Sensory Receptor Mechanisms and Somatic Sensations Flashcards
What does survival depend on?
Sensation & perception
What is sensation?
it is the ability to feel something physically, especially by touching
What is perception?
it is the conscious interpretation of the stimuli provided by sensations
What is a modality of sensation?
Each of the principal types of sensations that we can experience like touch, pain, sight, sound, etc.
Nerve fibers only transmit impulses so how do different nerve fibers transmit different modalities of sensation?
The type of sensation felt when a nerve fiber is stimulated is determined by the termination point in CNS. i.e unique neurons in the CNS capable of decoding specific modalities.
What is mechanoreceptive sensation?
it is stimulated by mechanical displacements
tactile sensation (skin): touch, pressure, vibration, tickle & itch Proprioceptive sensation (position of muscle and joint in space): muscle stretch sense, joint position sense
What is nociceptive sensation?
It detects pain (tissue damage)
What is thermoreceptive sensation?
detects heat & cold
What are the different somatosensory receptors?
Mechanoreceptors
Thermoreceptors
Nociceptors
What are all the different types of mechanoreceptors?
free nerve endings
expanded tip receptors (Merkel’s discs)
Encapsulated endings ( Meissner’s corpuscle, Pacinian corpuscle, Krause’s corpuscle, Ruffini’s end-organ)
Hair end-organ
What is receptor potential?
When a stimulus excites a receptor, the change in the membrane electrical potential of the receptor is called a receptor potential
What are the mechanisms of stimulation of the receptors?
Mechanical deformation which stretches the membrane
Application of chemicals
Change in temperature
Tissue damage
Define the adaptation of receptors.
When a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally, the rate of APs decreases to very few or none at all
Give two examples of rapidly adapting receptors.
Pacinian corpuscle
Hair receptor
Give two examples of slow adapting receptors.
Muscle spindles
joint receptors
What is the difference between rapidly and slowly adapting receptors?
Rapidly adapting receptors are best at detecting rapidly changing signals while slowly adapting receptors are capable of detecting a long continuous signal
What is the receptive field of a neuron?
It is the area on a surface, like the skin that a stimulus must reach in order to activate that neuron
How does the area of the receptive field vary?
It varies inversely with the density of receptors in the area, increased density= small receptive fields and vice-versa
Describe the receptive fields of the back and legs.
They have a low number of cutaneous receptors and therefore their receptive fields are large in size
Describe the receptive fields of the fingertips and tip of the tongue.
They have a large number of cutaneous receptors and therefore have very small receptive fields
What is two-point discrimination?
minimum distance at which two points of touch can be perceived as separate. This test is an indicator of tactile acuity
What do free nerve endings detect, where are they located, and are they fast or slow adapting?
Found everywhere in the superficial layer of the skin and some other tissues.
Can detect crude touch and pressure sensations
They are slowly adapting receptors
they can be specialized
What do Meissner’s corpuscles detect, where are they found, and are they fast or slow adapting
Elongated encapsulated nerve endings of a large myelinated sensory nerve fiber located in the superficial layers of the skin (non-hairy part).
Detects fine touch and low frequency vibrations
Rapidly adapting receptors
What do Merkel’s discs detect, where are they found, and are they fast or slow adapting receptors
Located in the superficial layers of the skin (epidermis)
detect touch and light pressure
Slowly adapting receptors
What do hair end-organs do, where are they found, and are they fast or slow adapting?
In contact with the root of skin hair
Detects hair movement
Rapidly adapting receptors
What do Ruffini’s end-organs do, where are they found, and are they fast or slow receptors?
Encapsulated endings located in the deeper layers of the skin
Detects heavy and prolonged touch and pressure signals
Slowly adapting receptors
Where are Pacinian corpuscles located, what do they detect, and are they fast or slow adapting?
Encapsulated endings located in the deeper layers of the skin
Detect tissue vibration or other rapid changes in the mechanical state of the tissues
Rapidly adapting receptors
When does pain sensation occur?
Occurs whenever tissue is being damaged
Why is pain sensation considered a protective mechanism?
It brings awareness of tissue damage and can help prevent further damage to the body
What are the two types of pain? describe
Fast pain (pricking pain): felt within 0.1 s of the stimulus and is sharp in character Slow pain (aching pain): begins after a second or more and is throbbing or aching in nature
What are pain receptors (nociceptors)?
