grade 8 Nervous System & Hormones

Nervous System & Hormones

When a hand hits a hot pot it jerks away at once: the body senses a stimulus and reacts fast. Neurons pass electrical signals through dendrite, cell body, axon, and terminal; reflexes go via the spinal cord, conscious acts via the brain. Hormones travel in the blood, acting slowly but longer, and with nerves they keep homeostasis. Compare neuron structure, reflex versus conscious paths, and key hormones and they line up.

Stimulus and Response

⚡ How Does the Body React to External Stimuli?

①Touch a hot pot → instantly pull your hand back

②The path: stimulus → receptor → nerve transmission → effector

③The nervous system is an electrical signaling system for fast information transfer

Structure of a Neuron

💡 Roles of Each Neuron Part

①Dendrites: receive signals from other neurons

②Cell body: contains the nucleus and processes information

③Axon: carries the electrical signal over long distances

④Axon terminal: passes the signal to the next neuron or muscle

Reflex vs Conscious Response

Reflex Pathway

Stimulus → Receptor → Sensory Nerve → Spinal Cord → Motor Nerve → Effector

Bypasses the brain so it is very fast (e.g., knee jerk, sneezing)

Conscious Response Pathway

Stimulus → Receptor → Sensory Nerve → Brain → Motor Nerve → Effector

The brain decides, then issues a command (e.g., writing, speaking)

Hormones — Chemical Signals

Features of Hormones

Endocrine gland → Blood → Target organ (slow but long-lasting)

Nerves: fast & brief | Hormones: slow but long-lasting

🧪 Major Hormones

①Growth hormone (pituitary): bone & muscle growth

②Insulin (pancreas): lowers blood sugar

③Adrenaline (adrenal): raises heart rate, stress response

④Thyroxine (thyroid): boosts metabolism

Summary

Nerves vs Hormones

Nerves: electrical (fast, short) | Hormones: chemical (slow, long)

They work together to maintain homeostasis

🎯 Exam Points

①Neuron: dendrites→cell body→axon→axon terminal

②Reflex: via spinal cord (fast) — knee, pupil reflex

③Conscious response: via brain — writing, speaking

④Hormones: travel through blood to target organs

⑤Homeostasis: keeping temperature, blood sugar, etc. stable

Examples and Unit Test

Example 1

How does the nerve path of pulling your hand away from a hot object differ from the path of crossing the street after seeing a traffic light?

Pulling the hand away is a reflex: stimulus → receptor → sensory nerve → spinal cord → motor nerve → effector, which skips the brain and is very fast.

Crossing at a light is a conscious response: along the same path the brain, not the spinal cord, makes the decision and then commands, so it is slower.

▸ Reflex = through the spinal cord (skips the brain, fast) / conscious response = through the brain (after judging, slower)

A reflex that avoids danger skips the brain to shorten the reaction time.

Example 2

When blood sugar rises, how does insulin act, and how does a hormone differ from a nerve in the way it sends signals?

Insulin is released from the pancreas, travels through the blood to target organs, and lowers blood sugar.

A nerve uses electric signals that act fast and briefly, while a hormone travels through the blood slowly and its effect lasts long.

▸ Insulin (pancreas) → blood → target organ → blood sugar down / hormones are slow but lasting, nerves are fast but brief

Nerves and hormones work together to keep homeostasis such as body temperature and blood sugar.

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

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).

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).

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Blood Circulation

Layers of the Atmosphere

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