ESO 3 B&G 3

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

CoordinationOne 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.
NeuroneOr 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 circuitIt 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.
GanglionIt 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.
NerveBundles 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).
HormoneChemicals 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 cellThe 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.


Nervous System


Introductory video to the role of neurones in the human body.

Produced by Cassiopeia Project

Nervous system (I)

Nervous system (I)

Short introductory video on the function of the nervous system.

Nervous system (II)

Nervous system (II)

Explore the nervous system through a series of animated sequences that trace the structure of neurons and illuminate biochemical impulses.

Produced by The Phoenix Learning Group

Brain anatomy

Brain anatomy

This video shows the anatomy and function of the brain using color coded areas.

Nucleus Medical Media

Backbone anatomy

Backbone anatomy

Cervical spine and intervertebral disc anatomy.

Brain 101

Brain 101

The brain constitutes only about 2 percent of the human body, yet it is responsible for all of the body's functions. Learn about the parts of the human brain, as well as its unique defenses, like the blood brain barrier.

National Geographic

Sensory Organs
Skin, nails and hair

Skin, nails and hair

Short introductory video on the function of the skin, the nails and the hair.



Short introductory video on the process of seeing.

Hearing and balance (I)

Hearing and balance (I)

Short introductory video on the senses of hearing and equilibrium.

Smelling and tasting

Smelling and tasting

Short introductory video on the senses of smelling and tasting.

Endocrine System
Endocrine system

Endocrine system

Short introductory video on the function of the endocrine system and the hormones.

Motor Apparatus
Motor apparatus

Motor apparatus

Short introductory video on the function and components of the motor apparatus.




The Hidden Brain