2. Afferent Division of the Nervous System
Sensory pathways (this chapter)
Efferent Division of the Nervous System
Motor pathways
Somatic Nervous system (this chapter)
Autonomic Nervous system (next chapter)

3. An Overview of Events Occurring Along the Sensory and Motor Pathways.
Immediate Involuntary Response
Motor Pathway (involuntary)
Processing centers in the spinal cord or brain stem may direct an immediate reflex response even before sensations reach the cerebral cortex.
Sensory Pathway
Arriving stimulus
Depolarization of Receptor
Action Potential Generation
CNS Processing
Propagation
A stimulus produces a graded change in the membrane potential of a receptor cell.
If the stimulus depolarizes the receptor cell to threshold, action potentials develop in the initial segment.
Axons of sensory neurons carry information about the type of stimulus (touch, pressure, temperature) as action potentials to the CNS.
Information processing occurs at every relay synapse. Sensory informa- tion may be distributed to multiple nuclei and centers in the spinal cord and brain.
Voluntary Response
Motor Pathway (voluntary)
Perception
The voluntary response, which is not immediate, can moderate, enhance, or supplement the relatively simple involuntary reflexive response.
Only about 1 percent of arriving sensations are relayed to the primary sensory cortex.

4. Sensory Receptors
Sensory Receptors are specialized cells or nerve endings that monitor specific conditions in the body or external environment
Connect our internal and external environment with the nervous system
When stimulated, a receptor passes information to the CNS
In the form of action potentials along the axon of a sensory neuron

5. 15-1 Sensation: Overview Sensation Perception
Interpretation of the sensory input occurs at a conscious or subconscious level
Sensation
the information carried by a sensory pathway to the CNS
May be interpreted at a conscious or subconscious level
CNS interprets the modality (type of sensation) according to the labeled line over which it arrives
For example: Touch, pressure, pain, temp sensations go to primary sensory cortex in parietal lobe of cerebrum
Perception
Conscious awareness of a sensation
Must be received by cerebral cortex

6. General Senses
General sensory receptors are simple in structure and widely distributed throughout the body (skin & internal organs)
Temperature
Pain
Touch
Pressure
Vibration
Proprioception

7. Special sensory receptors are complex in structure and located in sense organs only
Special Senses
Olfaction (smell)
Vision (sight)
Gustation (taste)
Equilibrium (balance)
Hearing

8. Figure 15-2 Receptors and Receptive Fields
The area is monitored by a single receptor cell is called the Receptive field
The larger the receptive field, the more difficult it is to localize a stimulus 8

9. Central adaptation occurs along sensory pathways within the CNS
Adaptation is the reduction in sensitivity in the presence of a constant stimulus
Peripheral Adaptation occurs when the level of receptor activity changes
Responds strongly at first, then gradually declines
Central adaptation occurs along sensory pathways within the CNS
Involves inhibition of nuclei along a sensory pathway

10. Adaptation, a reduction in sensitivity in the presence of a constant stimulus
Receptor
Labeled line
Arriving
stimulus
CNS processing center
Site of peripheral adaptation
Site of central adaptation
Figure Receptors for the general senses can be classified by function and by sensitivity
Figure 10

11. Two types of Receptors
Phasic receptors –usually off: detect a new stimulus or a change in stimulus
Tonic receptors - always active

12. Phasic receptors detect a new stimulus or a change in stimulus
Normally inactive
Become active only for a short time
Undergo peripheral adaptation: Fast-adapting receptors: Over time, their sensitivity to stimulus decreases:
Found in senses of smell, hair movement and cutaneous pressure
Tonic receptors
Are always active
Show little peripheral adaptation
Are slow-adapting receptors
Proprioceptors are among the most slowly adapting tonic receptors

13. Increased
Normal
Normal
Stimulus
Frequency of action potentials
Time a
Tonic receptors are always active and generate action potentials at a frequency that reflects the background level of stimulation. When the stimulus increases or decreases, the rate of action potential generation changes accordingly.

14. Increased
Normal
Normal
Stimulus
Frequency of action potentials
Time b
Phasic receptors are normally inactive, but become active for a short time in response to a change in the conditions they are monitoring.

