Chapter 12: The Somatic Sensory System

Homunculus

A somatotopic map is sometimes called a *homunculus*. This is a map of the relative size of the cortex devoted to each body part and it correlated with the density of sensory input received form that part. The importance of an input, and the size of its representation in cortex, are also reflections of how often it is used

Summary of Gate Theory of Pain

Activity in the pain axon alone maximally excites the projection neuron, allowing nociceptive signals to rise to the brain. However, if the large mechanoreceptive axons(touch) fires concurrently, it activates the interneuron and suppresses nociceptive(pain) signals

Function of Area 3a

Area 3a is concerned with the sense of body position(joints and muscles)(proprioceptive information) rather than touch

Area 3b as the Primary Somatic Sensory Cortex

Area 3b (1) receives dense inputs from the VP nucleus of the thalumus; (2) its neurons are very responsive to the somatosensory stimuli(but not to other stimuli); (3) lesions here impair somatic sensation; and (4) when electrically stimulated, it evokes somatic sensory experiences

Function of Areas 1 and 2

Areas 1 receive texture inputs from area 3b. Area 2: recieves size and shape info input from 3b

Primary Afferent Axons

Axons bringing information from the somatic sensory receptors to the spinal cord or brain stem are the *primary afferent axons* of the somatic sensory system. The skin is richly innervated by these axons and make their way to the CNS

Primary Somatosensory Cortex (S1 = 3b)

Brodmann's area *3b* is regarded as the primary somatosensory cortex(*S1*), and it lies on the postcentral gyrus(right behind central sulcus). So S1 contains area 3b as well as 3a, 1, and 2.

Hyperalgesia

Can be a reduced threshold for pain, an increased intensity of painful stimuli, or even spontaneous pain

Chemical Stimulus for Pain

Damaged cells at the site of injury can release a number of substances that cause ion channels on nociceptor membranes to open. Examples of release substances are 1.proteases(enzymes that digest proteins), 2.ATP), K^+, bradykinin, lactic acid,

Frequency Sensitivity of Types of Mechanoreceptors ( not important)

Different mechanical sensitivities of mechanoreceptors mediate different sensations . Pacinian receptors are more sensitive to vibrations of about 200-300 Hz, while Meissner's corpuscles respond best to around 50 Hz

Itch

Disagreeable sensation that induces desire or reflex to scratch

Segmental Organization of the Spinal Cord

Each segment of the spinal cord is named after the vertebra adjacent to where the nerves originate; cervical(C)1-8, thoracic(T) 1-12, lumbar(L) 1-5, and sacral(S) 1-5

Somatic Sensation

Enables our body to feel, to ache, to sense hot or chill, and to know what its parts are doing. It is very sensitive to stimuli

Endorphin-Containing Neurons

Endorphin-containing neurons in the spinal cord and brain stem prevent the passage of nociceptive signals through the dorsal horn and into higher levels of the brain where the perception of pain is generated

The Cortex and Pain Pathway

From the thalamus, pain and temperature information is projected to various areas of the cerebral cortex

Follicles

Hairs grow from *follicles* embedded in the skin; each follicle is richly innervated by free nerve endings - the terminations of single axons - that either wrap around the follicle or run parallel to it.

Comparison of Spinal Cord Deficits Effect on Pathways for Touch and Pain

If half of the spinal cord is damaged, certain deficits of mechanosensitivity occur on the *same side* as the spinal cord damage. Deficits in pain and temperature sensitivity will show up on the side of the body *opposite* the cord damage

Spinothalamic Pain Pathway

Information about pain(as well as temperature) in the body conveyed from the spinal cord to the brain via the *spinothalamic pathway*. The axons of the second-order neurons *immediately decussate* and ascend through the spinothalamic tract running along the ventral surface of the spinal cord

Comparison of Pathways for Touch and Pain

Information about touch ascends *ipsilaterally*, while information about pain(and temperature) ascends *contralaterally*

