Session 3 Flashcards
Classify sensory receptors according to their sensory endings
Encapsulated receptors - specialised nerve endings consisting of non-neural components, contributing to the functional properties of the nerve terminals. E.g. Pacinian corpuscles, Ruffini endings.
Free nerve endings - receptors which have unmyelinated terminal branches, which spread through the dermis and into the epidermis.
What is sensory transduction?
When a stimulus impinges upon a receptor, it causes a change in its membrane potential, which is proportional to stimulus intensity. This change fires action potentials encoding information about the intensity and duration of the stimulus.
How is sensory transduction achieved?
- Frequency coding: strength determined by rate of action potential stimulus.
- Activation of neighbouring cells: stronger stimulus activated neighbouring cells to a greater degree
Describe the two types of adaptation
1) Slowly (tonic) - may keep firing action potentials as long as the stimulus lasts, e.g. joint and pain.
2) Rapidly (phasic) - respond maximally and briefly to a stimulus. E.g. touch. No information on the duration of the stimulus.
What are the special senses?
All carried in the cranial nerves and are olfaction, vision, taste, hearing and vestibular function.
What are the general senses?
Somatic and visceral, including touch, pressure, pain and temperature, as well as posture and movement. Carried in spinal nerves, except when it is from the head, in which case it is carried via the trigeminal nerve.
What is sensory acuity?
Precision by which a stimulus can be located. Determined by lateral inhibition, two-point discrimination and synaptic convergence and divergence.
What is lateral inhibition?
The first order nerve fibre closest to the point of stimulation will produce more action potentials than those on the periphery.
They are connected to inhibitory interneurones, reducing the action potentials spread to second order neurones at the periphery.
What is two-point discrimination?
The minimal distance required to perceive two simultaneously applied skin indentations. The distance is determined by the density of receptors and the size of the specific neuronal receptive field.
Fingertips is 2mm apart.
Forearm is 40mm apart.
What do the terms convergence and divergence mean in terms of sensory acuity?
Convergence = decreases acuity. Divergence = causes amplification.
What is somatotopic distribution?
The point-for-point correspondance of an area of the body to a specific point on the CNS. Typically, the area of the body corresponds to a point on the primary somatosensory cortex (postcentral gyrus), represented as a sensory homunculus.
Which regions of the brain are required to ‘feel’ a sensation?
Thalamic level - for crude localisation and distribution of stimuli, and produces highly organised projections to the cortex.
Somatosensory cortex - receives projections from the thalamus and acts to localise and recognise the qualities of the modalities received (post-central gyrus).
Give three examples of cutaneous mechanoreceptors.
1) Krausse (cold)
2) Ruffini’s end organ (heat)
3) Meissner’s corpuscle (texture, slow vibration)
4) Pacinian corpuscle (deep pressure, fast vibration)
5) Merkel’s disc (sustained touch and pressure)
6) Free nerve endings
What is tactile discrimination?
The ability to differentiate information received through the sense of touch, e.g. between sharp and dull objects.
What is the role of the Dorsal column - medial lemniscus pathway?
Fine touch and conscious proprioception.
