The Nervous System and Endocrine System
The nervous and endocrine systems domain tests rapid neural signaling, slower hormonal regulation, and the integration of both systems. Questions often ask you to follow an action potential, identify a reflex arc, or interpret a feedback loop involving the hypothalamus and pituitary.
Core rules and concepts
- Neurons have dendrites, a cell body, an axon and axon terminals. Myelinated axons conduct faster by saltatory conduction. The resting membrane potential is maintained mainly by potassium leak and the sodium-potassium pump.
- An action potential opens voltage-gated sodium channels, causing depolarization, then potassium channels open and repolarize the membrane. The refractory period prevents backward conduction.
- At a synapse, calcium entry triggers neurotransmitter release. The neurotransmitter binds postsynaptic receptors and produces excitation or inhibition. Acetylcholine is used at the neuromuscular junction and in the autonomic nervous system.
- The sympathetic division prepares for activity; the parasympathetic division supports rest and digestion. Most organs receive dual autonomic input.
- Hormones can be peptide, steroid or amine. Steroid hormones are lipid-soluble and act on intracellular receptors; peptide hormones usually act through surface receptors and second messengers.
- The hypothalamus controls the anterior pituitary via releasing and inhibiting hormones. Tropic hormones stimulate target glands, and negative feedback keeps hormone levels stable.
Worked example
A patient has high thyroid hormone levels and low thyroid-stimulating hormone. The high thyroid hormone is not due to low thyroid output; instead, negative feedback from the thyroid gland is suppressing TSH. This pattern suggests a primary thyroid problem rather than a pituitary problem. If TSH were high with low thyroid hormone, the defect would more likely be thyroid failure with failed negative feedback.
Traps that cost marks
Do not say the sympathetic division always uses norepinephrine; sweat glands use acetylcholine. Do not confuse the anterior and posterior pituitary: the posterior pituitary stores and releases oxytocin and ADH but does not make them. Do not treat all hormones as lipid-soluble. Remember that negative feedback opposes a change, while positive feedback amplifies it.
Worked example
In a reflex arc, the receptor detects a stimulus, the sensory neuron carries the signal to the integration center, and the motor neuron carries the response to an effector.