Organismal Biology





Key Concepts




Chapter  15 - Animals -
Internal Controls






Hormonal Systems




Glands produce a large number of different secretions for animals.  When those secretions are released into blood or hemolymph, the glands are considered to be endocrine glands, and most of the secretions are called hormones.  Exocrine glands produce other kinds of secretions, usually conducted through dedicated ducts.

Somewhere out in the animal's tissues are a hormone's target cells, which have receptors for that particular hormone.  Protein-based hormones typically attach to receptors embedded in the cell membrane, where the attachment sends a message through the membrane, second messenger molecules get released into the cell, and a response is generated.  Lipid-based hormones, having a character similar to a lipid cell membrane, typically enter the cell, often then move into the nucleus and attach to chromosomal proteins that directly activate the genes needed for a response.  There are also amine hormones, modified amino acids, and a few derived from fatty acids, which are parts of lipid molecules.

These systems generally are controlled through a negative-feedback response.  Monitoring systems track either existing hormone levels or effects mediated by the hormones;  as those levels move off the proper measures, the hormone levels rise to bring the response back to the proper level.  A thermostat works this way:  too-low temperatures activate a heating response that will raise the temperature to the proper level, where the heat turns off.

In many invertebrates, hormones are a product of neurosecretory cells, sometimes found in clusters rather than glands.  These clusters may float free in the hemolymph of open circulation systems.

A characteristic of vertebrates is an entire endocrine system, with a number of specific glands producing a wide array of hormones.  Some glands do their own monitoring, such as the pancreas tracking glucose levels that are controlled by insulin (target cells in the liver, muscles, and fat are compelled to absorb and store the glucose as starches) and glucagon (those same target cells are told to release some of that glucose).  Many glands are controlled from a system in the head:  the hypothalamus area of the brain does the monitoring, sending releasing factors or hormones to the hypophysis (also known as the pituitary gland), which releases tropic hormones whose target cells are other endocrine glands.  The hypophysis also produces non-tropic hormones such as growth hormone and antidiuretic hormone.

Hormones are involved in a great many internal control systems.  They may help shift and stabilize metabolic rates.  Some control circulating levels of materials such as minerals and nutrients.  Antidiuretic hormone mentioned above helps stabilize the dilution level of the blood, keeping it balanced with cell interiors to prevent osmotic flow into or out of them.  Sex hormones control reproductive processes.  Digestive hormones control levels of digestive molecules and hunger sensations.  Inflammation reactions, which generally have a positive feedback / amplification response, need to be counteracted by antiinflammatory hormones.   Epinephrine, also known as adrenaline, is the famous "fight or flight" hormone that maximizes responses to potential danger, but also reacts to any number of daily stresses.






Quick Response - the Nervous System



Input of stimulus can provoke a rapid response - this is the response of the nervous system.  Neurons conduct impulses, action potentials, from place to place in the body, along axons (incoming) and dendrites (outgoing) with a supportive cell body to mark the directionality.  Input begins at sense receptors, which convert environmental stimuli into impulses.  That message travels into the system along afferent (sensory) neurons, which connect to a network of interneurons.  Interneurons are the processors, where information is analyzed, stored (for use in later analysis), and responses are decided upon and constructed.  Those response message travel along efferent (motor) neurons to the muscles and glands that produce the responses.

Axons and dendrites are typically bundled into nerves, and may have a whole system of glial cells that provide various types of support.  Some glial cells called schwann cells, full of a protein called myelin, are literally wrapped around axons in an insulating sheath, to be discussed below.






Nerve Impulses



Along an axon or dendrite, various ions have been pumped to set up a condition, a resting potential, where there is a positive overall charge inside the membrane and a negative charge outside.  This sets up not just a concentration gradient but an opposite-charge component.  When an action potential initiates at the beginning on an axon, gates open in the membrane and ions flush through, depolarizing the membrane as the charges balance out and opening more gates "downstream."  An action potential has a particular position and speed at any given moment, and a particular axon has a set speed limit.  The limit can increase two ways:  bigger axons conduct faster impulses, with more membrane surface involved, and myelin-wrapped / insulated axons,  which trap the ions very close to the membrane, causes the impulses to quickly zip from the edge of one schwann cell to the next.  After the impulse has passed by, the gates close and a recovery period turns on the pumps and resets the resting potential.  This can be done quickly, but still may limit the responses in some systems.

