1. FUNDAMENTALS OF ANATOMY AND PHYSIOLOGY
Mrs. Willie Grant, Instructor
(210)

3. An Introduction to Sensory Pathways and the Somatic Nervous System
Learning Outcomes
15-1 Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system.
15-2 Explain why receptors respond to specific stimuli, and how the organization of a receptor affects its sensitivity.
15-3 Identify the receptors for the general senses, and describe how they function.
15-4 Identify the major sensory pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body.
15-5 Describe the components, processes, and functions of the somatic motor pathways, and the levels of information processing involved in motor control.

5. 15-1 Sensory Information Sensory Receptors
Specialized cells that monitor specific conditions in the body or external environment
When stimulated, a receptor passes information to the CNS in the form of action potentials along the axon of a sensory neuron
Sensory Pathways
Deliver somatic and visceral sensory information to their final destinations inside the CNS using: Nerves Nuclei Tracts
1 Specialized cells that monitor specific conditions in the body or the external environment are called ____________.

6. 15-1 Sensory Information
Somatic Motor Portion of the Efferent Division
Controls peripheral effectors
Somatic Motor Commands
Travel from motor centers in the brain along somatic motor pathways of:
Motor nuclei
Tracts
Nerves
Somatic Nervous System (SNS)
Motor neurons and pathways that control skeletal muscles

7. 15-2 Sensory Receptors
The Detection of Stimuli
Receptor specificity—each receptor has a characteristic sensitivity
Receptive field—area is monitored by a single receptor cell
The larger the receptive field, the more difficult it is to localize a stimulus

8. Adaptation is the reduction in sensitivity of a constant stimulus
15-2 Sensory Receptors
Adaptation is the reduction in sensitivity of a constant stimulus
Your nervous system quickly adapts to stimuli that are painless and constant
Tonic receptors (always active, slow-adapting receptors that show little peripheral adaptation)
Remind you of an injury long after the initial damage has occurred
Phasic receptors (normally inactive, fast-adapting receptors that become active whenever a change occurs)
Provide information about the intensity and rate of change of a stimulus
2 A reduction in sensitivity in the presence of a constant stimulus is __________________.

10. 15-2 Sensory Receptors
General Senses
Describe our sensitivity to:
Temperature Pressure
Pain Vibration
Touch Proprioception
Sensation
The arriving information from these senses
Perception
Conscious awareness of a sensation

11. 15-2 Sensory Receptors
Special Senses
Olfaction (smell)
Vision (sight)
Gustation (taste)
Equilibrium (balance)
Hearing
The Special Senses
Are provided by special sensory receptors located in sense organs

12. 15-2 Sensory Receptors
The Interpretation of Sensory Information
Arriving stimulus reaches cortical neurons via labeled line (link between peripheral receptor and cortical neuron)
Takes many forms (modalities)
Physical force (such as pressure)
Dissolved chemical
Sound
Light
Sensations
Taste, hearing, equilibrium, and vision provided by specialized receptor cells
Communicate with sensory neurons across chemical synapses

13. 15-3 Classifying Sensory Receptors
Exteroceptors provide information about the external environment
Proprioceptors report the positions of skeletal muscles and joints and provide a purely somatic sensation.
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).
Interoceptors monitor visceral organs and functions
General Sensory Receptors (divided by nature of the stimulus)
Nociceptors (pain) Thermoreceptors (temperature) Mechanoreceptors (physical distortion) Chemoreceptors (chemical concentration)
3 Receptors that monitor visceral organs and functions are simply classified as ______.
_________

14. 15-3 Classifying Sensory Receptors
1 Nociceptors (Pain Receptors)
Are common in the superficial portions of the skin, joint capsules, periostea of bones and around walls of blood vessels
May be sensitive to: Temperature extremes, mechanical damage, dissolved chemicals, such as chemicals released by injured cells
Are free nerve endings with large receptive fields.
Two types of axons - Type A (myelinated for fast pain or prickling pain) Type C fibers (slow pain or burning and aching pain)
2 Thermoreceptors
Also called temperature receptors
Are free nerve endings located in: The dermis, Skeletal muscles, the liver, and the hypothalamus
Temperature sensations carry pain sensations sent to: The reticular formation, the thalamus, and the primary sensory cortex

