The hypothalamic pituitary adrenal (HPA) axis is our central stress response system. The HPA axis is an eloquent and every-dynamic intertwining of the central nervous system and endocrine system.
This system works in a fairly straightforward manner. The HPA axis is responsible for the neuroendocrine adaptation component of the stress response. This response is characterized by hypothalamic release of corticotropin-releasing factor (CRF). CRF is also known as CRH or corticotropin-releasing hormone. When CRF binds to CRF receptors on the anterior pituitary gland, adrenocorticotropic hormone (ACTH) is released. ACTH binds to receptors on the adrenal cortex and stimulates adrenal release of cortisol. In response to stressors, cortisol will be released for several hours after encountering the stressor. At a certain blood concentration of cortisol this protection is ostensibly achieved and the cortisol exerts negative feedback to the hypothalamic release of CRF and the pituitary release of ACTH (negative feedback). At this point, systemic homeostasis returns.
With repeated exposure to stressors, the organism habituates to the stressor with repeated and sustained HPA axis activation. Therefore, it is important to support healthy cortisol levels in order to ensure the hypothalamus and pituitary glands maintain the appropriate level of sensitivity to the negative feedback of cortisol. Secretion of alarm chemicals such as epinephrine and norepinephrine from the adrenal medulla, as well as HPA axis activation persists along with the secretion of CRF, ACTH, and cortisol. Interestingly, with aging, the hypothalamus and pituitary are less sensitive to negative feedback from cortisol and both ACTH and cortisol levels rise as we age.1 Older women secrete more cortisol in response to stress than do older men. Young women, however, produce lower levels of cortisol in response to stress than do young men.
Under conditions of normal exposure to cortisol, our tissues only experience fleeting glimpses of the alarm catecholamines and cortisol. As we are addressing the various health consequences of stress, it is imperative to also address the axis of response itself. Restoring homeostasis to the HPA axis is the primary goal of integrative care.
Although the term “Stress” is generally claimed as something negative, it is in reality also a positive driver. In order to perform well, a certain degree of positive stress (called EUSTRESS) is needed. Positive Stress can be experienced when someone is well focused on a specific task, motivated, feeling confident and also excited about the result he/she is hoping to achieve. It is a typical short term feeling.
Negative stress (called DISTRESS) occurs when a person feels unable to perform or to cope with situation. This feeling can be short or long term. It causes anxiety or concern and can lead to mental and physical problems. The causes (called STRESSORS) for the negative feelings of stress do not always lie with external situations. Internal feelings (i.e. fear of doing something), thoughts (i.e. continuous worrying) and certain behaviours (i.e. procrastination) can also lead to negative stress.
The underneath chart shows the relation between stress (positive and negative) and performance efficiency:The physiological stress can be measured by combining physiological variables whilst the perceived stress can be indicated using questionnaires. Whether the stress in negative or positive, the physiological process in your body is the same. In order to deal with the stressor, the body goes in the‘fight’ mode and biochemical reactions are taking place. These reactions can ask a lot of metabolic energy from the body. BioRICS continuously measures key variables in the individual energy equation.By using real-time algorithms, the metabolic energy that the body consumes for mental tasks iscalculated.
Firstly, let’s debunk one myth: stress is not necessarily a ‘bad’ thing. Without this brilliant ability to feel stress, humankind wouldn’t have survived. Our cavemen ancestors, for example, used the onset of stress to alert them to a potential danger, such as a sabre-toothed tiger.
Stress isn’t always bad. In small doses, it can help you perform under pressure and motivate you to do your best. But when you’re constantly running in emergency mode, your mind and body pay the price. If you frequently find yourself feeling frazzled and overwhelmed, it’s time to take action to bring your nervous system back into balance. You can protect yourself — and improve how you think and feel — by learning how to recognize the signs and symptoms of chronic stress and taking steps to reduce its harmful effects.
What is stress?
