The Nervous and Endocrine Systems, Heredity and Environment

Two control systems, two speeds

The nervous system sends electrical signals down fixed wiring: fast, precise, over in milliseconds. The endocrine system releases hormones into the bloodstream: slower to start, but the effect lingers for minutes or hours. Both are coordinated by the hypothalamus, which is why a single frightening moment produces an instant jolt and an hour of feeling shaky.

How the nervous system divides

• Central nervous system (CNS) — brain and spinal cord.
• Peripheral nervous system (PNS) — everything else, split in two:
• Somatic — carries sensory input in and controls voluntary skeletal muscle.
• Autonomic — runs organs automatically, and itself splits into the sympathetic branch (arousal: heart rate up, pupils dilate, digestion halted — fight or flight) and the parasympathetic branch (calming: rest and digest).

A reflex arc bypasses the brain entirely: sensory neuron → interneuron in the spinal cord → motor neuron. Your hand is already off the hot pan before the brain registers pain.

The endocrine system

• Pituitary — the master gland, directed by the hypothalamus; releases growth hormone and oxytocin.
• Adrenal glands — epinephrine and norepinephrine for the surge, cortisol for sustained stress.
• Thyroid — metabolic rate. Pancreas — insulin and blood sugar. Ovaries and testes — sex hormones.

Nature working through nurture

Twin studies compare identical (monozygotic) with fraternal twins; adoption studies compare children with biological versus adoptive parents. Heritability is the proportion of variation in a group traceable to genes — never a statement about one person. Epigenetics shows the environment can switch genes on and off without changing the DNA sequence.