Stimuli and Response

• A stimulus is a detectable change in the internal or external environment of an organism leading to a response
• Receptors are specific to and detect one stimulus
• Coordinators conduct a response to the stimulus, usually in the brain or spinal chord
• Effectors produce a response to a stimulus via a muscle or gland
• Hormones move slower than nerve impulses as they move through the bloodstream
• Stimulus -> receptor -> coordinator -> effector -> response
• A taxis is a directional response to a stimulus
• A positive taxis causes organisms to move towards it, a negative taxis causes organisms to move away from it
• Phototaxis is a response to light
• Chemotaxis is a response to a chemical eg, glucose
• Kinesis is a random change in speed and rate of change of direction toward a stimulus, an organism will move more rapidly and in more directions in unfavourable conditions to increase likelihood of reaching favourable conditions
• Tropisms are the response of a plant's growth to a stimulus
• Phototropism is a response to light
• Gravitropism or geotropism is a response to gravity
• Hydrotropism is a response to water
• Indoleacetic acid (IAA) is an auxin, it will promote cell elongation in shoot tips and restrict cell elongation in root tips
• Cells in the shoot tips produce IAA where it diffuses down the shoot, it will gather on the shaded side of the tip, causing cell elongation and the bending of the shoot tip toward the light
• In roots, IAA will be evenly distributed then build up in the bottoms of the root tips due to gravity, this causes less cell elongation on the upper side due to the high concentration of IAA inhibiting elongation, causing root tip cells to elongate on the upper side, meaning roots grow downwards
• IAA increases cell plasticity due to the acid growth hypothesis, hydrogen ions are actively transported into the cell wall, increasing plasticity allowing elongation by increased cell turgidity
• Plasticity is the ability of a cell to stretch
• IAA is easily made synthetically and incorporated into plants making it ideal to be made in industry as a herbicide as they cause death in high concentrations to some plants
• The nervous system is made of 2 main components, the central nervous system (spinal chord and brain) and the peripheral nervous system
• The peripheral nervous system is made from sensory and motor neurones
• The motor neurones have the somatic nervous system which is voluntary control, and the autonomic nervous system which is automatic control
• The autonomic nervous system has the parasympathetic and sympathetic nervous system
• The parasympathetic nervous system is most active in times of PEACE, it permits digestion and a slower heartbeat
• The sympathetic system occurs in times of STRESS, it involves the fight or flight response and decreases rate of digestion, increasing heart rate
• A reflex arc occurs when a sensory neurone detects a stimulus, an action potential travels down the neurone where it reaches an intermediate neurone, from here, the action potential reaches the motor neurone, where an effector creates a response
• The intermediate neurone is positioned in the grey matter of the spine
• Reflex arcs are rapid and involuntary responses to a stimulus, saving time by skipping the somatic nervous system, allowing for faster responses to danger such as hot surfaces, reducing harm
• Stimulations of a receptor produce a generator potential, if this reaches the threshold potential, an action potential will be generated, travelling along the sensory neurone
• Sensory receptors act as transducers, converting the energy of stimuli such as pressure and vibration into electrical energy
• Thermoreceptors detect changes in blood temperature in the hypothalamus and skin
• Photoreceptors detect light
• Chemoreceptors detect chemicals and pH changes due to carbon dioxide concentrations
• Baroreceptors detect blood pressure
• Pacinian corpuscles are sensory neurones surrounded by connective tissue, they have stretch mediated sodium ion channels which open when pressure is applied, allowing sodium ions to diffuse into the neurone, depolarising it and causing a generator potential
• The more pressure applied to a pacinian corpuscle, the more action potentials are generated, a higher frequency of electrical impulses results in a greater stimulation
• Rod cells contain rhodopsin and are involved in low light perception
• Cone cells contain iodopsin and are involved in high light intensity perception