Stress is your body’s way of responding to any kind of demand or threat. When you sense danger—whether it’s real or imagined—the body’s defenses kick into high gear in a rapid, automatic process known as the “fight-or-flight” reaction or the “stress response.”The stress response is the body’s way of protecting you. When working properly, it helps you stay focused, energetic, and alert. In emergency situations, stress can save your life—giving you extra strength to defend yourself, for example, or spurring you to slam on the brakes to avoid an accident.
Stress can also help you rise to meet challenges. It’s what keeps you on your toes during a presentation at work, sharpens your concentration when you’re attempting the game-winning free throw, or drives you to study for an exam when you’d rather be watching TV. But beyond a certain point, stress stops being helpful and starts causing major damage to your health, your mood, your productivity, your relationships, and your quality of life.Fight-or-flight response: what happens in the body.
Stress is primarily a physical response. When stressed, the body thinks it is under attack and switches to ‘fight or flight’ mode, releasing a complex mix of hormones and chemicals such as adrenaline, cortisol and norepinephrine to prepare the body for physical action. This causes a number of reactions, from blood being diverted to muscles to shutting down unnecessary bodily functions such as digestion.Through the release of hormones such as adrenaline, cortisol and norepinephrine, the caveman gained a rush of energy, which prepared him to either fight the tiger or run away. That heart pounding, fast breathing sensation is the adrenaline; as well as a boost of energy, it enables us to focus our attention so we can quickly respond to the situation.
In the modern world, the ‘fight or flight’ mode can still help us survive dangerous situations, such as reacting swiftly to a person running in front of our car by slamming on the brakes.The challenge is when our body goes into a state of stress in inappropriate situations. When blood flow is going only to the most important muscles needed to fight or flee, brain function is minimised. This can lead to an inability to ‘think straight’; a state that is a great hindrance in both our work and home lives. If we are kept in a state of stress for long periods, it can be detrimental to our health. The results of having elevated cortisol levels can be an increase in sugar and blood pressure levels, and a decrease in libido.
Signs and symptoms of stress overload
The most dangerous thing about stress is how easily it can creep up on you. You get used to it. It starts to feel familiar — even normal. You don’t notice how much it’s affecting you, even as it takes a heavy toll. That’s why it’s important to be aware of the common warning signs and symptoms of stress overload.
Causes of stress
The situations and pressures that cause stress are known as stressors. We usually think of stressors as being negative, such as an exhausting work schedule or a rocky relationship. However, anything that puts high demands on you can be stressful. This includes positive events such as getting married, buying a house, going to college, or receiving a promotion.
Of course, not all stress is caused by external factors. Stress can also be internal or self-generated, when you worry excessively about something that may or may not happen, or have irrational, pessimistic thoughts about life.
Finally, what causes stress depends, at least in part, on your perception of it. Something that’s stressful to you may not faze someone else; they may even enjoy it. While some of us are terrified of getting up in front of people to perform or speak, for example, others live for the spotlight. Where one person thrives under pressure and performs best in the face of a tight deadline, another will shut down when work demands escalate. And while you may enjoy helping care for your elderly parents, your siblings may find the demands of caretaking overwhelming stressful.
Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord.
All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 2) dedicated to processing that particular sense.
How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species is largely similar, owing to the inherited similarity of their nervous systems; however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ.
Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae(singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance (Figure 2): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.
Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.
Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 3). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.
The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.
The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.
Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure 4). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.
Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.
The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure 10). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.
The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure 11). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.
Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.
Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.
If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors.
Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint.
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 12). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.
Structural Receptor Types
The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 1). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.
Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce.
Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell.
Osmoreceptors respond to solute concentrations of body fluids.
Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor.
Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor.
Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.
Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.
Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away (some baleen whale sounds can propagate for many kilometers). For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances.
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 1) possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Recall that in the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.
Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.
Encoding and Transmission of Sensory Information
Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus, and this segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus—a sound.
The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals.
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction.
Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern.
Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.
The glandular system regulates the activities of the body by secreting chemical substances (hormones) into the bloodstream. These secretions come from a variety of glands which influences almost every cell, organ, and function of our bodies. The endocrine system is instrumental in regulating mood, growth and development, tissue function, metabolism, sexual function and reproductive processes.The endocrine system uses chemicals to “communicate” to and from various parts of the body. These chemicals are known as hormones. Each hormone’s shape is specific and can be recognized by the corresponding target cells.Low concentrations of a hormone will often trigger the gland to secrete. Once the concentrations of the hormone in the blood rise this may cause the gland to stop secreting, until once again hormone concentrations fall. This feedback mechanism (which is characteristic of most glands) causes a cycle of hormone secretions.Once hormones have served their function on their target organs/tissues they are destroyed. They are either destroyed by the liver or the actual tissues of the target organs. They are then removed by the kidneys.
The major glands that make up the human endocrine system
The hypothalamus is located in the lower central part of the brain.
It is the main link between the endocrine and nervous systems.
Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary. It is the gland that monitors what is happening in the body and then instructs the pituitary gland on what needs to be done.
Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system.
It’s often called the “master gland” because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons.
To accomplish this, the hypothalamus provides information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.
The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe.
The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands.
The posterior lobe of the pituitary releases antidiuretic hormone, which helps control the balance of water in the body.
The posterior lobe also produces oxytocin, which triggers the contractions of the uterus in a woman having a baby.
The pituitary also secretes endorphins, chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.
The pineal gland is located in the middle of the brain and is stimulated by nerves from the eyes.
The pineal gland secretes melatonin at night when it’s dark, thus secretes more in winter when the nights are longer.
Melatonin promotes sleep and also affects reproductive functions by depressing the activity of the gonads.
Additionally, it affects thyroid and adrenal cortex functions.
The thyroid, located in the front part of the lower neck, is shaped like a butterfly and produces the thyroid hormones thyroxine and triiodothyronine.
These hormones control the rate at which cells burn fuels from food to produce energy.
The production and release of thyroid hormones is controlled by thyrotropin, which is secreted by the pituitary gland.
The more thyroid hormone there is in a person’s bloodstream, the faster chemical reactions occur in the body.
Thyroid hormones regulate metabolism, therefore body temperature and weight. The thyroid hormones contain iodine, which the thyroid needs in order to manufacture these hormones.
If a person lacks iodine in his/her diet, the thyroid cannot make the hormones, causing a deficiency. In response to the body’s feedback loops calling for more thyroid hormones, the thyroid gland then enlarges to attempt to compensate (The body’s plan here is if it’s bigger it can make more, but that doesn’t help if there isn’t enough iodine.). This disorder is called goitre.
Under-secretion of thyroid hormone in children produces cretinism; the children show stunted growth (dwarfism) and fail to develop mentally. Under secretion in adults results in a low metabolic rate.
Over secretion in adults gives rise to exophthalmic goitre and the metabolic rate is higher than usual. Such persons may eat well but burn up so much fuel that they remain thin. This is usually accompanied by a rapid pulse rate.
This gland, therefore, has a profound influence on both mental and physical activity.
The Parathyroid Glands
Attached to the thyroid are four tiny glands that function together called the parathyroids.
They release parathyroid hormone, the function of which is to raise the blood calcium as well as maintain the balance of calcium and phosphorus in both the blood and bone structures.
Under secretion gives rise to a condition known as tetany in which the muscles go into spasm, and over secretion causes calcium to be lost to the blood from the bones giving rise to softened bones, raised blood calcium and a marked depression of the nervous system.
Calcium is important, not only for bones and teeth, but also for nerve functioning, muscle contractions, blood clotting and glandular secretion. If we don’t have enough calcium for these functions, the body will take it from the bones, causing them to easily fracture.
It may also cause twitching, spasms, convulsions and even death. Too much calcium may cause a weakening of muscle tone and kidney stones.
