Interaction in Humans

What to Learn

The meaning of interaction and coordination in multicellular beings.
Interaction and coordination in humans: stages involved (from stimuli to receptors); organ systems involved (receptors, nervous system, endocrine system and motor apparatus).
Receptors: internal and external; types according to the stimuli detected, how the eye works; how the ear works; the skin as a receptor.
The nervous tissue: neurones (and their structure) and glial cells, nerves, nerve impulse, synapse, neurotransmitters.
The nervous system: structure, main organs (and their functions), types of nerves, types of nervous circuits.
The endocrine system: endocrine glands, hormones (main kinds and their effects), target organs and target cells, stages and types of endocrine coordination, endocrine glands as effectors.
Homeostasis: how the human body controls the internal temperature and the levels of fluids and metabolites.
The motor apparatus: main muscles and bones; the muscles as effectors.

Key Information

Vocabulary

Coordination	One of the main three vital functions. It refers the ability of one living being to be aware of the events happening inside or outside itself and react to them accordingly. It requires (a) the detection of the stimuli by some receptor, such as the sensory organs, (b) the transmission of that sensory information to some control centre, (c) interpreting that sensory information and generating the response by the mentioned control centre, (d) transmitting the motor information that refers the response, and (e) performing the response by some effector, which, in the human body, is a muscle or a gland. In the human body, the aforementioned stages are performed by the following structures: (a) by sensory organs or disperse sensory cells; (b) by the afferent nerves; (c) by the Central Nervous System; (d) by the efferent nerves, the endocrine glands and the hormones; (e) by the muscles and the glands.
Neurone	Or neuron (Am), or nerve cell. It is the main type of cells in the nervous system. They work in groups and communicate between each other by transmitting nerve impulses. There are three types: (a) sensory neurones, which transmit sensory information from the receptors to the CNS, (b) motor neurones, which transmit the motor information from the CNS to the effectors, and (c) relay neurones, that occur in the CNS, and are the ones that decide the responses once they have interpreted the sensory information generated by a stimulus.
Nervous circuit	It is a circuit formed by a sequence of neurones that connect a receptor with an effector, in order to trigger an appropriate response to the stimulus that has been detected. It consists of, at least, one sensory neurone, one relay neurone, and one motor neurone.
Ganglion	It is a cluster of somas and dendrites of a group of neurones. They belong to the PNS, and often interconnect with other ganglia to form a more complex cluster known as a plexus.
Nerve	Bundles of bundles of axons of many neurones packed together. They belong to the PNS and (a) convey sensory information from the receptors to the CNS (afferent nerves), or (b) motor information from the CNS to the neurones (efferent nerves), or (c) both (mixed nerves).
Hormone	Chemicals secreted by the endocrine glands into the bloodstream, that act as chemical messengers, this is, they trigger certain responses in the target cells that are meant to react to some variation of the external medium (as when a secretion of adrenaline helps you to face succesfully some sort of threat) or some variation of the internal medium (as when a secretion of insulin helps you to keep an adequate level of sugar in your blood).
Target cell	The target cells (which belong to the so called target organs) are called like that because they are the specific target of an specific hormone. This means that, although the hormones are released into the bloodstrem, and therefore reach every cell in the organism, only a group of cells are going to react to the arrival of any particular hormone: these are the target cells, and what makes them be such, is the possession in the surface of their membranes of molecules that act as specific receptors to one specific hormone. It is the coupling of a hormone to those receptors what triggers the final response accomplished by the target cells (opening up your pupils, lowering the levels of glucose, etc.).

Structural Organisation of the Nervous System

Central Nervous System

Brain (= encephalon). Enclosed by the skull and the meninges. Main organs: cerebrum, cerebellum and brain stem.
Spinal cord. Enclosed by the backbone (= spinal column, = spine) and the meninges.

Peripheral Nervous System

Nerves: bundles of neurones' axons. Functional types: afferent, efferent or mixed. Structural types: cranial (12 pairs) or spinal (31 pairs).
Ganglia: clusters of neurones' somas.

Facts About Neurones

They occur only in the Nervous System, but they are not the unique type of cells in it: there are, also, the glial cells, which assist the neurones in several tasks (nutrition, disposal of wastes, defense, regeneration…).
They have two parts: the cell body or soma, and the nerve fibers: these are prolongations of the soma that can be two kinds: the axon (single, long, branched only at the end) and the dendrites (usually many, short and highly branched). The dendrites may be lacking.
Their function is transmitting an electric current called nerve impulse along circuits that connect the sensory information collected by the receptors with the responses performed by the effectors.
The nerve impulse is transmitted always in the same direction: from the dendrites (if any) to the soma, and from the soma to the axon. The axon terminals will make connections with other neurones or, at the end of the circuit, with an effector.
Many axons (the ones of the PNS and the ones that make up the white matter of the CNS) are wrapped by Schwann cells, that make up the myelin sheath, which helps speeding up the transmission of the nerve impulse.
The demyelination of myelinated axons is characteristic of some serious diseases such as multiple sclerosis.
The regeneration of damaged neurones in the CNS is not possible, but the myeline layer helps regenerate the damaged axons of the PNS (only).
Two consecutive neurones in a nervous circuit do not touch each other, and so, the nerve impulse has two "jump" over that gap (the synapse); this is acomplished by means of certain molecules called neurotransmitters (e.g. dopamine, endorphin).
The low production of certain neurotransmitters (or the inhability to use them) is the hallmark of several diseases such as the Parkinson's disease.

