Hormones vs. Neurotransmitters: Key Differences

Hormones and neurotransmitters, while both vital for cell communication, exhibit critical distinctions in their operational mechanisms. Endocrine glands, functioning as key components of the endocrine system, primarily secrete hormones into the bloodstream, which then facilitates widespread distribution throughout the body, influencing various physiological processes. In contrast, neurotransmitters are typically released locally at the synapse, the specialized junction between neurons, enabling rapid and targeted communication within the nervous system. A comprehensive understanding of signal transduction pathways is essential to discern how these chemical messengers initiate cellular responses, either via hormone receptors or neurotransmitter receptors. Given these fundamental differences in secretion, distribution, and action, determining what is the difference between hormones and neurotransmitters becomes crucial for elucidating the complexities of intercellular signaling and its impact on overall health and disease states, as elaborated in the groundbreaking work of researchers like Dr. Candace Pert.

Unveiling the Chemical Messengers: Hormones vs. Neurotransmitters

Biological systems are characterized by intricate communication networks, relying on diverse signaling methods to orchestrate physiological processes. Among these, chemical signaling stands out as a fundamental mechanism. It enables cells to interact and coordinate activities across the organism.

At the heart of this chemical communication lie two primary classes of signaling molecules: hormones and neurotransmitters.

Chemical Signaling: The Foundation of Biological Communication

Chemical signaling involves the release of molecules by one cell that then travel to and interact with another cell, eliciting a specific response.

This process is crucial for maintaining homeostasis, coordinating growth and development, regulating metabolism, and orchestrating complex behaviors. The efficiency and accuracy of chemical signaling are paramount to health and survival.

Dysregulation of these signaling pathways can lead to a wide range of diseases. These diseases include metabolic disorders, neurological conditions, and endocrine dysfunctions.

Hormones and Neurotransmitters: Defining the Divide

While both hormones and neurotransmitters act as chemical messengers, they differ significantly in their mode of action, signaling range, and target specificity.

Hormones are typically produced by endocrine glands and secreted into the bloodstream, allowing them to reach distant target cells throughout the body. Their influence is often slower and longer-lasting, affecting diverse physiological processes.

Neurotransmitters, on the other hand, are synthesized and released by neurons at specialized junctions called synapses. They act locally and rapidly, primarily influencing the activity of adjacent neurons. This contributes to fast, precise communication within the nervous system.

A Historical Perspective: Pioneering Discoveries

The understanding of hormones and neurotransmitters has evolved over more than a century of scientific investigation.

Ernest Starling's discovery of secretin in 1902 marked a turning point. It established the concept of chemical messengers coordinating digestion and laid the groundwork for the field of endocrinology.

Henry Dale and Otto Loewi's experiments in the early 20th century provided definitive evidence for neurotransmission. They demonstrated that nerve impulses transmit signals through the release of chemical substances, rather than electrical impulses alone.

Loewi's famous experiment involved stimulating the vagus nerve of a frog heart and transferring the fluid surrounding that heart to another heart. This demonstrated the presence of a chemical substance, later identified as acetylcholine, that mediated the nerve's effects.

Thesis Statement

This comparative analysis will delve into the intricacies of hormonal and neurotransmitter signaling.

It examines their mechanisms of action, functional characteristics, regulatory pathways, and physiological impacts within the endocrine and nervous systems. By highlighting the key differences and similarities between these two essential communication systems, a deeper understanding of their individual and collective roles in maintaining physiological homeostasis will be achieved.

Unveiling the Chemical Messengers: Hormones vs. Neurotransmitters

Having established the foundational differences between hormonal and neurotransmitter signaling, it is now crucial to dissect the intricate mechanisms by which these chemical messengers exert their influence. Understanding these processes, from synthesis to cellular response, provides a deeper appreciation for the complexity and precision of biological communication.

This section delves into the step-by-step pathways that define hormonal and neurotransmitter action, highlighting the key molecules, receptors, and signal transduction cascades involved.

Hormonal Signaling: A Cascade of Events

Hormonal signaling involves a multi-stage process that begins with hormone synthesis and culminates in a change in cellular function. This process is characterized by specific pathways for hormone production, transport, and receptor interaction.

Hormone Synthesis: Building the Messengers

Hormones are categorized into three primary classes based on their chemical structure: peptides, steroids, and amines. Each class follows a distinct synthesis pathway.

