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