15. 15-3 Classifying Sensory Receptors
3 Mechanoreceptors
Sensitive to stimuli that distort their plasma membranes
Contain mechanically gated ion channels whose gates open or close in response to: Stretching, Compression, Twisting, and Other distortions
Three Classes of Mechanoreceptors
Tactile receptors (provide the sensations of touch, pressure, vibration, shape, texture)
Pressure sensations indicate degree of mechanical distortion
Vibration sensations indicate pulsing or oscillating pressure
Baroreceptors (detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts)
Proprioceptors (monitor the positions of the joints and muscles)
The most structurally and functionally complex of general sensory receptors

16. 15-3 Classifying Sensory Receptors
Tactile Receptors
Fine touch and pressure receptors
Are extremely sensitive. Have a relatively narrow receptive field.
Provide detailed information about a source of stimulation including exact location, shape, size, texture, movement.
Crude touch and pressure receptors
Have relatively large receptive fields. Provide poor localization. Give little information about the stimulus.

17. 15-3 Classifying Sensory Receptors
Six Types of Tactile Receptors in the Skin
1 Free nerve endings which are sensitive to touch and pressure situated between epidermal cells. The nerve endings providing touch sensations are tonic receptors with small receptive fields.
2 Root hair plexus nerve endings monitor distortions and movements across the body surface whereever hairs are located
3 Tactile discs (Merkel discs) are fine touch and pressure receptors that are extremely sensitive to tonic receptors and have very small receptive fields.

18. 15-3 Classifying Sensory Receptors
Six Types of Tactile Receptors in the Skin
4 Tactile corpuscles (Meissner’s corpuscles) perceive sensations of fine touch, pressure and low-frequency vibration. They adapt to stimulation with 1 second after contact.
Most abundant in the eyelids, lips, fingertips, nipples, and external genitalia
5 Lamellated corpuscles (Pacinian corpuscles) are sensitive to deep pressure. They are fast-adapting and are most sensitive to pulsing or high frequency vibrating stimuli
6 Ruffini corpuscles
Also sensitive to pressure and distortion of the skin
Located in the reticular (deep) dermis
Tonic receptors that show little if any adaptation

19. 4 Fine touch and pressure receptors are called __________.
Figure 15-3 Tactile Receptors in the Skin
4 Fine touch and pressure receptors are called __________.
5 Temperature receptors are called ___________________.

20. 15-3 Classifying Sensory Receptors
Baroreceptors—Monitor change in pressure
Consist of free nerve endings that branch within elastic tissues within walls of distensible organs (blood vessels). Respond immediately to a change in pressure, but adapt rapidly.
Proprioceptors
Monitor position of the joints, tension in the tendons and ligaments as well as the state of muscular contraction
Three Major Groups of Proprioceptors
Muscle spindles (monitor skeletal muscle length and trigger stretch reflexes)
Golgi tendon organs (receptors in tendon that monitors the external tension during muscle contraction)
Receptors in joint capsules (receptors that detect pressure, tension, and movement at the joint).
6 Proprioceptors that monitor skeletal muscle length and trigger stretch reflexes are the _____________.

21. 15-3 Classifying Sensory Receptors
Chemoreceptors
Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid
Receptors that monitor pH, carbon dioxide, and oxygen levels in arterial blood are located in: Carotid bodies (within carotid arteries) and Aortic bodies (between branches of the aortic arch)

22. Somatic Sensory Pathways
Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs
Three major somatic sensory pathways
The spinothalamic pathway (provides conscious sensations of poorly localized (“crude”) touch, pressure, pain and temperature)
The posterior column pathway (carries sensations of highly localized (“fine”) touch, pressure, vibration and proprioception).
The spinocerebellar pathway (sends proprioceptive information about the position of skeletal muscles, tendons, and joints to the cerebellum).
7 Conscious sensations of poorly localized “crude” touch, pressure, pain and temperature are provided by the __________.

