Mammals coordinate their organs with two systems. Hormones travel in the blood and act slowly but for a long time; nerve impulses travel along neurones and act in milliseconds. This lesson compares them, then follows a stimulus from a receptor cell in a taste bud to the first action potential in a sensory neurone.
The endocrine system is made of ductless glands that secrete hormones into the blood; only target cells with the matching receptor respond. Examples: ADH from the posterior pituitary acts on collecting duct cells; insulin (beta cells) and glucagon (alpha cells) from the islets of Langerhans in the pancreas act on liver and other cells. The nervous system sends electrical impulses along neurones to specific effectors: it is faster, more precise and shorter-lived. A sensory neurone carries impulses from a receptor to the CNS; an intermediate (relay) neurone connects it to a motor neurone, which carries impulses to an effector. Sensory receptor cells are transducers: a stimulus changes the permeability of their membrane, producing a receptor potential; if this is big enough, an action potential starts in the sensory neurone. In a taste bud, Na+ from salt enters a chemoreceptor cell, depolarising it, so Ca2+ channels open and neurotransmitter is released onto a sensory neurone.
Fig. 1: a motor neurone (cell body in the CNS) and a sensory neurone (cell body in a ganglion outside the CNS)
Section 01
Hormones or nerves?
Click each part of the motor neurone (top, yellow) and the sensory neurone (bottom, green). The panel names it and gives its job. Notice where each cell body sits, and which parts are wrapped in myelin.
Feature
Endocrine system
Nervous system
signal
chemical: a hormone
electrical impulse along a neurone, chemical across synapses
route
carried in the blood plasma all round the body
along neurones to one particular effector
speed
slow (seconds to hours)
very fast (milliseconds)
effect
often long-lasting, widespread, on all target cells with the receptor
short-lived and localised
examples
ADH and water reabsorption; insulin and glucagon and blood glucose
reflex withdrawal; contraction of a single muscle
Endocrine glands: have no ducts; they secrete hormones straight into the blood. The posterior pituitary releases ADH; the islets of Langerhans in the pancreas release insulin (beta cells) and glucagon (alpha cells).
Target cells: a hormone reaches every cell, but only cells with a complementary receptor respond. ADH acts on collecting duct cells; insulin on liver, muscle and fat cells; glucagon on liver cells.
Motor neurone: cell body in the brain or spinal cord with many dendrites, a long axon and motor end plates on an effector (muscle or gland). The cell body has many mitochondria and much rough ER to make neurotransmitter and proteins.
Sensory neurone: a long dendron carries impulses from the receptor to a cell body in a ganglion just outside the CNS; a short axon carries them into the CNS.
Intermediate (relay) neurones: lie entirely in the CNS and connect sensory neurones to motor neurones.
Myelin sheath: layers of Schwann cell membrane wrapped round the axon, with gaps called nodes of Ranvier.
Examiner trap
Do not say hormones travel only to their target organ. Hormones are carried in the blood to every tissue; only target cells respond, because only they have receptors with a shape complementary to the hormone.
Section 02
A receptor cell in a taste bud.
Press Next step to follow salt from the tongue to the brain. Watch the receptor potential reading change from -70 mV to about -40 mV as Na+ flows in, then see Ca2+ enter and vesicles release their neurotransmitter.
Sensory receptor cells detect a stimulus and convert its energy into a change in membrane potential: they act as transducers. Each type responds to one kind of stimulus: chemoreceptors (taste, smell), photoreceptors (light), mechanoreceptors (touch, pressure).
Step 1: salt (NaCl) dissolves in saliva; Na+ ions reach the microvilli of a chemoreceptor cell in a taste bud.
Step 2: Na+ ions diffuse into the cell through Na+ channels in its membrane, down their electrochemical gradient.
Step 3: the inside becomes less negative: the cell depolarises. This is a receptor potential; a stronger stimulus gives a bigger one.
Step 4: depolarisation opens voltage-gated Ca2+ channels and Ca2+ ions diffuse in.
Step 5: Ca2+ makes vesicles of neurotransmitter fuse with the membrane and release it by exocytosis onto a sensory neurone.
Step 6: the neurotransmitter depolarises the sensory neurone. If threshold is reached, an action potential is set up and travels to the brain.
Examiner trap
A receptor potential is graded: its size depends on the stimulus strength. An action potential is all-or-nothing. Only when the receptor potential pushes the sensory neurone past threshold does an action potential start.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the neurones simulator, where is the cell body of the sensory neurone?
Quick check+10 XP
What does the simulator say the myelin sheath is made of?
Quick check+10 XP
In the taste receptor simulator, what happens straight after Na+ enters the receptor cell?
Quick check+10 XP
Which ion entering the taste receptor cell triggers release of neurotransmitter?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which feature is true of endocrine control but not of nervous control?
Exam style+20 XP
Insulin and glucagon are both secreted by the pancreas. Which statement is correct?
Exam style+20 XP
A stronger salt stimulus on the tongue leads to which change?
Exam style+20 XP
Which describes the route of a reflex arc correctly?
Skill unlocked
Nervous and endocrine systems, mastered.
This skill is now recorded on your profile. Keep the chain going.