1. Sensory Pathways and the General Senses
Chapter 15
Sensory Pathways and the General Senses
(“TOUCH”)

2. Sensory Pathways /Somatic Nervous System
Sensory receptors- Specialized sensory nerve endings that responds to a stimulus in the internal or external environment of an organism and monitor conditions in the body or external environment
Afferent Division of the Nervous System
Receptors; Sensory neurons; Sensory pathways
Sensory Pathways deliver somatic and visceral sensory information to their final destinations inside the CNS using: Nerves, Nuclei, Tracts
Somatic and visceral sensory information often travel along the same pathway. Somatic sensory information is distributed to either the primary sensory cortex of the cerebral hemispheres or appropriate areas of the cerebellar hemispheres.
Only about 1% of the sensory information is relayed to the primary sensory cortex.
Visceral sensory information is distributed primarily to reflex centers in the brain stem and diencephalon.
Efferent Division of the Nervous System
Somatic motor commands travel from motor centers in the brain along motor pathways via motor nuclei, tracts, and nerves
Conscious and subconscious motor functions

4. Sensory Receptors
Sensation - The arriving information from these senses
Perception - Conscious awareness of a sensation
General Senses -somesthetic
Visceral: conditions of internal organs
Somatic: external stimuli affecting the skin and its accessory structures as well as proprioceptive information from the joint
Describe our sensitivity to temperature, pain, touch, pressure, vibration, proprioception
Special Senses -Are located in sense organs protected by surrounding tissues
Olfaction (smell)
Vision (sight)
Gustation (taste)
Equilibrium (balance)
Hearing

5. Sensory RECEPTORS RECEPTOR type
Unencapsulated nerve ENDINGS-(sensory dendrites NOT wrapped in connective tissue)
Encapsulated nerve ENDINGS - wrapped in glia cells or connective tissue; for touch sensations and vibration. Enhances sensitivity to stimulus; are more selective
Each receptor has a characteristic sensitivity called receptor specificity.
The area monitored by a single receptor cell is called a receptive field.
The larger the receptive field, the more difficult it is to localize a stimulus
Sensory receptor transmissions based on the following:
Location- AREA OF SENSATION called receptor fields
Rate of Adaptation - tonic or phasic; measures:
Intensity – sound: loud/soft; light: bright/dim
Duration – LENGTH OF TIME of the stimulus
Modality- TYPE OF STIMULUS – pain, chemical, temperature, physical distortion

6. Encapsulated / Capsulated Nerve Endings

7. 1) LOCATION of Transmission Stimulus
Exteroceptors: interpret stimuli that come from the external environment at or near the surface of the body. Touch, pressure, vibration, temperature, pain, hearing, vision, smell, taste.
Interoceptors: interpret stimuli that are produced internally. Found in ORGANS such as blood vessels and the viscera. Perceived as pain or pressure
Proprioceptors: receptors that recognize the position of the body and about body position, muscle tension or activity of joints. Stretch receptors in muscles, tendons, and joints. Equilibrium receptors in inner ear.
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!!

8. 2) ADAPTATION
Adaptation- REDUCTION IN SENSITIVITY of a constant stimulus Receptors lose ability to respond and develop a diminished sensitivity to the stimulus. Continued exposure causes the brain cells to pay less attention to the stimulus and decreases the reaction to the particular sensation. Your nervous system quickly adapts to stimuli that are painless and constant
Tonic receptors (ex: nociceptors-pain receptors; joint capsule)
Always active; conveys information about stimulus DURATION
Adapt slowly to a stimulus and continues to produce action potentials over the duration of the stimulus.
Remind you of an injury long after the initial damage has occurred
Phasic receptors (ex: Ruffini corpuscle. thermoreceptors – temperature)
Provide information about the INTENSITY and rate of change of a stimulus
Receptor responds strongly at first; potential gradually decreases: response of the cell diminishes very quickly and then stops
Fast-adapting receptors

9. 3) Modality-TYPE Nociceptors - Pain Receptors
Nociceptors: respond to physical or chemical damage to tissues. Results in pain perception
Located in superficial portions of the skin; joint capsules; bones; walls of blood vessels
Free nerve endings (dendrites) are unencapsulated. They are the most common type of nerve ending, and are most frequently found in the skin; have large receptive fields. Located in every tissue of the body EXCEPT THE BRAIN.
In the meninges they play a role in headaches
Sensitive to temperature extremes, mechanical damage; dissolved chemicals, such as chemicals released by injured cells
Nociceptors have two different types of AXONS.
Myelinated TYPE A pain fibers
Conduct rapid sensations of fast pain, acute, sharp, or prickly such as that caused by an injection or a deep cut
Unmyelinated TYPE C pain fibers
Conduct sensations of slow pain, or burning and aching pain; pain begins several seconds after the stimulus and increases in intensity. Described as chronic, burning, aching, or throbbing

10. Encapsulated / Capsulated Nerve Endings

11. Phantom pain: sensations of pain in a limb that has been amputated
A) Brain interprets sensations from the REMAINING PART of the limb as sensations from the amputated part
B) neurons in the brain that RECEIVED INPUT FROM THE MISSING LIMB are still firing
Referred pain: sensation in one region of body that is not the source of the stimulus.
Pain in an internal organ is thought to be coming from the skin.
Types of Pain

12. Referred Pain

13. Modality Thermoreceptors
Thermoreceptors: respond to changes in temperature
Free nerve endings in
the dermis
skeletal muscles
liver
hypothalamus
Conducted along the same pathways that carry pain sensations
Quickly adapt to changing environments
Cold receptors 3-4x more numerous than warm
Modality Thermoreceptors
The adequate stimulus for a warm receptor is warming, which results in an increase in their action potential discharge rate. Cooling results in a decrease in warm receptor discharge rate.

