Biology: Organism Response Mechanisms

Organisms Responding to Internal and External Changes

• A stimulus is a change in the internal or external environment.
• A receptor detects the stimulus.
• A coordinator processes the stimulus and formulates a response.
• An effector carries out the response.
• Receptors are specific to one type of stimulus.
• Nerve cells transmit impulses rapidly, short-lived, and localized, releasing chemical messengers directly onto target cells.
• Hormones work via the blood system, producing slow, long-lasting, widespread responses.
• Plant responses use hormone-like growth substances.

Stimuli, Internal and External, Lead to Responses

• Organisms respond to environmental changes to increase survival chances.
• In flowering plants, growth factors move, influencing growth directionally.
• Indoleacetic acid (IAA) affects cell elongation, causing gravitropism (growth in response to gravity) and phototropism (growth in response to light).
• Taxes and kineses help mobile organisms maintain favorable environments.
• Simple reflexes, exemplified by three-neuron reflexes, provide protective responses.

Receptors

• Pacinian corpuscles are receptors that respond only to specific stimuli.
• Stimulation of a receptor initiates a generator potential.
• Pacinian corpuscles have a specific structure.
• Deformation of sodium channels in Pacinian corpuscles creates a generator potential.
• The human retina has rods and cones with different sensitivities to light and color, explaining visual acuity. Optical pigments and connections in the optic nerve are important aspects of vision.

Control of Heart Rate

• Myogenic stimulation and electrical activity in the heart; sinoatrial node (SAN), atrioventricular node (AVN), Purkyne tissue, and bundle of His.
• The roles of chemoreceptors and pressure receptors, and the autonomic nervous system in heart rate control.

Nervous Coordination: Nerve Impulses

• Myelinated motor neuron structure.
• Resting potential is related to differential membrane permeability and electrochemical gradients, notably sodium and potassium ions.
• Changes in membrane permeability lead to depolarization and action potential generation.
• The "all-or-none" principle describes action potentials.
• Action potential passage along myelinated and unmyelinated axons.
• Refractory period limits impulse frequency.
• Factors influencing impulse speed include myelination, saltatory conduction, axon diameter, and temperature.

Synaptic Transmission

• Synapse structure (including neuromuscular junction).
• Sequence of events in cholinergic synapses (unidirectionality, temporal and spatial summation, inhibition).
• Comparison between cholinergic synapse and neuromuscular junction.
• Effects of drugs on synapses.

Skeletal Muscle Contraction

• Muscles act antagonistically to move skeletons.
• Gross and microscopic skeletal muscle structure, including myofibril ultrastructure.
• Actin, myosin, calcium ions, and ATP drive myofibril contraction.
• Calcium, tropomyosin, and troponin involvement in the actin-myosin bridge cycle.
• ATP and phosphocreatine roles in muscle contraction.
• Slow and fast skeletal muscle fibres and their properties.