Types of Stimuli - Stimulus is a detectable change in the physical or chemical structure of an organism's environment - Light from a lamp functions as a detectable change in the plant's environment - Plants exhibit phototropism, which is directional growth toward the light stimulus - Sensory receptors can receive information from outside and inside the body - Stimulus transduction can elicit reflexes and trigger homeostatic control systems

Internal Stimuli - Homeostatic imbalances are monitored by receptors and sensors in the body - Mechanoreceptors, chemoreceptors, and thermoreceptors respond to pressure, chemical changes, and temperature changes - Internal stimuli include nutrient and ion levels, oxygen levels, and water levels - Deviations from homeostasis can generate homeostatic emotions like pain, thirst, or fatigue - Behavior is motivated to restore the body to stasis, such as withdrawal, drinking, or resting

External Stimuli - Sensory feelings, especially pain, elicit a large response and cause neurological changes - Sensory receptors on the skin detect pain and transmit signals to the central nervous system - Nociceptors are pain receptors, including A-fiber and C-fiber nociceptors - Absolute threshold for touch is the minimum amount of sensation needed to elicit a response - Vision allows the brain to perceive and respond to changes through light stimuli

Sensory Systems 4.1 Vision - Light enters the retina and excites photoreceptor cells - Local graded potential starts in the photoreceptor and passes along neurons to the CNS - Action potentials are created for the signal to reach the CNS - Visual information is processed in the occipital lobe's primary visual cortex - Absolute threshold for vision is the minimum amount of sensation needed to elicit a response from photoreceptors

4.2 Smell and Taste - Smell allows recognition of chemical molecules in the air through inhalation - Olfactory organs contain olfactory receptor cells and interact with odorant-binding proteins - Odorants bind to G protein-coupled receptors, activating adenylate cyclase - Absolute threshold for smell is the minimum amount of sensation needed to elicit a response from receptors in the nose - Taste records flavoring of food and other materials that pass across the tongue and through the mouth

Neuronal Communication and Responses - Excitatory neurons release excitatory neurotransmitters, causing a response in the postsynaptic neuron - Neurons communicate with thousands of other receptors and target cells through complex dendritic networks - Localised graded potentials trigger action potentials that travel along nerve axons to specific cortexes of the brain - Inhibitory neurons release inhibitory neurotransmitters, leading to an inhibitory postsynaptic potential - Neurons can be either excitatory or inhibitory depending on the type of stimulus

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