Flashcards in Somatosensory System Deck (135):
What is Somatosensation?
sensory information from the skin and musculoskeletal systems
Sensory information from the skin is _____ and sensory information from the musculoskeletal system is _____.
The speed in which somatosensory information proceeds from the receptor to the brain depends on what 3 things?
- Diameter of the axons
- Degree of axonal myelination
- Number of synapses in the pathway
What is the difference between sensory information
- sensory information includes nerve impulses generated from the original stimuli
- sensation includes awareness of stimuli from the senses
What is perception?
an active process of interaction between the brain and the environment
Where are sensory receptors located?
At the distal ends of peripheral neurons
What are the 3 types of somatosensory receptors?
A subset of each type of somatosensory receptors is classed as _______.
What are nociceptors?
Receptors that are sensitive to stimuli that damages or threatens to damage tissue
Stimulation of nociceptors results in what?
sensation of pain
Receptors that respond as long as a stimulus is maintained are called _____ receptors.
Receptors that adapt to a constant stimulus and stop responding are called ___ receptors.
Where are the cell bodies of most somatosensory peripheral sensory neurons located?
Outside the spinal cord in dorsal root ganglia or outside the brain in cranial ganglia
Describe the 2 axons of a peripheral sensory neuron
- Distal: Conduct messages from the receptor to the cell body
- Proximal: Project from the cell body into the spinal cord or brainstem
Peripheral axons are aka what?
Afferents (peripheral axons) are classified according to axon diameter. What are these 6 classifications?
- Ia, Ib, II,
- A(beta), A(delta), C
Larger-diameter axons transmit information faster than smaller-diameter axons, with a velocity in the range of _ -__ m/sec
2 - 130 m/sec
Which afferents have the largest diameter? Which have the smaller axon diameter?
- Ia and II have the largest
- followed by A(beta)
- then A(delta)
- and lastly C
The area of skin innervated by a single afferent neuron is called the _____ field for that neuron
Receptive fields tend to be _____ distally and _____ proximally.
Do distal or proximal regions of the body have a greater density of receptors?
Therefore proximally we have _____ receptive fields and _____ density of receptors. Why is this significant?
This helps us distinguish between 2 closely applied stimuli on a fingertip, as compared to the back
What are the 3 things cutaneous receptors respond to?
- pain/noxious stimuli
Touch is categorized as ____ or ____.
Superficial fine touch receptors have ___ receptive fields
What are a few examples of superficial fine touch receptors? What are each sensitive to?
- Meissner's corpuscles (sensitive to light touch and vibration)
- Merkel's disks (sensitive to pressure)
- Hair follicle receptors (sensitive to hair displacement)
Subcutaneous fine touch receptors have ___ receptive fields
What are a few examples of subcutaneous fine touch receptors? What are each sensitive to?
- pacinian corpuscles (sensitive to touch and vibration)
- Ruffini's corpuscles (sensitive to stretch of the skin)
All fine touch receptors transmit information on __ afferents
What is coarse touch mediated by?
free endings throughout the skin
Coarse touch nerve endings provide information about what?
Pleasant touch or pressure and the sensation of tickle and itch
What is the area of skin innervated by axons of cell bodies in a single dorsal root called?
What is the special sensory organ at the core of muscle that gives feedback to the CNS based on its state of elongation called?
Quick and tonic stretch of the muscle spindle is registered by what type of afferents?
Tonic stretch of the muscle spindle is registered by what type of afferents?
Describe intrafusal fibers
They are the specialized muscle fibers inside the spindle that contract only at their ends
What are the 2 types of intrafusal fibers?
- nuclear bag fibers
- nuclear chain fibers
Describe the difference between nuclear bag fibers and nuclear chain fibers
- nuclear bag fibers are clumps of nuclei
- nuclear chain fibers are nuclei arranged in single file
What are the 2 types of sensory endings required for spindles to monitor muscle length and rate of change in length?
