This document describes the somatic sensory and motor pathways in the nervous system. It discusses the different types of sensory receptors for touch, pain, temperature, and proprioception. The major somatic sensory pathways are the posterior column pathway for fine touch and proprioception, and the anterolateral pathway for crude touch, pain, and temperature. The spinocerebellar pathway relays proprioceptive information to the cerebellum. Somatic motor commands originate from motor areas of the brain and travel via pathways like the corticospinal tract to lower motor neurons innervating skeletal muscles. The basal ganglia and cerebellum provide feedback to coordinate voluntary muscle contractions.

1. Chapter 15:
Neural Integration I:
Sensory Pathways and the Somatic
Nervous System

2. General Senses
• Describe our sensitivity to:
– Temperature, pain, touch, pressure, vibration, &
proprioception
• Sensation - The arriving information from
these senses
• Perception - Conscious awareness of a
sensation

3. Special Senses
• Olfaction (smell)
• Vision (sight)
• Gustation (taste)
• Equilibrium (balance)
• Hearing

4. Free Nerve Endings
• The simplest of our sensory receptors
• Branching tips of dendrites
• Not protected by accessory structures
• Can be stimulated by many different stimuli

5. Figure 15–2
Receptive Field
• Area is monitored by a single receptor cell
• The larger the receptive field, the more difficult
it is to localize a stimulus

6. Adaptation
• Reduction in sensitivity of a constant stimulus
• Tonic Receptors - Are always active
• Phasic Receptors - Are normally inactive &
become active for a short time whenever a
change occurs
• Fast-Adapting Receptors - Response
characteristic of phasic receptors (smell & taste)
• Tonic Receptors - Called slow-adapting
receptors (proprio- & nociceptors)
– Remind you of an injury long after the initial damage
has occurred

7. Location of stimulus
• Exteroceptors- sensitive to stimuli arising outside the
body
– Touch, pressure, pain, special senses
• Interoceptors- (visceroceptors)- respond to stimuli
from inside the body (viscera/BV’s)
– Chemical changes, stretching of tissues,
temperature
– We are typically unaware of these receptors except
for pain, discomfort, hunger, & thirst
• Proprioceptors- respond to internal stimuli
– Location is only in skeletal muscle, tendons, joints,
ligaments, & CT coverings of bones & muscles

8. Stimulus type
• Mechanoreceptors- deformed by force
–Touch, pressure (BP), vibration, stretch, itch
• Thermoreceptors- changes in temperature
• Photoreceptors- light energy
• Chemoreceptors- chemicals in solution
–Smell, taste, blood chemistry
• Nociceptors- pain
All receptors can interpret pain if overstimulated!

9. Nociceptors
• Are common in the:
– superficial portions of the skin
– joint capsules
– within the periostea of bones
– around the walls of blood
vessels
• Free nerve endings with large
receptive fields
• May be sensitive to:
– extremes of temperature
– mechanical damage
– dissolved chemicals, such as
chemicals released by injured
cells Figure 15–2

10. Type A and Type C Fibers
• Type A Fibers - Carry sensations of fast pain,
or prickling pain, such as that caused by an
injection or a deep cut
– Sensations reach the CNS quickly and often trigger
somatic reflexes
– Relayed to the primary sensory cortex and receive
conscious attention
• Type C Fibers - Carry sensations of slow pain,
or burning and aching pain
– You become aware of the pain but only have a
general idea of the area affected

11. Thermoreceptors
• Also called temperature receptors
• Are free nerve endings located in:
– the dermis
– skeletal muscles
– the liver
– the hypothalamus
• Conducted along the same pathways that
carry pain sensations

12. 3 Classes of Mechanoreceptors
• Tactile receptors:
– provide the sensations of touch, pressure, and
vibration
• 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 joints and muscles

13. • Fine Touch and Pressure Receptors - Are
extremely sensitive & have a relatively narrow
receptive field
–Provide detailed information about a source
of stimulation, including: its 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

14. Tactile Receptors
• Range in complexity
from free nerve
endings to
specialized sensory
complexes with
accessory cells and
supporting structures
Figure 15–3

15. Figure 15–3a
6 Types of Tactile Receptors in the
Skin
• Free nerve endings:
– sensitive to touch and pressure
– situated between epidermal
cells
• Root hair plexus :
– monitor distortions and movements
across the body surface wherever hairs
are located
– adapt rapidly, so are best at detecting
initial contact and subsequent
movements

16. Figure 15–3c
6 Types of Tactile Receptors in the Skin
• Tactile discs:
– also called Merkel’s discs
– fine touch and pressure
receptors
• Tactile corpuscles:
– also called Meissner’s corpuscles
– perceive sensations of fine touch,
pressure, and low-frequency vibration
– most abundant in the eyelids, lips,
fingertips, nipples, and external
genitalia

17. Figure 15–3e
6 Types of Tactile Receptors in the Skin
• Lamellated corpuscles:
– also called Pacinian corpuscles
– sensitive to deep pressure
– fast-adapting receptors
• Ruffini corpuscles:
– also sensitive to pressure and
distortion of the skin
– located in the reticular (deep) dermis

