Sensory Physio- Lecture 7 Flashcards
Somatic vs Special senses
Somatic senses: Touch/pressure, temperature, pain and proprioception
Special senses: vision, hearing, taste, smell, equilibrium
Conscious vs unconscious senses
Unconscious senses are divided into 2 categories: somatic and visceral
Conscious senses are divided into 2 categories: somatic and special
How are sensory signals transduced, coded and processed?
Transduction: the receptor/primary sensory neuron detects the stimulus
Coding and processing: the primary sensory neuron passes the information to the secondary sensory neuron. Then the secondary sensory neuron passes the information to the tertiary neuron
Perception: the tertiary sensory neuron passes the information to the sensory cortex and we finally become consciously aware of the stimulus.
Types of sensory receptors (primary sensory neurons)
- chemoreceptor: sensitive to pH, 02, organic molecules
- mechanoreceptor: sensitive to vibration, acceleration, sound
- photoreceptors: sensitive to light
- Thermoreceptors: sensitive to temperature
- Noicireceptors: sensitive to tissue damage (pain)
Simple vs complex vs special sensory receptor cell
Receptor vs generator potential
Receptor potential: is for special senses. In special sensory cells, they don’t fire APs, so the stimulus causes the cell to depolarize but they lack the Na channels to fire an AP. So, all the changes in their membrane potential are forms of graded potential called receptor potential. The depolarization of these cells lead to the opening of Ca channels, which allows these cells to release NT which diffuse across the synapse and actvates receptors on the dendritic part of the secondary afferent. S, they can produce AP in that secondary afferent. But, everything that’s taking place at the receptor cell itself is a graded potential ( which can vary in their amplitude and duration.
Generator potential: Is for somatic senses. The depolarization that is taking place in response to the stimulus. if it reaches threshold, it can activate coltage gated Na channels and result in an AP. But, the changes in the membrane potential that lead up to the AP are examples of graded potentials called generator potentials
Modality
Modality is basically our senses, vision, hearing, touch, taste, smell. If you are the first neuron in the chain, you ask yourself, what aspects of the stimulus are important? What do I have to code? What information is necessary for me to know basically what’s happening out there?
It depends upon the specific stimulus interacting with a unique receptor that is tuned for that stimulus. For example, electromagnetic radiation is detected by rods and cones, known as photoreceptors. We experience color, music, taste...It is basically a construct- a virtual reality that we're constantly updating and constantly interacting with. That's what gives rise to a subjective experience. People have different perceptions of electromagnetic radiation. For example, colorblindness. People who are color blind have a very different subjective experience than someone who has full awareness of the visual spectrum. Some organisms can extend beyond the visual spectrum. For example, insects can see UV rays and snakes can see IR rays- which is why they can strike accurately in the dark. Electromagnetic radiation that we have evolved to detect are colors: the visible spectrum. We experience different wavelengths of electromagnetic radiation and have given them the label of color. We also experience different chemicals as taste and smells, and different mechanical frequencies as tone.
Labeled line code
How we keep track of modality?- as we ascent through sensory systems to conscious awareness of the stimulus, the specificity of a response that we see in the sensory receptor itself underlies a phenomenon called the labeled line code.
All of these receptors are mixed up together and are present in the same patch of skin in order to give you a uniform sensory experience. So, how do you distinguish a light tough for a painful or damaging touch?- Each of these receptors is present, we have receptors that are sensitive to light tough but and not sensitive to pain, and then we have nearby receptors that will only be active when damage has occurred- sometimes these are pain receptors that are sensitive to the intracellular cellular contents or to inflammatory compounds. These sensory receptors are the first step in a unique and separate pathwaythat is kept segregated as it ascends from the skin all the way to the brain.
So, we have a touch pathway, and a separate pain pathway. They do NOT cross
Location
True spatial awareness depends on 3 different perceptual abilities: location, discrimation and resolution.
Receptive fields
Every sensory neuron has it’s own unique receptive field. It is that patch of skin that that sensory receptor is monitoring.
