Chp 12
Chapter 12: The Somatic Sensory System
Learning Objectives
By the end of this chapter, you should be able to:
Describe the 4 major types of somatosensation.
Discuss the concept of a receptive field in the somatosensory system.
Describe how receptive fields vary with somatosensory afferent type and location on the body surface and how its size relates to the spatial acuity of touch.
Describe the four different types of mechanosensory cells found in glabrous skin, the types of tactile stimuli they detect, and the types of mechanoreceptor neurons with which they interact.
Describe the process of sensory transduction in mechanoreceptors.
Explain what proprioception is and what it is used for.
Identify the various neural pathways that convey touch information from the body and face to the brain.
Describe at what “level” mechanoreception and nociception crosses in the body.
Describe the somatotopic organization of touch stimuli present at various stages in the somatosensory circuits.
Discuss how the brain’s representation of the body surface is plastic and changes in response to altered input and experience.
Define nociception and explain how pain and itch are regulated and communicated to the brain and body.
Describe thermoreceptors, their pathway, and explain why peppers make us feel “hot” and menthol makes us feel “cold.”
Discuss how somatosensory dysfunction contributes to various diseases and disorders.
Important Terminology
Somatic Sensation: Enables the body to feel various stimuli, including touch, temperature, and pain.
Mechanoreception: Refers to the sense of touch detected through mechanoreceptors in the skin.
Proprioception: The body’s ability to perceive its position and movement in space.
Thermoreception: The perception of temperature through sensory receptors.
Nociception: The sensory process that provides signals that trigger pain responses.
Touch: Mechanoreceptors of the Skin
The skin is the largest sensory organ of the human body, and it is divided into two types:
Hairy skin: Contains hair follicles.
Glabrous skin: Lacks hair and is typically found on the palms and soles of the feet.
Skin is comprised of two main layers:
Epidermis: The outer layer that protects against environmental elements.
Dermis: The inner layer that contains connective tissue, blood vessels, and mechanoreceptors.
Mechanoreceptors
Mechanoreceptors are specialized cells sensitive to physical distortions such as bending, stretching, pressure, and vibration. They have unmyelinated axon branches that facilitate the sensation of touch.
Types of Mechanoreceptors
Pacinian Corpuscle:
Location: Found deep in the skin.
Function: Selective for high-frequency vibrations.
Meissner’s Corpuscle:
Location: Located on the ridges of glabrous skin.
Function: Detects edges, contours, and braille-like stimuli.
Ruffini’s Endings:
Location: Present in both hairy and glabrous skin.
Function: Detects stretching and force against the skin.
Merkel’s Disks:
Location: Situated in the epidermis.
Function: Detects form and roughness.
Krause End Bulbs:
Location: Found on the border regions of dry skin and mucous membranes.
Receptive Fields of Mechanoreceptors
Mechanoreceptors have varying sizes of receptive fields:
Small receptive fields: Meissner’s corpuscles and Merkel’s disks, allowing for high spatial acuity in touch.
Large receptive fields: Pacinian corpuscles and Ruffini’s endings, which are less sensitive to precise spatial differences.
Adaptation Rates
Rapidly Adapting Mechanoreceptors: Includes Meissner’s corpuscles and Pacinian corpuscles; they respond quickly but stop firing after a stimulus is applied.
Slowly Adapting Mechanoreceptors: Includes Merkel’s disks and Ruffini’s endings; they generate a sustained response throughout the duration of a stimulus.
Somatic Sensory Transduction
Mechanoreceptors possess unmyelinated axon terminals with mechanosensitive ion channels that convert mechanical force into changes in ionic current. These channels can open in three different ways, leading to action potentials.
Two-Point Discrimination
This concept measures spatial resolution, defined as the minimum distance at which two stimuli applied simultaneously are perceived as one. The paperclip test exemplifies two-point discrimination, revealing that our fingertips have the highest spatial resolution due to a high density of mechanoreceptors.
Primary Afferent Axons
Primary afferent axons transport information from somatic sensory receptors to the spinal cord via dorsal roots. The speed of signal conduction is influenced by the axon diameter and presence of myelin.
The Spinal Cord
The spinal cord contains 30 spinal segments (dorsal + ventral roots) categorized into four groups:
Cervical: 8 segments
Thoracic: 12 segments
Lumbar: 5 segments
Sacral: 5 segments
Dermatome: An area of skin innervated by a single spinal segment.
Pathway to the Brain
Initially, axons enter the ipsilateral dorsal column of the spinal cord. At the dorsal column nuclei in the brainstem, somatic information crosses to the opposite side of the body, wherein one side of the brain interprets sensations from the opposite side of the body.
Dorsal Column-Medial Lemniscal Pathway
Information regarding touch from the skin is carried along this pathway.
