Coordination in Biology PDF

# **15 Control and coordination** ## **15.1 Nervous communications and neurones** On these pages you will learn to: - Outline the roles of the nervous system in coordinating homeostatic mechanisms. - Compare the nervous and endocrine systems as communication systems that co-ordinate responses to...

# **15 Control and coordination** ## **15.1 Nervous communications and neurones** On these pages you will learn to: - Outline the roles of the nervous system in coordinating homeostatic mechanisms. - Compare the nervous and endocrine systems as communication systems that co-ordinate responses to changes in the internal and external environment. - Describe the structure of a sensory neurone and a motor neurone. ### **Stimulus and response** The ability to react to stimuli is a basic characteristic of all living organisms. The stimuli may occur internally or externally and they lead to a response from the organism. The ability to respond to a stimulus increases the chances of survival for an organism. For example, to be able to detect and move away from harmful stimuli such as predators, extremes of temperature and pH, or to detect and move towards a source of food clearly aid survival. | **Endocrine system** | **Nervous system** | |---|---| | Communication is by chemicals called hormones | Communication is by nerve impulses (and neurotransmitters) | | Transmission is by the blood system | Transmission is by nerve fibres | | Transmission is usually relatively slow | Transmission is very rapid | | Hormones travel to all parts of the body, but only target organs respond | Nerve impulses travel to specific parts of the body | | Effects are widespread | Effects are localised | | Response is slow | Response is rapid | | Response is often long lasting | Response is short lived | | Effect may be permanent and irreversible | Effect is temporary and reversible | Those organisms that survive have a greater chance of raising offspring and of passing their alleles to the next generation. There is always, therefore, a selection pressure favouring organisms with better responses. Stimuli are received by receptors and the response is carried out by effectors. Receptors and effectors are often some distance apart and a form of communication is therefore needed between them if the organism is to respond effectively. This communication may be relatively slow via the endocrine system, which uses hormones (Topic 14.7), or rapid via the nervous system, which uses nerve impulses. Further differences between the endocrine and nervous system are given in Table 1. As animal species became more complex and the number of receptors and effectors increased, it became more efficient to link each receptor and effector to a central control centre. This is the central nervous system, consisting of the brain and spinal cord. The inter-relationships of all these various components are shown in Figure 1. ### **The structure of neurones** Neurones (nerve cells) are specialised cells adapted to rapidly carry electrochemical changes called nerve impulses from one part of the body to another. Mammalian neurones are made up of: - **A cell body** that contains a nucleus, mitochondria and large amounts of rough endoplasmic reticulum grouped to form Nissl's granules. These are associated with the production of proteins and neurotransmitters. - **Dendrons** - small extensions of the cell body that sub-divide into smaller branched fibres called dendrites that carry nerve impulses towards the cell body. - **Axon** - a single long fibre that carries nerve impulses away from the cell body (the dendron of the sensory neurone is sometimes also termed a peripheral or afferent axon). Many axons are surrounded by Schwann cells, which protect and provide insulation, act as phagocytes to remove cell debris and play a part in peripheral nerve regeneration. These Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around the axon. These membranes are rich in a lipid known as myelin and so form a covering to the axon called the myelin sheath. The space between adjacent Schwann cells lacks myelin, forming gaps 2-3 µm long, called nodes of Ranvier, which occur every 1-3 mm in humans. Neurones with a myelin sheath are called myelinated neurones and transmit nerve impulses faster than neurones without the myelin sheath (unmyelinated neurones) (Topic 15.6). The structure of a typical neurone is illustrated in Figure 3. Neurones can be classified according to their function: - **Sensory neurones** transmit nerve impulses from a receptor to a relay or motor neurone. They have one afferent dendron that brings the impulse towards the cell body and one axon that carries it away from the cell body. - **Relay neurones (intermediate neurones)** transmit impulses between neurones, e.g. from sensory to motor neurones. They have numerous short processes (extensions). - **Motor neurones (effector neurones)** transmit nerve impulses from a relay or sensory neurone to an effector such as a gland or a muscle. They have a long axon and many short dendrites. The three different types of neurone are illustrated in Figure 4. ## **15.2 Sensory receptors** On these pages you will learn to: - Outline the roles of sensory receptor cells in detecting stimuli and stimulating the transmission of nerve impulses in sensory neurones. The central nervous system receives sensory information from its internal and external environment through a variety of sensory receptors that detect different types of stimuli. These receptors include sense organs and sensory receptor cells, often found within sense organs. Each type of sensory receptor cell detects a specific stimulus. Sensory reception is the function of these sense organs, whereas sensory perception involves making sense of the information from the receptors. This is largely a function of the brain. In this topic we shall look