15. A Functional Classification of General Sensory Receptors
Nociceptors
Thermoreceptors
Chemoreceptors
Mechanoreceptors
Pain
receptors
Temperature
receptors
Respond to
water-soluble
and lipid-
soluble
substances
dissolved in body
fluids
Sensitive to
stimuli that
distort their
plasma
membranes
Figure Receptors for the general senses can be classified by function and by sensitivity
Myelinated Type A fibers
(carry sensations of
fast pain)
Unmyelinated
Type C fibers (carry
sensations of slow
pain)
Proprioceptors
(monitor the
positions of joints
and muscles)
Baroreceptors
(detect pressure
changes)
Tactile receptors (provide the
sensations of touch, pressure,
and vibration) 15

16. Nociceptors (Pain Receptors)
Nociceptors are free nerve endings (branching tips of dendrites) with large receptive fields
Can be stimulated by many different stimuli
Located in every organ of the body except the brain
Superficial portions of the skin, joint capsules, periostea of bones, around the walls of blood vessels
Notifies brain about injuries or changes that may harm the body, used as a sign during diagnosis of disease or injury.
Tonic receptors: Adaptation is slight if at all

17. Nociceptors (Pain Receptors)
Fast pain
Acute, sharp, prickly pain-Such as from injection or deep cut
Carried by myelinated Type A fibers
Quickly reach CNS and trigger fast reflexive responses
Relayed to primary sensory cortex for conscious attention
Stimulus can be located to an area within a few cm
Slow pain
Chronic, burning or aching pain-such as back pain
Carried by unmyeleinated Type C fibers
Individual aware of pain but only general location

18. Phantom pain: sensations of pain in a limb that has been amputated
Brain interprets sensations from the remaining part of the limb as sensations from the amputated part or
Neurons in the brain that received input from the missing limb are still firing

19. Organ pain usually referred to skin
Referred pain: sensation of pain in one region of body that is not the source of the stimulus.
Organ pain usually referred to skin
Both the organ and that region of the skin input to the same spinal segment, activate the same interneurons and converge on the same ascending neurons
For example; pain of heart attack
frequently felt in medial left arm

20. Heart Liver and gallbladder Stomach Small intestine Ureters Appendix
Figure Referred Pain
Heart
Liver and gallbladder
Stomach
Small intestine
Ureters
Appendix
Colon 20

21. Thermoreceptors (temperature receptors) are free nerve endings
Located in the skin and hypothalamus
Maintain body temperature
Phasic (fast-adapting) receptors

22. Detect changes in the concentration of specific chemicals
Respond only to water-soluble and lipid-soluble substances dissolved in body fluids (interstitial fluid, plasma and CSF)
Monitor pH, CO2 and O2 levels in the blood
Information sent to autonomic control centers in brainstem: we do not perceive this sensation
Receptors exhibit peripheral adaptation over period of seconds; Central adaptation may also occur
Chemoreceptors

23. The body’s chemoreceptors, which play important roles in the
reflexive control of respiration and cardiovascular function
Chemoreceptors in Respiratory
Centers in the Medulla Oblongata
Trigger reflexive
adjustments in
depth and rate of
respiration
Respond to the concentrations of
hydrogen ions (pH) and carbon
dioxide (PCO2) in cerebrospinal fluid
Chemoreceptors of Carotid Bodies
Via cranial
nerve IX
Sensitive to changes in the pH,
PCO2 , and PO2 in arterial blood
Trigger reflexive
adjustments in
respiratory and
cardiovascular
activity
Chemoreceptors of Aortic Bodies
Via cranial
nerve X
Figure Baroreceptors and chemoreceptors initiate important autonomic reflexes involving visceral sensory pathways
Sensitive to changes in the pH,
PCO2, and PO2 in arterial blood 23

24. Mechanoreceptors
Stimulated by mechanical stimulus (touch, pressure, stretching, vibration) that distort the plasma membrane
Mechanically gated ion channels whose gates open or close in response to stimuli

25. Three Classes of Mechanoreceptors
Tactile receptors
Provide the sensations of touch, pressure, and vibration
Six Types of Tactile Receptors in the Skin
Baroreceptors
Detect pressure changes in the walls of blood vessels and in portions of digestive, reproductive and urinary tract
Proprioceptors
Monitor the positions of joints, tendons and muscles

26. Mechanoreceptors: A. Tactile Receptors
Meissner’s (Tactile) Corpuscles
Capsule
Dendrites
Tactile
corpuscle
Hair
Dermis
Afferent
fiber
Free Nerve Endings
Pacinian (Lamellated) Corpuscles
Free nerve
endings
Layers of
Collagen fibers
and fibroblasts
Sensory
nerve
Dendrite
Dermis
Root Hair Plexus
Hair shaft
Ruffini Corpuscles
Root hair
plexus
Sensory nerves
Capsule
Module 9.1 General sensory receptors in the skin vary widely in form and function
Dendrites
Afferent
fiber
Merkel (tactile) discs
Merkel cells
Merkel disc

