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Sensory Function and Neurophysiology
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Covers the neurophysiology in different animals, and the sensory function of different tissues.
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Comparative Anatomy and Physiology of AnimalsLecture 27 Sensory function and neurophysiology 23/02/21
The link between sensing and action
- Neural networks consist of neurones that are all interconnected. There are two key
networks- the central nervous system consists of the brain and spinal column in vertebrates.
- In invertebrates, there are different nerve ganglia and coordinating neurones.
- The peripheral nervous system pervades the rest of the body and consists of sensory and
motor neurones that transmit sensory information to the CNS, or response information
away from the nervous system, respectively.
- Sensory receptor cells convert some kind of stimulus energy into an electrical signal via a
system of receptor molecules (often proteins embedded in enlarged cell membranes) that
are sensitive to a particular stimulus, such as temperature (this process is known as signal
transduction).
- This change leads to a change in receptor potential which may cause
the propagation of an action potential to the CNS.
- Whether an action potential is fired depends on the strength of the
stimulus and whether it crosses a threshold.
- Sensory receptor cells are classified by their mechanism of
transduction – ionotropic or metabotropic. In the ionotropic
transduction process, a cell membrane receptor binds a
neurotransmitter as the stimulus and this causes a molecular change
in shape, and so opens an ion channel across the cell membrane.
- This is known as a ‘ligand-gated channel’. The movement of, for
example, sodium ions across the membrane causes a membrane
depolarisation.
Polarised membranes
- A plasma membrane around a cell separates two liquids
that contain ionised molecules.
- If one liquid has a higher concentration of sodium ions
then there will be a small difference in electrical potential
across the membrane, which has a polarity.
- Any chemical change that allows the movement of sodium
ions into the cells causes a depolarisation of the
membrane.
Metabotropic transduction
- Receptor molecule acts as a neurotransmitter to activate a metabotropic cascade.
, - Receptor molecule activates a G protein, which activates a second effector molecule in the
cell that then affects ion permeability of the membrane.
The link between sensing and action
- Sensory receptor cells have two roles: transduction and encoding information.
- Transduction: turns the stimulus into an electrical signal – this generates a receptor
potential.
- Encoding information about the stimulus – usually a series of action
potentials generated by the magnitude of the stimulus.
- An animal’s perception of the outside world comes only from the
encoded signals of the peripheral nervous system.
- Sensory axons (nerves) go to specific parts of the central nervous
system that specialise in particular types of sensory system.
Labelled-lines principle
- Even though all receptors use action potentials to encode stimuli, the
physical destination of the sensory action deals with each type of
sensory receptor.
- A poke in the eyes is still perceive as light, even though it is a response
to pressure. This is because the physical forces stimulates the
photoreceptors and generates action potentials that are perceive as
light because this is what the photoreceptors normally register.
Sensory receptor function – the insect bristle (sensillum)
- Hollow shaft protrudes through the exoskeleton and is attached
to the end of a dendrite, which is supported by sheath and socket
cells.
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