organ systems: somatosensory part 1

this lecture was an overview of the different types of sensory receptors and pathways involved in somatosensory reception. the first section introduced the concept of sensory transduction in general: how organisms transduce different forms of energy from the environment such as vibration, heat, light, etc. into neuronal action potentials in order to receive information about the environment which allows them to adapt and survive/reproduce. when a sensory receptor is stimulated, it produces a local graded potential, several of which might add up to elicit an action potential if the threshold is reached.

there are several characteristics that are common to all sensory reception systems; first is that the frequency of the action potentials produced is directly proportional to the intensity of stimulation. the axons that propogate the action potentials have varying degrees of myelination as well, which leads to different conducting velocities for different neurons. measuring the "compound action potential" can differentiate between the different velocities of conduction of different neuron bundles within a nerve- the further away the point of measurement is from the stimulus, the more the different bundles are distinguishable via their velocity.

another way that sensory receptors are distinguished is how they adapt to the stimulus. fast adapting receptors produce action potentials at the beginning and end of the stimulus, and fade quickly in between-- signalling that the event starts and ends. slow adapting receptors produce action potentials all throughout the stimulus, signalling that the event is occuring. the underlying mechanism for this differentiation is through the control of calcium and sodium channel permeability.

there are a whole variety of particular types of sensory receptors: see chart for more detail. exteroceptors detect outside stimuli and can be divided roughly into two categories: 1) discriminative touch, and 2) non-discriminative touch, pain, and temperature. discriminative touch receptors are covered or modified axonal endings, have faster conducting axons, and project into the CNS via the dorsal lemniscal system (more later). the second category uses free nerve endings as receptors, has slower conducting axons, and project into the CNS using the anterolateral system. interoceptors detect internal stimuli; either from visceral organs or from other internal somatic structures.

there are two distinct pathways by which somatosensory information can travel up to the CNS [see comparison chart]. the dorsal column lemniscal system takes in discriminative touch, proprioception, and some visceral information via first order neurons, which then ascend in the dorsal funiculus, forming the cuneate and gracilis nuclei. in the medulla, they synapse with second order neurons, which decussate (cross over) and form the medial lemniscus, which then travel to the thalamus, where they synapse with the third order neurons, which then project to the somatosensory cortex.

the other pathway for receiving somatosensory input is the anterolateral system, which receives pain, temperature, and non-discriminative touch information. although the distinction between first, second, and third order neurons is not as clear cut as in the lemniscal system, there are still some defining characteristics: first order neurons form vertical tracts of lissaeur, which can span several spinal levels before entering the dorsal horns. first order neurons synapse with second order neurons much lower than in the lemniscal system, which then decussate and project to the anterolateral tract within the lateral funiculi. the second order neurons then project to the thalamus, where they synapse with the lateral thalamic neurons (which project to the primary somatosensory cortex), and the medial thalamic neurons (which project to the cingulate gyrus and insula).

questions
introduction...
1. how is the process of sensation related to survival?
2. describe how our sense organs transduce energy in the environment.
3. sensory receptors are either...
4. what are exteroceptors and interoceptors?

action potentials...
5. describe the production of action potentials by sensory receptors.
6. how does myelination affect the velocity of action potential propagation?
7. what are compound action potentials?
8. how does compound action potential change when the point of measurement is moved further from the point of stimulus?
9. what is the system of nomenclature used to specify motor/sensory nerves within a compound action potential?
10. what are some examples of the assignments of function within the compound action potential naming system?

sensory coding...
11. what are the four attributes of coding that are common to all sensory systems?
12. describe intensity coding of sensory reception.
13. describe the two different mechanisms of "sensory adaption".
14. in general, what is the underlying mechanism for sensory adaptation?
15. what is a cutaneous receptive field?
16. in general, densely innervated RF's are...
17. size and density of RF's provide means for CNS to...
18. what type of proteins are sensory receptor proteins?