They are specialized free nerve endings.
Where are nociceptors located?
Widespread in many locations: the superficial layer of the skin internal tissues arterial walls bones, joints, muscle surfaces
How can nociceptors be stimulated?
Mechanical (stretch, tissue damage)
Thermal (> +/- 45 celsius)
Chemical (lactic acids
Are nociceptors fast or slow adapting receptors?
Slowly adapting receptors
What are thermoreceptors?
Free nerve endings that detect changes in temperature primarily in the non-harmful range
What senses temperatures likely to damage an organism?
Nociceptors
How do thermoreceptors respond to change in temperature?
They respond with a phasic component (rapidly adapting) followed by a tonic component (slowly adapting) (think cold pool)
What does the medial Lemniscal Pathway do?
Transmits tactile sensations and proprioceptive sensation
Does the Medial Lemniscal Pathway transmit information with a high level of fidelity?
Yes it does
What are the three-order neurons in the Medial Lemniscal Pathway?
1st order neurons synapse with 2nd order neurons at the dorsal column nucleus (medulla)
2nd order neurons decussate at the level of the medulla
2nd order neuron synapse with 3rd order neuron in the thalamus
3rd order neuron transmit info to the primary somatosensory cortex
Where does the Medial Lemniscal Pathway decussate
2nd order neuron decussates at the level of the medulla
What does the Antero-Lateral Pathway do?
Transmits a broad spectrum of modalities, signal originates from free nerve endings receptors
Uses smaller myelinated and unmyelinated fiber for slow transmission
What degree of spatial fidelity (orientation) does the Antero-Lateral Pathway have?
Low degree of spatial fidelity
What are the three-order neurons in the Antero-Lateral Pathway
1st order neurons synapse with 2nd order neurons in the Substantia Gelatinosa
2nd order neurons decussate at the level of the spinal cord
2nd order neurons synapse with 3rd order neuron in the thalamus
3rd order neurons transmit info to the primary somatosensory cortex
Where does the Antero-Lateral Pathway decussate?
At the spinal cord level in the Substantia Gelatinosa
What is the neospinothalamic tract?
The tract where fast pain fibers are transmitted.
Fast pain fibers terminate at the somatosensory cortex. Fast sharp pain can be highly localized if it is simultaneously activated with tactile sensation
What is the paleospinothalamic tract?
The tract where slow pain fibers are transmitted.
Slow pain fibers terminate at the level of the thalamus or below
Slow pain is poorly localized
Where is the Primary somatosensory cortex localized?
In the post-central gyrus (parietal lobe)
What is the somatosensory cortex
It is highly organized with distinct spatial orientation, each area of the cortex is devoted to a given body part. each side of the cortex receives information from the other side of the body
Who did the mapping of the somatosensory cortex?
Dr. Wilder Penfield
What is Penfield’s humonculus?
Unequal representation of the body, lips have the greatest representation followed by the face, thumb, fingers, hand.
The trunk and lower body have the least area
What is the relationship between receptors and representation area in the somatosensory cortex?
The more a region has receptors the greater the area of representation in the somatosensory cortex
What is the relationship between receptors and representation area in the somatosensory cortex?
The more a region has receptors the greater the area of representation in the somatosensory cortex
The somatosensory cortex is composed of how many cellular layers?
6
Within the cellular layers of the somatosensory cortex, how are neurons arranged?
In vertical columns, each column serves a specific sensory modality
True or False. The degree to which we react to pain does not vary from person to person.
False it varies tremendously
How does the brain suppress inputs of pain signals?
By activating the analgesia system
What are the components of the analgesia system?
1) the periaqueductal gray of the mesencephalon and upper pons
2) the raphe magnus nucleus located in the lower pons
3) the pain inhibitory complex located in the dorsal horns of the spinal cord
What is the main neurotransmitter involved in the analgesia system and how does it work?
Enkephalin (release on all three levels), it releases Cl- which inhibits the neurons
How do tens work?
They excite the interneuron and release more enkephalin
What is referred pain?
Pain in internal organs is often sensed on the surface of the body
What is Congenital Insensitivity to Pain with anhidrosis (CIPA) characterized by?
- inability to feel pain and temperature
- inability to sweat (anhidrosis)
- it is also known as hereditary sensory and autonomic neuropathy type IV