The Thymus Gland
This gland lies in the lower part of the neck and attains a maximum length of about 6 cm.
It is made up of two lobes that join in front of the trachea.
Each lobe is made of lymphoid tissue, consisting of tightly packed white blood cells and fat.
The thymus enlarges from about the 12th week of gestation until puberty.
After puberty, the thymus begins to atrophy so that in the adult only fibrous remnants is found.
Its secretion is thought to act as a brake on the development of sex organs so that as the thymus atrophies, the sex organs develop.
Its function is to transform lymphocytes (white blood cells developed in the bone marrow) into T-cells (cells developed in the thymus).
These cells are then transported to various lymph glands, where they play an important part in fighting infections and disease.
Swelling of lymph glands and fever are a signal that immune cells are multiplying to fight off invaders of the body: bacteria, fungi, viruses or parasites.
The body has two triangular adrenal glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function.
The outer part, the adrenal cortex, produces hormones called corticosteroids. There are three main types: those which control the balance of sodium salt and potassium in the body (salt and water levels); those which raise the level of sugar in the blood; and sex hormones. This cortex, or outer, yellow layer, takes its instructions from the pituitary hormone ACTH.
The smaller, inner region is part of the sympathetic nervous system and is the body’s first line of defense and response to physical and emotional stresses. This inner, reddish brown layer makes two types of hormones and takes all its instructions from the nervous system, producing chemicals which react to fear and anger and are sometimes called “fight or flight” hormones called catecholamines.
One of these, called epinephrine or adrenaline, increases blood pressure and heart rate when the body experiences stress.
The pancreas is a long, narrow, lobed gland located behind the stomach.
This organ has two functions.
It secretes digestive enzymes into the small intestine, which break down fats, carbohydrates, proteins and acids.
It also secretes bicarbonate, which neutralizes stomach acid as it enters the duodenum.
It also has endocrine cells which are arranged in clusters throughout the Pancreas, these are known as Islets of Langerhans.
They secrete insulin and glucagon to regulate the blood sugar level.
Glucagon tells the liver to take carbohydrate out of storage to raise a low blood sugar level.
Insulin tells the liver to take excess glucose out of circulation to lower a blood sugar level that’s too high.
If a person’s body does not make enough insulin (and/or there is a reduced response of the target cells in the liver), the blood sugar rises, perhaps out of control, and we say that the person has diabetes mellitus.
Male Hormone System
The entire male reproductive system is dependent on hormones, which are chemicals that stimulate or regulate the activity of cells or organs.
The primary hormones involved in the functioning of the male reproductive system are follicle-stimulating hormone (FSH), luteinizing hormone (LH) and testosterone.
FSH and LH are produced by the pituitary gland.
FSH is necessary for sperm production (spermatogenesis), and LH stimulates the production of testosterone.
Testosterone is important in the development of male characteristics, including muscle mass and strength, fat distribution, bone mass and sex drive.
Testosterone as well as sperm cells, are produced in the testes.
The prostate is a gland of the male reproductive system that produces fluid for semen, which helps to transport sperm during the male orgasm.
The prostate is made up of about 30% muscular tissue; the rest is glandular tissue.
The prostate is about the same size and shape as a walnut and is located in front of the rectum and just below the bladder.
The prostate wraps around the urethra, which is the tube that carries urine out from the bladder through the tip of the penis.
Female Hormonal System
The hypothalamus produces gonadotropin-releasing hormone, which stimulates the pituitary gland to produce luteinizing hormone and follicle-stimulating hormone.
These hormones stimulate the ovaries to produce the female sex hormones, estrogen and progesterone, and some male sex hormones (androgens).
(Male sex hormones stimulate the growth of pubic and underarm hair at puberty and maintain muscle mass in girls as well as boys.) After childbirth, the hypothalamus signals the pituitary gland to produce prolactin, a hormone that stimulates milk production.
