Sensory System Flashcards
Define “receptive field” for a sensory neuron (1)
The receptive field of a neuron is the area on a sensory surface (such as skin or tissue), that a stimulus must reach to activate that neuron.
Explain why the topographic representation (homunculus) of the body surface in the somatosensory cortex is an altered map of actual anatomic space (5) State the purpose of this feature (1)
The neural maps of the body surface are distorted because of the disproportionate representation of different areas of the body (e.g., the hand and face areas) in the postcentral gyrus. That is, as the neurons representing the hand and face have small receptive fields, a greater number of neurons are required to represent the hand and face. This distortion reflects that of the DCN (dorsal column nuclei), e.g., the skin of the back has a small representation because of low afferent density and because of the high convergence (and large receptive fields) in DCN neurons where many afferents converge onto a single DCN neuron, whereas the skin on the fingertip has a high afferent density and only a few afferents converge on a single DCN neuron. Thus, many DCN neurons are required to represent a given area of the skin on the fingertip.
Purpose: Certain body regions that are especially important for function (hands, face) are over-represented; i.e., receive disproportionate cortical allocation within the body map.
Give a concise definition of sensory transduction (2). Discuss the following factors in terms of how they influence information coding in sensory systems: (a) the quality and strength of a stimulus, (b) sensory receptor adaptation and threshold, and (c) receptive fields for primary neurons and neurons in the cortex (8) [10]
Sensory transduction is the conversion of the physical (or chemical) energy stimulus into an electrical signal in a sensory neuron. Factors which influence sensory information coding are the quality and strength of a stimulus, adaptation and sensory threshold of a receptor, and the receptive field. The quality of a stimulus is encoded by the identity of the activated peripheral receptor. Different axonal endings respond to restricted sets of sensory stimuli (selectivity). Selectivity is explained by the morphological specializations of receptor endings and properties of the ionotropic channels in receptor membranes. This selectivity is conveyed and preserved in parallel pathways in the CNS. The strength of a stimulus is encoded by the rate of action potentials in the afferent axons, and the temporal pattern of action potentials in afferent axons. All receptors adapt (decrease their firing rate) to the persistent presence of a stimulus; some adapt slowly and display tonic firing patterns as long as a stimulus is present (more static qualities), whereas others adapt rapidly and display phasic firing patterns (more dynamic qualities). All receptors have a sensory threshold for firing action potentials: in the somatic sensory system, “threshold” is the strength of mechanical deformation necessary for producing a generator potential of sufficient amplitude to elicit an action potential; some receptors have a low threshold (e.g., encapsulated endings) while others have a high threshold (e.g., free nerve endings). For primary neurons, receptive field size is inversely proportional to the density of their peripheral processes (receptors). For neurons in the cortex, receptive field size is directly proportional to the degree of convergence in sensory pathways.
In the somatosensory system, two stimulus parameters include the quality or type (for example, pressure and vibration) and intensity (strength) of a stimulus.
Use an example of a cutaneous mechanoreceptor to explain how:
The properties of that peripheral receptor together with their associated primary sensory afferent nerve endings encode the quality of a stimulus (3)
The quality of a stimulus refers to the type of energy transmitted by a stimulus and is encoded by the identity of the activated peripheral receptor. Different axonal endings respond to restricted sets of sensory stimuli (selectivity). Selectivity is explained by the morphological specializations of receptor endings and properties of the ionotropic channels in receptor membranes. This selectivity is conveyed and preserved in parallel pathways in the CNS.
