16.1 Nervous and Endocrine Systems

The nervous and endocrine systems work together to control and coordinate bodily functions. Neurons transmit electrical signals, while hormones act as chemical messengers traveling through the bloodstream. Both systems maintain homeostasis and regulate processes like growth, metabolism, and reproduction.

The nervous system produces fast, short-lived responses, while the endocrine system produces slower but longer-lasting effects. Understanding how these two systems interact is central to understanding human physiology and the body's ability to adapt to changing conditions.

Nervous System Components

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Structure and Function of Neurons

Neurons are the basic functional units of the nervous system. Each neuron has three main parts:

• Cell body — contains the nucleus and most organelles; serves as the metabolic center of the neuron
• Dendrites — branching extensions that receive signals from other neurons and carry them toward the cell body
• Axon — a long projection that transmits electrical signals (action potentials) away from the cell body toward other neurons or effector cells like muscles

An action potential is a rapid change in voltage across the neuron's membrane that travels down the axon. When it reaches the axon terminals, it triggers the release of neurotransmitters into the synapse.

Many axons are wrapped in a myelin sheath, produced by Schwann cells in the PNS and oligodendrocytes in the CNS. Myelin acts as insulation, forcing the action potential to "jump" between gaps in the sheath (called nodes of Ranvier), which dramatically increases signal speed. When myelin is damaged, as in multiple sclerosis, signal transmission slows and motor and sensory functions deteriorate.

Communication Between Neurons

Neurons don't physically touch each other. Instead, they communicate across tiny gaps called synapses.

Here's how synaptic transmission works:

1. An action potential arrives at the axon terminal of the presynaptic neuron.
2. Voltage-gated calcium channels open, and calcium ions flow into the terminal.
3. Calcium triggers synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft.
4. Neurotransmitters cross the cleft and bind to specific receptors on the postsynaptic neuron.
5. This binding either excites or inhibits the postsynaptic neuron, depending on the type of neurotransmitter and receptor.

Some key neurotransmitters to know:

• Acetylcholine — stimulates muscle contraction at neuromuscular junctions; also involved in memory
• Dopamine — associated with reward, motivation, and motor control
• Serotonin — helps regulate mood, sleep, and appetite
• Glutamate — the main excitatory neurotransmitter; increases the likelihood of an action potential in the postsynaptic neuron
• GABA — the main inhibitory neurotransmitter; decreases the likelihood of an action potential

The balance between excitatory and inhibitory signals determines whether the postsynaptic neuron fires.

Organization of the Nervous System

The nervous system is divided into two major parts:

• Central nervous system (CNS) — the brain and spinal cord. The CNS integrates and processes incoming information and coordinates responses.
• Peripheral nervous system (PNS) — all the nerves outside the brain and spinal cord. The PNS connects the CNS to the rest of the body using sensory (afferent) neurons that carry information to the CNS and motor (efferent) neurons that carry commands from the CNS to effectors.

The PNS is further divided into:

• Somatic nervous system — controls voluntary movements by stimulating skeletal muscles. If you decide to raise your hand, that's somatic.
• Autonomic nervous system — controls involuntary functions like heart rate, digestion, and breathing. You don't have to think about these; they happen automatically.

The autonomic nervous system has two opposing divisions:

These two divisions work in opposition to fine-tune the body's internal state.

Endocrine System Components

Hormones and Their Functions

Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream. They travel throughout the body but only affect target cells that have the correct receptors.

There are two main categories of hormones, and the distinction matters because they work through different mechanisms:

• Steroid hormones (e.g., testosterone, estrogen, cortisol) are derived from cholesterol and are lipid-soluble. Because they can pass through the phospholipid bilayer of the cell membrane, they bind to intracellular receptors, often acting as transcription factors that directly influence gene expression. Their effects tend to be slower but longer-lasting.
• Peptide hormones (e.g., insulin, growth hormone) are water-soluble and cannot cross the cell membrane. They bind to receptors on the cell surface, which triggers intracellular signaling cascades (often involving second messengers like cAMP). Their effects tend to be faster but shorter-lived.

Hormones regulate a wide range of processes: growth and development, metabolism, blood sugar levels, reproduction (including the menstrual cycle), and stress responses.

Major Endocrine Glands and Their Hormones

• Hypothalamus — a region of the brain that links the nervous and endocrine systems. It produces releasing hormones (e.g., TRH, GnRH) and inhibiting hormones that control what the pituitary gland secretes. This is the main point of integration between the two systems.
• Pituitary gland — often called the "master gland" because its hormones regulate many other endocrine glands. The anterior pituitary secretes hormones like TSH (targets the thyroid), ACTH (targets the adrenal cortex), FSH and LH (target the gonads), and growth hormone. The posterior pituitary releases oxytocin and ADH (antidiuretic hormone).
• Thyroid gland — produces T3 and T4, which regulate metabolic rate across nearly all body tissues, and calcitonin, which lowers blood calcium levels by promoting calcium storage in bones.
• Adrenal glands — sit on top of the kidneys. The adrenal cortex produces cortisol (a steroid hormone involved in the stress response and metabolism) and aldosterone (regulates blood pressure by controlling sodium reabsorption in the kidneys). The adrenal medulla produces epinephrine and norepinephrine, which reinforce the sympathetic nervous system's fight-or-flight response.
• Pancreas — functions as both an endocrine and exocrine gland. Its endocrine role involves the islets of Langerhans, which contain beta cells that secrete insulin (lowers blood glucose by promoting cellular uptake) and alpha cells that secrete glucagon (raises blood glucose by stimulating glycogen breakdown in the liver). Dysfunction of this system leads to diabetes mellitus: Type 1 results from autoimmune destruction of beta cells, while Type 2 involves insulin resistance in target cells.

Regulation of Hormone Secretion

Most hormone levels are controlled by negative feedback loops, which work to maintain homeostasis by reversing a change when a variable moves away from its set point.

A clear example is the hypothalamic-pituitary-thyroid (HPT) axis:

1. The hypothalamus detects low levels of thyroid hormones and releases TRH (thyrotropin-releasing hormone).
2. TRH stimulates the anterior pituitary to release TSH (thyroid-stimulating hormone).
3. TSH stimulates the thyroid gland to produce and release T3 and T4.
4. As T3 and T4 levels rise, they inhibit further release of TRH and TSH, reducing thyroid hormone production.
5. If thyroid hormone levels drop again, the cycle restarts.

This loop keeps thyroid hormone levels within a narrow, functional range.

Positive feedback is less common and amplifies a response rather than reversing it. The classic example is childbirth: pressure from the baby's head on the cervix stimulates the release of oxytocin, which increases uterine contractions, which increases pressure, which triggers more oxytocin release. This cycle continues until delivery, at which point the stimulus is removed and the loop ends.

Endocrine disorders often result from abnormal hormone levels:

• Hyposecretion (too little hormone) — e.g., hypothyroidism causes fatigue, weight gain, and slowed metabolism
• Hypersecretion (too much hormone) — e.g., hyperthyroidism causes weight loss, rapid heart rate, and anxiety