26. Feeling Pain (Lateral Spinothalamic Tract)
15-4 Sensory Pathways
Feeling Pain (Lateral Spinothalamic Tract)
Referred pain (Pain felt at a site other than where the cause is situated)
The pain of a heart attack is frequently felt in the left arm
The pain of appendicitis is generally felt first in the area around the navel and then in the right, lower quadrant
8 Where is the pain of a heart attack felt?
9 Which visceral organ has the broadest area for referred pain?

27. The Somatic Nervous System (SNS)—the somatic motor system
15-4 Sensory Pathways
Sensory Information
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
The Somatic Nervous System (SNS)—the somatic motor system
Controls contractions of skeletal muscles
The Autonomic Nervous System (ANS)—the visceral motor system
Controls visceral effectors, such as smooth muscle,
cardiac muscle, and glands
10 What is ALS?

28. Levels of Processing and Motor Control
All sensory and motor pathways involve a series of synapses, one after the other. Along
the way, the information is distributed to processing centers operating at the subconscious
level.
Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that
preserve homeostasis over the short term.
Voluntary responses are more complex and require more time to prepare and execute.
Cranial and spinal reflexes control the most basic motor activities. Integrative centers in the
brain perform more elaborate processing.
As you move from the medulla oblongata to the cerebral cortex, the motor patterns become
more complex and variable. The primary motor cortex of the cerebral hemispheres directs
the most complex and variable motor activities.

29. Clinical Case Living with Cerebral Palsy
Name the principal descending motor pathways that carry information that would be
Affected by the brain damage causing Michael’s cerebral palsy.
Besides aspiration pneumonia, what other difficulties could result from poor motor
Control of the muscles used in swallowing?

31. Figure 15-5 Somatic Sensory Pathways (Part 4 of 6)
POSTERIOR COLUMN PATHWAY
SPINOCEREBELLAR PATHWAY
The posterior column pathway carries sensations of highly localized (”fine”) touch, pressure, vibration, and proprioception. This pathway, also known as the dorsal column-medial lemniscus, begins at a peripheral receptor and ends at the primary sensory cortex of the cerebral hemispheres.
The cerebellum receives proprioceptive information about the position of skeletal muscles, tendons, and joints along the spinocerebellar pathway. The posterior spinocerebellar tracts contain axons that do not cross over to the opposite side of the spinal cord. These axons reach the cerebellar cortex via the inferior cerebellar peduncle of that side. The anterior spinocerebellar tracts are dominated by axons that have crossed over to the opposite site of the spinal cord.
Ventral nuclei in thalamus
PONS
Midbrain
Cerebellum
Nucleus gracilis and nucleus cuneatus
Medial lemniscus
Medulla oblongata
Medulla oblongata
Spinocerebellar pathway
Fasciculus gracilis and fasciculus cuneatus
Posterior spinocerebellar tract
Spinal cord
Anterior spinocerebellar tract
Dorsal root ganglion
Fine-touch, vibration, pressure, and proprioception sensations from right side of body
Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsules 31

32. Figure 15-5 Somatic Sensory Pathways (Part 1 of 6)
SPINOTHALAMIC PATHWAY
The spinothalamic pathway provides conscious sensations of
poorly localized (“crude”) touch, pressure, pain, and temperature. In
this pathway, the axons of first-order neurons enter the spinal cord
and synapse on second-order neurons within the posterior gray
horns. The axons of these interneurons cross to the opposite side of
the spinal cord before ascending to the thalamus. The third-order
neuron synapses in the primary sensory cortex.
KEY
Axon of first- order neuron
Second-order neuron
Third-order neuron
A Sensory Homunculus
A sensory homunculus
(“little human”) is a functional
map of the primary sensory
cortex. The proportions are
very different from those of
any individual because the
area of sensory cortex devoted
to a particular body region is
proportional to the number of
sensory receptors it contains.
Midbrain
Midbrain
The anterior spinothalamic tracts of the spinothalamic pathway carry crude
touch and pressure sensations.
The lateral spinothalamic tracts of the spinothalamic pathway carry pain and temperature sensations.
Medulla oblongata
Medulla oblongata
Anterior spinothalamic tract
Lateral spinothalamic tract
Spinal cord
Crude touch and pressure sensations from right side of body
Pain and temperature sensations from right side of body 32