What are the two classification schemes by which peripheral nerves are classified?
1) Their contribution to a compound action potential (A, B, and C waves)
2) Based on fiber diameter, myelin thickness, and conduction velocity (classes I, II, III, IV)
What are the features of the group Ia or Aα peripheral axons (i.e., fiber diameter, myelination conduction velocity, and receptor supplied)?
- 13-20 μm (large)
- 80-120 m/s (fast)
- Heavily myelinated
- Receptor supplied: primary muscle spindles, Golgi tendon organs
*These are the alpha-motorneurons
What are the features of the group IV or C peripheral axons (i.e., fiber diameter, myelination, conduction velocity, and receptor supplied)?
- 0.2-1.5 μm (small)
- 0.5-2 m/s (slow)
- Receptor supplied: skin mechanoreceptors, thermal receptors, and nociceptors (slow pain)
An appropriate stimulus applied to a somatosensory receptor produces a ________ that, when large enough, leads to action potentials that can be carried over a considerable distance into the CNS.
An appropriate stimulus applied to a somatosensory receptor produces a generator potential that, when large enough, leads to action potentials that can be carried over a considerable distance into the CNS.
What is the Weber-Fechner Law; how much stimulus required for noticeable difference?
- There exists a logarithmic relationship between stimulus and perception
- A 10% difference is usually required for conscious perception of change
How does perceived intensity differ amongst muscles and cutaneous stimuli?
- Muscle perceived intensity matches the actual intensity very closely
- Cutaneous perceived intensity may diverge from the actual intensity substantially
When a stimulus persists unchanged for several minutes without a change in position or amplitude, what occurs to the neural response and sensation; this is called what?
- The neural response diminishes and sensation is lost
- This is receptor adaptation
Receptors that respond to prolonged and constant stimulation are classified as?
Slowly adapting receptors
Receptors that respond only at the beginning or end of a stimulus are classified how; what activates them?
- Rapidly adapting receptors
- Only active when the stimulus intensity increases or decreases
What are the 4 mechanoreceptors?
1. Meissner corpuscle
2. Pacinian corpuscle
3. Merkel disk
4. Ruffini ending
What is a receptive field?
Individual mechanoreceptor fibers convey information from a limited area of skin
Tactile acuity is highest and lowest where and how does this related to the size of the receptive field?
- Tactile acuity is highest in fingertips and lips (smallest receptive field)
- Tactile acuity is lowest in calf, back and thigh (largest receptive field)
What is an afterdischarge?
- In some cases of receptor adaption, the removal of the stimulus triggers AP's as the ending "reforms."
- The persistance of a sensation after the stimulus eliciting the discharge has been removed
What is Pre-synaptic inhibition; what kind of synapse; what is the end result?
- Axo-axonal synapse
- The post-synaptic cell is a pre-synaptic terminal
- The end result of pre-synaptic transmission is reduced NT released from the inhibited pre-synaptic terminal
What are the 4 steps in pre-synaptic inhibiton (what's released and the end result)?
1. Pre-synaptic terminal of neuron C synspases on the pre-synaptic terminal of neuron A, when acitvated neuron C releases GABA causing influx of Cl- into neuron A
2. Results in hyperpolarization of pre-synaptic terminal of neuron A
3. Less Ca2+ enters cytosol
4. Leads to less NT released and reduced probability of AP's in neuron B
Pre-synaptic inhibition occurs where in the pathways involved in central processing of the senses; what impact does it have on the brain?
- Occurs between neighboring receptors at the first synapse in their pathway
- Every synapse along the way represents a chance to modify or respond to the stimulus
- Increases the brain's ability to localize the signal
What is the most powerful form of inhibitory control in all primary afferent fibers?
The somatosensory cortex has how many layers, how are the neurons arranged, and what is the importance of this arrangement?
- Has 6 layers
- Arranged in columns
- Each column deals with one sensory modality in one part of the body
Which layers of the primary sensory cortex houses the columns which are the main site of termination of axons from the thalamus?
Which are the main output neurons from the primary sensory cortex?
Where is the primary somatosensory cortex (S1) located, which Brodmann's areas, and what is this part of the brain involved in?
- Located in post-central gyrus
- Brodmann's 1, 2, and 3
- First stop for most cutaneous senses
- Involved in the integration of information for postion sense as well as size, shape discrimination
Where is the secondary somatosensory cortex (S2) located, whre does it receive input from and what is this part of the brain involved in?
- Wall of the sylvian fissure
- Receives input from S1
- Cognitive touch
- Comparisons between objects, different tactile sensations and determining whether something becomes a memory
Do the primary and secondary somatosensory areas (S1 and S2) have somatotopic representation?
- S1 has detailed somatotopic representation
- S2 has somatopic representation, BUT is NOT as detailed as S1
What is the function of the parieto-temporal-occipital (PTO) association cortex?
- High level interpretation of sensory inputs
- Receives input from multiple sensory areas, including S1 and S2
- Analyzes spatial coordinates of self in enviornment, names objects, and has many functions
What are Corticofugal signals, where are they transmitted between and their function?
- Transmitted back from cortex to lower relay stations in the thalamus, medulla, or SC
- Controls the intensity of sensory sensitivity
- Typically inhibitory and suppresses sensory input
What is the Doctrine of specific nerve energies, tells you what?
- No matter where along the afferent pathway is stimulated, the sensation that will occur is determined by the nature of the sensory receptor in the periphery connected to that pathway
- Tells you the modality (i.e., what kind of receptor stimulated)
What is the Law of Projections, tells you what?
- No matter where along the afferent path we stimulate, the perceived sensation is always referred back to the area of the body in which the receptor is located
- Tells you where you feel it (location)
What are the 4 ways pain can be characterized by location?
1) Deep pain (i.e., bone pain)
2) Muscle pain
3) Visceral pain
4) Somatic/cutaneous pain
Nociceptors are bare nerve endings used for sensing noxious stimuli, what are the 2 types of fibers involved and characteristics of each?
Aδ fibers: small, sparsely myelinated. Fast, sharp pain
C fibers: unmyelinated fibers associated with dull pain (slow pain)
What are the different types of nociceptors (i.e., what are they sensitive to)?
- Sensitive to both thermal and mechanical stimuli (majority)
- Sensitive only to thermal or only to mechanical
- Silent/sleeping: not active during initial injury, but can be "woken up."