sensory receptors Flashcards
sensory transduction
process by which sensory receptors convert one kind of signal energy into another
Receptor neurons are modified axons that project to and synapse on specific destinations in CNS. The distal part (the receptor) converts stimuli into a neural event called generator or receptor potential. The receptor neuron conducts AP to the CNS. The receptor neuron encodes information from the receptive field. With the special senses a separate cell is the receptor which synapses on the sensory neuron.
labeled line principles
(parallel pathway) - receptor field in periphery is connected to an area in the brain
Each receptor generates information in response to a stimulus that conveys info about location and modality. A given pathway btwn a receptor and its destination in the cortex only transmits AP from one spot and for one modality.
receptive field
Restricted region of receptive surface - the area of skin where stimulation excites the receptor
absolute sensory threshold
lowest intensity stimulus that can be detected in 50% of trials
Sensation threshold coincides with the neuron threshold.
Changes in sensory threshold can often be due to changes in CNS
Weber’s Law
The larger the stimulus, the larger the increase has to be in order to be “noticed.”
Discrimination is best at low intensities
Adaptation
A process that occurs while a stimulus is in place. In the face of a continuous stimulus AP frequency may quickly or gradually change depending on the type of receptor.
tonic receptors
Slowly adapting
i.e. pain receptors, baroreceptors, muscle spindles
Keeps CNS up-to-date about events in the periphery (nociceptor)
phasic receptors
Deep touch receptor
i.e. Pacinian corpuscle
Rapidly adapting
Rate coding, reporting changes in stimuli
2-point discrimination
Spatial discrimination.
2 harp points of a caliper are applied to areas of the skin while subject is looking away. With high 2-point discrimination, the subject can distinguish two closely placed stimuli as two points rather than one. Areas with higher concentrations of receptors have a greater 2-point discrimination (i.e. the finger tips)
temporal and spatial summation
temporal - The build-up of an electrical charge on a membrane to create an AP as a result of more than one small successive inputs
spatial - the ability to achieve an AP potential in a neuron which receives input from several cells.
Mechanoreceptor
touch, pressure, vibration muscle and joint position (proprioception), hearing and balance.
Ex. Pacinian corpuscles, Ruffini corpuscles, Merkel disks
Thermoreceptors
changes in temperature and some chemicals
Nociceptors
pain from harmful or potentially harmful stimuli - tissue damage
Electromagnetic receptors
photoreceptors - detect light striking the retinas
Four main somatic modalities
tactile, proprioception, thermal, pain
Receptor generator potential vs. Action potential
RPs are a local potential. They are graded responses such that the greater the stimulus, the greater the receptor potential.
The stimulus (in the correct modality) opens or closes transduction gated channels. The channel event results in potential changes that can be either a depolarization or a hyperpolarization.
For an action potential to occur, the membrane HAS to depolarize (in order to reach threshold), but after action potential, there’s a short time where the membrane hyperpolarizes
In the somatic nervous system, the receptor is the specialized distal end of the axon. When the RP reaches the threshold of the adjoining neuron, AP occur in the neuron.
Transmission
generator potentials resulting in AP going to the CNS
Frequency coding
Greater increases in RP beyond threshold, result in greater frequencies of AP going to the CNS. Frequency coding is any increased stimulus intensity that increases AP
Size of receptive fields in 2-point discrimination
Finger tip vs. Palm vs. forearm
Fingertip - 1 mm
Palm - 10 mm
Forearm - 20 mm
A-neurons
large, myelinated with high conducting velocities
alpha, beta, delta, gamma
alpha neurons
innervate skeletal muscle
large, myelinated A neurons
Fast
Carry fast pain
Beta neurons
medium sized neurons with medium conducting velocity which are associated with autonomic pathways
C- neurons
small, unmyelinated neuron with slow conducting velocities which are associated with pain, itch, temp, crude touch pathways
General sensory pathway
First order neurons transmits AP from the skin and muscles to the CNS (spinal cord or medulla)
Second order neurons cross on the midline (contralateral representation) and transmit AP to the thalamus
Third order neurons project to the parietal lobe (primary somatosensory cortex). Sensory info from the left side of the body goes to the right side of the brain.
How does info get back to the CNS?
The somatic NS communicates with the CNS via spinal nerves. Spinal nerves are mixed nerves meaning they carry both sensory and motor information. The somatic sensory info returns to the spinal cord via the dorsal root.
Dermatome
Area of skin innervated by a single dorsal root
Some overlap
Orderly arrangement of sensory circuitry
Spinal cord - white and gray matter
White matter - myelinated axons
Gray matter - nerve cell bodies and synapses
Dorsal column pathway/medial leminiscal
large fibers, high conduction velocity
- First order neurons ascend spinal cord in dorsal columns
Information from the lower body enters the sacral and lumbar spinal levels, travels in the fasciculus gracilis tracts in the medial dorsal column, ascend the spinal cord and synapse in the brainstem nucleus gracilis.