The adrenal glands produce small amounts of female and male sex hormones.
At puberty, when the ovaries and uterus are mature enough to respond to hormonal stimulation, certain stimuli cause the hypothalamus to start secreting gonadotropin-releasing hormone.
This hormone enters the blood and goes to the anterior pituitary gland where it stimulates the secretion of follicle-stimulating hormone and luteinizing hormone.
These hormones, in turn, affect the ovaries and uterus and the monthly cycles begin.
A woman’s reproductive cycles last from menarche to menopause.
Menopause occurs when a woman’s reproductive cycles stop.
This period is marked by decreased levels of ovarian hormones and increased levels of pituitary follicle-stimulating hormone and luteinizing hormone.
The changing hormone levels are responsible for the symptoms associated with menopause.
bout 50 neurotransmitters have been discovered so far. Out of these, a few of the most important neurotransmitters and their functions are discussed below.
This neurotransmitter was discovered in the year 1921, by Otto Loewi.
It is mainly responsible for stimulating muscles.
It activates the motor neurons that control the skeletal muscles.
It is also concerned with regulating the activities in certain areas of the brain, which are associated with attention, arousal, learning, and memory.
People with Alzheimer’s disease are usually found to have a substantially low level of acetylcholine.
Dopamine is the neurotransmitter that controls voluntary movements of the body, and is associated with the reward mechanism of the brain. In other words, dopamine regulates the pleasurable emotions.
Drugs like cocaine, heroin, nicotine, opium, and even alcohol increase the level of this neurotransmitter.
A significantly low level of dopamine is associated with Parkinson’s disease, while the patients of schizophrenia are usually found to have excess dopamine in the frontal lobes of their brain.
Serotonin is an important inhibitory neurotransmitter, which can have a profound effect on emotion, mood, and anxiety.
It is involved in regulating sleep, wakefulness, and eating.
It plays a role in perception as well.
The hallucinogenic drugs like LSD actually bind to the serotonin receptor sites, and thereby block the transmission of nerve impulses, in order to alter sensory experiences.
A significantly low level of serotonin is believed to be associated with conditions like depression, suicidal thoughts, and obsessive compulsive disorder. Many antidepressants work by affecting the level of this neurotransmitter.
Gamma-aminobutyric Acid (GABA)
GABA is an inhibitory neurotransmitter that slows down the activities of the neurons, in order to prevent them from getting over excited.
When neurons get over excited, it can lead to anxiety. GABA can thus help prevent anxiety.
It is a non-essential amino acid, that is produced by the body from glutamate.
A low level of GABA can have an association with anxiety disorders.
Drugs like Valium work by increasing the level of this neurotransmitter.
Glutamate is an excitatory neurotransmitter that was discovered in 1907 by Kikunae Ikeda of Tokay Imperial University.
It is the most commonly found neurotransmitter in the central nervous system. Glutamate is mainly associated with functions like learning and memory.
An excess of glutamate is however, toxic for the neurons.
An excessive production of glutamate may be related to the disease, known as amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease.
Epinephrine and Norepinephrine
Epinephrine (also known as adrenaline) is an excitatory neurotransmitter, that controls attention, arousal, cognition, and mental focus.
Norepinephrine is also an excitatory neurotransmitter, and it regulates mood and physical and mental arousal.
An increased secretion of norepinephrine raises the heart rate and blood pressure.
Endorphins are the neurotransmitters that resemble opioid compounds, like opium, morphine, and heroin in structure.
The effects of endorphins on the body are also quite similar to the effects produced by the opioid compounds. In fact, the name ‘endorphin’ is actually the short form for ‘endogenous morphine’.
Like opioids, endorphins can reduce pain, stress, and promote calmness and serenity. The opioid drugs produce similar effects by attaching themselves to the endorphin receptor sites.
Endorphins enable some animals to hibernate by slowing down their rate of metabolism, respiration, and heart rate.