lecture 16 - animal nervous systems 3 Flashcards
What is sensory transduction?
Conversion of physical or chemical stimuli into nerve impulses
Sensory transduction usually involves the opening of ion channels in the cell membranes of the sensory receptors (the whole neuron is referred to as the sensory receptor). This can lead to a depolarization or hyperpolarization of the membrane. Sensory receptors are named according to the stimuli they respond to (eg chemoreceptors, mechanoreceptors, thermoreceptors, photoreceptors).
Sensory receptors either produce action potentials themselves, or release neurotransmitters without the generation of action potentials.
Describe the different types of sensory receptor
Simple sensory receptors have free nerve endings
Complex receptors are enclosed in connective tissue capsule (sense pressure)
Many special sense receptors are cells that release neurotransmitters to sensory neurons. They often don’t generate action potentials themselves. Special senses are senses that have specialized organs devoted to them.
Describe the coding of signal strength by firing rate and sensory receptor recruitment
Action potentials are generated in an ‘all-or-nothing’ fashion, so individually cannot convey information about the strength of the incoming signal (e.g. brightness of light, intensity of odor etc.).
The strength of the signal is coded by the firing rate of action potentials, with weak signals resulting in a lower firing rate than strong signals.
- Stronger stimuli activate more sensory receptors than weak stimuli.
Describe how stimulus type and location is determined
- Each sensory receptor only responds to one type of stimulus, and the information received is transmitted to a specific location in the CNS. Thereby, the activation of a sensory receptor conveys information both about the type and the location of the stimulus.
Describe how localization of stimulus depends on receptive field characteristics
The receptive field of a sensory neuron is the part of the body or organ surface in which a stimulus will trigger the firing of that neuron.
Small receptive fields give higher precision of signal localization than large receptive fields. Receptive fields can also overlap, increasing the precision with which the signal can be localized.
For precision of stimulus localization, not only the receptive fields of the sensory receptors are relevant, but also the degree of convergence of the sensory receptors. If several sensory receptors stimulate the same neuron, stimuli cannot be related to the different receptive fields of the sensory receptors.
How does lateral inhibition sharper sensations?
Lateral inhibition increases the contrast between signals received from the centre of the stimulus and the periphery of the stimulus. Stimulated sensory receptors inhibit the signal transmission by neighbouring sensory receptors (via inhibitory interneurons).
Describe how human sense of smell works
The human sense of smell is an example of chemoreception.
The olfactory sensory receptors are bipolar cells. The axons of these cells form the olfactory nerve (only sensory axons, no motor neuron axons). The olfactory nerve consists of several filaments.
Each sensory receptor contains one type of membrane receptor. A total of around 1000 odorant receptor molecules exist in humans.
Binding of odorant to G-protein coupled receptors leads to opening of sodium and calcium channels. Movement of the positive ions into the cell leads to a depolarization of the membrane.
The increased calcium concentration in the cytoplasm of the neuron leads to an inhibition of the cation channels - desensitization of sensory receptors to odorants. This is referred to as olfactory adaptation (adaptation: decrease of sensory receptor firing rate over time in response to a continuous stimulus).
Similar adaptation processes occur in many sensory systems.
Describe how human sensing of sound works
Human sensing of sound is an example of mechanoreception. Sound waves that reach the tympanic membrane are converted into fluid pressure waves in the cochlea (via the bones in the middle ear) that lead to the bending of the basilar membrane. The basilar membrane has variable sensitivity to sound wave frequency along its length; waves with different frequencies lead to the bending of the membrane in different regions of the cochlea.
Bending of the basilar membrane leads to the bending of the hair cell stereocilia
opening of ion channels
membrane depolarization.
The hair cells do not fire action potentials themselves, but release neurotransmitters that lead to the generation of action potentials in signal-receiving sensory neurons of the cochlear nerve.
