Lecture 2 Somatosensory Receptors and Pathways

Question 1

What is the purpose of somatosensation and proprioception?

Answer 1

Explore, identify, increase feedback, and prevent injury.

Question 2

What is sensory perception?

Answer 2

Perception is the awareness of stimuli. We can interpret sensory information into meaningful forms. It is an active and ongoing process as perception involves acting within the environment.
Sensory information is sent from peripheral receptors. They promote adaptations in posture and movement.

Question 3

What are the different types of cutaneous sensory receptors?

Answer 3

Mechanoreceptors detect pressure, discriminative vs crude touch, vibration, and proprioception.
Thermoreceptors detect heat and cold.
Nociceptors detect pain.

Question 4

Describe Meissner corpuscles

Answer 4

Meissner corpuscles are for discriminative touch (two point discrimination) and low frequency vibrations (30-50hz) (a flutter).
Found in glabrous skin and are highly concentrated in the finger tips. They have small receptive fields around 2-4mm. At the fingertips, we are able to discriminate between 1 point and 2 points (two point discrimination). They are sensitive to velocity (2-40mm/s).

Question 5

Describe Pacinian corpuscles

Answer 5

Pacinian Corpuscles respond to high frequency vibrations (100-300 Hz) (tickle) and pressure (deep) over large spaces.
Found in the subcutaneous layer and highly concentrated in fingertips. They have larger receptive fields (>4mm)

Question 6

Describe Merkel’s Discs

Answer 6

Merkel’s Discs are found in the superficial skin and of greater concentration in the finger tips.
They are also found in the dermis and hair follicles.
They have a small receptive field (2-4mm) and respond to pressure, specifically for discrimination of tips/edges/shapes.

Question 7

Describe Ruffinis Corpuscles

Answer 7

Ruffinis Corpuscles are found in subcutaneous skin and can detect skin stretch (direction and force). They have large receptive fields (>4mm).

Question 8

Describe free nerve endings

Answer 8

Free nerve endings are found throughout the skin and visera.
Founds in the epidermis.
Can detect temperature, pain, tickles, and itch.

Question 9

What are the types of pain? (nociceptive vs neuropathic)

Answer 9

Nociceptive pain results from the activation of pain receptors in the body due to tissue injury or inflammation. It is a normal pain response.
Neuropathic pain is from direct injury to neural tissue which results in a burning sensation that radiates and follows the peripheral nerve.

Question 10

What are the different nociceptor types?

Answer 10

Myelinated A-delta are afferent fibers that carry sharp, stabbing, pricking pain, or cold. They are high-threshold receptors that sense immediate pain.

Unmyelinated C fibers are slower than A-delta and carry dull, achy pain. They are high threshold receptors. They are polymodal pain receptors (chemical, heat thermal, mechanical, hypoxia).

Question 10

Pain receptors in the skin vs pain receptors in organs

Answer 10

Pain receptors in the skin are plentiful and are capable of transmitting precise information(where is the injury located, What are the characteristics of the pain?) Pain receptors in organs are not precise.

Question 11

What are receptive fields and how do they differ in the body?

Answer 11

A receptive field is a specific region of sensory space in which an appropriate stimulus can drive an electrical response in a sensory neuron.
The skin of the fingertips has many sensory newtons, each with a small receptive field. At the finger tips, we have a two-point discrimination threshold of 3mm.

The skin of the forearm has fewer sensory neurons each with a large receptive field. At the forearm, we have a two-point discrimination threshold of 40mm.

Question 12

How does size impact axons?

Answer 12

The larger the size of the myelinated axon, the greater the speed of transmission
The speed of transmission from receptor to the first and second order neurons is dependent on the diameter of the sensory axon
A-delta and C fibers have a slower speed of transmission because they are relatively smaller in size
A-alpha (alpha motor neuron) and A-beta (all sensory neurons listed above) have a relatively larger size so they are faster

Question 13

What are the different types of proprioceptors in the body?

Answer 13

The muscle spindle (MS) is a sensory proprioceptor within the muscle belly.
The golgi tendon organ (GTO) is a sensory proprioceptor in the tendon
Joint receptors include ruffini, paciniform, and free nerve endings

Question 14

what is the muscle spindle? What is its purpose?