Large receptive fields: have low resolution. They are found on parts of the body where thiss isn’t as much of a need for fine discrimination (ex- the back). Even if the stimulus is made of two discrete points, if you apply it to a very large receptive field, you can’t resolve that information and you experience the stimulus as a single point.
Small receptive fields: have high resolution. They are found in sensitive areas like the fingertips and lips. Each point of the stimulus reaches a different cell, so therefore, we can perceive this as 2 separate points.
Receptive fields can interact with their neighbors and make a continuous fields with no section unmonitored.
Neurons A, B and C have these different receptive fields but they all synapse on a single sensory neuron. This secondary neuron also has a receptive field, however, its receptive field is the sum of all 3 receptive fields of the primary afferents, which is a very large receptive field. This process is known as convergence- when primary sensory neurons converge on a secondary sensory neuron.
Parts of a sensory unit
Convergence
What’s the next step in this series of neurons that leads to perception of the stimulus?- primary neurons have to sunapse on the secondary neurons
(a) all of thses primary sensory units converge on a single sencondary sensory neuron. So, when you apply the stimulus- even though you are activating 2 receptive fields, and only those 2 neurons are activated, the signal that goes to the brian is a single stimulus. So, you would not be able to discriminate that the stimulus wa actually made of two separate points.
(b) lack of convergence gives us fine resolution. We can consciously perceive as 2 separate points. We can discriminate because the information is not being lost at the level of the secondary sensory neuron.
Lateral inhibition
Supports our ability to very finely and accurately locate the position of a stimulus on the body’s surface. This only happens at the level of the secondary afferent. Happens in small and large receptive fields.
Intensity
How strong was the stimulus? Was it a gentile tap? Or was it painful?
If we increase the intensity of the stimulus, we increase the frequency of APs
Population coding
This falls under intensity. The number of receptors that are being activated. Depends upon how many sensory unites are involved and how much of their receptive fields are activated. Increased population coding leaves to increased frequency coding.
The stronger the stimulus is, the more of the receptive field it activates when single sensory units are stimulated.
When multiple sensory units are stimulated, the stronger the stimulus, the more it can override lateral inhibition and can send the information along each pathway of the neurons involved. The weaker stimulus activates a smaller portion because it is affected by lateral inhibition.
Duration
How long does the stimulus last?
Adaptation monitors the duration.
Adaptation
Tonic vs phasic receptors
Every sensory system exhibits a mechanism of adaptation. Adaptation is important because it allows the sensory system to adapt to ongoing stimuli so that it can remain sensitive to new stimuli.
Tonic receptors: slowly adapting. They are sustained for the entire stimulus application- it never gets to zero while the sitmulus is applied. It relaxes a little bit, but remains depolarized for as long as the stimulus is present. APs appear as soon as the stimulus is applied, and they continue to be fired for as long as the stimulus is impinging on the receptor.
Phasic receptors: rapidly adapting. They keep track of the changing portion of the stimulus. These are silent for the tonic portion of the stimulus because they rapidly adapt- they depolarize and they go back to rest. Start and end. There’s a burt in APs when the stimulus starts, and a burts when it is removed, with nothing in between.
Label where each part of the pathway to perception of stimulus takes place
Types of touch receptors in the skin and where they are. (subtypes of mechanoreceptors)
- Merkel’s disk
- Meisser’s corpuscle
- Pacinian corpuscle
- Ruffini’s ending (Ruffini’s corpuscle)
Pacinian corpuscle
- Complex receptor
- Detects gross pressure changes in the skin and vibrations. (vibrations are continually oscillating at a particular frequency)
- Found in deep layers
- Phasic receptor (rapidly adapting)
Categorize the touch receptors by size of the receptive field and phasic/tonic
The receptors with the small receptive fields are found more superficially, and have a finer resolution.
The receptors with the larger receptive fields are found deeper and have a coarser representation on the environment.