Afferent Regulation of Pain

Light touch can evoke pain via hyperalgesia. However, pain evoked by activity in nociceptors can also be reduced by simultaneous activity in low-threshold mechanoreceptors(Aβ)

Meissner's Corpuscles

Mechanoreceptors that are about 1 tenth the size of Pacinian corpuscles and are located in the ridges of glabrous skin(the raised parts of your fingerprints, for example)

Ruffini's Endings

Mechanoreceptors that are found in both hairy and glabrous skin and are slightly smaller than Pacinian corpuscles

Merkel's Disks

Mechanoreceptors that are located in the epidermis which each consist of a nerve terminal and a flattened, non-neural epithelial cell (the Merkel cell)

Krause End Bulbs

Mechanoreceptors which lie in the border regions of dry skin and mucous membrane(around the lips and genitals, for example), and have nerve terminals look like knotted balls of string

Rapidly Adapting Receptors

Mechanoreceptors, such as Meissner's and Pacinian corpuscles, that tend to respond quickly at first to stimulus but them stop firing even through the stimulus continues

Slowly Adapting Receptors

Mechanoreceptors, such as Merkel's disks and Ruffini's endings, that generate a more sustained response during a long stimulus

Receptive Field of Human Sensory Receptors

Meissner's corpuscles and Merkel's disks had small receptive fields, only a few mellimeters wide, while Pacinian corpuscles and Ruffini's endings had large receptive fields that could cover an entire finger or half the palm

Somatosensory Cortex

Most of the cortex concerned with the somatic sensory system is located in the parietal lobe

The Spinal Cord

Most peripheral nerves communicate with the CNS via the spinal cord, which is encased in the bony vertebral column

Thermoreceptors

Neurons that are exquisitely sensitive to temperature because of specific membrane mechanisms. These receptors contribute to our perception of temperature

Temperature Sensations

Nonpainful temperature sensations originate from receptors in the skin(and elsewhere), and they depend on the neocortex for their conscious appreciation

Opiod Receptors

Opiods act by binding tightly and specifically to several types of *opiod receptors* in the brain, and that the brain itself manufactures endogenous morphine-like substances, collectively called *endorphins*

Other Cortical Areas that Process Somatic Sensory Information

Other cortical areas that process somatic sensory information include areas 3a, 1, and 2 on the *postcentral gyrus*, and areas 5 and 7 on the adjacent *posterior parietal cortex*

Two-Point Discrimination on the Body Surface

Our ability to discriminate the detailed features of a stimulus varies tremendously across the body ( to test the spatial resolution) reason for the difference 1.receptive field density 2.receptive field size 3.computing power 4.special neural mechanisms

Difference Between Nociception and Pain

Pain is a feeling or perception. Nociception is the sensory process that provides signals that trigger pain

The Trigeminal Pain Pathway

Pain(and temperature) information from the face and head takes a path to the thalamus that is analogous to the spinal path called the *trigeminal pain pathway*. The axon of these cells cross and ascend to the thalamus in the *trigeminal lemniscus*

Various Sizes of Primary Afferent Axons

Primary afferent axons sizes correlate with the type of sensory receptor to which they are attached. Axons from skin sensory receptors are usually designated Aα( group 1), -proprioceptors of skeletal muscle Aβ,( group II) - mediates touch sensations (mechno receptor of skin) Aδ ( group III), -pain and temperature C ( group IV)- fibers only mediate pain, itch and temperature the groups mean axon from muscules Group C(or IV) axons are, by definition, unmyelinated axons, while all the rest are myelinated

Opiods

Produce profound analgesia when taken systemically

Endorphins

Relatively small proteins, or peptides. Endorphins and their receptors are widely distributed in the CNS, but are concentrated in areas that process or modulate nociceptive information

Columnar Organization of S1's Area 3b

S1 neurons with similar inputs and responses are stacked vertically into columns that extend across the cortical layers

