8b.docx

The narrow cross-section of axons lessens the metabolic expense of carrying action potentials; however, thicker axons convey the impulses more rapidly. In order to minimize metabolic expense, yet maintain a rapid conduction velocity, neurons of vertebrates and higher invertebrates have insulating sh...

The narrow cross-section of axons lessens the metabolic expense of carrying action potentials; however, thicker axons convey the impulses more rapidly. In order to minimize metabolic expense, yet maintain a rapid conduction velocity, neurons of vertebrates and higher invertebrates have insulating sheaths of myelin around their axons. These sheaths are formed by gill cells, oligodendrocyte in the central nervous system and Schwann cells in the peripheral nervous system. The sheath enables the action potentials to travel faster than in unmyelinated axons of the same diameter, whilst simultaneously spending less energy to 'recharge' the action potential afterwards. Brain While all vertebrates have a brain, most invertebrates have either a centralised brain or collections of individual ganglia. Some animals, such as cnidarians and echinoderms, do not have a centralised brain; instead they have a decentralised nervous system, while animals such as sponges lack both a brain and nervous system entirely. Unlike the spinal cord, which has changed little in structure during vertebrate evolution, the brain has altered dramatically. A primitive linear brain (fishes and amphibians) expanded to form the deeply fissured and enormously intricate brain in the mammal. Its greatest complexity is found in humans, which contains around 35 billion nerve cells, each of which has connection via tens of thousands of synapses at the same time. The ratio between an animal's spinal cord and brain is a useful determinant of intelligence. In fish and amphibians, this ratio is 1:1, whilst in humans it is 55:1. Although the human brain is neither the largest (the sperm whales brain is seven times heavier), nor the most convoluted (the porpoise brain is even more folded) it is the best in overall performance. Brains of early vertebrates had three principle divisions, a forebrain, (prosencephalon) a midbrain (mesocephalon) and a hindbrain (rhombcephalon). Each part was concerned with one or more specialised senses, the forebrain with olfaction, the midbrain with sight, and the hindbrain with hearing and balance. These primitive features have, in some instances, been intensified, and in others reduced as evolutionary processes emphasised or de-emphasised different senses. Sensory input is processed by the brain to recognise danger, find food, identify potential mates, and perform more sophisticated functions. Visual, touch, and auditory sensory pathways of vertebrates are routed to specific nuclei of the thalamus and then to regions of the cerebral cortex that are specific to each sensory system; the visual system, the auditory system, and the somatosensory system. Olfactory pathways are routed to the olfactory bulb, then to various parts of the olfactory system. Taste is routed through the brainstem and then to other portions of the gustatory system. To control movement, the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brainstem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone. The brain also produces a portion of the body\'s hormones that can influence organs and glands elsewhere in the body. Conversely, it also reacts to hormones produced elsewhere in the body. In mammals, the hormones that regulate hormone production throughout the body are produced in the brain by the pituitary gland. Functions of the various areas of the cerebrum have been localised by the direct stimulation of exposed brains of people and experimental animals, and post mortem examinations of people suffering various brain lesions and surgical removal of specific brain areas of experimental animals. Specific areas co ordinate muscle and sensory information, other 'silent regions' called association areas are concerned with memory, judgement, reasoning, and other integrative functions. However, the brain does not always function correctly; an interesting example of incorrect 'routing' of information within the brain is Synesthesia. This is a neurologically-based phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. In one common form of synesthesia, letters or numbers are perceived as inherently coloured, or numbers, days of the week and months of the year evoke personalities. Peripheral nervous system The peripheral nervous system (PNS) resides or extends outside the central nervous system (CNS), which consists of the brain and spinal cord, to serve the limbs and organs. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. Both of these have sensory (afferent) components and motor (efferent) components. The somatic nervous system is responsible for coordinating body movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control. The autonomic nervous system is then split into the sympathetic division, and the parasympathetic division. The sympathetic nervous system responds to impending danger or stress, and is responsible for an increase in heart rate and blood pressure, and a decrease in digestive function, allowing maximum energy to be available for strenuous activity, e.g. fight or flight response. The parasympathetic nervous system is active when the animal is resting, and is responsible for slowing the heart rate, dilation of the blood vessels, and the stimulation of activity in the digestive and genitourinary systems. Sense organs Animals require a constant inflow of information from the environment to regulate their lives. Sense organs are specialised receptors designed for detecting environmental status and change. A stimulus is a form of energy, electrical, mechanical, and chemical, heat or light. A sense