Human Feedback Systems

Homeostasis attempts to relate the uniformity of the samples to the other major variables in the state of the animal such as activity or location. It is crucial to note that the samples must be taken over the full range of possible associated conditions. One way to achieve stability is by giving up freedom to invade certain areas or by curtailing the range of activities. An extreme instance of this is hibernation. All activity is traded for the minimal activity required for survival. In an engineering concept, feedback refers to a type of connectivity in which the consequences of an action produced are returned in some fashion to participate in the causes of the mechanical action. A system which has been devised to see what has been done, contrast it to what it should do, and act according to the distinction between the two is a feedback system. First, a quantitative estimate of the error is made, then the responsive mechanism depends on the error.

There is an inherent difference in the terms, homeostasis and feedback. Homeostasis deals with the concept of stability and freedom and feedback deals more with the organization of processes.

Several areas of the cardiovascular system containing specialized nerve endings summon cardiovascular reflexes when stimulated. These regions are located in the bifurcation of the common carotid artery, at the arch of the aorta, and, to a lesser degree, in the heart, large veins, and pulmonary vasculature. The impulses from these specialized regions, known as baroreceptors and chemoreceptors, travel by afferent nerves to the medullary centers and then through efferent nerves, back to the heart and blood vessels when they evoke changes in heart rate, contractility, vascular resistance, and compliance. As their names imply, baroreceptors respond to changes in the arterial pressure, and chemoreceptors respond to changes in the chemical concentration of the blood. Both receptors operate through the principle of negative feedback.

Negative feedback systems operate in such a way as to maintain a control variable at a fixed value, often called a set point. For example, consider the control of mean systemic arterial blood pressure. Let us assume its normal values 100 mm Hg. If for some reason, say hemorrhage, the mean systemic arterial blood pressure falls to 88 mm Hg, an error signal is created. The error signal is the difference between the set point (100 mm Hg) and the actual value (80 mm Hg).

The peripheral receptors sense this error and feed information back to the central nervous system such as to return the mean systemic arterial pressure back to its set point. It does this by initiating autonomic adjustments to increase systemic arterial resistance and/or cardiac output. When the signal ends, the adjustments stop.

In order to derive the transfer function for the following closed loop system:

Ea(s) = R(s) - B(s)

Substituting for B(s), we obtain:

Ea(s) = R(s) - H(s) C(s)

Similarly, an expression for C(s) can be obtained:

C(s) = G(s) Ea(s)

Substituting for Ea(s) using the above relation:

C(s) = G(s) [R(s) - H(s) C(s)]

Rewriting the equation reults in:

C(s) [1 + G(s) H(s)] = G(s) R(s)]

Which provides us with the transfer function:

C(s) / R(s) = G(s) / [1 + G(s) H(s)]

Drug Delivery System

Control theory techniques are currently being applied to develop a closed loop drug delivery system (CLDD). CLDD systems are developed for theraputic and diagnostic purposes. They assist the clinician in administering a drug to achieve specific clinical objectives.

The Senses

What is the difference between sensation and perception ?
We perceive the world in specific ways because of differing sensory abilities:

Sensory receptors ----->

Detect a stimulus

Transduce it into electrochemical form

Generate a receptor potential

Stimulate the AP in the sensory neuron

Transmit information to the brain

Brain - Integrates and interprets

The sensation to perception

Sensory Systems

1) Chemoreception - Detect chemical stimuli - perceive them as smell/taste.
Locate and identify food. Select mates, and breeding sites

2) Photoreception - Detect light: Rhodopsin

3) Mechanoreception - Responds to physical stimuli
Detects touch and sound (sound waves).

Human Sensory System
1) Chemoreception

• smell - A particle is dissolved in the mucous layer covering the cilia. In the nose, protein molecules pick up the dissolved odor and send them to the receptors. Part of the olfactory receptor cell passes through the skull and synapses with neuron’s in the brain’s olfactory bulb. Then a message is relayed to the cerebral cortex, where the message is interpreted as an odor and identified.
• taste - taste receptors detect chemical stimuli. Taste sensations are centered around the mouth, where we have about 10,000 tastebuds. Each human taste bud contains 60 to 100 receptor cells, which generate action potentials when food molecules bind to them. The receptor cells live for about three days and then are replaced by new ones.

2) Photoreception

• visual system - photoreceptors are cells that detect light. The human eye is fluid filled and contains three layers:

  1. Sclera (outermost) - curves to form the cornea which bends incoming light rays to focus an image.
  2. Choroid coat (middle) - includes the lens, iris, and pupil.
  3. Retina - subdivided into layers

  • photoreceptive rods and cones, bipolar neurons, and ganglion cells.
  • vitreous humor
  • aqueous humor

3) Mechanoreception
Convert mechanical energy to action potentials to produce hearing balance and touch. Hearing results when sound waves from a vibrating object stimulate receptor cells, which produce action potentials in a nerve leading to the brain. They impinge upon our tympanic membrane. In the inner ear, vibrations cause hair cells to push against tectorial membrane, stimulating the auditory nerve. This nerve transmits action potentials to the brain, where the vibration is perceived as a particular sound.

Electronic Nose

It is expected that artificial olfactory devices will "replace or complement human sesory test in many fields, such as food, drink, cosmetic, and environmental control". By mimicking biological mechanisms of olfaction it is believed that artificial sensing systems can be created and actual bilogical subjects will become obsolete. Concentrating on the sence of smell, a artificial sensor can eventually be made to pick up certain smells and process them like we do. The recognition of odarants occur in the cilia of the olfactory receptor neurons. The cilia is considered the scaffoldig for the chemosensory membrane, wich greatly exands the surface area. The odorant interacts with receptor proteins wich cause a multistep reaction wich ampligies the olfactory signal wich leads to electrical response of the semsory neurons. So the process starts out as a chemical signal wich is then changed to a electrical symbol where it is then processed in the olfactory bulb and in higher brain centers. The odor binding proteins bond with the odorus hydrophobic molecules and shutle them toward the chemosensory cilia. It has also been said that the proteins may act as scavengers by binding wich then deactivates the hydrophobic molecules.

Enzymes play a large role in the clearing out of old stimuli. If the old stimuli is not removed the subject will keep smelling the same thing over and over. By a proces known as biotransformation where detoxification enzymes neutralize the odor.

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