Peptide hormones are synthesized as preprohormones, which undergo processing in the endoplasmic reticulum and Golgi apparatus to yield the active hormone. Examples include insulin and growth hormone.

Steroid hormones are derived from cholesterol and synthesized in the adrenal glands, ovaries, and testes. The synthesis involves a series of enzymatic reactions that modify the cholesterol molecule to produce hormones like cortisol, estrogen, and testosterone.

Amine hormones are synthesized from single amino acids, such as tyrosine or tryptophan. Examples include epinephrine (adrenaline), norepinephrine, and thyroid hormones.

Hormone Secretion: Releasing the Signal

Endocrine glands, including the pituitary, hypothalamus, adrenal, thyroid, and pancreas, are responsible for hormone secretion. The release of hormones is tightly regulated by various factors, including neural signals, feedback loops, and circulating levels of other hormones or metabolites.

For example, the pituitary gland releases hormones that control growth, reproduction, and metabolism, while the adrenal glands secrete hormones involved in stress response and electrolyte balance.

The pancreas secretes insulin and glucagon, which regulate blood glucose levels. The hypothalamus plays a central role in coordinating endocrine function by releasing hormones that control the pituitary gland.

Hormone Transport: Delivering the Message

Once secreted, hormones enter the bloodstream and travel to target cells throughout the body. Many hormones, particularly steroid and thyroid hormones, are transported bound to carrier proteins. These proteins protect the hormone from degradation and increase its half-life in circulation.

The binding to carrier proteins also ensures a steady supply of hormone to target tissues. Only unbound, or "free," hormone can diffuse across the capillary walls and interact with receptors on target cells.

Hormone Receptors and Signal Transduction: Initiating the Response

Hormones exert their effects by binding to specific receptors on target cells. These receptors can be located either on the cell surface or within the cell (intracellular receptors).

Cell surface receptors are typically used by peptide and amine hormones, which cannot easily cross the cell membrane. Binding to these receptors activates intracellular signaling pathways, such as the cyclic AMP (cAMP) pathway or the phosphoinositide pathway.

Intracellular receptors, found in the cytoplasm or nucleus, are used by steroid and thyroid hormones, which can diffuse across the cell membrane. Hormone binding to these receptors often leads to changes in gene expression.

Altering Cellular Function: The Final Impact

The ultimate outcome of hormone signaling is a change in cellular function. Hormones can alter gene expression, leading to increased or decreased synthesis of specific proteins.

They can also affect enzyme activity, either by directly binding to and modulating enzyme function or by altering the expression of genes encoding enzymes. Additionally, hormones can influence membrane permeability, affecting the transport of ions and other molecules across the cell membrane.

Neurotransmitter Signaling: Rapid and Precise Communication

Neurotransmitter signaling, in contrast to hormonal signaling, is characterized by rapid and localized communication between neurons. This process involves the synthesis, storage, release, and inactivation of neurotransmitters at the synapse.

Neurotransmitter Synthesis: Building Blocks of Neural Communication

Neurotransmitters are synthesized in the presynaptic neuron through a variety of enzymatic pathways. The specific pathway depends on the type of neurotransmitter being produced.

For example, acetylcholine is synthesized from choline and acetyl-CoA by the enzyme choline acetyltransferase. Catecholamines (dopamine, norepinephrine, and epinephrine) are synthesized from tyrosine through a series of enzymatic steps.

Serotonin is synthesized from tryptophan, and gamma-aminobutyric acid (GABA) is synthesized from glutamate.

Neurotransmitter Storage: Ready for Release

Once synthesized, neurotransmitters are stored in synaptic vesicles within the presynaptic neuron. These vesicles protect the neurotransmitters from degradation and provide a mechanism for regulated release.

The vesicles are concentrated at the presynaptic terminal, ready to fuse with the cell membrane and release their contents into the synaptic cleft.

Neurotransmitter Release and Diffusion: Crossing the Synaptic Cleft

The arrival of an action potential at the presynaptic terminal triggers the opening of voltage-gated calcium channels. The influx of calcium ions promotes the fusion of synaptic vesicles with the presynaptic membrane, leading to the release of neurotransmitters into the synaptic cleft.

