The Physiology of Integration
Integration is a broad term that refers to processes such as summarisation and coordination that result in incoherence and harmonious process. The term “integration” refers to combining sensory, endocrine, and central nervous system impulses to ensure the appropriate functioning of the animal.
Whole animal integration consists of many systems but mainly nerve and endocrine cells, leading to smooth, coordinated movements.1.5.1 NervousSystem
The nervous system is essentially a massive, complex, bodywide communication system. To demonstrate, stimuli are relayed to the Central Nervous System (CNS) through sensory neurons; further detected by a receptor, which sends the electrical impulses along a sensory neuron to the CNS. The CNS then relays the message by motor neurons to effectors that respond, such as stimulating animals’ legs to run when they are about to get hunted.
Nerve cells are known as neurons (the basic unit of the nervous system). The nervous system consists of two major systems: the central nervous system (CNS) and the peripheral nervous system (PNS). CNS comprises the brain and spinal cord, and it is rich with cell bodies (axons and dendrites) of neurons. Within CNS, there are three different types of neurons: sensory, intermediate or relay, and motor neurons. These specialised cells carry information as tiny electrical impulses and make up the nervous system. Sensory neurons carry signals from receptors to the spinal cord and then to the brain. For instance, the eyes send data to the brain about the environment. The intermediate or relay neurons carry messages from one part of the CNS to another and the motor neurons carry signals from the CNS to effectors. The PNS is a division of the nervous system that deals with all the nerves outside of the CNS. The complexes of nerves that make up the PNS are axons or bundles of axons from nerve cells or neurons.
It ranges from microscopic to large size that can be easily visible to the human eye. Further divisions of PNS are the somatic nervous system (SNS), which controls voluntary movements like skeletal muscles, and the autonomic nervous system (ANS), which takes over the striated and non-striated muscles. All neurons have three main components; a cell body with a nucleus, dendron, and dendrites which are the neuron’s inputs. They receive information from other neurons or the external environment and transfer it to the cell body, and other axons carry the signal away from the cell body. Nervous systems mainly consist of neurons and glial cells, connective tissue cells, and circulatory system cells.Previously, these cells were identified through a simple microscope. Later internal structures were acknowledged through electron microscopy, and presently, different crosssection imaging technologies like functional magnetic resonance imaging (fMRI), computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) have revolutionised neurology, where physiologists study the functioning of the nervous system. For example, common nervous neoplasia in dogs and cats are meningiomas and gliomas. These two neoplastic cells compress brain parenchyma. Demonstrating this neoplasia through MRI and CT scans is very reliable and easy compared to old techniques practised.
1.5.2 EndocrineSystem
Endocrine system, also known as the hormone system, comprises many tissues that regulate animals’ internal environment by releasing a chemical substance called hormones into circulation to act on the target organ for the desired action. Endocrine tissues are typically ductless glands (e.g. pituitary, thyroid) that secrete hormones via capillaries that permeate the tissue (Fig. 1.2). These glands receive an abundant supply of blood. However, non-typical endocrine tissues contribute significant amounts of hormones to circulation, for example, secretion of the atrial natriuretic peptide from the heart, erythropoietin from the kidney, insulin-like growth factor from the liver leptin from fat.
Fig. 1.2 Description of the complete endocrine system and the different constituents in cattle. (Courtesy: BioRender)
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The endocrine system controls physiological processes like growth, metabolism, reproduction, behaviour, body regulation, development, fluid, and water balance to maintain homeostasis in most animals. For instance, cows during oestrous phase, various endocrine signals cause characteristic signs like frequent micturition, bellowing, swollen vulva, and mucus discharge. These signs act as visual signs to attract ox, leading towards promoting reproduction. Nevertheless, another class of hormones named pheromones is also responsible for the mentioned example. Pheromones are chemical signals that act within species produced within the animal and then released into the environment. Pheromones also influence physiological functions like the onset of puberty and oestrous.
Several hormones are secreted by the pituitary gland located right below the hypothalamus in the brain. According to the stimuli, the hypothalamus instructs the pituitary gland to secrete related hormones. The secreted hormones make their way via the bloodstream to the target organs. They either elicit a specific reaction directly or stimulate or cause the target organ to secrete hormones. We can think of this as a post office system. Hormones or posts must reach the correct target organs or addresses for the proper response to occur. For example, when hypothalamus detects low water levels in the blood, it signals the pituitary gland to release the antidiuretic hormone (ADH) into the bloodstream. ADH travels to the kidneys, the target organ, and causes water to be absorbed, so urine becomes more concentrated and decreases output. Water consumption elevates the water content in the blood, which subsequently causes the hypothalamus to instruct the pituitary gland to secrete less ADH.
Less ADH means kidneys will absorb less water, causing urine to become less concentrated and increase output.Presently, endocrinologists play around with different hormones to get the best and fast animal stocks to fulfil the global food demand. For example, hormones like androgens and oestrogen are implanted beneath the ear skin. These implants release growth promoters over time into the bloodstream. Animals attain growth much faster compared to traditional methods, thus filling the gap. Along with the development, we need to look into welfare too. Due to urbanisation, various chemicals accumulate in environments that act as hormone agonists or antagonists, finally disrupting hormonal imbalance, leading to the endocrine disruptor hypothesis. This condition is particularly harmful to aquatic, young, and unborn animals. However, the long-term impact of this on humans is still up for debate. Developments in endocrinology depend on emerging technologies like Omics and structural biology. Looking back, one might question them; will we ever fully comprehend how hormones act at the cellular level. The quick answer is: No. Our present understanding of endocrinology at any given time will influence future hormone research.
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