Sensory NS Flashcards
What are receptors?
Receptors are transducers (convert various forms of energy to action potentials)
How do you classify receptors?
Adequate stimulus
Location
Duration
What is the structure of a receptor?
Receptors can either be specialised endings of afferent neurons or a separate receptor cell
The difference between the two is that the separate receptor cell acts as an intermediary between the sensory input and a neuron through something similar to synaptic transmission whilst specialised afferent ending neurons do not have this intermediary step and are directly stimulated by stimuli
What is a graded potential?
Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none.
The main difference between graded potential and action potential is that graded potentials are the variable-strength signals that can be transmitted over short distances whereas action potentials are large depolarizations that can be transmitted over long distances.
Graded potential – dissipates as it spreads out; AP propagates
Pacinian Corpuscle
Sensitive to fast vibrations and deep pressure
Unmyelinated dendrite endings of a sensory nerve fibre
Encapsulated by concentric lamella of CT and located deep in dermis/subcutaneous tissue of all skin
Fast adapting and has big receptive fields
4 elementary attributes of a stimulus
Modality
Location
Intensity
Duration
What is a sensory unit?
A sensory unit is defined as a single sensory axon and all its peripheral branches.
What is a receptive field?
The receptive field of a sensory unit is the spatial distribution from which a stimulus produces a response in that unit.
What is lateral inhibition?
Information from neurons whose receptors are at the peripheral edge of the stimulus is inhibited, compared to information from the sensory neurons at the centre of a stimulus.
What is two-point discrimination?
Two-point discrimination is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one
It is assumed to reflect how finely innervated an area of skin is
Two factors determine two-point discrimination: density of sensory receptors, and size of neuronal receptive fields
The higher the number of sensory receptors in a region, the more accurate the sensory perception of the region
Weber-Fechner Law
The amount of change needed for sensory detection to occur increases with the initial intensity of stimulus, and is proportional to it
Describe stimulus intensity.
Intensity is determined by the amplitude of the stimulus applied to the receptor
An increased magnitude of the stimulus increases the firing rate in the sensory axon and hence an increase in the number of APs reaching the CNS
In addition to the increase in firing rate in an axon, a great intensity of stimulation will also recruit more receptors into the receptive field
Rapidly adapting (phasic) receptors
Pacinian and Meissner’s corpuscles
Slow adapting (tonic) receptors
Muscle spindles, nociceptors, Merkel cells and Ruffini corpuscles
Describe sensory adaptation.
Maintained stimulus reduces frequency of APs in sensory nerve fibres over time (adaptation)
When a mechanoreceptor receives a stimulus, it begins to fire impulses or action potentials at an elevated frequency (the stronger the stimulus, the higher the frequency)
The cell, however, will soon “adapt” to a constant or static stimulus, and the pulses will subside to a normal rate
Doctrine Of Specific Energies
When the nerve pathways from a particular sense organ are stimulated, the sensation evoked is that for which the receptor is specialised no matter how or where along the pathway the activity is initiated.
For example, if you are punched in the eye, an AP can be generated but will be interpreted in the brain as light despite the stimuli being mechanical.
Law of Projection
If we stimulate a sensory pathway along its course to the sensory cortex, the conscious sensation produced is perceived to be from the location of receptors.
If the receptors are skipped and the axon is stimulated somewhere else along its length, the CNS will perceive the stimulus to be arising from the region the receptor is located in.
Phantom limb pain
Ongoing painful sensations that seem to be coming from the part of the limb that is no longer there.
Muscle spindle
Detects when muscle is stretched and is located throughout perimysium (CT around muscle cells)
Golgi tendon organ
Detects when tendon stretched and is located in tendons close to muscle insertion
Joint receptors
Detects joint position and motion and is located in joints
What information is carried by the medial lemniscal pathway?
Fine touch and proprioception
What type of fibres make up the medial lemniscal pathway?
Large myelinated fibres
Describe the medial lemniscal pathway.
1st and 2nd order neurons synapse in medulla – 1st synapse
2nd order neurons run to the medial lemniscus and decussate; run through pons and midbrain to the thalamus
2nd and 3rd order neurons synapse in thalamus – 2nd synapse
3rd order neurons run to the sensory cortex in the parietal lobe
3rd and 4th order neurons synapse in the sensory cortex – 3rd synapse
Some 1st order neurons synapse with interneurons at posterior horn
Axons run to anterior horn and synapse directly with motor neurons
Important for reflexes
Outline the difference fascicles of the medial lemniscal pathway.
Cuneate fascicle for arms and chest
Gracilis fascicle for trunk and legs
What information is carried by the spinothalamic pathway?
Pain, temperature and crude touch
What information is carried by small myelinated fibres
Sharp pain and cold temperature
What information is carried by non-myelinated fibres
Hot temperature, burning pain and crude touch
Describe the spinothalamic pathway.
1st and 2nd order neurons synapse in the posterior horn of the spinal cord – 1st synapse
Small myelinated fibres – enter through dorsal root, bend upwards and travel through two vertebral segments
Non-myelinated fibres – follow the same pathway but synapse with interneurons first (before reaching posterior horn)
2nd order neurons decussate and cross to the anterior horn through the central canal
Neurons are then carried through one of two tracts to thalamus
Lateral tract – carries information for pain, pressure, temperature through the lateral funiculus
Anterior tract – carries information for crude touch through the anterior funiculus
2nd and 3rd order neurons synapse in the thalamus – 2nd synapse
3rd order neurons run to the sensory cortex in the parietal lobe
3rd and 4th order neurons synapse in the sensory cortex – 3rd synapse