Pain

Selective activation of nociceptors can lead to the conscious experience of pain. Feelings of Sore, aching or throbbing

Stimulus and Nociceptors

Simple stretching or bending of the nociceptor membrane activates mechanically gated ion channels that cause the cell to depolarize and generate action potentials

Lesions in Areas 1 and 2

Small lesions in area 1 or 2 produce predictable deficiencies in discrimination of texture, size, and shape

Nociceptors

Somatic sensation depends strongly on *nociceptors*, the free, branching, unmyelinated nerve endings sensory process provides signals that trigger pain Transduction of pain types of noiceptros : polymodal: response to mechanical thermal and chemical stimuli

Somatotopic Maps

Somatotopic maps roughly resemble a body with its legs and feet at the top of the postcentral grus and its head at the opposite, lower end of the gyrus

Multiple Somatic Maps

Somatotopy in the cerebral cortex is not limited to a single map. The somatic sensory system has several maps of the body.

Segregation of Touch and Pain Systems

Some of the axons of the spinothalamic tract and trigeminal lemniscal axons(both pain) terminate in the VP nucleus, just as the medial lemniscus(touch) but the touch and pain systems still remain segregated.

Descending Regulation of Pain

Strong emotion can powerfully suppress feelings of pain. One zone of neurons that has been implicated in pain suppression in the midbrain is called the periventricular and *periaqueductal gray matter (PAG)*. Electrical stimulation of the PAG can cause a profound analgesia that has sometimes been exploited clinically

First and Second Pain

The activation of skin nociceptors produces two distinct perceptions of pain: 1. fast, sharp, first pain followed by a duller, longer lasting second pain. First pain is caused by the activation of Aẟ fibers; second pain is caused by the activation of C fibers

Organization of the Spinal Cord

The arrangement of the paired dorsal and ventral roots is repeated 30 times down the length of the human spinal cord. Each spinal cord passes through a notch between the vertebrae and there are as many spinal nerves as their are notches. The 30 *spinal segments* are divided into four groups

Cauda Equina

The bundles of spinal nerves streaming down within the lumbar and sacral vertebra. These course down the spinal column within a sack of dura with cerebrospinal fluid(CSF)

Referred Pain

The cross-talk between visceral and cutaneous nociceptors gives rise to the phenomenon of *referred pain*, where visceral nociceptor activation is perceived as a cutaneous sensation

Agnosia

The inability to recognize objects even though simple sensory skills seem to be normal which results from damage to the posterior parietal cortex

Dermis

The inner layer of skin

S1 of Rodents

The large facial vibrissae(whiskers) of rodents receive a huge share of the territory in S1 and the sensory signals from each vibrissae follicle go to one clearly defined cluster of the S1 neurons; such clusters are called barrels. The five rows of cortical barrels precisely match the five rows of facial vibrissae

Pacinian Corpuscle

The largest and best studied mechanoreceptor which lies deep in the dermis. Each human hand has 2500 Pacinian corpuscles, with the highest densities in the fingers

Mechanosensitive Ion Channels

The mechanoreceptors of the skin all have unmyelinated axon terminals, and the membranes of these axons have *mechanosensitive ion channels* that convert mechanical force into a change of ionic current

Second-Order Sensory Neurons

The neurons that receive sensory input from primary afferents are called *second-order sensory neurons*. Most of the second-order sensory neurons of the spinal cord lie within the dorsal horn

Epidermis

The outer layer of skin

Dorsal Column Medial Lemniscal Pathway

The pathway serving for touch. 1.The ascending branch of the large sensory axons(Aβ) enters the ipsilateral *dorsal column* of the spinal cord(carry tactile sensation and limb position toward brain) and terminate in the *dorsal column nuclei*. 2.The axons of the dorsal column nuclei decussate(cross-over) and ascend within the *medial lemniscus*, whose axons then synapse upon neurons of the *ventral posterior (VP) nucleus* of the thalumus. 3.Thalamic neurons of the VP nucleus then project to specific regions of the *primary somatosensory cortex*, or *S1*