at how chemoreceptors in taste buds act as sensory receptor cells. ### **The role of chemoreceptor cells of human taste buds as sensory receptor cells** Taste buds are onion-shaped structures located in the epithelium of the tongue. Within each taste bud there are 50-100 chemoreceptor cells that detect the presence of chemicals associated with taste. Each one has microvilli that project up through an opening at the top of the taste bud, called the taste pore. The microvilli provide a large surface area to allow chemicals dissolved in saliva to contact the chemoreceptor cell. The structure of a taste bud and a chemoreceptor cell are shown in Figure 1. Chemoreceptor cells arc thought to detect the chemicals associated with just four tastes - salt, sour, bitter and sweet - although savoury has been suggested as a fifth. As with all sensory receptors, taste chemoreceptor cells: - **are specific to a single type of stimulus** - in this case to dissolved chemicals only - **act as transducers** - they convert the energy of the stimulus into a receptor potential, which is a change in the potential difference that exists across the membrane of the chemoreceptor cell (Topic 15.4). Different types of chemoreceptor may have a slightly different sequence of events occurring when a stimulus is present. For some, the chemical binds to a specific membrane receptor, whereas for others the chemical enters the cell through specific membrane transport proteins. Whichever mechanism, the events lead to a change in the cell surface membrane to create the receptor potential. A receptor potential also leads to the release of a chemical transmitter from the end of the chemoreceptor that forms a synapse with a sensory neurone (Figure 1(b)). The stronger the stimulus, the greater the receptor potential and the more chemical transmitter is released (Topic 15.7). - **produce a generator potential** - as a result of the release of the chemical transmitter (neurotransmitter), the receptor potential of the chemoreceptor cell may be enough to create a generator potential in the sensory neurone with which it synapses (in very close contact). - **give an all-or-nothing response** - the greater the intensity of the stimulus, the greater the size of the generator potential. If the generator potential reaches or exceeds the set threshold level, an action potential is generated in the sensory neurone. Anything less than the threshold level, and no action potential is generated. Anything more than the threshold level, and the same action potential is generated, regardless of by how much the level is exceeded (Topic 15.4). - **become adapted** - if exposed to a steady stimulus over a period of time, there is a slow decline in the frequency of generator potentials produced and so action potentials in the sensory neurone become less frequent and eventually stop. This is adaptation and prevents the nervous system becoming overloaded with unimportant information. ## **EXTENSION ** ### **Rods and cones as light receptors** Rods and cones are photoreceptor cells found in the retina of the mammalian eye. The structure of a single rod cell is illustrated in Figure 2. Both rods and cones are secondary receptors. There are around six million cones, often with their own separate sensory neurone, in each human eye. The rods are more numerous, with 120 million in each eye, but up to 150 of them may share a single sensory neurone. As they share sensory neurones they cannot resolve very well, i.e. they have low visual acuity. Rods cannot distinguish different wavelengths of light and therefore produce images only in black and white. Cones, by contrast, need much higher light intensities to respond, but have high visual acuity and detect colour. ### **Transduction in rod cells** Although a rod cell is used in this account, the basic mechanism of transduction is the same in rods and cones. Each rod cell possesses up to a thousand vesicles in its outer segment. These contain the photosensitive pigment called rhodopsin, which is made up of the protein opsin and a derivative of vitamin A, called retinal. The process of transduction in the rod cell is as follows: - Light reaching a rod cell changes one isomer of retinal into another. - This causes the rhodopsin to split into opsin and retinal - a process called bleaching. The splitting causes a chain of reactions that make the cell surface membrane of the rod cell less permeable to sodium ions. - As sodium ions cannot now easily diffuse back into the rod cell, but continue to be actively pumped out of it, they accumulate outside, making this positive relative to the inside. - This redistribution of sodium creates hyperpolarisation, which acts as the generator potential. - If the threshold level is reached or exceeded by this change, then an action potential will be generated in the sensory neurone, which is connected to the brain via the optic nerve. - Mitochondria found in the inner segment of the rod cell, generate ATP, which provides the energy necessary to recombine retinal and opsin into rhodopsin. The process in a cone cell is very similar except that the pigment here is iodopsin. This is less sensitive to light and so a greater light intensity is required for it to breakdown and so create an action potential in the sensory neurone. ### **SUMMARY TEST 15.2** Receptors are cells and organs that respond to different stimuli. Making sense of the information provided by sensory receptors is called sensory (1). Chemoreceptor cells of taste buds respond to the stimulus of (2), converting the energy of the stimulus into a (3) potential as such they act as (4). In turn a (5) potential is set up in the attached associated sensory neurone. If this potential equals or exceeds a threshold level, then an (6) is produced, which is the same regardless of how much the threshold level is exceeded. This is known as an (7) response.