Twenty Two Flashcards
What do sensory receptors do? What are 3 major categories of external stimuli transduced by our CNS?
For our appreciation of the outside world, of our ‘environment’, we have receptive tissues called the neuroepithelium, made up of sensory receptor cells and non-neural supporting cells. The receptors convert, or transduce, one form of energy into another form, which can then be appreciated by our brain. There are three major categories of external stimuli transduced for analysis by our central nervous system.
- Electromagnetic and thermal energy
- Mechanical energy (skin touch/pressure and joint movement/position).
- Chemical agents
What are 5 touch and pressure receptors? 2 joint position receptors? What receives the signal for heat/cold/pain?
Touch and pressure: Pacinian Corpuscle, Meissner’s Corpuscle, Ruffini ending, Merkel disks, nerve endings on hair follicles in the skin
Joint position: Golgi Tendon Organ and muscle spindles
Heat, cold, pain: free nerve endings in the skin
What are 4 features of physical stimuli that we can appreciate?
What features of the physical stimuli can we appreciate?
A. Modality (is the stimulus cold or noisy?)
B. Intensity (how cold? how noisy?)
C. Adaptation (is the stimulus still felt?)
D. Location (is the stimulus on the body; where on the body? where outside the body?)
What is meant by modality? What does the anterolateral system relay? How is it localized? What effects does it have on affect? Same questions for lemniscal pathway. What are 3 categories of somatosensory modalities with examples?
A. Modality. No matter how much a sensory fiber is stimulated, it will always relay the same sensation, e.g. stimulation in the skin of “cold” fibers (free nerve endings) no matter by what – even hot probe – these will relay the sensation of cold (paradoxical cold). Within the CNS, somatosensory axons segregate in two paths. The anterolateral system relays pain and temperature and is characterized by poorly localized sensations that have significant effects on affect (mood, general feeling). The lemniscal system is characterized by well-localized
skin sensations that lack emotional feelings. Changes in affect by the anterolateral system are perceived as positive (warm or sexual) or negative (cold or pain) and its phylogenetic origin is older than the lemniscal system.
Specific somatosensory modalities can be grouped into proprioceptive stimuli (muscle and joint) reflecting position and movement of the body, exteroceptive stimuli (touch, light pressure) that localize stimuli well, and interoceptive stimuli from internal visceral organs that mediate autonomic reflex responses with minimal conscious perception.
What is meant by intensity? What are some guidelines concerning intensity?
B. Intensity. The size of the generator potential is proportional to the intensity of the external stimulus. However, the generator potential is not propagated, but its size is usually relayed to the brain by the amount of neurotransmitter released by generator potential or by the frequency of action potentials traveling down an axon.
There is usually a wide range of intensities and the rule has been that there is an approximate logarithmic relationship between the two. Take the example of discrimination between two weights. One can easily perceive the difference between a 1 Kg and a 2 Kg weight, but not between a 50 Kg and a 51 Kg weight. Yet both sets differ by the same, 1 Kg. Ernst Weber (1834) first described this multiplicative relationship the smallest intensity difference in a stimulus that can be discriminated is linearly proportional to the intensity of the reference stimulus (Weber’s Law).
Gustav Fechner (1860) added to this law, showing that the subjective intensity of a stimulus is logarithmically proportional to the stimulus intensity (Weber-Fechner Law). In other words, the subjective sense of a given modality is non-linear (e.g. ‘ten violins do not sound ten times louder than a single violin’). This scaling factor was investigated by Stevens (1953). He found that the relationship of perceived intensity to stimulus intensity for a given modality was the physical intensity raised to a specific power (Stevens Power Law). The power value 'n' depended on the sensory modality that was being perceived (Fig. 2). Forexample, the increase of a few degrees in this room may subjectively feel twice as uncomfortable, while leaving this room to go outside into the sun would not be perceived as uncomfortable, even though your eyes are subjected to 10,000 fold more photons.
What is meant by adaptation? What is another name for rapidly adapting receptors? slowly adapting? How does the pacinian corpuscle adapt rapidly? What is meant by punctate or diffuse?
C. Adaptation. Sustained distortion of free nerve endings, with a generator potential and subsequent afferent action potentials, will encode the continued presence of any particular stimulus. However, some sensory nerve endings are ENCAPSULATED and add a non-neural component to the transduction process. Basically, adaptation is when there is a constant stimulus, but the generator potential decays rapidly. The response is not sustained.
An example is the Pacinian corpuscle, a rapidly-adapting receptor found deep in the skin that is sensitive to vibration (Fig 3). The intact receptor responds to both the onset and offset of a sustained application of pressure. The importance of the laminae comprising the capsule
is easily demonstrated when they are removed. Then the receptor responds to the same mechanical stimulus as in a slowly-adapting manner. The corpuscle is thus well suited to the periodic nature of vibrations. Another example of a non-neural mechanism necessary for encoding periodic vibrations is the cochlear apparatus in the ear (Organ of Corti).
Another name for rapidly-adapting receptors is ‘phasic’ receptors; slowly-adapting receptors are termed ‘tonic’ receptors (Fig. 4 below).
receptive field
Small field with sharp boundaries (Punctate), near epidermis
Large field with fuzzy boundaries (Diffuse), further from epidermis
What are the 4 basic skin receptors? Are they phasic or tonic? punctate or diffuse? What sensations do they relay?