exteroception: discriminative touch...
19. describe "discriminative touch".
20. what is the classification (slow adapting or fast adapting) of merkel cells and what are examples of what they are receptive to?
21. ...ruffini corpuscle?
22. ...meissner corpuscle?
23. ...pacinian corpuscle?
24. ...hair follicle?
25. what is a mechanoreceptor receptive field?
26. what is an example of how mechanoreceptor receptive fields aid in forming 3D representations?

exteroception: non-discriminative touch, pain, temperature...
27. what is unique about the sensory reception of non-discriminative touch, pain, and temperature?
28. what is the function and the compound action potential classification of mechanoreceptors for non-discriminative touch?
29. ...thermoreceptors?
30. nociceptors?
31. visceral receptors?

interoception...
32. what are the two types of structures that interoceptors can receive stimulus from?
33. what are visceral afferents?
34. what do visceral nociceptors detect and along which nerves do they travel?
35. describe the reception of pressure, stretch, tension, blood pressure, etc. from visceral organs.
36. what type of sensory information do proprioceptors mediate?
37. what type of receptors are proprioceptors?
38. the muscle receptor proprioceptors detect changes in...
39. the joint receptors detect...

modality...
40. how are the different somatosensory modalities distinguished?
41. describe the characteristics of the discriminative touch, vibration, and proprioception modality.
42. describe the characteristics of the non-discriminative touch, pain, and temperature modality.
43. describe the characteristics of the interoceptive modalities.

different sensory pathways...
44. describe the neuronal pathways that travel through the dorsal lemniscal system and the structure through which the information travels.
45. what are first order neurons?
46. what are second order neurons?
47. what are third order neurons?
48. what modalities travel through the anterolateral system?
49. what is lissauer's tract?
50. what neurotransmitters do first order neurons in the anterolateral system use to synapse with second order neurons?
51. describe the separation of second order neurons by modality within the rexes lamina.
52. where does the neospinothalamic tract project to and what is it involved with?
53. where does the paleospinothalamic tract project to and what is it involved with?
54. where are the third order neurons within the anterolateral pathway and what do they project to?
55. what is the reticular formation?
56. describe the pathway for sensory input and neuronal projection out of the reticular formation.
57. what is a unique anatomical characteristic of reticular formation neurons?
58. what is the physiological effect of the ascending reticular system?

answers
1. the survival of the organism depends on the organism having adequate information about its environment which is gleans via the sensory mechanisms; which provide information that allows the organism to adapt its behavior so as to facilitate survival.
2. our sense organs transduce energy in the form of light, pressure, vibration, sound, etc from the environment into neuronal signals.
3. modified non-neural tissue cells or axons themselves.
4. exteroceptors are sensory receptors that respond to external stimuli such as touch, pain, temperature. interoceptors are sensory receptors that respond to internal stimuli such as chemical change or tissue stretch.

5. stimulation of sensory receptors creates local graded potentials which translate into an action potential which propagates down the axon if the threshold is reached.
6. myelination increases the velocity of action potential propagation.
7. compound action potentials are the sum total of all neuronal activity at a given point due to one stimulation.
8. the further away from the site of stimulation that the compound action potential is measured, the more distinct peaks will be measured based upon velocity of conduction: because of the discrepancy of different axonal clusters based on myelination level and therefore velocity.
9. roman numerals for motor nerves and letters for sensory nerves. lower alphabet letters or lower numbers designate a faster conduction rate.
10. prioprioception and motor neurons are A-alpha. light touch are A-beta. fast pain is A-gamma. slow pain is C.

11. intensity, sensory adaptation, localization, modality.
12. increased intensity of sensory reception increases the frequency of action potential production.
13. rapid adapting receptors respond to stimuli with action potentials that rapidly fade in frequency after the onset or offset of stimuli, indicating that an event occured. slow adapting receptors respond with action potentials that are sustained through the stimuli; indicating that the event is still occuring.
14. calcium or sodium channel activation/inactivation.
15. an area of skin that is innervated by axonal branches off a single neuron.
16. smaller in area.
17. determine location of stimulus on body, distinguish size and shape of stimulus, and resolve spatial resolution.
18. they are transient receptor proteins: each protein responds maximally to a different type of stimulus.