Neurons very rarely connect directly to the next neuron in a way that allows the impulse to just flow through connecting membranes.  Usually there is a tiny gap, a synapse, that requires the release of neurotransmitter molecules from the first cell, and the activation of a certain number of receptors (a threshold response) on the next cell.  Neurotransmitters are quickly deactivated (often broken down) and reabsorbed into the first cell for reuse.  This may happen thousands of time a second.  In interneuron clusters such as brains, there may be whole areas affected by various neurotransmitter release.






Nervous System Organizations



There are two basic ways to set up a nervous system:  use a diffuse organization of interacting neurons, perhaps with many small interneuron-rich processing centers, or centralize the majority of the processors and run the system from there.  Diffuse systems are common in animals with radial symmetry, where stimuli could be coming from any direction.  Cnidarians have the simplest of these diffuse systems, a nerve net, where impulses may run in both directions on the same axons (this is always functionally possible, but other systems organize one-way systems).  Echinoderms have a diffuse pentaradial system.  Octopi have become famous for the complexity of processors in each of their arms, but theirs is still technically a centralized system.

Bilateral animals, with their tendency to always lead with / encounter new stiluli with a front end, show cephalization, a concentration of sense receptors and processors at the anterior end.  Since most major animal groups are bilateral, most have centralized systems, with outlying areas called a peripheral system.  When processors are somewhat small, each processor is called a ganglion;  larger processors are brains.  Systems tend to have clusters of processors distributed through the system, especially in areas that require their own processing, such as locomotory structures (for instance, insects have large ganglia near their legs and wings).  Processing seems to rise in complexity from simple linear systems, through nodal systems that handle clusters of processing in a somewhat linear order, to multiparallel systems that can handle many processing duties of one stimulus all at the same time. 

is a term with no clear definition, and so it is "tested" in many ways, looking for many different, occasionally contradictory, abilitiesIf one is looking for basic problem-solving capabilities, animals with higher-level intelligence are often social animals, which naturally live in groups whose individuals must be continually dealt with.  In the mollusks, the cephalopods have some very good problem solvers, even though an octopus is not particularly social.  In the arthropods, species that show particular intelligence are not only social but often behave as if the entire colony is itself a brain;  researchers studying ants or bees generally don't see high-level processing in individual insects, but find that they as groups interact in a way that may mimic how neurons interact in large brains.

Bilateral systems in larger animals almost always have a medial trunk of interneurons, nerve cords, running the length of the animal, doing some reflex action processing but also providing a maximum-speed highway relaying impulses in and out of the main processing centers.  In many invertebrates, such as the annelids and the arthropods, the nerve cords, in a pair, run down just inside the ventral surface of the animals.  In chordates, a single, technically hollow nerve cord runs just inside the dorsal surface of the animal, often associated with a skeletal vertebral column, where it is called the spinal cord.

Vertebrate central processors are commonly subdivided into substructures with particular specialties.  The spinal cord connects to a hindbrain, consisting of a medulla that coordinates basic functions such as heartbeat, breathing rate, and digestive movements, and a cerebellum, which coordinates responses the higher processors have decided are necessary.  The midbrain is involved in processing the primary senses, which vary from animal to animal, and sending somewhat analyzed imagery up the system.

The forebrain, which varies greatly among subgroups, includes the thalamus, sort of the central processor, to make sense of the sensory input, access memories for analysis and process new experiences for memorization.  It seems to be active in different ways during wakefulness and sleep, using sleep to move recent experiences into permanent memory.  The hypothalamus is said to process "primitive urges," such as hunger, fear, and sex, as well as being a major monitoring system for virtually any body factor which can be tracked through blood-borne factors.  It directly connects to the hypophysis / pituitary gland to control the endocrine system, as described above.

A large part of the forebrain, at least in mammals, is the cerebrum.  In this section is the paleocortex, also called the limbic system, where vital functions are monitored.  A lot of emotional reactions are developed here, and it often is said to be the source of the unconscious mind, processing that affects decisions but which the individual may not be fully aware of.  It interacts strongly with the hypothalamus.  The neocortex is the location of most of what's called "higher level" processing, where experiences are fully analyzed and responses decided upon to be sent out to the coordinating structures.









Although some receptors are classified by position, typically they are classified by the type of environmental stimulus they respond to.

Chemoreceptors pick up various types of atomic / molecular particles.  They may be distance chemoreceptors, sampling the environment for particles from afar, as in the olfactory / smell sense.  Some of these systems, such as the blood detectors of hunting sharks, the pheromone receptors of moths, or the tracking detectors of snakes and dogs, can be extremely sensitive, potentially capable of responding to single-molecule stimuli.  Contact chemoreceptors require a higher concentration of input, as in found in a taste sense.  Some pain receptors detect molecules in tissue fluid that spill from damaged cells.