Four basic skin receptors:
1) Meissner corpuscle axons are Rapidly Adapting - Punctate field fibers that relay Pressure that feels like Flutter.
2) Merkel disks are hooked to Slowly Adapting - Punctate field fibers that relay light touch.
3) Ruffini ending axons are Slowly Adapting - Diffuse field fibers that relay skin stretch.
4) Pacinian corpuscle axons are Rapidly Adapting - Diffuse field fibers that relay deep pressure that feels like Vibration.
What do muscle spindles encode? What is the result of their being excited? Where and how are they located? Same questions for golgi tendon organs. Why is it important that they’re tonic instead of phasic?
Two basic joint receptors:
1) Muscle Spindles are slowly adapting stretch receptors aligned in parallel with muscle fibers. These primary proprioceptors encode muscle length.
2) Golgi Tendon Organs are specialized mechanoreceptors found in the tendons that attach
muscle to bone so are aligned in series with muscle fibers. Their sensory dendrites are interwoven with the tendon’s collagen fibrils and encode muscle tension.
The tonic nature of both these kinesthetic receptors inform the brain of the joint position in space, even when the limbs are not moving. The awareness of our limbs and their orientationin space provides feedback that is essential for the accurate execution of body movement.
Unlike the stretch reflex in which spindle afferents send feedback signals centrally to tighten muscles when the limb is stretched, the Golgi tendon reflex has an opposite effect. Golgi Tendon Organs provide protective feedback that signals relaxation before the tendon tension becomes high enough to cause damage (Fig.5).
What makes the acuity of a system high or low? How is this tested clinically?
D. Location. Although receptor cells may be small, the area from which response can be evoked, its receptive field, can extend much further. Adequate stimulation of any part of the receptive field will set up an afferent action potential in the sensory nerve. If the receptor density is high and the receptive field is very small, the so-called ‘acuity’ of the system is high (great discrimination). If the receptor density is low and the size of the receptive field is very large, then acuity is low. For example, on the finger tips, receptor cells are packed densely and their receptor fields are very small, so a person can distinguish clearly when two stimuli occur very close to one another (two-point discrimination). Figure 6 on the right is a homunculus, a drawing showing a distorted skin surface that corresponds to the regions of higher acuity, where two close points can be discriminated.
What are 2 ways in which primary afferent signals are modulated in the spinal cord? What happens in each? How does this affect 2 point discrimination?
Processing at the first central synapses in the dorsal column nuclei. Modulation of afferent input in the lemniscal system improves the ability to discriminate distinct objects. This neural integration is due to feed-forward and feed-back inhibition. Figure 7.
1) Pre-Synaptic or Feed-Forward Inhibition. Primary afferent fibers activate interneurons in the dorsal column nuclei. These interneurons synapse on the nerve terminals of adjacent primary fibers in the dorsal column nuclei to inhibit the release of neurotransmitters from the nerve terminals of nearby axons. Due to topography, this results in reduction of activity in neurons of adjacent receptive fields, yet a strong response in the primary pathway.
2) Post-Synaptic or Feed-Back Inhibition.
Collateral branches of the primary afferent fibers activate interneurons in the dorsal column nuclei. These interneurons directly project to secondary neurons from adjacent pathways and inhibit them. This too will enhance the ability to discriminate the location of specific stimuli since difference between neurons representing adjacent receptive fields is enhanced.
How does complex spatio-temporal processing work? Where does it happen? What kinds of cells are used? Give examples of how it works.
Within the postcentral gyrus, further processing occurs that create the perception of the stimulus. Anatomically, the neurons in areas 3a and 3b of the PCG typically respond to simple stimuli. In contrast, neurons in areas 1 and 2 of the PCG respond to more complex patterns of input. These include Motion-sensitive cells (responding to movement in any direction); Direction-sensitive cells (responding to movement in specific direction better than
other directions) and Orientation - sensitive cells - respond to movement along specific axis. To create these complex spatiotemporal properties, temporal activation occurs from a series of rapidly- and slowly-adapting fibers in a sequential, topographic manner.
For example, rolling a pen across your hand evokes a sensation that can readily be interpreted. In this case, serial activation of rapidly adapting Meissner’s receptors plus memory results in the recognition of an object even without other sensory information. This integration of different sensory inputs is necessary to encode the complex features of an object (e.g., texture, movement), etc.
Figure 10 shows responses of a cortical cell whose receptive field responded better to specific shapes. The cell was maximally sensitive to a stimulus in the shape of a rod placed across the excitatory zone. A bottle and a piece of cloth produced a smaller increase in activity. In the next deeper layer in the cortex, cells were also selective to directionalchanges - i.e. a movement of a bar in one specific direction caused higher spike firing.
What determines the temperatures that thermoreceptors respond to? What is the range of temp for cold receptors? Warm?
Thermoreception. Although in the skin, thermoreceptors are NOT mechanoreceptors, and ideally do not respond to mechanical stimuli. Two sets of free nerve endings act as thermoreceptors: “Cold” and “Warm” fibers (Fig. 8). Cold fibers respond to decreases in temperature, while warm fibers respond to an increase in temperature. Each fiber expresses a specific protein that confers its preferred temperature. Preferred temperatures of cold fibers range from 15º C to 45º C, while preferred temperatures of warm fibers range from 30º C to 47º C. This overlap explains “paradoxical cold”. Jumping into a very hot bath transiently activates both hot and cold fibers, and for a short time, the bath feels both hot and cold.