- Primary endings of type Ia neurons
- Secondary endings of type II afferents
Where are primary and secondary sensory endings found?
- primary endings wrap around the central region of each intrafusal fiber
- secondary endings end mainly on nuclear chain fibers adjacent to the primary endings
Do primary endings respond phasically or toniccally? Explain...
They are both.
- phasic discharge is maximal during quick stretch and fades quickly
- tonic discharge in sustained during constant stretch
Do secondary endings respond phasically or toniccally?
In order to maintain the sensitivity of the spindle throughout the normal range of muscle lengths what happens?
Gamma motor neurons fire causing the ends of the intrafusal fibers to contract, which stretches the central region, thus maintain sensory activity from the spindle
Receptors at the interface of muscle and tendon that provide mechanosensory information to the CNS about muscle tension are called what?
Golgi Tendon Organs (GTOs)
GTOs are sensitive to how much of a change in tension?
SLIGHT TENSION (<1 g) exerted by both active contraction and passive stretch of muscle
Information via GTOs is transmitted to the spinal cord by what type of afferent?
type Ib afferents
What do joint receptors respond to?
mechanical deformation of the capsule and ligaments
Describe the roles of Ruffini's endings and paciniform corpuscles in the joint capsule
- Ruffini's endings in the joint capsule signal the extremes of joint ROM and respond more to passive than active movement
- paciniform corpuscles respond to movement, but not when joint position is constant
Free nerve endings in the joint capsule are most often stimulated by what?
What afferents are associated with ligament receptors?
What afferents are associated with ruffini's and paciniform endings?
In summary large-diameter afferents transmit information from where?
specialized receptors in muscle, tendons, and joints
In summary medium-diameter afferents transmit information from where?
joint capsules, muscle spindles, and cutaneous touch, stretch, and pressure receptors
In summary small-diameter afferents transmit information from where?
From both the musculoskeletal system and the skin
When describing pathways in the nervous system, only neurons with ____ axons that connect distant regions of the nervous system are counted.
Within the CNS a bundle of axons with the same origin and a common termination are called what?
Somatosensory pathways are often named after what?
for the origin and termination of the tract that contains the second neuron in the series (e.g., spinothalamic)
What are the 3 types of pathways that bring sensory information to the brain?
- Conscious relay
- Unconscious relay
Describe a conscious relay pathway
These pathways convey high-fidelity, somatotopically arranged information to the cerebral cortex
Describe a divergent relay pathway
These pathways convey information that is not somatotopically organized to many areas of the brainstem and cerebrum
Describe a unconscious relay pathway
These pathways convey movement-related information to the cerebellum
The pathways of consciousness travel upward in the spinal cord via what 2 routes?
- dorsal columns
- anterolateral tracts
What type of information travels in the dorsal columns?
sensory information about discriminative touch and conscious proprioception
What type of information travels in the anterolateral tracts?
discriminative pain and temperature information
In order to be aware of sensory information, where must this information reach?
For discriminative perception of stimuli localized with fine resolution, information must be processed by what?
the cerebral cortex
What is discriminative touch?
the localization of touch and vibration and the ability to discriminate between two closely spaced points touching the skin
What is conscious proprioception?
the awareness of the movements and relative position of body parts
What is Stereognosis?
the ability to use touch and proprioceptive information to identify an object; for example, a key in the hand can be identified without vision
Pathways for discriminative touch and conscious proprioception use a three-neuron relay. What are these 3 neurons and where do they convey information to?
- Primary conveys information from the receptors to the medulla
- Secondary conveys information from the medulla to the thalamus
- Tertiary conveys information from the thalamus to the cerebral cortex
The proximal axon of the primary neuron enters the spinal cord via the dorsal root, it then ascends to the brain how?
via the ipsilateral dorsal root
What is the anatomic name for the pathway that conveys discriminative touch and conscious proprioception?
the dorsal column/medial lemniscus system
Axons from the lower limb occupy the more medial section of the dorsal column called the fasciculus ______.