27. Mechanoreceptors: B. Baroreceptors
Detect changes in stretch or distension or pressure in walls of blood vessels and in portions of digestive, reproductive and urinary tract.
Consist of free nerve endings that branch within elastic tissues in the wall of the vessel/organ
Phasic receptors: Respond immediately to a change in pressure, but adapt rapidly
Involved, for example, in blood pressure regulation

28. Mechanoreceptors: B. Baroreceptors
Baroreceptors of Carotid
Sinus and Aortic Sinus
Provide information on blood
pressure to cardiovascular and
respiratory control centers
Baroreceptors of Lungs
Provide information on lung
stretching to respiratory
rhythmicity centers for control
of respiratory rate
Baroreceptors of
Digestive Tract
Provide information on
volume of tract segments,
trigger reflex movement of
materials along tract
Figure Baroreceptors and chemoreceptors initiate important autonomic reflexes involving visceral sensory pathways
Baroreceptors of
Bladder Wall
Baroreceptors of Colon
Provide information on volume
of fecal material in colon, trigger
defecation reflex
Provide information
on volume of urinary
bladder, trigger urination
reflex 28

29. Mechanoreceptors: C. Proprioceptors
Monitor the position of joints and muscles
Tonic receptors-Do not adapt to constant stimulation
Most proprioceptive information is processed at a subconscious level
No proprioceptors in the visceral organs of the thoracic and abdominopelvic cavities
You cannot tell where your spleen, appendix, or pancreas is at the moment

30. Somatic sensory pathways
Somatic sensory pathways relay information from skin and musculature receptors to CNS. Neural pathway involves
First-order neuron
From receptor to synapse in spinal cord posterior gray horn
Second-order neuron
From posterior gray horn, crosses spinal cord and reaches thalamus
Third-order neuron
If conscious perception occurs
From thalamus to primary sensory cortex
Right side of cerebral hemisphere receives sensory information from left side of body due to decussation along the pathway

31. Three major somatic sensory pathways The spinothalamic pathway
The posterior column pathway
The spinocerebellar pathway
Posterior column pathway
Fasciculus gracilis
Fasciculus cuneatus
Dorsal root
Dorsal root ganglion
Spinocerebellar tracts
Posterior spinocerebellar tract
Anterior spinocerebellar tract
Spinothalamic tracts
Ventral root
Lateral spinothalamic tract
Anterior spinothalamic tract

32. Sensory homunculus (“little man”) is a functional map of somatic sensations from body parts to discrete areas in cerebral cortex
Proportions very different from individual because:
Area of sensory cortex devoted to particular body region is not proportional to region’s size, but to number of sensory receptors it contains

33. Figure 15-5 Somatic Sensory Pathways
POSTERIOR COLUMN PATHWAY
Ventral nuclei in thalamus
Midbrain
Nucleus gracilis and nucleus cuneatus
Medial lemniscus
Medulla oblongata
Fasciculus gracilis and fasciculus cuneatus
Dorsal root ganglion
Fine-touch, vibration, pressure, and proprioception sensations from right side of body 33

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

35. 15-4 Sensory Pathways Visceral Sensory Pathways
Collected by interoceptors monitoring visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities
These interoceptors are not as numerous as in somatic tissues
Interoceptors include:
Nociceptors
Thermoreceptors
Tactile receptors
Baroreceptors
Chemoreceptors

36. Cranial Nerves V, VII, IX, and X
Carry visceral sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, and associated vessels and glands
Solitary nucleus
Large nucleus in the medulla oblongata
Major processing and sorting center for visceral sensory information
Extensive connections with the various cardiovascular and respiratory centers, reticular formation

37. 15-5 Somatic Motor Pathways
The Somatic Nervous System (SNS)
Also called the somatic motor system
Controls contractions of skeletal muscles (discussed next)
The Autonomic Nervous System (ANS)
Also called the visceral motor system
Controls visceral effectors, such as smooth muscle, cardiac muscle, and glands (Ch. 16)

38. 15-5 Somatic Motor Pathways
The Somatic Nervous System (SNS)
Also called the somatic motor system
Provides voluntary (conscious) control over skeletal muscles