18. 3 Major Groups of Proprioceptors
• Muscle spindles:
– monitor skeletal muscle length
– trigger stretch reflexes
• Golgi tendon organs:
– located at the junction between skeletal muscle and
its tendon
– stimulated by tension in tendon
– monitor external tension developed during muscle
contraction
• Receptors in joint capsules:
– free nerve endings detect pressure, tension, and
movement at the joint

19. Chemoreceptors
• Located in the:
– carotid bodies:
• near the origin of the internal carotid arteries on each
side of the neck
– aortic bodies:
• between the major branches of the aortic arch
• Receptors monitor Ph, carbon dioxide, and
oxygen levels in arterial blood

20. White Matter in the Spinal Cord
• Fibers run in three directions – ascending, descending,
and transversely
• Divided into three funiculi (columns) – posterior,
lateral, and anterior
• Each funiculus contains several fiber tracts
– Fiber tract names reveal their origin and destination
– Fiber tracts are composed of axons with similar functions
• Pathways decussate (cross-over)
• Most consist of two or three neurons
• Most exhibit somatotopy (precise spatial relationships)
• Pathways are paired (one on each side of the spinal
cord or brain)

21. Processing at the circuit level
• First order neurons (cell bodies in DRG or cranial nuclei)
– Conduct impulses from receptors/proprioceptors to
the cord or brain stem to synapse w/ 2nd
order
neurons
• Second order neurons (cell bodies in dorsal horn of cord or
medullary nuclei)
– Transmit impulses to the thalamus or cerebellum
where they synapse
• Third order neurons (none found in the cerebellum)
– Located in the thalamus & conduct impulses to the
somatosensory cortex of the cerebrum

22. 3 Major Somatic Sensory Pathways
1. The posterior column pathway
2. The anterolateral pathway
3. The spinocerebellar pathway

23. Posterior Column Pathway
• Fasciculus gracilis
• Fasciculus cuneatus
• Carries sensations of
highly localized
(“fine”) touch,
pressure, vibration,
and proprioception
Figure 15–5a

24. Ability to Determine Stimulus
• Precisely where on the
body a specific stimulus
originated depends on
the projection of
information from the
thalamus to the primary
sensory cortex
• Sensory Homunculus

25. The Anterolateral Pathway
• Provides sensations of
“crude” touch, pressure,
pain, and temperature
• Ascend within the anterior
or lateral spinothalamic
tracts:
– the anterior spinothalamic
tracts carry crude touch and
pressure sensations
– The lateral spinothalamic
tracts carry pain and
temperature sensations

26. Strong Visceral Pain aka Referred pain
• An individual can feel pain in uninjured part of body
when pain actually originates at another location
• Sensations arriving at segment of spinal cord can
stimulate interneurons that are part of anterolateral
pathway
• Activity in interneurons leads to stimulation of primary
sensory cortex, so an individual feels pain in specific
part of body surface:

27. The Spinocerebellar Pathway
• Cerebellum receives
proprioceptive
information about
position of skeletal
muscles, tendons, and
joints
Figure 15–7

28. Visceral Sensory Information
• Collected by interoceptors monitoring visceral tissues
and organs, primarily within the thoracic and
abdominopelvic cavities
• These interoceptors, not as numerous as in somatic
tissues, include:
– nociceptors
– thermoreceptors
– tactile receptors
– baroreceptors
– chemoreceptors

29. Somatic Motor Pathways
• Upper motor neuron:
– cell body lies in a CNS processing center
– synapses on the lower motor neuron
– activity in upper motor neuron may facilitate or inhibit
lower motor neuron
• Lower motor neuron
– cell body lies in a nucleus of the brain stem or spinal
cord
– triggers a contraction in innervated muscle:
– destruction of or damage to lower motor neuron
eliminates voluntary and reflex control over innervated
motor unit

30. Corticospinal Pathway
• Sometimes called the
pyramidal system
• Provides voluntary control
over skeletal muscles:
– system begins at pyramidal
cells of primary motor cortex
– axons of these upper motor
neurons descend into brain
stem and spinal cord to
synapse on lower motor
neurons that control skeletal
muscles
Figure 15–9

31. Motor Homunculus
• Primary motor cortex
corresponds point by
point with specific
regions of the body
• Cortical areas have
been mapped out in
diagrammatic form

32. Somatic Motor Commands
• Several centers in cerebrum, diencephalons,
and brain stem may issue somatic motor
commands as result of processing performed
at subconscious level

33. Basal Nuclei and Cerebellum
• Responsible for coordination and feedback control
over muscle contractions, whether contractions are
consciously or subconsciously directed
• Basal Nuclei - provide background patterns of
movement involved in voluntary motor activities
• Cerebellum - monitors:
– proprioceptive (position) sensations
– visual information from the eyes
– vestibular (balance) sensations from inner ear as
movements are under way