Axons from the upper limb occupy the lateral section of the dorsal column called the fasciculus _____.
Axons that ascend the fasciculus gracilis synapse with the second-order neurons where?
in the nucleus gracilis of the medulla
Axons that ascend the fasciculus cuneatus synapse with the second-order neurons where?
in the nucleus cunateus of the medulla
Throughout the dorsal column/medial lemniscal (DCML) system the primary neurons contain collateral branches that do what?
contribute to motor control, influence activity in neurons in other sensory systems, or influence autonomic regulation
Where are cell bodies of the secondary neuron of the column/medial lemniscal (DCML) system found?
in the nucleus gracilis or cuneatus
The second-order neurons of the DCML ascend to the thalamus as what?
the medial lemniscus
What is the area of the thalamus called in which the second-order neurons of the DCML end?
the ventral posterolateral (VPL) nucleus
Sensory innervation for the face is supplied by what nerve?
the 3 branches of the trigeminal
What is the area of the thalamus called in which the second-order neurons responsible for discriminative touch information from the face end?
the ventral posteromedial (VPM) nucleus
If there is a lesion in the peripheral or spinal regions, or in the lower medulla what will the effect on discriminative touch and conscious proprioception information be?
If there is a lesion in the cerebrum, midbrain, pons, or upper medulla what will the effect on discriminative touch and conscious proprioception information be?
As axons of the dorsal column ascend how are they arranged?
into a somatotopic arrangement
What is the area of the cerebral cortex devoted to discriminative somatosensation called?
primary sensory cortex
Where is the primary sensory cortex located?
in the postcentral gyrus
Areas of the cortex responding to somatosensory stimulation are indicated by what?
the homunculus (map)
What type of sensory information does the primary sensory cortex identify?
the size, texture, or shape of objects
What type of information do the somatosensory association areas analyze?
information from the primary sensory area and the thalamus and provide stereognosis and memory of the tactile and spatial environment
The anterolateral columns of white matter in the spinal cord contain axons that transmit discriminative information about what?
coarse touch, pain, and temperature
How is coarse touch transmitted?
via C-fiber unmyelinated axons from low-threshold mechanoreceptors
__ fibers carry information regarding cold and __ fibers carry information regarding heat
Fast pain, which is the initial and immediate sharp sensation that indicates the location of injury is aka what?
Slow pain, which is the dull, throbbing ache following fast pain that is not well localized is aka as what?
Pathways for fast, localized pain use a three-neuron system. What are these 3 neurons and where do they convey information to?
- Primary neurons bring information into the dorsal horn of the spinal cord
- Axons of secondary neurons cross the midline and project from the spinal cord to the thalamus
- Tertiary neurons project from the thalamus to the cerebral cortex
What is the primary neuron in the fast pain pathway that transmits information from free nerve endings in the periphery to the spinal cord?
a small myelinated A(delta) fiber
Where do the first and second order neurons that carry information regarding pain synapse?
in lamina I, II, or V of the dorsal horn
After the second-order neuron gets information from the first-order neuron in the laminae what does it do?
it crosses the midline in the anterior white commissure, then ascends to the thalamus via the spinothalamic tract
Where do most spinothalamic tract neurons end?
in the ventral posterolateral nucleus of the thalamus
After synapsing with the second-order neuron in the VPL nucleus, where does the third-order neuron transmit information?
the primary and secondary sensory cortex
What is the major difference between the DCML and spinothalamic systems (other than what type of information they transmit)?
In the dorsal columns the primary neuron axons ascend ipsilaterally, whereas in the anteolateral columns (spinothalamic) the secondary neuron axons ascend contralaterally.
Therefore the crossing of the midline occurs earlier (before the axons ascend) in the spinothalamic tract than they do in the dorsal column path (in the medulla)
What is analgesia?
the absence of pain in response to stimuli that would normally be painful
What is crossed analgesia?
a single lesion can cause pain sensation to be lost on the side of the face ipsilateral to the lesion and the contralateral side of the body
Compare and contrast slow and fast pain
Fast pain travels in large, myelinated A delta fibers with conduction velocities to the spinal cord of 0.03 seconds which results in sharp pain in a specific area.
