Relation and Coordination in Animals: An Encyclopedic Guide

The Function of Relation in Animals

The function of relation is a biological process that allows animals to interact with their environment and maintain internal homeostasis. This process unfolds through three fundamental phases: the reception of stimuli, coordination, and response. Reception involves capturing signals through specialized structures known as receptors, which can be internal or external. Coordination involves the integration of this received information to elaborate an appropriate reaction. Finally, the response is executed through effectors, which are the organs or tissues that carry out the dictated action.

Types of Stimuli and Receptors

Stimuli are defined as signals to which animals are sensitive, categorized primarily by their origin. External stimuli are detected by externoreceptors and include factors such as light, physical contact, and temperature. Internal stimuli, which provide information about the body's own internal state, are divided into two groups: physicochemical variables and proprioceptors. Physicochemical variables include pressure, temperature, and $pH$ , which are detected by internoreceptors. Proprioceptors are specialized receptors that provide information regarding the relative position of various body parts.

Receptors are further classified based on the nature of the stimulus they capture:

Photoreceptors: Sensitive to light.
Chemoreceptors: Sensitive to chemical signals such as smell and taste.
Mechanoreceptors: Sensitive to physical forces like touch and vibrations.
Thermoreceptors: Sensitive to changes in temperature.

Systems of Coordination

Coordination entails interpreting signals from receptors and integrating them into a decision-making system. This system then issues a command to an effector to produce a response, such as a muscular movement or glandular secretion. In the animal kingdom, two primary coordination mechanisms exist:

Chemical or Hormonal Coordination System: This system utilizes endocrine glands that secrete hormones, which serve as chemical messengers. These hormones are released into the bloodstream and travel throughout the body. However, they only elicit a response in specific "target organs" (órganos diana) that possess the appropriate receptors. Because it utilizes the circulatory system, this mechanism does not require additional "wiring."
Nervous Transmission System: Information in this system is transmitted via electrical impulses through specialized cells called neurons. The nerve impulse travels extremely rapidly through nerves, allowing for near-instantaneous responses.

Types of Animal Responses

Responses in animals are categorized based on the type of effector organ involved:

Muscular Movement: The effector is a muscle that contracts or relaxes to produce motion.
Secretion of Liquid Substances: The effector is a gland. These glands can be exocrine (secreting substances outside the blood-stream or body) or endocrine (secreting hormones directly into the blood).

Evolution of Hormonal Coordination

Hormonal coordination systems have increased in complexity throughout evolutionary history. In invertebrates, these systems often control fundamental life processes, such as molting in insects. In vertebrates, the hormonal system is significantly more complex and is deeply integrated with the nervous system, forming what is known as the neuroendocrine system. In these organisms, most endocrine glands are regulated by the hypothalamus, which acts to either stimulate or inhibit their function.

The Hypothalamus-Pituitary Complex and Endocrine Glands

The hypothalamus is a part of the brain that controls nearly the entire hormonal system. It contains large neurosecretory cells that produce neurohormones to regulate the pituitary gland (hypophysis). The specific hormones and glands involved include:

Somatotropin: Controls physical growth.
Prolactin: Activates the secretion of milk.
Vasopressin: Favors water reabsorption in the nephrons.
Oxytocin: Facilitates milk expulsion and stimulates uterine contractions during labor.
Thyrotropin (TSH): Regulates the secretion of thyroxine by the thyroid.
Adrenocorticotropin (ACTH): Controls the secretion of cortisol by the adrenal glands.
Gonadotropins (FSH, etc.): Stimulate the gonads.
Pineal Gland: Releases melatonin to control daily circadian rhythms based on light-dark cycles.
Thyroid and Parathyroid: Located between the larynx and trachea. The thyroid uses thyroxine to regulate cellular metabolism. Together, they control bone calcium levels: calcitonin from the thyroid inhibits calcium release, while parathormone from the parathyroid activates it.
Pancreas: Controls blood glucose concentration through the synthesis or hydrolysis of glycogen. Insulin reduces blood glucose, while glucagon increases it.
Adrenal Glands: The cortex produces cortisol (acting on molecule metabolism) and aldosterone (intervening in water and salt balance). The medulla produces adrenaline and noradrenaline, which manage the body's alert states.
Testicles: Produce testosterone, determining secondary male sexual characteristics.
Ovaries: Produce estrogens (female sexual characteristics) and progesterone (preparing the uterus for pregnancy).

