Neurophysiology 6 Flashcards
What are sensory modalities?
Refers to the way that information is encoded. Similar to the idea of transduction.
The main sensory modalities can be described on the basis of how each is transduced.
The chemical senses are taste and smell. Also includes chemical sensation in the form of nociception, or pain (part of the general sense of touch).
Pressure, vibration, muscle stretch, hearing, balance and the movement of hair by an external stimulus, are all sensed by mechanoreceptors.
Vision involves the activation of photoreceptors
Describe the classification of sensory modalities
Can be classified as either general or specific.
A general sense is distributed throughout the body and has receptors within the structures of other organs. E. g. mechanoreceptors in the skin, muscles, or the walls of blood vessels.
They often contribute to the sense of touch,proprioception(body position),kinaesthesia(body movement), or to avisceral sense (important to autonomic functions).
Aspecial senseis one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.
What are sensory sub modalities?
Sub-modalities are more specific categories of the larger sense.
For example, the general sense of touch, known assomatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
Describe the Pacinian corpuscles
Located in subcutaneous tissue below the dermis.
Dendritic endings of a sensory nerve fiber encapsulated by concentric lamellae of connective tissue.
The myelin sheath of the sensory nerve begins inside the corpuscle.
The first node of Ranvier is also located inside, whereas the second is usually near the point at which the nerve fiber leaves the corpuscle
Describe generator potentials based on Pacinian corpuscles
Recording electrodes can be placed on the sensory nerve as it leaves the corpuscle and graded pressure applied to the corpuscle.
When a small amount of pressure is applied, a non-propagated depolarizing potential resembling an EPSP is recorded.
This is called the generator potential or receptor potential.
Recordings of electrical responses to a pressure of:
1X
2X
3X
4X
Strongest stimulus which produced an APs in the sensory neuron
Receptor converts mechanical energy into an electrical response, the magnitude of which is proportionate to the intensity of the stimulus.
The GP, in turn, depolarizes the sensory nerve at the first node of Ranvier.
Once the threshold is reached, an AP is produced and the membrane repolarizes.
If the GP is great enough, the neuron fires again as soon as it repolarizes, and it continues to fire as long as the GP is large enough to bring the membrane potential of the node to the firing level
The APs are propagated along the axon and may synapse onto the next sensory neuron.
Thus, the first node converts the graded response of the receptor into APs, the frequency of which is proportionate to the magnitude of the applied stimuli.
What is sensory coding?
Refers to when a stimulus is converted to a recognizable sensation.
All sensory systems code for four elementary attributes of a stimulus:
Modality is the type of energy transmitted by the stimulus.
Location is the site on the body or space where the stimulus originated.
Intensity is signaled by the response amplitude or frequency of AP generation.
Duration refers to the time from start to end of a response in the receptor.
Describe modality as a stimulus attribute coded by the sensory systems
Modality is the type of energy transmitted by the stimulus
Humans have 4 basic classes of receptors based on their sensitivity to one predominant form of energy: mechanical, thermal, electromagnetic, or chemical.
The particular form of energy to which a receptor is most sensitive is called its adequate stimulus.
For example, the adequate stimulus for the rods and cones in the eye is light (an example of electromagnetic energy).
Receptors do respond to forms of energy other than their adequate stimuli, but the threshold for these nonspecific responses is much higher.
Pressure on the eyeball will stimulate the rods and cones, for example, but the threshold of these receptors to pressure is much higher than the threshold of the pressure receptors in the skin.
Describe location as a stimulus attribute coded by the sensory systems
Location is the site on the body or space where the stimulus originated
A sensory unit: a single sensory axon and all its peripheral branches.
These branches vary in number and may be numerous, especially in the cutaneous senses.
The receptive field of a sensory unit is the spatial distribution from which a stimulus produces a response in that unit.
Generally, the areas supplied by one unit overlap and interdigitate with the areas supplied by others.
One of the most important mechanisms that enable localization of a stimulus site is lateral inhibition.
Information from sensory neurons whose receptors are at the peripheral edge of the stimulus is inhibited compared to information from the sensory neurons at the center of the stimulus.
Thus, lateral inhibition enhances the contrast between the center and periphery of a stimulated area and increases the ability of the brain to localize a sensory input.
Lateral inhibition underlies two-point discrimination.
Describe integrity as a stimulus attribute coded by the sensory systems
Intensity is signaled by the response amplitude or frequency of AP generation
The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor.
For example, as a greater pressure is applied to the skin, the receptor potential in the mechanoreceptor increases, and the frequency of the APs in a single axon transmitting information to the CNS is also increased.
In addition to increasing the firing rate in a single axon, the greater intensity of stimulation also will recruit more receptors into the receptive field
Describe duration as a stimulus attribute coded by the sensory systems
Duration refers to the time from start to end of a response in the receptor.
When a maintained stimulus of constant strength is applied to a receptor, the frequency of the APs in its sensory nerve declines over time.
This phenomenon is known as adaptation or desensitization.
The degree to which adaptation occurs varies from one sense to another.
Receptors can be classified into:
rapidly adapting (phasic) receptors
slowly adapting (tonic) receptors
For example, mechanoreceptors found in the skin/muscle:
Receptors that do not sustain the depolarization for the duration of the mechanical distortion are called rapidly adapting. e.g. Pacinian corpuscles
Receptors that sustain the depolarization with minimal decrease in amplitude for the duration of a stimulus are called slowly adapting. e.g. Merkel cells, muscle spindles, nociceptors
Different types of sensory adaptation appear to have some value to the individual.
Light touch vs muscle spindles vs nociceptors
What is the law of specific nerve energies?
The sensation evoked by impulses generated in a receptor depends in part on the specific part of the brain they ultimately activate.
The specific sensory pathways are discrete from sense organ to cortex.
Therefore, when the nerve pathways from a particular sense organ are stimulated, the sensation evoked is that for which the receptor is specialized no matter how or where along the pathway the activity is initiated.
What is the law of projection with reference to sensation?
No matter where a particular sensory pathway is stimulated along its course to the cortex, the conscious sensation produced is referred to the location of the receptor.
Cortical stimulation experiments during neurosurgical procedures on conscious patients illustrate this phenomenon.
For example, when the cortical receiving area for impulses from the left hand is stimulated, the patient reports sensation in the left hand, not in the head
