stimulus and response 6a, 6b Flashcards
What is a stimulus?
A detectable change in the internal or external environment.
What is a receptor?
A cell or protein that detects stimuli and converts it into an electrical impulse.
What is an effector?
A muscle or gland that brings about a response to a stimulus.
What is the role of the nervous system?
To detect stimuli, process information, and coordinate responses via electrical impulses.
What are the three types of neurones?
Sensory neurones: carry impulses from receptors to CNS
Relay neurones: connect sensory and motor neurones in the CNS
Motor neurones: carry impulses from CNS to effectors
What is a reflex?
A rapid, automatic response to a stimulus that does not involve conscious processing.
Why are reflexes important?
They protect the body from harm by providing fast responses.
Describe the reflex arc.
Stimulus → receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector → response
What are tropisms?
Growth responses of a plant towards or away from a stimulus.
What is phototropism?
Growth of a plant in response to light (e.g., shoots grow towards light).
What is gravitropism (geotropism)?
Growth of a plant in response to gravity (e.g., roots grow downward).
What is the role of auxins in tropisms?
Auxins (e.g., IAA) promote cell elongation in shoots and inhibit it in roots, creating curvature towards/away from stimuli.
What is resting potential?
The voltage across a neurone membrane at rest, around -70 mV (inside is more negative).
How is resting potential maintained?
By the sodium-potassium pump and K⁺ channels — 3 Na⁺ out, 2 K⁺ in.
What is an action potential?
A temporary reversal of membrane potential caused by ion movement across the membrane.
Describe the sequence of an action potential.
Stimulus opens Na⁺ channels
Depolarisation: Na⁺ floods in
Repolarisation: K⁺ channels open, K⁺ leaves
Hyperpolarisation: too much K⁺ leaves
Resting potential restored
What is the refractory period?
A short time when a neurone cannot fire again — ensures one-way transmission.
What is a synapse?
A junction between two neurones.
How is a signal transmitted across a synapse?
Impulse arrives → Ca²⁺ channels open
Vesicles release neurotransmitter (e.g. acetylcholine) into synaptic cleft
Neurotransmitter binds to receptors → Na⁺ channels open → new impulse
How is the signal stopped?
Enzymes break down neurotransmitter in the synaptic cleft.
What is spatial summation?
Multiple neurones release neurotransmitters to trigger one post-synaptic neurone.
What is temporal summation?
One neurone releases neurotransmitter repeatedly in a short time to exceed threshold.
How do stimulants affect synapses?
Increase neurotransmitter release or inhibit breakdown — heightening responses.
How do depressants affect synapses?
Reduce neurotransmitter activity — slowing responses.
What is a sarcomere?
The contractile unit of a muscle, found between two Z-lines.
What proteins are involved in muscle contraction?
Actin (thin filament)
Myosin (thick filament)
Tropomyosin and troponin (regulatory proteins)
What is the sliding filament theory?
Muscles contract by actin and myosin filaments sliding past each other — the sarcomere shortens.
Outline the steps of muscle contraction.
Ca²⁺ released from sarcoplasmic reticulum
Ca²⁺ binds to troponin, causing tropomyosin to move and expose binding sites
Myosin heads bind to actin → form cross bridges
ATP hydrolysed → myosin head pivots, pulling actin
ATP binds again → myosin detaches and resets
What role does ATP play in muscle contraction?
Provides energy for cross-bridge movement and detachment.
What enzyme breaks down ATP in muscle cells?
ATPase, activated by Ca²⁺.
What are the three sources of ATP for muscle contraction?
Aerobic respiration – most efficient
Anaerobic respiration – produces lactate
Phosphocreatine system – quickly regenerates ATP (short bursts)
What is a neuromuscular junction?
A synapse between a motor neurone and a muscle fibre.
How is transmission at a neuromuscular junction different from a synapse between neurones?
Only excitatory
Uses acetylcholine
Causes muscle contraction, not another nerve impulse.