Answer 14

The muscle spindle is an encapsulated sensory receptor that lies within the muscle belly parallel with extrafusal muscle fibers.
It transmits information on the length of the muscles and the speed of the muscles. DOES NOT contribute significantly to force generation as that is the job of the GTO.
The muscle spindle plays a vital role in regulating the contraction of muscle

Question 15

what is a way to test the integrity of the muscle spindle?

Answer 15

Tendon reflexes is a way to assess the integrity of the MS.

Question 15

how does the muscle spindle interact with the cerebellum?

Answer 15

The muscle spindle will send information about the muscles to the cerebellum. The cerebellum calculates the joint position and other variables that are necessary to make the appropriate movements. The cerebellum houses motor memory. The “memory” comes from the MS.

A sustained stretch of a muscle will cause an afferent 1a neuron to discharge toward the CNS. A gamma will identify the efferent nerve that travels from the CNS to the MS. There is always a two-way transmission between the cerebellum and MS.

Question 16

What is the process in which the muscle spindle sends and receives information during a patellar tendon reflex test?
(LABEL AND DRAW DIAGRAM)

Answer 16

1. A stimulus, tendon tap, occurs
2. The receptor, the muscle spindle, receives the message
3. An afferent 1a sensory neuron is discharged and sends information to the spinal cord
4. After passing through the dorsal root ganglion and entering the gray matter of the spinal cord, the 1a sensory neuron synapses with interneurons
5. Two alpha motor neurons are discharged and sent to muscles
   One is sent to do contraction of the quads
   One is sent to perform reciprocal inhibition and inhibit the hamstrings

Question 17

What are order neurons?

Answer 17

A tract is a neural pathway that is located in the brain and spinal cord. Tracts are formed by neurons synapsing onto one another, and these neurons can be classified as first-order, second-order, or third-order neurons depending on their location and order in the tract.
Order neurons are used to describe the delivery of sensory information from the periphery to the cortex

Question 18

what is a first order neuron?

Answer 18

First-order neurons are afferent. Sensory input from receptors is sent through the peripheral nerve to the dorsal root ganglion. The body of the first-order neuron within the ganglia projects its axons to the posterior gray horn of the spinal cord.

Question 19

what is a second order neuron?

Answer 19

second order neurons synapse with first order neurons at the posterior gray horn of the spinal cord. Second order neurons ascend along the spinal cord.

Question 20

what is a third order neuron?

Answer 20

third order neurons are found in structures of the brain like the thalamus. They will pick up the neural impulse from second order neurons and carry it on to the cerebral cortex.

Question 21

What are neural pathways? (ascending vs descending)

Answer 21

Neural pathways are groups of nerve fibers which carry information between the various parts of the CNS. Neural pathways that connect the CNS and spinal cord are called tracts.
There are ascending pathways and descending pathways. Ascending tracts run from the spinal cord to the brain. Descending tracts run from the brain to the spinal cord (corticospinal tract)

Question 22

Explain the posterior column medial lemniscal pathways (PCML pathway) DRAW IT

Answer 22

The dorsal/ posterior columns are two ascending pathways located side-by-side in the posterior funiculus of the spinal cord.
Sensory information: vibration, joint position (proprioception), and fine touch
Together with the medial longitudinal fasciculus, these tracts form the posterior column medial lemniscus pathway.
DECUSSATE AT MEDULLA

Question 23

Explain the spinothalamic tract and DRAW IT

Answer 23

There are two spinothalamic tracts: anterior and lateral.
The anterior spinothalamic tract transports coarse touch and pressure sensations. It is located in the anterior funiculus of the spinal cord.
The lateral spinothalamic tract carries pain and temperature sensations. It is found in the lateral funiculus of the spinal cord.
Sensory information: pain, temperature, crude touch, and itch
DECUSSATE AT SPINAL CORD

Question 24

What are the functions of the thalamus?

Answer 24

The thalamus is a sensory relay center. It is the center for sensory integration.
The thalamus influences voluntary movements (basal ganglia and cerebellum) as the as the limbic system and memory
The thalamus is responsible for visual grasp, recognition of pain and temperature

Question 25

What are type of peripheral stimuli do each afferent neuron pick up?

Answer 25

A-beta will respond to non-noxious mechanical stimuli
A-delta will respond to noxious mechanical stimuli
C fibers will respond to noxious heat and chemical stimuli