Process:
Axons terminate in the dorsal column nuclei (junction of spinal cord and medulla).
Somatosensory receptor axons enter the ipsilateral dorsal column of the spinal cord and ascend through the dorsal columns.
Axons CROSS and ascend through the medial lemniscus.
Finally, axons synapse on neurons in the ventral posterior (VP) nucleus of the thalamus, which project to the primary somatosensory cortex (S1).
Trigeminal Touch Pathway
Somatic sensations of the face do not travel through the spinal cord; instead, they largely travel through the trigeminal nerves. From the pons, trigeminal nucleus axons CROSS and project to the VP nucleus of the thalamus. The cranial nerves enter the brain at the pons (ipsilateral), and VP thalamic information is then sent to the S1.
Somatosensory Cortex
Primary Somatosensory Cortex (S1): Also known as Brodmann’s Area 3b, is located in the parietal lobe on the postcentral gyrus.
It possesses dense inputs from the VP nucleus of the thalamus, with neurons that are highly responsive to somatosensory stimuli. Lesions to S1 impair somatic sensation, while electrical stimulation can evoke somatic sensory experiences localized to specific parts of the body.
Cortical Somatotopy
The surface of the body is represented on the surface of the somatosensory cortex, a phenomenon known as cortical somatotopy.
Cortical Map Plasticity
The dynamics of the somatotopic map can adjust based on sensory experience. An example includes studies on the owl monkey brain, which show reorganization following sensory deprivation.
In humans, examples such as amputees and professional violinists demonstrate cortical map plasticity, with adaptations in the brain's sensory representation according to experiences and input changes.
Posterior Parietal Cortex
This area integrates multiple sensory modalities (touch, texture, sound, taste, smell, and sight) to form a complete mental image of a stimulus. Damage to posterior parietal cortex can lead to neurological disorders such as agnosia (inability to recognize objects despite normal sensory skills) and neglect syndrome (a part of the body or the environment is ignored).
Pain
Pain is described as an adaptive response that is communicated by unmyelinated nerve endings called nociceptors. These send signals whenever body tissue is damaged or at risk.
Pain Definition: The perception of irritating, sore, stinging, aching, throbbing, or unbearable sensations arising from a damaged area.
Nociception: The sensory process that provides signals triggering the experience of pain.
Nociceptor Activation
Nociceptors activate in response to stimuli with potential to cause tissue damage. The membrane depolarizes through mechanically gated ion channels, generating action potentials.
Damaged cells release various pain signaling substances, which contribute to the so-called "inflammatory soup": these include proteases, ATP, and K+.
Hyperalgesia
Defined as increased sensitivity of a body part to nociceptive stimuli. The inflammatory soup can modulate the excitability of nociceptors, making them more sensitive to thermal or mechanical stimuli. Key ingredients of inflammation include bradykinin, prostaglandin, and substance P.
Pain Signaling
Pain can be perceived in two distinct phases:
Fast, sharp first pain: Activated by Aδ fibers responsible for the initial sharp sensation.
Dull, longer-lasting second pain: Activated by C fibers associated with more chronic pain sensations.
The primary neurotransmitter involved in these signaling pathways is glutamate.
Ascending Pain Pathways
Unlike touch information, pain and temperature sensations cross over at the level of the spinal cord immediately and then ascend through the spinothalamic tract which synapses in the thalamus and travels to the primary somatosensory cortex (S1).
Pain Regulation
Pain regulation involves both afferent and descending pathways.
Afferent regulation: Stimulating large-diameter sensory neurons activates interneurons that inhibit pain signals. This implicates mechanoreceptors and thermoreceptors in pain modulation.
Descending regulation: The periaqueductal gray matter (PAG) can send axons that depress pain. This modulation is mediated through opioid receptors and endorphins.
Gate-theory of pain: Proposes that pain receptor-generated neural impulses can be inhibited in the spinal cord by signals produced in the brain and by inhibitory interneurons.
Temperature Sensation
Thermoreceptors are specialized neurons that respond to nonpainful temperature changes. There are six distinct subtypes that respond to different temperature ranges.
The active ingredient in hot peppers, capsaicin, identifies the “hot” receptor protein known as TRPV1, while the active ingredient in mint, menthol, identifies the “cold” receptor protein TRPM8.
Thermoreceptors exhibit adaptation to prolonged stimulation, conveying information to the brain through the same fibers as pain: Aδ and C fibers.
Conclusion
By understanding the structure and function of the somatic sensory system, including the various receptors and pathways involved in touch, pain, and temperature, we gain insight into how our body perceives and interacts with the environment. These concepts underscore the importance of somatic sensations for survival and quality of life, as well as the implications of dysfunction in these systems for health and disease.