Mechanoreceptors are generally those associated with a sense of touch; some mechanically-activated receptors exist (you'll notice them elsewhere on this list) that are not generally classified as such.  Touch senses include pressure, and proprioception, where internal receptors are used to detect body position.  Temperature sensors, which are sensitive to flow of heat, are usually in the mechanoreceptor class.   Some pain receptors are mechanoreceptors with high threshold response, indicating input with potential to injure.  Hearing involves receptors sensitive to vibrations.

Light receptors respond to those frequencies of the electromagnetic spectrum that are available and useful in an environment.  Most vision systems respond to intensity and movement, telling an animal when moving threats are near them.  Other systems pick up particular variations:  colors often pick up cues developed by plants to show pollinators and/or seed spreaders where the "bribes" are located.  The color range may include ultraviolet, although that isn't in a human's range.  Infrared, a marker for heat, is useful for nocturnal or burrow hunters of homeothermic prey.  Some systems detect polarization of light, such as the system in bees that allow them to orient to sun position even on a cloudy day.  Detection of magnetic fields may be classified in this category.

Some vision systems use imaging systems with lenses to cast an image on a surface of receptors.  The two main types of imaging systems are single-lens systems and compound systems with many lenses.  Typically, a single lens system, common in vertebrates, is relatively large, with a need for some distance to the focal point; they may take up a lot of head space, but the receptor screen can be densely packed, providing good resolution.  The compound system, common in annelids and arthropods, has many small lenses with short focal lengths, and can "cover" a head without using much internal space, but the resolution of such systems is believed to be worse.  It's also more useful for looking in many directions at once.

Equilibrium systems (sometimes called balance) can be classified into two categories.  One system is basically a gravity receptor system, sometimes called static equilibrium.  It is very useful for most animals to know "which way is up."  The other system is for moving equilibrium, which helps animals adjust to the forces exerted when movement changes directions.  Not surprisingly, these systems are very sensitive in animals that can fly.

Hearing as a sense is somewhat tricky to categorize; it was included in mechanoreception above, but in air the system is so specialized that it deserves its own designation.  Sound is basically vibrations carried through the surrounding environment.  Water is very good at conducting such vibrations, so a lot of aquatic animals essentially hear with their sense of touch, hearing with their whole surfaces.  In air, the vibrations are much weaker and weaken with distance more quickly, so many terrestrial animals have specific structures that vibrate in response to air-conducted sounds.  Some terrestrial systems take advantage of the ability of sounds to reflect back from surfaces - these echolocation systems use the reflection times of animal-generated sounds to detect positions of objects around them.  Small nocturnal bats are well-known to use such systems.  In many whales, these systems function to locate prey, possibly even to stun prey, as well as for communication.

Some animals can detect electromagnetic fields.  This ability can be used in sharks to protect them while they close on prey (they "roll in" their eyes and use the electromagnetic fields of animals in salt water that they can detect when close enough).  An ability to detect the magnetic field of the Earth is used in many migrating animals to navigate, like using a compass.  Some animals such as electric eels use their own generated electromagnetic fields to  detect prey animals that enter it (and an amplified version for defense).






Click on term to go to it in the text.
Terms are in the order they appear.



 Endocrine Glands
Exocrine Glands
Target Cells
Hormone Receptors
Second Messengers
Protein Hormones
Lipid Hormones
Negative Feedback
Neurosecretory Cells
Endocrine System
Tropic Hormones
Positive Feedback
Hormone Actions
Action Potentials
Nerve Loop
Sense Receptors
Afferent Neurons
Efferent Neurons
Glial Cells
Schwann Cells
Resting Potential
Action Potential
Impulse Speed
Recovery Period
Threshold Response
Nervous System Types
Radial Symmetry & Nervous
Nerve Net
Peripheral System
Brain Organization (Processing)
Linear Processing
Nodal Processing
Multiparallel Processing
Nerve Cords
Vertebrate Brain


Limbic System

Olfactory / Smell
Pain (Chemical)

Temperature Sense
Pain (Touch)

Ultraviolet Vision
Infrared Vision
Polarization Vision

Imaging Systems
Single-Lens System
Compound System
Resolution in Vision

Static Equilibrium
Moving Equilibrium


Electromagnetic Sense
Magnetic "Compass"











Organismal Biology

Copyright 2001-2019, Michael McDarby.   e-mail Contact.

Reproduction and/or dissemination without permission is prohibited.


Hit Counter