6 1 Sensory Systems All cells must be able to respond to stimuli in their environments such as binding chemicals or changing shape to engulf food particles For unicellular animals these responses are sufficient to survive and reproduce Multicellular animals have evolved two cell types that are unique to animals neural cells and muscle cells These allow them to control their larger more complex bodies and coordinate multiple tissues Some cells in multicellular animals have specialized to become sensory receptor cells These cells have special proteins in their membranes that allow them to detect particular kinds of stimuli or modalities so that the animal can respond to changes in its internal and external environment Some sensory receptor cells are simple neurons that synapse onto other neurons but others are grouped together along with accessory structures into sensory organs There are many ways to approach sensory systems We will learn about some common principles in sensory receptor cells then go through several major classes of stimuli modalities For each class we will talk about the sensory receptor cells that detect them the membrane proteins involved some accessory structures and sensory organs where the cells are found and how the animal perceives the stimulus Note that detection of a stimulus and perception of a stimulus are separate processes One occurs at the sensory receptor cell the other is a more complex process in the central nervous system when the animal becomes aware of the stimulus The modalities we will cover are chemicals chemoreception mechanical pressure and movement mechanoreception electromagnetic radiation photoreception temperature thermoreception and magnetic and electric fields magnetoreception and electroreception For all stimulus types the sensory receptor cell transduces the stimulus into an electrical signal a graded change in membrane potential Sensory receptor cells are typically either neurons or modified epithelial skin cells that have excitable membranes and can release neurotransmitter like a neuron When the membrane receptor protein detects its particular stimulus it causes a conformational change in the protein that opens or closes ion channels in the membrane changing its potential and causing an EPSP or IPSP In sensory receptor cells that are neurons these graded potentials are called generator potentials because if it depolarizes the membrane to threshold it will generate an action potential The region where the axon begins in a sensory receptor neuron is called the trigger zone instead of the axon hillock These sensory receptor neurons are some of the sensory afferents that we talked about previously In sensory receptor cells that are modified epithelial cells these graded potentials are called receptor potentials The graded receptor potential spreads through the cell via electrotonic conduction just like in sensory receptor neurons Instead of triggering an action potential in the sensory receptor cell it causes a graded release of neurotransmitter onto the next cell which is the sensory afferent neuron Just like at a regular synapse neurotransmitter will binds its post synaptic receptors cause a graded potential and potentially cause an action potential In either case a stimulus is transduced into an electrical signal If the stimulus is sufficient it will cause a sensory afferent to fire an action potential Recall that this leads to the central nervous system where sensory input is integrated You are familiar with the concept of the five senses a classification system that can be traced back to Aristotle over 2000 years ago We will approach sensory systems from a perspective that is focused on physiology how does it work and allows us to compare them across animals The most useful classification system for comparative physiology is by modality or type of stimulus The five senses taste smell touch hearing and sight correspond to the modalities of chemoreception mechanoreception and photoreception Most if not all animals are able to respond to changes in chemicals pressure and electromagnetic radiation but this does not mean all animals have the same five senses that we do Two species may perceive the same stimulus differently one may not perceive it at all Not all animals have the five senses that we do and many of them have senses that we are lacking By approaching this diversity by modality we can more easily see the similarities and differences among taxa Most sensory receptor cells have a modality to which they are most sensitive and are thusly named For example photoreceptors are most sensitive to light Thermoreceptors are most sensitive to temperature That being said many receptors can also be activated by other stimuli if they are sufficiently intense If you push on your eye hard enough you will see light That is you will perceive light but it is clearly not due to detection of light Instead you are activating your photoreceptors with a stimulus of intense pressure Because these sensory receptor cells send input to regions of your brain associated with vision any activation will be perceived as light Nociceptors sensory receptor cells that lead to the perception of pain are unusually multi modal These cells can be activated by intense changes in several modalities including temperature pressure and chemicals Functionally these stimuli are united by the fact that they can cause tissue damage Some senses are a result of integrating multiple sensory receptor cell types and sensory modalities such as proprioception the ability to determine where parts of the body are in space relative to others Proprioception allows animals to position their limbs relative to the ground and walk prevents them from bumping into things and allows them to perform complex bodily movements such as tool use This ability requires mechanical input from the body surface mechanical input from stretch receptors in the muscles and joints and input from the inner ear regarding balance and gravity The CNS also integrates proprioception with visual input to modify movement based on where things appear in space Sensory afferents vary as to how much of the sensory surface they innervate Some sensory afferent neurons receive input from a large area while others receive input from only a small area The area that the neuron receives input from is referred to as its receptive field Neurons with large receptive fields are worse at precisely locating where a stimulus
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