Evolution of the Nervous System

The nervous system began with simple structures and gradually increased in complexity through a process called cephalization. The least evolved organisms, such as jellyfish (medusae), possess interconnected cells forming a nerve plexus (plexo nervioso). Evolutionary advancement led to the appearance of nerve centers or ganglia, as seen in arthropods. This progressed further into highly complex structures, such as the brains of octopuses.

The Vertebrate Nervous System

The vertebrate nervous system is organized into two main parts:

Central Nervous System (SNC): Comprising the brain (encéfalo) and the spinal cord (médula espinal).
Peripheral Nervous System (SNP): Comprising neurons and nerve extensions (nerves) located outside the SNC. The SNP is further subdivided into: - Afferent or Sensory Branch: Transmits information from receptors to the SNC. - Efferent or Motor Branch: Carries orders from the SNC to effectors. This is subdivided into the Somatic Nervous System (voluntary control, with exceptions like reflex arcs) and the Autonomous Nervous System (involuntary functions).

The Autonomous Nervous System consists of two antagonistic systems: the Sympathetic and the Parasympathetic.

Anatomy of the Vertebrate Brain

Comparing different vertebrates (bony fish, amphibians, reptiles, birds, and mammals), the brain (encéfalo) consistently includes several key structures protected by the cranium and bathed in cerebrospinal fluid (líquido cefalorraquídeo):

A: Olfactory Lobe (Lóbulo olfativo)
B: Cerebrum (Cerebro)
C: Optic Lobe (Lóbulo óptico)
D: Cerebellum (Cerebelo)
E: Medulla Oblongata (Bulbo raquídeo)
F: Spinal Cord (Médula espinal)

Additional internal structures include the corpus callosum (cuerpo calloso), hypothalamus, pituitary gland (hipófisis), and mesencephalon.

Cellular Structure: Neurons and Neuroglia

The neuron is the functional unit of the nervous system. While their shapes vary, the basic structure includes:

Soma or Cell Body: Contains the nucleus and most of the cytoplasm.
Dendrites: Extensions responsible for receiving information.
Axon: A long projection for the transmission of nerve impulses. Groups of axons form nerve fibers.

Neuroglia (glial cells) perform essential support functions, including defense, repair, and nutrition. Key types include:

Astrocytes: Provide structural support.
Oligodendrocytes: Responsible for the production of myelin.
Microglia: Act as the defense system for nervous tissue.
Schwann Cells: Associated with the axon structure.

In the CNS, nervous tissue is divided into two zones: Grey Matter (composed of neuronal bodies) and White Matter (constituted by axons).

Organization and Functioning of Neurons

The nerve impulse is a transmission signal that moves through neurons via an electrochemical process. The passage of this impulse from one neuron to another is called a synapse. This occurs across a gap known as the synaptic cleft (hendidura sináptica). The transmission is mediated by chemical substances called neurotransmitters, which are stored in synaptic vesicles within the presynaptic neuron and captured by receptors on the postsynaptic neuron. These synapses can be activated or inhibited by various mechanisms, and drugs often act by modifying these communications.

Integration and Control: Perceptions, Voluntary Actions, and Reflexes

Information processing for perception and conscious capacity occurs in the cerebral cortex. The pathway involves: Sensory organ -> Stimulus capture -> Nerve impulse via sensory pathway -> Specific point in the cerebral cortex -> Elaboration of response via complex neuronal connections -> Motor pathway -> Effector -> Response.

Involuntary regulation manages essential functions like heart rate, digestion, and water balance. Two notable examples are:

Regulation of Stress and Alert States: This survival mechanism involves the link: Hypothalamus → Pituitary → Adrenal Cortex. The hypothalamus receives sensory info and secretes CRH (Corticotropin-Releasing Hormone), which stimulates the pituitary to release ACTH (Adrenocorticotropic Hormone). This reaches the adrenal glands, stimulating the cortex to release cortisol. Simultaneously, the adrenal medulla, activated by cortisol and the sympathetic nervous system, releases adrenaline and noradrenaline to prepare the body for alert. If the stimulus ceases, the hormones exercise self-regulation (negative feedback) to inhibit further synthesis.
Reflex Acts: These are "short circuits" in the standard sensory-to-brain-to-motor pathway. For example, the patellar reflex (knee-jerk). Receptors receive a stimulus (a hit), and sensory neurons carry the impulse to the spinal cord. There, they synapse with interneurons (neuronas de asociación) that immediately send a contraction order through a motor nerve. While the impulse eventually reaches the brain (making the sensation conscious), the movement has already been performed by the time consciousness occurs.