The released neurotransmitters then diffuse across the synaptic cleft to reach the postsynaptic neuron. The synaptic cleft is a narrow space, typically only 20-40 nanometers wide, ensuring a high concentration of neurotransmitter in the vicinity of the postsynaptic receptors.

Neurotransmitter Receptors and Signal Transduction: Binding and Activation

Neurotransmitters exert their effects by binding to specific receptors on the postsynaptic neuron. These receptors can be classified as either ionotropic or metabotropic.

Ionotropic receptors are ligand-gated ion channels that open or close in response to neurotransmitter binding, leading to a rapid change in membrane potential. Examples include the nicotinic acetylcholine receptor and GABA_A receptor.

Metabotropic receptors are G protein-coupled receptors that activate intracellular signaling pathways upon neurotransmitter binding. These pathways can modulate ion channels, enzyme activity, and gene expression. Examples include the muscarinic acetylcholine receptor and adrenergic receptors.

Altering Postsynaptic Neuron Membrane Potential: Excitation and Inhibition

The binding of neurotransmitters to postsynaptic receptors leads to changes in the postsynaptic neuron's membrane potential.

Excitatory neurotransmitters, such as glutamate, depolarize the postsynaptic membrane, making it more likely to fire an action potential. Inhibitory neurotransmitters, such as GABA, hyperpolarize the postsynaptic membrane, making it less likely to fire an action potential.

Neuromodulation: Fine-Tuning Neuronal Responses

Neuromodulators are substances that can alter neuronal responses to neurotransmitters. They can act presynaptically to modulate neurotransmitter release or postsynaptically to modulate receptor function.

Neuromodulators include neuropeptides, such as substance P and endorphins, as well as other substances like adenosine and nitric oxide.

Neurotransmitter Inactivation: Termination of the Signal

To ensure precise and transient signaling, neurotransmitters must be rapidly inactivated after release. There are two primary mechanisms for neurotransmitter inactivation: reuptake and enzymatic degradation.

Reuptake involves the transport of neurotransmitters back into the presynaptic neuron or into nearby glial cells. This process is mediated by specific transporter proteins located on the cell membrane.

Enzymatic degradation involves the breakdown of neurotransmitters by enzymes present in the synaptic cleft or in the presynaptic neuron. For example, acetylcholine is degraded by the enzyme acetylcholinesterase.

Functional Characteristics: Speed, Specificity, and Signaling Distance

Hormonal and neurotransmitter signaling, while both integral to biological communication, exhibit stark differences in their functional characteristics. These differences pertain to the speed and duration of their effects, the specificity with which they act, and the distance over which they operate. These parameters dictate the roles each system plays in coordinating physiological processes.

Speed and Duration of Action: Temporal Dynamics of Signaling

One of the most noticeable distinctions between hormonal and neurotransmitter signaling lies in the temporal dynamics of their actions. Hormonal effects are generally characterized by a slow onset and prolonged duration, while neurotransmitter effects are typically rapid and transient.

Hormonal Signaling: Sustained and Gradual Effects

The relatively slow onset of hormonal effects stems from several factors, including the time required for hormone synthesis, secretion, transport through the bloodstream, and interaction with target cell receptors. Furthermore, many hormonal effects involve alterations in gene expression, a process that inherently requires time for transcription, translation, and protein synthesis.

Once initiated, hormonal effects can persist for extended periods, ranging from minutes to days, or even longer. This prolonged duration is due to the relatively long half-lives of many hormones in circulation, as well as the sustained nature of the cellular changes they induce.

Neurotransmitter Signaling: Swift and Transient Effects

In contrast, neurotransmitter signaling is characterized by its rapid onset and short duration. This is because neurotransmitters are synthesized and stored in synaptic vesicles, ready for immediate release upon arrival of an action potential.

The diffusion of neurotransmitters across the synaptic cleft is a rapid process, and their binding to postsynaptic receptors elicits immediate changes in membrane potential. Moreover, neurotransmitters are quickly inactivated through reuptake or enzymatic degradation, ensuring that the signal is terminated promptly. This rapid inactivation is crucial for maintaining the temporal precision of neural circuits.

Specificity of Action: Targeting Precision

The specificity of hormonal and neurotransmitter signaling also differs significantly. Hormones can affect multiple target tissues throughout the body, provided those tissues express the appropriate receptors. Neurotransmitters, on the other hand, primarily affect the postsynaptic neuron in close proximity to the synapse.