Endocrine Control

In many cases, when an endocrine gland releases a hormone, it is upon request of some controlling organ, which, at the end of the hierarchical chain, is always the brain. But who tells the brain to tell the hypothalamus (the so called master gland), to tell the hypophisis to tell the breasts (via the secretion of oxytocin) to secrete milk? The sensory cells (receptors) that detect the baby's suckling do. They send nerve impulses to the brain informing of this event, and then the brain starts the chain of orders.

There are also cases in which it is the same gland that produces the hormone the one that detects the stimulus that will finally lead to the secretion of the hormone. It is the pancreas itself the organ that learns about the rise of the level of sugar in blood, and responds to it by secreting insulin. And if the pancreas notices a low level of glucose in blood, it, without asking anyone, will release glucagon, which will help to take more glucose into the blood. Insulin and glucagon are antagonist hormones, because they do opposite things. The have in common who secretes them (the pancreas) and their target organs (chiefly the liver and the muscles).

As more glucose in blood leads to less glucose in blood (through the action of the insulin) and less glucose in blood leads to more glucose in blood (via glucagon), these two are examples of negative feedback in endocrine control. But secreting milk when the baby suckles leads to keep on secreting more milk: the stimulus empowers itself, and this is called positive feedback.

External Links

I i U

Nervous System

The nervous system at Faqs.org.

A wide overall view, with text and images, on the neurons, the organs of the nervous system and their functions, the nervous disorders, a glossary and additional readings.

Neuroscience for kids.

Absolutely comprehensive site to learn everything about the neurones, the nervous system, the brain habilities, drugs, diseases… With an extensive table of contents at the top.

The brain from top to bottom.

Amazing resource that covers all sorts of topics related to the human brain: organisation, memory, emotions, senses, body movement... All of them in three different levels of deepness.

100 fascinating facts you never knew about the human brain.

How it works, how it develops, what it controls, how it affects sleep, dreams and memory, and more.

Brain pictures.

A set of 9 pictures that tell different stories about the way your nervous system works.

Inside Bill Moorier's head.

In his own words: "In September 2009 my doctor recommended an MRI to rule-out a couple of potential conditions. The scan came back completely normal, which was a great relief! I decided to do something with the images, and spent a fun weekend writing this MRI explorer".

5 ways your brain is messing with your head.

It turns out that the ways in which your head is being truly messed up are not coming from advertisers, politicians or magicians: they are coming from inside.

How scratching can stop an itch.

A scientific study has discovered why scratching helps relieve the irritation of an itch.

Sensory Organs

The special senses at Faqs.org.

Good and wide overview, with text and images, on the eye, the ear, the sense of smell, the sense of taste, and the way they all work. Also with a description af the main sensorial pathologies, a glossary and additional readings.

Anatomy of the eye.

Highly informative interactive animation to learn the parts of the eye and their functions.

Magenta ain't a colour.

Which comes to prove that your brain is tricking you. Learn the difference between reality and the image your brain composes of it through this interesting article on colours.

Endocrine System

The flight or fight response.

Travel inside the body and see how cell signaling brings about physiological changes during the fight or flight response.

The endocrine system at Faqs.org.

A wide overall view, with text and images, on all the endocrine glands and the way they work, the hormones and their effects, the endocrine disorders, a glossary and additional readings.

Sorry, you're just not my (testosterone's) type.

Learn how hormones condition the type of person to which you are attracted.

Motor Apparatus

eSkeletons.

eSkeletons isn't only about understanding human anatomy. It is an interactive comparative tool that enables a student to view the bones of both human and non-human primates and to learn more about them. Selecting the human skeleton on the homepage starts the anatomical study. A mouseover on the skeleton selects a specific bone for a closer look.

The muscular system at Faqs.org.

A wide and good overall view, with text and images, on the muscular system, the muscular cells, the main muscles, and the way they all work. Also with a description of the main muscular disorders, a glossary and additional readings.

The skeletal system at Faqs.org.

A wide and good overall view, with text and images, on the skeletal system, the main bones, the joints, and the way they all work. Also with a description of the main skeletal disorders, a glossary and additional readings.

Learn the skeletal system.

Interactive activity to check your knoledge on the human skeleton.

Videos

I U

b f d 0