39. Somatic Nervous System (SNS)
Always involve at least two motor neurons
Upper motor neuron
Lower motor neuron

40. Upper Motor Neuron
Pyramidal neurons of Precentral gyrus of frontal lobe
All of these neurons are located in CNS
Descending tracts containing fibers of these neurons go to brainstem or the spinal cord
Fibers decussate: Hence each precentral gyrus controls muscles on the contralateral side of the body
Once reach brainstem or spinal cord, upper motor neuron synapses on the lower motor neuron

41. Lower Motor Neuron
Cell body lies in a nucleus of the brain stem or spinal cord
Axon of lower motor neuron extends outside CNS
Innervates a single motor unit in a skeletal muscle (effector); triggers contraction of muscle
May also be controlled by reflexes based in spinal cord
Destruction of or damage to lower motor neuron eliminates voluntary and reflex control over innervated motor unit

42. Somatic Nervous system (SNS)
Upper motor
neurons in
primary motor
cortex
BRAIN
Somatic motor
nuclei of brain
stem
Skeletal
muscle
Lower
motor
neurons
Spinal cord
Figure 14 Section 1 The Functional Anatomy and Organization of the Autonomic Nervous System (ANS)
Somatic
motor
nuclei of
spinal cord
Skeletal
muscle 42

43. Conscious and Subconscious Motor Commands control skeletal muscles by traveling over three integrated motor pathways
Corticospinal pathway
Medial pathway
Lateral pathway

44. Motor Homunculus
Map of primary motor cortex showing control of skeletal muscles
Like sensory homunculus, corresponds point by point with specific regions of the body
Motor area devoted to a specific area is proportional to number of motor units involved in the region’s control

45. Figure 15-8 The Corticospinal Pathway
Motor homunculus on primary motor cortex of left cerebral hemisphere
KEY
Axon of upper motor neuron
Lower motor neuron
To skeletal muscles
Corticobulbar tract
Motor nuclei of cranial nerves
Cerebral peduncle
To skeletal muscles
Midbrain
Motor nuclei of cranial nerves
Medulla oblongata
Decussation of pyarmids
Pyramids
Lateral corticospinal tract
Anterior corticospinal tract
To skeletal muscles
Spinal cord 45

46. Levels of Processing and Motor Control
All sensory and motor pathways involve a series of synapses, one after the other
Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over short term
Control the most basic motor activities
Voluntary responses are more complex and require more time to prepare and execute
Nervous system motor disorders may result from problems with neurons, pathways, or a combination of the two

47. Multiple levels of somatic motor control
The flow of information as an individual
begins a movement
Primary
motor
cortex
Motor
association
areas
Cerebral
cortex
Basal
nuclei
As the movement proceeds,
the cerebellum monitors
proprioceptive and vestibular
information and compares
the arriving sensations with
those experienced during
previous movements. It then
adjusts the activities of the
upper motor neurons
involved.
The basal nuclei adjust patterns
of movement in two ways:
1. They alter the sensitivity of the
pyramidal cells to adjust the
output along the corticospinal
tract.
2. They change the excitatory or
inhibitory output of the medial
and lateral pathways.
Other nuclei of
the medial and
lateral pathways
Cerebellum
Corticospinal
pathway
Lower
motor
neurons
Motor activity 47

48. When substantia nigra neurons are damaged or secrete less dopamine
Parkinson disease
When substantia nigra neurons are damaged or secrete
less dopamine
The substantia nigra from individuals with and
without Parkinson disease
Normal substantia nigra
Diminished substantia
nigra in Parkinson patient
Figure Nervous system disorders may result from problems with neurons, pathways, or a combination of the two 48

49. Amyotrophic Lateral Sclerosis (ALS)
Commonly known as Lou Gehrig’s disease
Progressive degenerative disorder
Affects upper and lower motor neurons in the spinal cord, brain stem and cerebrum
Atrophy of associated skeletal muscles
Stephen Hawkings

50. Cerebral palsy (CP)
Refers to a number of disorders that affect voluntary motor performance
Appears during infancy or childhood and persists throughout life
Cause may be:
Trauma associated with premature or stressful childbirth
Maternal exposure to drugs (including alcohol)
Genetic defect that causes improper motor pathway development

51. Acknowledgements: Visual Anatomy & Physiology: Martini/Ober/Nath
Pearson 2011 51