Slow pain travels on small unmyelintaed C fibers with conduction velocities to the spinal cord of 0.5 seconds which results in dull aching pain
When do many responses to nociception depend on?
a divergent ascending network of neurons called the medial pain system
What kind of responses does activity of the medial pain system elicit?
affective, motivational, withdrawal, arousal, and autonomic responses
Can slow pain information from the medial pain system be precisely localized? Why or why not?
No, because it is not somatotopically organized
Does the medial pain system use a three-neuron pathway?
No, it uses several pathways with variable numbers of projection neurons, not a three-neuron pathway like fast pain.
Describe the first neuron of the medial pain system for slow pain
It is a small, unmyelinated C fiber
What do the receptors of the first neuron of the medial pain system for slow pain respond to?
noxious heat, chemical, or mechanical stimulation
Describe why it is painful when someone gently touches sunburnt skin
The high-threshold C fiber endings have become sensitized with repeated stimulation, so after injury these neurons can be fired with less stimulation than is usually required
Where does sensory information from free nerve endings in the periphery travel to?
the dorsal root ganglion where it then enters the spinal cord, branches in the dorsolateral tract, and then synapses with interneurons in lamina I, II, or V of the dorsal horn
What are the 3 tracts in the anterolateral spinal cord in which slowly conducted nociceptive information from the body travels?
How do the Spinomesencephalic, Spinoreticular, and Spinolimbic travel through the anterolateral spinal cord?
they ascend parallel to one another
Information in the spinomesencephalic nd spinoreticular tract is perceived as what?
arousal, motivational, and reflexive functions and/or activates descending projections that control the flow of sensory information
Where does the spinomesencephalic tract carry information to?
Two areas in the midbrain: the superior colliculus and periaqueductal gray
What is the spinomesencephalic tract involved in?
turning the eyes and head toward the source of noxious input and in activating descending tracts that control pain
Where do the ascending neurons of the spinoreticular tract synapse?
In the reticular formation
What is the reticular formation?
A neural network in the brainstem that includes the reticular nuclei and their connections
What is modulated by the reticular formation?
arousal, attention, and sleep-waking cycles
Information in the spinolimbic tract is perceived as what?
Where do axons of the spinolimbic tract transmit slow pain information to?
the medial and intralaminar nuclei in the thalamus
Eventually, spinolimbic information projects to areas of the cerebral cortex involved with what?
Emotions, sensory integration, personality, and movement
Slow pain from the face is transmitted via what pathway?
Describe the path slow pain from the face takes as it travels through the trigeminoreticulolimbic pathway
The first neurons are C fibers in the trigeminal nerve that synapses in reticular formation, then ascending projection neurons that project to the intralaminar nuclei in the thalamus, these neurons then project to many areas of the cerebral cortex
Information from proprioceptors and information about activity in spinal interneurons are transmitted to the cerebellum via what tracts?
Relay information from spinocerebellar tracts is critical for adjusting what?
movements and posture
What are the 2 tracts that relay high-fidelity, somatotopically arranged information to the cerebellar cortex?
- Posterior spinocerebellar pathway
- Cuneocerebellar pathway
Where does the posterior spinocerebellar pathway transmit information from?
the legs and the lower half of the body
Where does the cuneocerebellar pathway transmit information from?
the arms and upper half of the body
What are the 2 internal feedback tracts that monitor the activity of spinal interneurons and of descending motor signals from the cerebral cortex and brainstem?
- Anterior spinocerebellar tract
- Rostrospinocerebellar tract
Where does the anterior spinocerebellar tract
transmit information from?
the thoracolumbar spinal cord
Where does the anterior rostrospinocerebellar tract
transmit information from?
the cervical spinal cord to the ipsilateral cerebellum