Primary Afferent Axons Entering the Spinal Cord

The primary afferent axons enter the spinal cord through the dorsal roots; their cell bodies lie in the dorsal root ganglia

Interneuron of Gate Theory of Pain

The projection neurons is also inhibited by an interneuron, and the interneuron is both excited by the large sensory axon and inhibited by the pain axon

Somatotopy

The receptive fields of many S1 neurons produce an orderly map of the body on the cortex. The mapping of the body's surface sensations onto a structure in the brain is called *somatotopy*

Dermatone

The segmental organization of spinal nerves and the sensory innervation of the skin are related. The area of skin innervated by the right and left dorsal roots of a single spinal segment is called the *dermatome*; thus, there is a one-to-one correspondence between dermatomes and spinal segments. ex) shingles

Selectivity of Mechanoreceptor

The selectivity of a mechanoreceptive axon depends primarily on the structure of its special endings.

Two Major Types of Skin

The sensation of touch begins at the skin. The two major types of skin are called *hairy* and *glabrous*(hairless)(palm of our hand)

Spinal Connections of Nociceptive Axons

The small-diameter fibers have their cell bodies in the segmental dorsal root ganglia, and they enter the dorsal horn of the spinal cord. The fibers branch out immediately, travel a short distance up and down the spinal cord in a region called the *zone of Lissauer*, and then synapse on cells in the outer part of the dorsal horn in a region known as the *substantia gelatinosa*

Sensory Organization of the Spinal Cord

The spinal cord is composed of an inner core of gray matter, surrounded by a thick covering of white matter tracts that are often called *columns*. Each half of the spinal gray matter is divided into a *dorsal horn*, an *intermediate zone*, and a *ventral horn*

2nd Difference Between Touch and Pain Pathway

They differ with respect to the diameter of their axons. The touch pathway is swift, using fat, myelinated Aβ fibers; the pain pathway is slow, using thin, lightly myelinated Aδ fibers and unmyelinated C fibers

3rd Difference Between Touch and Pain Pathway

They differ with respect to their connections in the spinal cord. Branches of Aβ axons terminate in the deep dorsal horn; the Aδ and C fibers branch run within the zone of Lissauer, and termiante within the substantia gelatinosa

1st Difference Between Touch and Pain Pathway

They differ with respect to their nerve endings in the skin. The touch pathway is characterized by specialized structures in the skin; the pain pathway has only free nerve endings

Types of Sensitivity of Mechanosensitive Ion Channels

This is the explanation of the image in the previous card

Projection Neuron of Gate Theory of Pain

This theory suggests that certain neurons(projection neurons) of the dorsal horns, which project an axon up the spinothalamic tract, are excited by both large-diamter sensory axons(touch axons) and unmylelinated pain axons.

Painful Stimuli

Tissue damage can result from strong mechanical stimulation, extremes in temperature, oxygen deprivation, and exposure to certain chemicals, among other causes. The membranes of nociceptors contain ion channels that are activated by these types of stimuli

Somatic Sensory System's Four Senses

We can think of this system as a group of at least four senses rather than a single one: the senses of touch, temperature, pain, and body position

Organization of Dermatone

When mapped, the dermatomes delineate a set of bands on the body surface

Vibration and Pacinian Corpuscle ( not important)

When the capsule like ending of a Pacinian corpuscle is compressed, energy is transferred to the nerve terminal, its membrane is deformed, and mechanosensitive channels open, stimulating an action potential. When the pressure is released, the events reverse themselves; the terminal depolarizes again and may fire another action potential

Mechanoreceptors

most somatic sensory receptors are mechanoreceptors, sensitive to physical distortion , feel contact with the skin, pressure in the heart and blood vessels, stretching of bladder.