Hormonal Specificity: Receptor-Mediated Targeting

The ability of hormones to affect multiple target tissues is conferred by the widespread distribution of their receptors. A single hormone can elicit diverse effects in different tissues, depending on the specific receptors expressed and the intracellular signaling pathways activated.

This broad specificity allows hormones to coordinate physiological processes across multiple organ systems, such as metabolism, growth, and reproduction.

Neurotransmitter Specificity: Localized and Precise Signaling

Neurotransmitters exhibit a high degree of spatial specificity, primarily affecting the postsynaptic neuron directly adjacent to the synapse. This localized action is essential for maintaining the precise communication required for neural circuits to function properly.

While neurotransmitters can diffuse to some extent beyond the synapse, their effects are generally limited to the immediate vicinity due to rapid inactivation mechanisms and the presence of glial cells that remove excess neurotransmitters from the extracellular space.

Distance of Signaling: Range of Communication

The distance over which hormones and neurotransmitters operate represents another key difference in their functional characteristics. Hormones are transported through the bloodstream to distant target cells, enabling systemic communication. Neurotransmitters, in contrast, act locally across the synaptic cleft, facilitating rapid communication between adjacent neurons.

Hormonal Signaling: Systemic Communication via Bloodstream

The transport of hormones through the bloodstream allows them to reach virtually any tissue in the body. This systemic distribution enables hormones to coordinate physiological processes across multiple organ systems, ensuring that distant tissues respond in a coordinated manner to maintain homeostasis.

Neurotransmitter Signaling: Localized Communication via Synapses

Neurotransmitters operate over very short distances, typically across the synaptic cleft, which is only a few nanometers wide. This localized action allows for rapid and precise communication between neurons, enabling the complex information processing that underlies neural function.

While neurotransmitter signaling is primarily localized, it is important to note the existence of paracrine signaling, in which neurotransmitters or other signaling molecules diffuse to nearby cells that are not directly connected by a synapse. This form of signaling allows for a degree of communication beyond the immediate postsynaptic neuron.

Regulatory Pathways and Feedback Mechanisms: Maintaining Balance

Hormonal and neurotransmitter signaling, while distinct in their operational mechanisms and physiological roles, share a common imperative: the maintenance of homeostasis. Both systems are subject to intricate regulatory pathways and feedback mechanisms that ensure appropriate levels of signaling activity, preventing both deficiency and excess. These regulatory controls are essential for maintaining a stable internal environment and coordinating adaptive responses to changing conditions.

Hormonal Regulation: Orchestrating Endocrine Activity

Hormone secretion is not a static process but rather a dynamic one, finely tuned by a variety of factors including feedback loops, higher-level neural control, and external cues.

Feedback Loops: Balancing Hormone Levels

Negative feedback loops are the predominant regulatory mechanism in the endocrine system. In this system, the hormone itself, or a downstream product of its action, inhibits further hormone release. For instance, elevated levels of thyroid hormone (T3 and T4) inhibit the release of thyroid-stimulating hormone (TSH) from the pituitary gland, thus reducing further thyroid hormone production.

This self-limiting process ensures that hormone levels remain within a physiological range, preventing excessive hormone secretion. Positive feedback loops, while less common, amplify hormone secretion in response to a stimulus. A classic example is the surge of luteinizing hormone (LH) during the menstrual cycle, triggered by increasing estrogen levels, which leads to ovulation. Positive feedback loops are typically short-lived and are essential for specific physiological events.

The Hypothalamus-Pituitary Axis: A Hierarchical Control System

The hypothalamus and pituitary gland form a critical control center for the endocrine system. The hypothalamus secretes releasing and inhibiting hormones that regulate the secretion of hormones from the pituitary gland.

The anterior pituitary then secretes tropic hormones that control the activity of other endocrine glands, such as the adrenal glands, thyroid gland, and gonads. This hierarchical arrangement, exemplified by the hypothalamic-pituitary-adrenal (HPA) axis, allows for coordinated regulation of multiple endocrine pathways in response to stress or other stimuli.

Influence of External Stimuli, Circadian Rhythms, and Developmental Stage

Hormone secretion is also influenced by a variety of external stimuli, such as light, temperature, and stress. For example, exposure to light can influence the secretion of melatonin from the pineal gland, regulating sleep-wake cycles.

