Neruoscience Lecture 2: Peripheral Sensation and Pain Flashcards
What two neural mechanisms provide the afferent sensory information?
Somatosensory system and special senses
What is the role of afferent sensory information?
Bring about awareness of our internal and external world
What are the special senses?
- Visual
- Auditory
- Vestibular
- Gustatory
- Olfactory
What does the somatosensory system consist of?
- Touch
- Proprioception
- Temperature
- Pain
What is the role of sensory receptors?
Change energy from the external world into graded potentials that can generate action potentials which can be interpreted by the CNS.
Define sensory transduction
The process by which a stimulus is transformed into an electrical response.
What is the axon size principle?
The larger the diameter of the axon, the faster the conduction velocity because larger axons tend to be more heavily myelinated.
How does the transduction of an action potential occur?
- Involves opening and closing of ion (Na/K) channels
- Gating of these channels allows an ion influx across the membrane
- This can produce a change in the membrane potential in the afferent neuron
What is primary sensory coding?
Conversion of stimulus energy into signal that conveys relavent sensory information to the CNS.
What are important characteristics of a stimulus?
- Location
- Type of energy it represents
- Intensity of energy
What is a neuron’s receptive field?
The area of body that, when stimulated, leads to activity in a particular afferent neuron.
Define acuity
The precision with which we can locate the site of stimulation.
What factors affect acuity?
- Size of receptive field - larger fields, less acuity
- Density of receptors - greater receptor density, greater the acuity
- Amount of receptor overlap - greater overlap, greater acuity
- Amount of convergence of neuronal input - greater convergence, less acuity
What is two-point discrimination?
Minimum distance that two stimuli can be separated and be perceived as two stimuli.
Define dermatomes
Specific cutaneous territory Innervated by a given dorsal root ganglion and associated spinal nerve.
What are the types of sensory receptors?
- Mechanoreceptors
- Thermoreceptors
- Photoreceptors
- Chemoreceptors
- Nociceptors
What do mechanoreceptors respond to?
Mechanical stimuli such as bending of hair, deep pressure, vibrations, stretch, and superficial touch.
What do thermoreceptors respond to?
Temperature of the skin and are active over broad ranges (cold receptors, 10-38 C; warm receptors, 32-45 C)
What do photoreceptors respond to?
Particular light wavelengths
What do chemoreceptors respond to?
Binding of particular chemicals. Some respond to changes in internal environment (ie. oxygen in blood), while others respond to external environment (ie. taste and olfactory)
What do nociceptors respond to?
Responds to different types of stimuli that cause pain, or are on the verge of causing pain. Responds to excessive mechanical deformation, excessive temperatures, and many chemicals.
What sensory receptors respond to pain due to extreme temperatures?
Thermal nociceptors
What are 3 factors that affect stimulus intensity?
- Threshold
- Frequency coding
- Population coding
Define threshold
The lowest stimulus intensity that can be reliably detected. Thresholds can increase of decrease.
Define frequency coding
Stimulus intensity is coded by frequency of action potentials. Stronger stimulus produces greater action potential frequency in a single receptor.
Define population coding
Stronger stimuli affect larger areas which activate similar receptors of other afferent neurons.
What are the types of receptor adaptation?
Rapidly adapting receptors and slowly adapting receptors
Define rapidly adapting receptors
Respond very rapidly at stimulus onset, but after initial burst of activity, fire slowly or stop.
Define slowly adapting receptors
Maintain their response at or near the initial level of firing regardless of stimulus duration.
Define sensation
Sensory information reaches consciousness
Define perception
Person’s understanding of the sensation’s meaning.
Define pain
A sensory and emotional experience associated with real or potential injuries, or described in terms of such injuries.
Define algesia and analgesia
- Algesia: sensitivity to pain
2. Analgesia: selective suppression of pain without effects in consciousness or other sensations
What are the three pain states?
- First (fast) pain
- Second (slow) pain
- Third (neuropathic) pain
What are characteristics of first (fast) pain?
- Processing of stimulus that activates nociceptors
- Acute
- Well localized
- Signals current of impending tissue damage
- Is protective
What is second (slow) pain?
- Prolonged noxious stimulated
- Associated with inflammation
- Outlasts the duration of initial stimulus
- NSAIDS decrease second pain
- Opioid-like drugs decrease second pain
What is third (neuropathic) pain?
- Abnormal pain that is chronic, non-protective, and pathologic
- Results from direct injury to nerves
- Has a burning or electric sensation
- Associated with end-stage cancer, neuralgias, peripheral neuropathies, phantom limb pain, and causalgia
- Can evoke fear, depression, and anxiety
What are the dimensions of pain?
Sensory component of pain and motivational-affective responses
What is the sensory component of pain?
- Processing of nociceptor information at the spinal cord or higher brain centers
- Type of pain: prickling, burning, shooting, or aching
- Location of painful stimulus
- Intensity of pain: affected by surrounding conditions, same stimulus different responses
- Duration of pain: acute or chronic
What are motivational-affective responses?
- Nociceptors are activated by noxious (damaging or potentially damaging) stimulation of peripheral tissues
- Nociception may, or may not, lead to the experience of pain
- Pain perception is product of brain’s processing of sensory input
What do motivational-affective responses to pain include?
- Attention and arousal
- Somatic and autonomic reflexes
- Endocrine responses
- Emotional changes
What are characteristics of nociceptor anatomy?
Bipolar cell with free nerve endings and release glutamate or substance P
What are classes of nociceptors?
- Thermal nociceptors
- Mechanical nociceptors
- Polymodal nociceptors
Define thermal nociceptors
- Activated by extreme temperatures (>45 C or <15 C)
- Small-diameter
- Thinly myelinated Adelta fibers
Define mechanical nociceptors
- Activated by intense pressure applied to skin
- Small-diameter
- Thinly myelinated Adelta fibers
Define polymodal nociceptors
- Activated by high-intensity mechanical, chemical, or thermal (both hot and cold) stimuli
- Small-diameter
- Non-myelinated, slow-conducting C fibers
What types of nociceptor fibers cause first and second pain?
- First pain is caused by Adelta fibers
2. Second pain is caused by C fibers
What is referred pain?
Pain perceived as coming from and area remote from its actual origin. Caused by convergence of somatic and visceral nociceptive input onto same dorsal horn secondary neuron.
What is primary hyperalgesia?
Increase in magnitude of pain evoked by noxious stimuli due to nociceptor sensitization or recruitment of additional nociceptors
What is secondary hyperalgesia?
Undamaged skin surrounding site of injury becomes tender due to sensitization of spinal cord neurons. Noxious stimulus applied to this tender area now evokes excessive pain.
What is allodynia?
Non-noxious stimulus, such as gentle touch, now evokes pain.
What is peripheral sensitization and what are some examples?
Sensitization of nociceptors after injury or inflammation results from release of a variety of chemicals by damaged cells in the vicinity.
Ie. bradykinin
What is central sensitization?
During severe and persistent tissue injury, C fibers will constantly be stimulated which release SP and glutamate causing the dorsal horn neurons to become hyperexcitable. This can cause long-term changes in physiological function of these neurons and SP can diffuse to other dorsal horn neurons and activate them as well.
