Chapter 12 Somatic Sensory System
Axons ascending via the medial lemniscus to synapse onto
3rd order neurons in the ventral posterior (VP) nucleus of the thalamus
The Trigeminal Pathway: Axons from the trigeminal nucleus decussate and ascend to the VP nucleus of the thalamus, synapsing onto
3rd order neurons whose axons project to the somatosensory (S1) cortex
The Spinal Cord: Can be divided into
4 groups
First and Second Pain: A fast, sharp first pain caused by
Agamma fiber activation
Somatic Receptors in the Skin- Skin: 2 layers
Epidermis Dermis
Sensory Receptors and Activation- Different receptors mediating somatic sensations can be activated by a variety of ways:
Mechanical deformation application of a chemical by a change in temperature by electromagnetic radiation (such as observed when light strikes the visual receptors)
Mechanoreceptors of the Skin: Rapidly Adapting Small Receptive Fields
Meissner's corpuscles
Mechanoreceptors of the Skin: Slowly Adapting Small Receptive Fields
Merkel's disks
Mechanoreceptors of the Skin: Rapidly Adapting Large Receptive Fields
Pacinian corpuscles
Mechanoreceptors of the Skin: Slowly Adapting Large Receptive Fields
Ruffini's endings
Somatic sensations include
Sensations associated with touch, pain, temperature, and proprioception
Ascending Pain Pathways- (The Anterolateral System):
Spinoreticular tract (SRT) Spinomesencephalic tract (SMT) Trigeminothalamic tract
The purpose of all sensory receptors is to mediate different
Types of stimuli into a change in membrane potential
The Trigeminal Pathway: Axons from the trigeminal nucleus decussate and ascend to the
VP nucleus of the thalamus
Two-Point Discrimination: Fingertips have
a higher density of mechanoreceptors
Sensory Interpretation: Each nerve fiber, carrying information related to
a specific sensory modality
The Segmented Spinal Cord: Dorsal roots carry
afferent (sensory) fibers
The Dorsal Column-Medial Lemniscal (DCML) Pathway: After entering the ipsilateral dorsal column medial to the dorsal horn; Abeta fibers ascend
along with 2nd order axons from the spinal gray matter
Transduction of Somatic Sensations: If depolarizing current is large enough,
an action potential will be generated
Ascending Pain Pathways- (The Anterolateral System): Spinothalamic tract (STT) includes the
anterior STT around the lateral STT (cells from the tracts originate in different lamina)
In the DCN, axons decussate before
ascending via the medial lemniscus
The Segmented Spinal Cord: Each spinal nerve upon entering the spinal column
bifurcates into a dorsal root and a ventral root
The Dorsal Column-Medial Lemniscal (DCML) Pathway: Tactile (touch) information is transmitted to the
brain by a pathway known as the DCML pathway
Do all mechanoreceptors respond the same?: A single stimulus
can, and often activates several receptors
Sensory Transduction: Activation of a sensory receptor
causes a change in membrane potential called a receptor potential
Somatosensory Cortex: Much of the complex processing associated with somatosensations occurs in the
cerebral cortex; specifically the primary somatosensory cortex (S1) (aka, Brodmann's area 3b) in the parietal lobes
Mechanoreceptors of the Skin: Make up the largest
collection of sensory receptors in the somatic sensory system
Pain: Although Abeta-type fibers play a role in neuropathic pain, their role in normal pain is somewhat
controversial
Somatosensory Cortex: The figures (caricature) is not scaled according to the human body, but representative of the
density of the sensory receptors from that particular area; which reflects the importance of sensory input from that part of the body
Dermatomes: There is a one-to-one correspondence between
dermatomes and spinal segments
Sensory Receptors and Activation- Types of sensory receptors: Thermoreceptors
detect changes in temperature
Sensory Receptors and Activation- Types of sensory receptors: Chemoreceptors
detect chemicals that may or may not be irritating