Many hormones exhibit circadian rhythms, with secretion varying throughout the day and night. Cortisol levels, for instance, are typically highest in the morning and lowest at night. Finally, hormone secretion can change dramatically across the lifespan, with distinct patterns of hormone secretion during development, puberty, and aging.

Neurotransmitter Regulation: Fine-Tuning Synaptic Transmission

Neurotransmitter signaling is subject to a different set of regulatory mechanisms, focused on maintaining the precision and efficiency of synaptic transmission. These mechanisms include autoreceptors, postsynaptic neuron activity, and neuromodulation.

Autoreceptors: Presynaptic Control of Neurotransmitter Release

Autoreceptors are located on the presynaptic neuron and bind to the neurotransmitter released by that neuron. This binding typically inhibits further neurotransmitter release, providing a negative feedback mechanism that prevents excessive neurotransmitter accumulation in the synapse.

Autoreceptors can also regulate neurotransmitter synthesis and storage. This mechanism allows the presynaptic neuron to self-regulate its neurotransmitter output, ensuring that synaptic transmission remains within an optimal range.

Postsynaptic Neuron Activity: Influencing Presynaptic Release

The activity of the postsynaptic neuron can also influence neurotransmitter release from the presynaptic neuron. For example, retrograde signaling involves the release of signaling molecules from the postsynaptic neuron that travel back to the presynaptic neuron, influencing its activity.

These retrograde messengers can either enhance or inhibit neurotransmitter release, depending on the specific signaling pathways involved. This type of feedback allows the postsynaptic neuron to communicate its needs to the presynaptic neuron, fine-tuning synaptic transmission to meet the demands of the neural circuit.

Neuromodulation: Fine-Tuning Neuronal Responses

Neuromodulation refers to the ability of certain neurotransmitters or substances to alter the effects of a primary neurotransmitter. For example, neuropeptides, such as substance P and neuropeptide Y, can modulate the activity of neurons involved in pain perception, stress response, and appetite regulation.

Neuromodulators can act at multiple sites within the synapse, affecting neurotransmitter release, receptor binding, and postsynaptic neuron excitability. This type of regulation allows for a high degree of flexibility in neural circuits, enabling them to adapt to changing conditions and perform complex computations.

Physiological Impacts and Integration: System-Wide Effects

Hormonal and neurotransmitter signaling exert profound effects on a multitude of physiological processes. These effects span from the regulation of fundamental metabolic functions to the orchestration of complex cognitive processes. Understanding the individual and integrated actions of these signaling systems is crucial for deciphering the intricate mechanisms that govern health and disease.

Endocrine System Effects: A Symphony of Systemic Regulation

The endocrine system, with its diverse array of hormones, plays a pivotal role in maintaining metabolic homeostasis. It is responsible for coordinating growth and development, regulating reproductive functions, and orchestrating the body's response to stress. The impact of these effects is systemic and far-reaching.

Metabolic Regulation

Hormones are essential for regulating nutrient utilization, energy storage, and glucose homeostasis. Insulin, secreted by the pancreas, facilitates glucose uptake by cells, lowering blood sugar levels. In contrast, glucagon, also from the pancreas, stimulates glucose release from the liver, raising blood sugar levels.

Thyroid hormones (T3 and T4) regulate the body's metabolic rate, influencing energy expenditure and heat production. Dysregulation of these hormones can lead to metabolic disorders such as diabetes and thyroid dysfunction.

Growth and Development

Hormones are critical determinants of growth and development throughout the lifespan. Growth hormone (GH), secreted by the pituitary gland, promotes tissue growth and protein synthesis. Sex hormones, such as estrogen and testosterone, are critical for the development of secondary sexual characteristics.

These hormones also contribute to bone growth and maturation. Other hormones, such as thyroid hormones and insulin-like growth factor 1 (IGF-1), also play important roles in regulating growth and development, acting synergistically with GH and sex hormones.

Reproductive Function

The endocrine system governs reproductive function through the coordinated action of sex hormones. In females, estrogen and progesterone, produced by the ovaries, regulate the menstrual cycle, prepare the uterus for pregnancy, and maintain pregnancy.

In males, testosterone, produced by the testes, regulates sperm production and the development of male secondary sexual characteristics. Dysregulation of sex hormone production can lead to infertility, menstrual irregularities, and other reproductive disorders.

