stimulus and receptors

• Heart rate at rest in a typical adult human is 60-80 beats per minute
• During exercise, the heart rate increases to supply increased blood flow and oxygen to muscle cells
• Heart rate is controlled by the autonomic nervous system, which has two branches: sympathetic and parasympathetic
• The medulla oblongata in the brain controls heart rate:

  • Inhibits effectors and slows down heart rate
  • The cardiac centre in the medulla has two parts: Acceleratory centre linked to SAN by the sympathetic nervous system, and Inhibitory centre linked to SAN by the parasympathetic nervous system
• Two types of receptors important in heart rate control are found in the aorta and carotid artery:

  • Chemoreceptors detect changes in CO₂ levels affecting blood pH
  • Pressure receptors respond to changes in blood pressure
• The eye has photoreceptor cells in the retina: rods and cones
• Rods:

  • Found mainly at the periphery
  • Do not allow color vision
  • Higher sensitivity to light, low visual acuity
• Cones:

  • Found mainly in the fovea
  • Allow color vision
  • Lower sensitivity to light, high visual acuity
• Light causes the pigment in rods and cones to break down, causing depolarization and a generator potential, leading to the transmission of action potentials to the brain forming an image
• The nervous system consists of the CNS (Central Nervous System) - the brain and the spinal cord, and the Peripheral nervous system - nerves that cover the rest of the body
• A reflex is a rapid, involuntary response to a stimulus
• The nervous system is divided into the voluntary nervous system (under voluntary control) and the autonomic nervous system (involuntary, subconscious control)
• A reflex is a rapid, involuntary response to a stimulus, involving a pathway of 3 neurones known as a reflex arc
• Reflex actions are important because they are rapid (due to a short pathway and only 3 synapses), innate (do not have to be learned), prevent damage to tissues, help escape from predators, and have a role in homeostasis
• Kinesis is non-directional movement to a stimulus, involving increasing/decreasing random movements, speed, and direction changes
• Woodlice spend more time in humid areas where dehydration is less, move faster in low humidity with less direction changes, and move slower in high humidity with more direction changes
• Positive taxis is towards the stimulus, negative taxis is away from the stimulus
• Taxes involve directional movement towards or away from a stimulus to find a favorable environment
• Phototropism is a response to light, gravitropism is a response to gravity, and chemotropism is a response to chemicals like salt
• IAA growth factor causes growth in shoots by cell elongation, diffusing through plant tissue to bring about tropisms
• In shoots, IAA is produced in tips of roots and shoots, diffusing to shaded parts for phototropism and underside for geotropism
• In roots, IAA inhibits elongation of root cells, diffusing towards the lower side for directional growth away from light and towards gravity