Sensory Receptors and Activation- Types of sensory receptors: Electromagnetic receptors
detect light such as found in the retina of the eye
Sensory Receptors and Activation- Types of sensory receptors: Nociceptors
detect painful
Sensory Receptors and Activation- Types of sensory receptors: Mechanoreceptors
detect physical distortion like that associated with touch and pressure
Sensory Transduction: Different sensory receptors have
differing sensitivities to different types of sensory stimuli
Two-Point Discrimination: The ability to
discriminate 2 points of stimuli as being separate- that is, spatial resolution
The Dorsal Column-Medial Lemniscal (DCML) Pathway: After ascending, Abeta fibers terminate in the
dorsal column nuclei (DCN) in the medulla
Descending Pain Pathways: Many (if not most) of the targets of descending input are the
dorsal horns of the spinal cord as well as the trigeminal nucleus
Herpes, Shingles, and Dermatomes: Following the initial infection (chicken pox)...The virus remains in the
dorsal root ganglia
The Segmented Spinal Cord: Ventral roots carry
efferent (motor) fibers
Two-Point Discrimination: It varies across the body, with the
fingertips having the greatest ability to discriminate (the highest resolution)
Pain: Nociceptors are mostly
free-ending small unmyelinated C-type fibers, but also include Agamma-type fibers which also transmit pain
Somatic Receptors in the Skin- Skin: 2 types
hairy hairless (glabrous)
Herpes, Shingles, and Dermatomes: Varicella (chicken pox) is caused by
herpes zoster virus
Sensory Interpretation: Each nerve fiber related to a specific sensory modality terminates
in a specific part of the CNS; that is, different parts of the brain receive and interpret different sensations
Herpes, Shingles, and Dermatomes: Reactivation affects the skin (dermatome) supplied by the
infected DRG (however, the rash can involve more than one dermatome)
Pain: Somatic sensory receptors called nociceptors transmit
information about pain to the CNS
Pain: Transduction involves
ion channels that are activated by a variety of stimuli
The Dorsal Column-Medial Lemniscal (DCML) Pathway: Abeta fibers enter the
ipsilateral dorsal column (white matter) medial to the dorsal horn
The Trigeminal Pathway: These axons synapse onto 2nd order neurons in the
ipsilateral trigeminal nucleus in the pons
Mechanoreceptors of the Skin: These receptors are mostly innervated by
large-diameter myelinated A-type fibers, while some are innervated by small-diameter unmyelinated C-type fibers
Dermatomes: Cutting a single dorsal root does not mean
losing all sensation to that corresponding dermatome
Primary Afferent Axons (PAFs): Peripheral nerves
made up of many nerve fibers (axons) some are afferent fibers and some are efferent fibers
Do all mechanoreceptors respond the same?: Different mechanical sensitives of
mechanoreceptors mediate different sensations
Each somatic sensation is a
modality of sensation
Descending Pain Pathways: Signals from the PAG play a large role in
modulating pain signals in the dorsal horn
Descending Pain Pathways: These pathways exert
much control over pain signals and its perception
Sensory Transduction: All sensory fibers transmit
nerve impulses in the form of action potentials
Pain: The International Association for the Study of Pain (IASP) defines "nociception" as the
neural process of encoding noxious stimuli
Pain and Hyperalgesia: Substance P is synthesized by
nociceptors
Pain and Hyperalgesia: Tissues damage results in
numerous substances being released that act to modulate nociceptor excitability
Dermatomes: Adjacent spinal nerves innervate
overlapping areas
Dermatomes: Each spinal nerve innervates a
particular "segment" of skin called a dermatome
The Segmented Spinal Cord: From the spinal nerve to the sensory receptor is considered the
peripheral nerve
Pain: Types of receptors include
polymodal mechancal thermal chemical
The Dorsal Column-Medial Lemniscal (DCML) Pathway: 3rd order neurons project to the
primary somatosensory cortex (S1) (most terminating the postcentral gyrus)