Stress Response

The endocrine system plays a central role in the body's response to stress, primarily through the hypothalamic-pituitary-adrenal (HPA) axis. Cortisol, released from the adrenal glands, mobilizes energy stores, suppresses inflammation, and modulates immune function.

Adrenaline (epinephrine), also from the adrenal glands, increases heart rate, blood pressure, and alertness, preparing the body for "fight or flight." Chronic stress and HPA axis dysregulation can have detrimental effects on physical and mental health.

Nervous System Effects: Orchestrating Sensation, Movement, and Cognition

The nervous system, with its intricate network of neurons and neurotransmitters, governs sensory perception, motor control, and cognitive function. Neurotransmitters mediate communication between neurons at synapses, enabling rapid and precise control of these processes.

Sensory Pathways

Neurotransmitters play a crucial role in transmitting sensory information from the periphery to the brain. Sensory neurons express receptors that respond to specific stimuli (e.g., light, sound, touch, pain).

When activated, these receptors trigger the release of neurotransmitters that transmit signals to higher brain centers for processing and interpretation. Glutamate is a major excitatory neurotransmitter involved in sensory transmission, while GABA is a major inhibitory neurotransmitter that helps regulate sensory processing.

Motor Neuron Function and Muscle Contraction

Neurotransmitters are essential for initiating and controlling movement. Motor neurons release acetylcholine (ACh) at the neuromuscular junction. ACh binds to receptors on muscle fibers, triggering muscle contraction.

Disruptions in neurotransmitter signaling at the neuromuscular junction can lead to muscle weakness, paralysis, and movement disorders.

Learning, Memory, and Emotional Processing

Neurotransmitters play a central role in learning, memory, and emotional processing. Glutamate is critical for synaptic plasticity, the ability of synapses to strengthen or weaken over time, which underlies learning and memory.

Dopamine is involved in reward and motivation, while serotonin regulates mood, sleep, and appetite. Dysregulation of these neurotransmitter systems has been implicated in psychiatric disorders such as depression, anxiety, and schizophrenia.

Integration of Endocrine and Nervous Systems: A Harmonious Dialogue

The endocrine and nervous systems do not operate in isolation but rather engage in a complex interplay to maintain homeostasis and coordinate physiological responses. The hypothalamus serves as a critical interface between these two systems, integrating neural and hormonal signals.

Neuroendocrine Integration by the Hypothalamus

The hypothalamus receives input from various brain regions and peripheral sensory receptors, allowing it to monitor the body's internal state and external environment. In response to these inputs, the hypothalamus releases hormones that regulate the pituitary gland, which in turn controls the activity of other endocrine glands.

This hierarchical arrangement allows the hypothalamus to orchestrate coordinated hormonal responses to maintain homeostasis.

Hormonal Influence on Brain Function

Hormones can exert profound effects on brain development, synaptic plasticity, and neurotransmitter release. For example, sex hormones can influence brain structure and function, contributing to sex differences in behavior and cognition.

Stress hormones, such as cortisol, can impair synaptic plasticity and memory formation. Thyroid hormones are critical for brain development and cognitive function.

Neurotransmitter Regulation of Hormone Secretion

Neurotransmitters can also regulate hormone secretion from endocrine glands. For example, dopamine inhibits prolactin secretion from the pituitary gland, while serotonin stimulates cortisol release from the adrenal glands.

This reciprocal communication between the nervous and endocrine systems ensures that hormonal responses are appropriately coordinated with neural activity.

The Blood-Brain Barrier

The blood-brain barrier (BBB) is a highly selective barrier that protects the brain from harmful substances in the bloodstream while allowing essential nutrients and signaling molecules to enter. The BBB is formed by specialized endothelial cells that line the brain's capillaries.

These cells are tightly connected by tight junctions, which restrict the passage of molecules between cells. The BBB plays a critical role in maintaining the brain's internal environment and preventing exposure to toxins, pathogens, and inflammatory molecules.

So, there you have it! While they both act as chemical messengers in the body, the key difference between hormones and neurotransmitters boils down to hormones traveling longer distances through the bloodstream for a widespread effect, while neurotransmitters work locally and quickly across synapses. Understanding this fundamental distinction can really shed light on how our bodies regulate everything from mood to metabolism.