14. Modality Mechanoreceptors
Responds to mechanical pressure or distortion: touch, pressure, vibration, proprioception, hearing and equilibrium
Sensitive to stimuli that distort plasma membranes
Contain mechanically gated ion channels whose gates open or close in response to stretching, compression, twisting, other membrane distortions
Three Classes of Mechanoreceptors
Tactile receptors
Provide the sensations of touch (shape or texture)
Pressure (degree of mechanical distortion)
Vibration (indicate pulsing or oscillating pressure)
Baroreceptors - Detect pressure changes in the walls of blood vessels, the digestive, reproductive, and urinary tracts
Proprioceptors - Monitor the positions of joints and muscles. In a dark room able to transmits signals to the brain of body movements without benefit of light.

15. Mechanoreceptors- Tactile SKIN Receptors
Free nerve endings – unspecialized, afferent nerve endings; bare dendrites no encapsulation; Sensitive to touch and pressure; -- pain and temperature, mucous membranes in mouth, vagina, anus
Most common type of nerve ending
Root hair plexus nerve endings; no encapsulation
Monitor distortions and movements across the body surface wherever hairs are located
Tactile discs - Merkel discs - touch and texture; sensitive to low vibration frequencies-small receptor field; dermis; no encapsulation
Tactile corpuscles - Also called Meissner’s corpuscles - fine touch and pressure receptors; -– fingers and toes; adapt to sensation with 1 second after contact; encapsulated
Perceive sensations of fine touch, pressure, and low-frequency vibration (eyelids, lips, fingertips, nipples, and external genitalia)
Lamellated corpuscles - Also called Pacinian corpuscles- sensitive to deep pressure; stretch, tickle, itch, and higher vibrations- hypodermis; encapsulated
Ruffini corpuscles – In the reticular (deep) dermis; sensitive to the cutaneous stretching produced by digit or limb movements; account for 20% of receptors located in the hand; encapsulated

16. Sensory Receptor Type

17. MODALITY Chemoreceptors Transmissions
Chemoreceptors: chemical concentrations - oxygen, carbon dioxide, taste, smell
Some respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid
smell - odor molecules
taste molecules
Carotid bodies and Aortic bodies are receptors that monitor pH, carbon dioxide, and oxygen levels in arterial blood.

18. General Senses- SENSORY PATHWAYS
From receptor to final destination in the brain, most somesthetic signals travel by way of three neurons
1st order neuron fibers (afferent neuron)- can DETECT and transmit/conduct stimulus information
from body, enter the dorsal horn of spinal cord via spinal nerves
from head, enter pons and medulla via cranial nerves
Fibers for touch, pressure and proprioception are large, fast, myelinated axons
Fibers for heat and cold are small, unmyelinated, slow fibers
2nd order neuron – cannot detect simply conduct information received from 1st order neuron/ most neurons are 2nd order.
decussation to opposite side in spinal cord, medulla, or pons
end in thalamus, except for proprioception, which ends in cerebellum
3rd order neuron
Complete the signal route from the thalamus to primary cortex of cerebrum

19. Sensory Pathways
Sensory pathways consist of the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations.
Each somatosensory pathway is named after a major tract or nucleus in the pathway.
The Spinothalamic Pathway – conscious sensation
Information transmitted from spinal cord to thalamus. Provides conscious sensations of poorly localized (thinly myelinated and unmyelinated nerve fibers) touch, pressure, SHARP PAIN, DROP IN TEMPERATURE
Posterior Column Pathway – conscious sensation
Carries well localized (heavily myelinated) sensations of (“FINE”) touch, pressure, vibration, and proprioception; undergo decussation at the 2nd neuron. From medulla nucleus gracilis & cuneatus
The Spinocerebellar Pathway – unconscious sensation
Cerebellum purkinje cells receives proprioceptive information about joint position, muscle, tendons from spinal cord. This data does NOT reach our conscious awareness
Visceral Sensory Pathways unconscious sensation
Collected by interoceptors monitoring visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities

21. Somatic Motor Pathways
The Somatic Nervous System (SNS)
Also called the SOMATIC MOTOR SYSTEM
Controls contractions of SKELETAL MUSCLES
The Autonomic Nervous System (ANS)
Also called the VISCERAL MOTOR SYSTEM
Controls visceral effectors, such as SMOOTH MUSCLE, CARDIAC MUSCLE, AND GLANDS