The somatosensory component of the nervous system provides awareness of body sensations like. . .

 touch, temperature, body position and pain--

(the four major modalities)

Three types of sensory neurons include. . .

General somatic

Special somatic

General visceral

General somatic afferent neurons. . .

have branches with distinct type of receptors

that result in sensations

such as touch, temperature and pain

Special somatic afferen neurons have. . .

receptors located primarily

in the muscles, tendons and joints

that sense position and movement

General visceral afferent neurons. . .

have receptors on various visceral structures

that sense fullness and discomfort

First-order neurons. . .

•   transmit sensory info from the periphery to the CNS

or

•   from the sensory receptors to the dorsal horn neurons

     in the spinal cord

Second-order neurons. . .

communicate with various

reflex networks and sensory pathways in the spinal cord

and travel directly to the thalamus

or

communicate with various reflex circuits and transmit information to the thalamus

Third-order neurons. . .

relay information from the

thalamus to the cerebral cortex

Organizational framework for the

Somatosensory System. . .

• Sensory units

• Ascending pathways

• Central processing centers in the thalmus and cerbral cortex

Interneurons. . .

process and modify the sensory information

at the level of the second- and third-order neurons

The somatosensory experience arises from info provided by a variety of receptors distributed thorughout the body.

These receptors montior four types or modalities of sensation which are. . .

• stimulus discrimination

• tactile sensation

• thermal sensation

• position sensation

All somatosensory info from the

limbs to the trunk

share a common clase of sensory neurons called. . .

dorsal root ganglion neurons

Somatosensory information from the face and carinial structures is transmitted by. . .

the trigeminal sensory neurons

A sensory unit is comprised of. . .

• the cell body of the dorsal root ganglion

• its peripheral branch (which innervates a small area of the periphery)

• its cental axon (which projects to the CNS)

Three types of nerve fibers. .  .

• A fibers

• B fibers

• C fibers

Aδ fibers. . .

• have the fastest rate

• are myelinated

• convey cutaneous pressure and touch sensation, cold sensation, mechanical pain and heat pain

B fibers . . .

• are myelinated

• transmit info from cutaneious and subcutaneous mechanoreceptors

C fibers. . .

•  are the slowest

• are unmyelinated

• convey cutaneous pressure and touch sensation, cold sensation, mechanical pain and heat pain

• NOTE: The lower urinary tract has both Aδ and C fibers

Dermatome. . .

• The region of the body wall that is supplied by a single pair of dorsal root ganglion

• 33 paired spinal nerves provide sensory and motor innervation of the body wall, the limbs, and the viscera.

• Neighboring dermatomes overlap one another suffciently so that a loss of one dorsal root or too ganglion results in reduced but not total loss of sensory innervation of a dermatome

On entry into the spinal cord, the central axons fo the somatosensory neuron branch extensively and project to neruons in the sponal cord gray matter.

Some cental axon branches become involved

in local spinal cord reflexes and directly initiate motor reflexes such as. . .

the flexor-withdrawal reflex

The two parallel pathways that carry info from the

spinal cord to the thalamic level of sensation,

(each taking a different route through the CNS)

are. . .

• discrimitnative pathway

• anterolateral pathway

These pathways relay information to the brain for three purposes:

1. perception
2. arousal
3. motor control

The descriminative pathway crosses at the. . .

Medulla

The anterolateral pathway crosses. . .

within the first few segments

of entering the spinal cord

Advantages of having two pathways,

descriminative and anterolateral,

include. . .

1. Sensory information can be handled in two different ways
2. If on pathway is damages, the other still can provide input.

The descriminative pathway,

AKA The dorsal column-medial leminiscal pathway. . .

• is used for the rapid transmission of sensory info such as discriminative touch

• contains branches of primary afferent axons that travel up the ipsilateral (i.e. same side) dorsal comumns of the spinal cord white matter and synapse with highly evolved somatosensory input assoication neuorns in the medulla

• See figure 18.4 (A), p. 426

Distinct features of the discriminative pathway. . .

1. It relays precise info regarding spatial orientation

2. The only pathway taken by the sensations of muscle and joinn movement, virbration and delicate discrimiative touch, e.g. "two-point descrimination"

3. Integrates input from multiple receptors (stereognosis)

Stereognosis. . .

• The sense of shape and size of an object in absense of visualization (screwdriver vs. knife)

• This complex interpretive perception implies that both the discriminative and higher-order parietal association cortex are funtioning properly.

Astereognosis. . .

If the discriminative somatosensory pathway is functional

but the parietal association cortex has become discretely

damaged, the person can correctly describe the object but

does not recognize that it is a screwdriver

The anterolateral pathway

(anterior and lateral spinothalamic pathways)

• consist of bilateral, multisynaptic, slow-conducting tracts

• transmit sensory info such as pain, thermal sensations, crude touch and pressure that does not require discrete localization

• See figure 18.4 (B), p. 426

Anatomy of the anterolateral pathway. . .

• Fibers of the anterolateral pathway originate the dorsal horn, cross at the anterior commisure to the opposit anterolateral pathway where they ascend upward toward the brain

• The spinothalamic tract fibers. . .

1. synapse with several nuclei in the thalamus, but en route they give off numerous branches that travel to the reticular activating system of the brain stem
2. These projections provide wakefullness or awareness after strong somatosensory stimulation like a startle
3. Are responsible for autonomic nervous system responses like ↑ HR and BP, pupil dialation, etc.