organ transforms this energy into nerve impulses. As all nerve impulses are qualitatively alike, how do animals perceive and distinguish between impulses arising from different stimuli? The answer is that perception occurs in discrete areas of the brain, which are specialised to receive and process stimuli from specific senses (see brain section above). Receptors are traditionally classified by their location; those near the external surface (exteroceptors) keep an animal informed about its external environment. Internal parts of the body have interoceptors, and receive stimuli from internal organs. Muscles, tendons and joints have proprioceptors, which are sensitive to changes in the tension of muscles, providing an organism with a sense of body position. More commonly, receptors are classified according to the stimulus they react to, which is the system used in this Diploma. Chemoreception Chemoreception is the most primitive and universal sense in the animal kingdom, and probably guides the activity of animals more than any other sense. Unicellular forms use contact chemoreceptors to locate food, adequately oxygenate water and avoid noxious substances. Most Metazoans have specialised distance chemoreceptors, which often have a remarkable degree of sensitivity. Distance chemoreception is usually called olfaction/smell, and guides feeding behaviour, location, mate selection, territorial marking, and the alarm responses of many species. Social insects and many other animals (including mammals) produce species-specific compounds called pheromones, which constitute a highly developed communication system. Pheromones are a group of organic compounds that animal's release which then affects the behaviour/physiology of a member of the same species. Ants produce several pheromones, including alarm and trail markers, and primer pheromones, which alter endocrine and reproductive systems of different 'castes' within a colony. Insects have a number of specific © OXL/CN/AN 2022 147 Student No: PD24-51917-ZOCIE15 Email: walkergrace116\@gmail.com Name: Grace Walker pheromone receptors on their integument, as well as receptors for other chemicals. Many terrestrial vertebrates possess an additional vomeronasal organ (Jacobson organ), which responds to pheromones. Recent research has discovered pheromone action in humans in that menstruating women establish cycle synchronisation when in long-term proximity of each other. How this occurs, however, is not currently known. In all vertebrates and in insects, the senses of taste and smell are clearly distinguishable, and although there are similarities between taste and smell receptors, taste is more restrictive in response, and is less sensitive than smell. Central nervous system centres for olfaction and taste perception are in different parts of the brain. In vertebrates, taste receptors are found in the mouth cavity, especially the tongue, as taste buds consist of a cluster of receptor cells surrounded by supporting cells. Although the mechanisms differ for each basic taste sensation, the specific chemical depolarises receptor cells, and an action potential is produced. Taste discrimination depends on assessment by the brain of the relative activity of the various receptors. Olfactory endings are located in a special epithelial layer covered by a fine film of mucous, positioned deep in the nasal cavity. Each olfactory neuron has several cilia protruding from the free end. Odour molecules bind to receptor proteins on the cilia and action potentials are produced. Mechanoreception Mechanoreceptors are sensitive to quantitative forces such as touch, pressure, stretching, sound, vibration, and gravity. Touch Pacinian corpuscles are relatively large mechanoreceptors that register deep touch and pressure in mammalian skin. They occur in deep skin layers, connective tissue surrounding muscles and tendons, and abdominal mesenteries. Pressure at any point on the cell causes distortion and a graded action potential. Potentially stronger stimulation activates an action potential. A second action potential is triggered when the stimulus is removed. This response is called adaptation, which explains why many touch receptors are able to detect sudden changes, but readily adapt to changes, e.g. we are aware of the feel of clothes when getting dressed, but we are not aware of the feel of the cloth all day. The simplest touch receptors are bare nerve fibre terminals found in the skin, but there are a wide variety of other kinds of touch receptors of varying shapes and sizes, for example hair follicles have many touch sensitive cells, and the base of mammalian whiskers are particularly sensitive. Touch receptors are present all over animal bodies, but they are concentrated in particular areas making them more sensitive. The diagram is an artist's impression of a cat's body, in proportion to its sensitivity. Pain is a sign of distress from the body, signalling some noxious stimulus or internal disorder. Although there is no cortical pain centre, particular areas have been found in the brainstem where pain messages from the periphery terminate. These areas contain endorphins, which have analgesic properties. Pain receptors are relatively unspecialised nerve fibre endings that respond to a variety of stimuli signalling possible or real tissue damage. These free nerve endings also respond to other stimuli such as mechanical movement of a tissue or temperature change. Pain fibres respond to small peptides, which are released from injured cells. This type of response is known as slow pain. In contrast, fast pain responses (e.g. a pin prick or burn) are a more direct response of nerve endings to mechanical or thermal stimuli. Lateral line system This is a specialised group of cells found in fishes and amphibians, which detects wave vibrations and water currents. Receptor