors.
Secondary neurons A B C
Frequency of action potentials
Inhibition of lateral neurons enhances perception of stimulus.
Tertiary neurons
Tonic level A B C
Figure 10-6

32. 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

33. 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)

34. Tonic Receptors Always active Signal at different rate when stimulated
Monitor background levels
Figure 10-8a

35. 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

36. 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)

37. Processing at the circuit Level3
Perceptual level (processing in
cortical sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Cerebellum
Pons
Medulla 2
Circuit level
(processing in
ascending pathways)
Spinal
cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle 1
Receptor level
(sensory reception
and transmission
to CNS)
Joint
kinesthetic
receptor
Figure 13.2

38. 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

39. 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

40. 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

41. 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

42. 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

44. 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

46. 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

48. Pathway
Sensation
1st order
2nd order
3rd order
Final destination
Spinothalamic pathway
Lateral spinothalamic
Pain and temperature
Dorsal root ganglion
Posterior horn
Thalamus
Primary sensory cortex (opposite side)
Anterior spinothalamic
Crude touch and pressure
Posterior column pathway
Fasciculus gracilis
Proprioception, fine touch and pressure from inferior half of the body
Medulla oblongata
Fasciculus cuneatus
Proprioception, fine touch and pressure from superior half of the body
Spinocerebellar pathway
Anterior and posterior
Proprioception
Not present
Cerebellar cortex

49. Somatic Senses Pathways4 4
Sensations are perceived in the primary somatic sensory cortex. 3 3
Sensory pathways synapse in the thalamus.
THALAMUS
MEDULLA 2 2
Fine touch, vibration, and proprioception pathways cross the midline in the medulla.
Fine touch, proprioception, vibration
KEY 1 1
Pain, temperature, and coarse touch cross the midline in the spinal cord.
Nociception, temperature, coarse touch
Primary sensory neuron
Secondary sensory neuron
Tertiary neuron
SPINAL CORD
Figure 10-9, steps 1–4

50. Processing at the Perceptual Level3
Perceptual level (processing in
cortical sensory centers)
Motor
cortex
Somatosensory
cortex
Thalamus
Reticular
formation
Cerebellum
Pons
Medulla 2
Circuit level
(processing in
ascending pathways)
Spinal
cord
Free nerve
endings (pain,
cold, warmth)
Muscle
spindle 1
Receptor level
(sensory reception
and transmission
to CNS)
Joint
kinesthetic
receptor
Figure 13.2

51. 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)

52. 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

53. 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)

54. 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

55. 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

56. The main Sensory Areas in the cerebral cortex
Figure 12.8a

57. 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

58. 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

59. 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

60. 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

61. 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
(minute 13.41)

62. 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

63. 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

64. 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

65. 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)

66. Referred Pain Skin (usual stimulus) Primary sensory neurons
Kidney (uncommon stimulus)
Secondary sensory neuron
Ascending sensory path to somatosensory cortex of brain
(b)
Figure 10-13b

67. Sensory Pathways Figure 10-4 Primary somatic sensory cortex
Gustatory cortex
Olfactory cortex
Olfactory bulb
Auditory cortex
Visual cortex 1
Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex.
Eye 2
Cerebellum 2
Most sensory pathways project to the thalamus. The thalamus modifies and relays information to cortical centers. 1
Nose
Thalamus
Sound
Brain stem
Equilibrium 3 3
Equilibrium pathways project primarily to the cerebellum.
Tongue
Somatic senses
Figure 10-4