19. a venue of exteroception which is mediated by fast or slow adaptive mechanoreceptors in the CT or around hairs in the epidermal layer.
20. SA1- form and texture, such as fingers scanning a surface. (a murky slow scanner)
21. SA2- skin stretch- perception of hand shape and position. (a dog doing slow taichi)
22. FA- skin movement and slip- for grip control. (mice quickly grip)
23. FA- vibration. (pacman vibrating fast)
24. motion/direction of tactile stimuli. (hair directing a fast picture)
25. a sensory receptive field which contains mechanoreceptors that branch off of a single neuron.
26. the CNS can form a 3D representation of an object held in the hands through the combined contributions of the distinct mechanoreceptive fields from separate digits.

27. they are all mediated by free nerve endings, have slower conducting, A-gamma or C type axons, and are slow adapting.
28. tap, squeeze, rub, skin stretch function, with A-gamma and C axons.
29. hot or cold sensation, A-gamma and C axons.
30. mechano-thermal: mechanical or thermal tissue damage sensation, A-gamma axon. polymodal nociceptor: heat, chemical, tissue damage, C axon.
31. nociception (sympathetic), physiological (parasympathetic), C axon.

32. visceral structures or somatic structures such as the muscles and CT.
33. the afferent nerve fibers that have free nerve ending receptors that mediate pain, pressure, temperature, chemical, and stretch reception in organs and blood vessels.
34. tissue damage or irritation; sympathetic nerves.
35. takes place via physiologic or specialized receptors in smooth muscle, mucosae, hypothalamus- axons travel with parasympathetics.
36. muscle and joint position / movement.
37. fast conducting/large diameter axons, A-alpha,beta
38. length with muscle spindle receptors, and tension with golgi tendon organs.
39. stretch of CT with pacinian and ruffini receptors.

40. through different receptors, conduction velocity / axonal thickness, and the location of their ascending pathway.
41. a more quantitative, localized modality, with sensitive mechanoreceptors, rapid AP conduction, and axons that travel through the lemniscal system within the dorsal column.
42. a more qualitative modality that involves free nerve endings, and axons with slower AP conduction that travel through the anterolateral system.
43. proprioception ascends in the dorsal columns, while visceral sensory information ascends in both dorsal column and anterolateral systems.
44. the dorsal lemniscal system conveys touch, vibration, proprioception, and some visceral information to the cortex by means of first, second, and third order neurons which synapse in the spinal cord, thalamus, and medulla.
45. sensory neurons that form dorsal columns, then synapse onto the gracilis and cuneate nuclei in the medulla.
46. neurons that synapse with first order neurons in the gracilis and cuneate nuclei in the medulla, which then decussate, and ascend to the thalamus, where they synapse with third order neurons.
47. the neurons that synapse with second order neurons in the thalamus and project to the primary somatosensory cortex into the post central gyrus.
48. non-discriminative touch, pain, temperature
49. the ascending and descending of first order neurons within the dorsolateral fasciculus before entering several spinal levels of the gray matter.
50. substance P, glutamate, NO.
51. lamina I and II are associated with pain signals and lamina IV is associated with touch sensation.
52. lateral thalamus and somatosensory cortex, involved in localization of sensation.
53. reticular formation, medial thalamus and cortex- involved in qualitative aspects of pain, temperature, non-discriminative touch.
54. the third order neurons in the lateral thalamus project to the primary somatosensory cortex and the neurons in the medial thalamus project to the cingulate gyrus and insula.
55. a collection of nuclei within the medulla, pons, and midbrain that projects to the cortex and thalamus and is involved in a variety of physiological functions such as alertness, wakefulness, attention.
56. sensory input is received through the raphe and lateral nuclei and is projected upwards through the medial nuclei into the cortex, as well as down into the spinal cord.
57. reticular formation neurons have particularly long axons which allow them to exert an influence over a large area of brain structures.
58. the ascending reticular formation generates alertness and wakefulness.