cells (neuromasts) are located on the body's surface. In many fishes, they are located within canals running beneath the epidermis, with the canals opening at intervals to the surface. Each neuromast is a collection of hair cells with sensory cilia embedded in a gelatinous wedge shaped mass (cupula). These hair cells are similar to the ones detecting sound and balance in terrestrial vertebrates. Hearing An ear is a specialised receptor for detecting sound waves in the surrounding environment. Most invertebrates do not posses sound receptors. Only certain arthropod groups (crustaceans, spiders, and insects) have true sound perception. Amongst insects, only the grasshopper group, and moths, possess ears, and these are simple, a pair of air pockets each enclosed by a tympanic membrane that passes sound vibration to sensory cells. The moth's ability to detect the ultrasonic frequencies used by hunting bats is a particularly useful adaptation for them. Vertebrate ears started off as a balance organ or labyrinth. In all jawed vertebrates, the labyrinth has a similar structure consisting of two small chambers (saccule and utricle) and their semi circular canals. The avian and mammalian cochlea is derived from a small pocket in the saccule. The human ear is representative of mammalian ears; the outer ear (pinna) collects sound waves and funnels them through an auditory canal to an eardrum (tympanic membrane) lying next to the middle ear. The middle ear is an air filled chamber containing three tiny bones (ossicles) known as the hammer (malleus) anvil (incus) and stirrup (stapes). These bones transform sound waves into mechanical action when the stapes contact the oval window of the inner ear. Within the inner ear is the cochlea (coiled in mammals), it is divided longitudinally into three tubular canals running parallel with each other, which contain sensory hair cells. When a sound wave strikes the ear, its energy is transmitted through the ossicles of the middle ear to the oval window, which oscillates back and forth. This disturbs the fluid in the cochlea, which moves the sensory hair cells, setting up action potentials. © OXL/CN/AN 2022 149 Student No: PD24-51917-ZOCIE15 Email: walkergrace116\@gmail.com Name: Grace Walker Photoreception Cells sensitive to light are called photoreceptors. These range from simple light sensitive cells scattered randomly on the body surface of many invertebrates, to the highly developed eyes of cephalopods and vertebrates. However, even some unicellular forms (such as Nematodinium) have quite advanced photoreceptors, with a lens, a light gathering chamber, and a photoreceptive cup. The dermal light receptors of most invertebrates are far less specialised, but serve well as locomotary orientation and photoperiodic coordination of reproductive function (so that all members of the population are ready to reproduce at the same time, e.g. in the spring). More highly organised eyes are capable of excellent image formation, and are based on one of two principles. Firstly, there is a single lens 'camera like' eye (e.g. cephalopods and vertebrates), secondly a multifaceted (compound) eye found in arthropods. Arthropod compound eyes are formed from multiple independent visual units called ommatidia. Eyes of insects, such as bees, have approximately 15,000 of these units, each of which views a discreet narrow segment of the visual field. These eyes form a mosaic of images, so in comparison to the vertebrate eye, resolution (ability to see objects sharply) is poor. For example, a fruit fly (drosophila) must be closer than 3cm to see another fruit fly clearly. However, the compound is especially acute at perceiving motion. The eyes of vertebrates, some annelids and molluscs have the same basic structure, containing a light chamber, and lens system, which focuses an image on a light sensitive surface (retina) in the back. The spherical eyeball has three main layers, the outer white sclera, which provides protection, the middle chroid coat, which contains blood vessels for nourishment, and the inner retina, which contains the light sensitive cells. The cornea is a transparent anterior modification of the sclera layer. A pigmented curtain (iris) controls the size of the opening (pupil). The lens (transparent, elastic oval disc) is positioned just behind the iris. Cilliary muscles can change the curvature of the lens, and bend the light rays to focus an image on the retina. In terrestrial vertebrates, the cornea does most of the bending of the light rays, whereas the lens adjusts focus for objects at various distances. Between the cornea and lens is an outer chamber filled with a watery fluid (aqueous humour). Between the lens and the retina is a much larger inner chamber filled with more viscous fluid, (vitreous humour). The retina is composed of several layers of cells. (See diagram left) The outer most layer close to the sclera is composed of pigment cells, (not shown). Next to this layer are the photoreceptors, rods and cones. In the human eye there are approximately 125 million rods and one million cones. In various other animals, the ratios between the two types of cells vary considerably. Cones are primarily concerned with colour vision in bright light, and rods with colourless vision in dim light. Next is a layer of intermediate neurones (bipolar, horizontal and amacrine cells) that process and relay visual information from the receptor cells to the ganglion cells of the optic nerve. This network allows convergence, especially for rod cells. Information from several- hundred rod cells may impact on a single ganglion cell, an adaptation, which greatly increases the effectiveness of rods in dim light. Cones, in contrast, show little convergence. By coordinating activity between the ganglion cells, and adjusting the sensitivity of bipolar, horizontal and amacrine