Pain: Nociception is the process that
provides the information that results in pain
Herpes, Shingles, and Dermatomes: After the chicken pox virus is dormant for years, can become
reactivated, resulting in shingles
Transduction of Somatic Sensations: A stimulus causes channels to open
resulting in the movement of ions (current flow) and the generation of a receptor potential (which is depolarizing)
Descending Pain Pathways: Two of these systems are the
rostral ventromedial medulla (RVM) and the dorsolateral pontine tegmentum (DLPT)
First and Second Pain: First pain is followed by a duller, longer-lasting
second pain; caused by C fiber activation
The Segmented Spinal Cord: Each spinal nerve upon entering the spinal cord is
segmented into a "spinal segment" that corresponds to each pair of spinal nerves
Two-Point Discrimination: More cortex is devoted to
sensory information from the fingertip than elsewhere
Somatic Receptors in the Skin- Skin: The largest
sensory organ
Pain and Hyperalgesia: Substance P can sensitize nearby nociceptors around the
site of injury resulting in secondary hyperalgesia
Herpes, Shingles, and Dermatomes: Reactivation affects the
skin (dermatome)
Dermatomes: Sensory innervation of the
skin and the segmental organization of spinal nerves are related
Two-Point Discrimination: Receptors in the fingertips have
small receptive fields
Somatosensory Cortex: The body's surface sensations can be mapped onto the brain giving a
somatotopic map (this reflects that a specific surface sensation area is associated with a particular area of the primary somatosensory cortex)
Pain and Hyperalgesia: Tissues damage results in numerous substances being released such as
substance P bradykinin prostaglandins histamine ATP
Pain and Hyperalgesia: Secondary hyperalgesia occurs in the
supersensitive region surrounding the damaged area
The Spinal Cord: 1st order neurons (PAFs) are able to
synapse with other neurons (2nd order neurons) that ascend to higher centers in the spinal cord and the brain
Ascending Pain Pathways- (The Anterolateral System): Spinothalamic tract (STT) fibers terminate in several nuclei of the
thalamus, including the posterior portion of the ventral medial nucleus (VMpo)
Pain and Hyperalgesia: Primary hyperalgesia occurs within
the area of tissue damage
Dermatomes: There is much overlap and
the boundaries are approximate
The Spinal Cord: Is connected to
the brain via the brain stem
The Segmented Spinal Cord: The DRG contains
the cell bodies (somas) of PAFs
Primary Afferent Axons: Carry informations from
the periphery (PNS) to the spinal cord and brain (CNS)
The Spinal Cord: Peripheral nerves connect (communicate) with
the spinal cord through 31 paired spinal nerves
Primary Afferent Axons (PAFs): Their diameters vary as well as
their conduction velocities
Pain and Hyperalgesia: Hyperalgesia is a reduced
threshold to pain- increased pain- that results from a normally noxious stimulus
Two-Point Discrimination: Possible neural mechanisms devoted
to high-resolution discriminations
The sensory receptors' change in membrane potential is transmitted
to the spinal cord and higher brain centers in the form of action potentials
The Trigeminal Pathway: Facial sensations are largely supplied by the
trigeminal nerve (CN V), with some sensory information of the head being supplied by other cranial nerves
First and Second Pain: Nociceptor activation produces
two different perceptions of pain
Pain: Is defined as the perception of
unpleasant sensory or emotional experience that is associated with actual or potential damage
Primary Afferent Axons (PAFs): Peripheral nerves associated sensory receptors in the skin are named
using the letters "A" and "C" and Greek letters; while those associated with muscles are classified into groups, using Roman numerals
The Segmented Spinal Cord: Most 2nd order neurons receiving sensory input lie
within the dorsal horn
The Segmented Spinal Cord: The cell bodies of efferent motor fibers lie
within the ventral horns