Information from the upper body enters the thoracic and cervical levels, travels in the fasiculus cuneatus tracts in the lateral dorsal columns, ascend the spinal cord and synapse in the brainstem nucleus cuneatus.
- Second order neurons cross at medulla and the fibers travel in bundles through medial lemniscus and project to the thalamus
- Third order neurons project to the somatosensory cortex in the parietal lobe and information is mapped in a sensory homunculus in the cortex
Anterolateral system/Spinothalamic tract
Smaller, slower fibers
- First order neurons enter the dorsal root and synapse in the dorsal horn.
- Second order neuron crosses and ascends spinal cord in spinothalamic tract, projecting to the thalamus and RAS
- Third order neuron projects from thalamus to the somatosensory cortex in the parietal lobe - information also mapped in sensory homunculus but the mapping is not as precise as the dorsal column pathway
Modality conveyed in dorsal pathway
fine touch, vibration, position, mechanoreceptors
High spatial organization, discrete mechanical stimulation, highly localized
Info/modality convey in anterolateral pathway
pain and temp, crude touch, pressure, tickling, itching, sexual
Brown-Sequard syndrome
caused by damage to one half of the spinal cord, resulting in paralysis and loss of proprioception on the same (or ipsilateral) side as the injury or lesion, and loss of pain and temperature sensation on the opposite (or contralateral) side as the lesion.
Stereogenesis
Ability to pick up an object (without looking at it) and figure out what it is
If a person can’t figure out the object, indicates degermation in parietal lobe
Why is the thalamus a filter?
Sensory information thus travels to the thalamus and is routed to a nucleus tailored to dealing with that type of sensory data. Then, the information is sent from that nucleus to the appropriate area in the cortex where it is further processed. making sure that the information gets sent to the right place
Reticular activating system
piece of the brain that starts close to the top of the spinal column and extends upwards around two inches. It has a diameter slightly larger than a pencil. All of your senses (except smell, which goes to our brain’s emotional center) are wired directly to this bundle of neurons that’s about the size of your little finger.
it’s the gatekeeper of information that is let into the conscious mind. This little bit of brain matter is responsible for filtering the massive amounts of information your sensory organs are constantly throwing at it and selecting the ones that are most important for your conscious mind to pay attention to.
Lateral inhibition and resolution
Inhibits the sides the most, intensifying the center, thereby allowing for edge detection.
Not present at the level of the receptor but starts at the dorsal column nuclei and is found at each subsequent relay step.
In the CNS, the center cells are exited and are surrounded by a rind of inhibited cells. This helps with resolution, precise location and edge detection.
Where is the somatosensory cortex?
Anterior part of the parietal lobe
What is involved in stimulus perception?
Sensation with context
How are the body areas represented in the cortex?
Sensory cortex is laterally mapped to body regions called somatotopic representation
The map is represented according to two point discrimination.
Somatic regions with a higher two-point discrimination have greater representation in the cortex (hands and face).
Homunculus
How is information in the cortex localized?
Fast pain is more localized via fast nuclei
A delta fibers
In what ways does the somatosensory cortex interpret sensory information?
The cortex has 6 layers of cells that permit connections with other brain areas. Different modalities are from a given area of the body are grouped together in modality specific pathways
The mapping in the cortex enables the CNS to localize the information well
- Location of a stimulus via labeled line theory
- Intensity determined by frequency of AP
- Modality determined by pathway to the cortex
- Resolution - density of receptive fields
What is the function and cause of pain?
Pain is a protective mechanism
Acute pain is a physiologic adaptation that ells the organism something is wrong. Acute pain is classified as somatic, visceral or referred.
Somatic pain can be fast or slow.
Nociceptors
Tonic receptors that adapt very little. They can actually become more sensitive (hyperalgesia)
unencapsulated or free nerve endings found in skin, periosteum, vessel and gut walls, joint surfaces, and dura btwn brain hemispheres.
Polymodal - they can be stimulated by more than one kind of stimulus
Extreme heat or cold, tissue damage and release of certain chemicals, ischemia, or mechanical stimuli like muscle spasms
Compare/contrast fast and slow pain in terms of pathway, speed of transmission, interpretation at the CNS
FAST PAIN
Stimulus - Noxious levels of thermal or mechanical stimulation
Receptor - A delta fibers - myelinated and fast
Conduction velocity - Conducts AP at 5-30 m/sec
NTZ - glutamate
Evolutionary name - neospinothalamic
Pathway - First order neuron enters the spinal cord and synapses at the level of entry. The second order neuron crosses at the midline and ascends via the contralateral spinothalamic tract. Most fibers project to the thalamus where the info is filtered. The third order neuron relays the info to the cortex.
Characteristics - sharp, electric, stabbing pain that is well localized
IF the somatosensory cortex were missing or damaged, could a person still sense pain?
Ppl engage in guarding behavior, elevated HR with painful stimuli even with severe cerebral damage.
What is the role of endogenous opiates in pain modulation?
Work at all levels of pain processing
Nociceptor inhibited through binding of endogenous opioids (e.g. enkephalin). The release of substance P is prevented.