cells, overall contrast and quality of a visual image can be varied. The fovea is the region of sharpest vision and is located in the centre of the retina, in direct line with the centre of the lens and cornea. It contains only cones, a vertebrate specialisation for diurnal (daytime) vision. Acuity of an animal's vision depends on the density of cones in the fovea. Human (and lion) foveae contain approximately 150,000 per square millimetre. However, many birds have up to one million cones per square millimetre. At the peripheral part of the retina, only rods are found. Rods are highly sensitive receptors for dim light. In low light conditions, the cone-filled fovea is unresponsive, and humans become functionally colour blind. Both rods and cones contain light sensitive pigments (rhodopsins). Molecules within the pigment change shape when a quantum of light hits it, causing excitation of the cell. Cones require 50 to100 times as much stimulation as rods to initiate an impulse. Consequently, night vision is almost totally rod vision. Unlike humans, who have both day and night vision, some vertebrates specialise in one or the other. Strictly nocturnal animals (bats/owls) have pure rod retinas and are therefore colour blind. Purely diurnal forms (grey squirrels and some birds) have only cones, and are therefore virtually blind at night. The perception of different colours is thought to be due to the presence of three types of cones, red, blue and green. Each of these responds to a different wavelength of light. Colour vision occurs in some members of all vertebrate groups with the exception of amphibians. Bony fishes and birds have particularly good colour vision, most mammals are colour blind, exceptions include primates and squirrels. Endocrine system Endocrinology is a huge subject in its own right, and unfortunately, it is not possible to explore these issues in any depth in a course of this length. The following is a brief overview, although students are encouraged to pursue the subject in more detail, particularly the hormonal control of mammalian reproductive cycles. The endocrine system is the second way in which animal's co- ordinate internal activities. In contrast to the nervous system, control is by chemical messengers called hormones. Hormones are chemical compounds released into the blood in tiny amounts and transported to the target organs/cells by the circulatory system, where they cause physiological changes. Hormones are generally secreted by endocrine glands (small, vascularised, ductless glands). As endocrine glands have no ducts, they communicate with the rest of the body via the blood, receiving raw materials from it, and secrete the finished hormonal product back into it. In contrast, exocrine glands have ducts for discharging their products (e.g. sweat, salivary and digestive glands). Compared with nervous systems, the endocrine system is slow acting, due to the passage of the chemical across various cellular membranes and through the circulation system. Hormonal responses also differ from nervous systems, as effects are generally longer lasting (minutes to days) as opposed to milliseconds to minutes. Endocrine control tends to occur when a sustained effort is needed, such as in growth and reproductive processes. Despite their differences, the nervous and endocrine systems function together. Endocrine glands often receive direct stimulation from the brain, and many hormones act on the nervous system and affect a wide range of animal behaviours. Neurosecretory cells are specialised nerve cells capable of synthesising and secreting hormones and are present in all metazoan groups. An example of hormones in invertebrates is the linked processes of growth and metamorphosis in insects. Moulting and metamorphosis are primarily controlled by the interaction of two hormones. One, ecdysone (moulting hormone) is produced by the prothoracic gland, and juvenile hormone is produced by the copora allata area of the brain. At intervals during juvenile growth, ecdysone favours the development of adult features, resulting in moulting, however, secretion of juvenile hormone means that juvenile features remain. As the output of juvenile hormone decreases, as the insect ages, production of ecdysone results in a metamorphosing moult. Most mammalian hormone secretion is controlled (directly or indirectly) by the pituitary gland at the base of the brain; it is divided into two distinct areas, the anterior pituitary, which secretes the following: Thyroid stimulating hormone -- stimulates thyroid hormone production. Follicle stimulating hormone -- stimulates follicle maturation and oestrogen production in ovaries and sperm production in males. Luteinising hormone -- stimulates ovulation and corpus luteum formation and testosterone in males. Prolactin -- stimulates mammary gland growth, milk synthesis in mammals and parental behaviour in lower vertebrates. Growth hormone - stimulates growth and metabolism of glycogen and fat. Adrenocorticotropic hormone - stimulates adrenal cortex to produce glucocortizone. The posterior lobe of the pituitary produces oxytocin, which causes milk ejection and uterine contractions, is an anti diuretic hormone, causing water re-absorption in the kidneys, and vasotocin, which decreases water reabsorption. The pineal gland (known as the third eye) is an area of glandular and photoreceptive cells in ectothermic vertebrates, and functions to control light-dark biological rhythms. In many amphibians, this organ contains structure analogous to the lens and cornea. In birds and mammals, it is an entirely glandular structure, which secretes melatonin, and is responsible for seasonal behaviours such as hibernation and reproduction. There are several other important hormone glands in higher vertebrates such as the adrenal glands (adrenalin) and the pancreas (insulin).