Touch a hot pot and your hand jerks away before you even feel the heat. This reflex is fast because the spinal cord gives the command directly, without going through the cerebrum.
Signals are carried by nerve cells called neurons, which receive a signal on their dendrites and pass it along the axon to the next cell.
Switching between reflex and conscious response in the second diagram shows the deciding center change from the spinal cord to the cerebrum.
Where nerves work quickly with electrical signals, hormones spread slowly through the blood and act for a long time, keeping the body's conditions steady.
Stimulus and Response
⚡ How Does the Body Respond to a Stimulus?
①Sense organs such as the eyes, ears and skin take in stimuli like light, sound and heat
②Sensory nerves send those signals to the spinal cord or brain
③Commands from the spinal cord or brain travel along motor nerves to effectors such as muscles
④All of these signals move along neurons as very fast electrical signals
Structure of a Neuron
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Dendrites, cell body, axon and axon terminals of a neuron, with the direction of the signal
💡 What Each Part of a Neuron Does
①Dendrites: receive signals from other neurons or sense organs
②Cell body: holds the nucleus and keeps the cell alive
③Axon: carries the signal all the way to its far end
④Axon terminals: pass the signal on to the next neuron or a muscle
Reflexes and Conscious Responses
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Reflex pathway commanded by the spinal cord compared with the conscious pathway decided by the cerebrum
Spinal reflex pathway
Stimulus → sense organ → sensory nerve → spinal cord → motor nerve → effector
Very fast because it skips the cerebrum. Examples: knee-jerk reflex, pulling your hand from something hot
Conscious response pathway
Stimulus → sense organ → sensory nerve → cerebrum → motor nerve → effector
Slower than a reflex because the cerebrum decides first. Examples: catching a ball, writing
Hormones: Signals Carried by the Blood
Features of hormones
Endocrine gland → blood → target organ (slow but long-lasting)
Nerves act quickly and briefly; hormones act slowly but widely and for longer
🧪 Key Hormones
①Growth hormone (pituitary gland): helps bones and muscles grow
②Insulin (pancreas): lowers blood sugar
③Adrenaline (adrenal gland): speeds up the heartbeat and raises blood sugar
④Thyroxine (thyroid gland): speeds up metabolism such as cellular respiration
Summary
Nerves vs Hormones
Nerves: electrical signals (fast, brief) | Hormones: chemicals in the blood (slow, lasting)
Together the two systems keep conditions such as body temperature and blood sugar steady (homeostasis)
④Hormones are released from endocrine glands into the blood and act only on target organs
⑤Homeostasis is the way nerves and hormones together keep body temperature and blood sugar steady
Examples and Unit Review
Example 1
How do the nerve pathways differ between pulling your hand off a hot object and crossing the street after seeing a traffic light?
1
Pulling your hand away is a reflex along sense organ → sensory nerve → spinal cord → motor nerve → effector; it skips the cerebrum and is very fast.
2
Crossing at a traffic light needs the cerebrum to judge the signal from the sensory nerve before giving a command, so it is slower than a reflex.
▸ Pulling the hand away is a reflex commanded by the spinal cord (fast); crossing the street is a conscious response judged by the cerebrum (slower)
By skipping the judging step, a reflex shortens the reaction time, which helps protect the body from danger.
Example 2
How does insulin act when blood sugar rises, and how does hormonal signaling differ from nervous signaling?
1
When blood sugar rises, the pancreas releases insulin, which travels in the blood to target organs such as the liver and muscles and makes them take up and store glucose, lowering blood sugar.
2
Nerves act quickly and briefly with electrical signals, while hormones travel slowly in the blood and their effects last longer.
▸ Insulin is released from the pancreas into the blood and lowers blood sugar; hormones are slower than nerves but their effects last longer
When blood sugar falls, the pancreas releases glucagon, which raises it. The two hormones work as a pair to keep blood sugar steady.
Unit Test
Which statement about the nervous system and hormones is correct?
①A reflex happens after the brain makes a judgment
②In a neuron the signal travels from the axon terminal toward the dendrites
③Hormones act on target organs through the blood
④Nerve signals are slower and last longer than hormones
⑤Insulin is a hormone that raises blood sugar
▸ ③ Hormones act on target organs through the blood
1
A reflex passes through the spinal cord without the brain (① wrong), and a neuron signal runs dendrite → cell body → axon → axon terminal (② wrong).
2
Nerve signals are fast and brief (④ wrong), and insulin lowers blood sugar (⑤ wrong); hormones act on target organs through the blood (③ correct).