Coordination 15.1-15.5 PDF

# 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 t...

# 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 are 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. ## 15.3 The reflex arc On these pages you will learn to: - Describe the functions of sensory, relay and motor neurones in a reflex arc The simplest type of nervous response is a **reflex arc**. Before considering how a spinal reflex works, it is helpful to understand how the millions of neurones in a mammalian body are organised, and to know the structure of the spinal cord. ### The spinal cord The **spinal cord** is a column of nervous tissue running along the back within the vertebral column for protection. There is a small canal, the **spinal canal**, at its centre. The central region, called **grey matter**, comprises neurone cell bodies, synapses and unmyelinated relay neurones. Around the grey matter are many myelinated neurones running along the spinal cord. The **myelin** gives this region a lighter appearance and it is therefore known as **white matter**. The structure of the spinal cord is shown in Figure 1. ### Nervous organisation The nervous system has two major divisions: the **central nervous system** (CNS), which is made up of the brain and spinal cord, and the **peripheral nervous system** (PNS), which is made up of pairs of nerves that originate from either the brain or the spinal cord. The peripheral nervous system is divided into: - The **sensory** (afferent) nervous system, which carries nerve impulses towards the central nervous system. - The **motor** (efferent) nervous system, which carries nerve impulses away from the central nervous system. The motor nervous system can be further sub-divided into: - The **somatic nervous system**, which carries nerve impulses to skeletal muscles and is under **voluntary control**. - The **autonomic nervous system**, which carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control, i.e. it is **involuntary**. A summary of nervous organisation is given in Figure 2 and the way its components interact is shown in Figure 3. An involuntary response that follows a sensory stimulus is called a **reflex**. The pathway of neurones involved in a reflex is known as a **reflex arc**. The simplest forms of reflex arc, such as the knee jerk reflex, involve only a sensory and a motor neurone. More complex ones, like the withdrawal reflex, also involve a relay neurone. If a reflex involves the spinal cord but not the brain it is known as a **spinal reflex**. The main stages of a spinal reflex arc such as withdrawing the hand from a hot object are: - **stimulus** - heat from the hot object - **receptor** - temperature receptors in the skin of the back of the hand. If the threshold value of the temperature receptor is exceeded, a **generator potential** is established - **sensory neurone** - the generator potential leads to an action potential passing along the sensory neurone to the spinal cord - **relay (intermediate) neurone** - links the sensory neurone via synapses to the motor neurone within the grey matter of the spinal cord - **motor (effector) neurone** carries an action potential away from the spinal cord to the biceps muscle in the forearm - **effector** - the biceps muscle of the forearm is stimulated to contract - **response** - the hand is raised away from the hot object. These events are shown in Figure 4. ### Adaptive value of reflex arcs Any action that aids survival is said to have an adaptive value. Reflexes are involuntary - the actions they control do not need to be 'considered', because there is only one obvious course of action, e.g. remove the hand from the hot object. The adaptive value of reflex actions include: - Being involuntary, they do not need the decision-making powers of the brain, leaving it free to carry out more complex responses. In this way the brain is not overloaded with situations in which the response is always the same. Some impulses are sent at the same time to the brain, so that it is informed of what is happening and can over-ride (prevent) the reflex if necessary. - They protect the body from dangerous stimuli. They are effective from birth as they do not have to be learned. - They are fast, because the neurone pathway is short with very few, typically one or two, synapses (which are the slowest link in a neurone pathway). This is important in withdrawal reflexes. ## 15.4 The nerve impulse On these pages you will learn to: - Describe and explain the transmission of an action potential and its initiation from a resting potential A nerve impulse may be defined as a self-propagating wave of electrical disturbance that travels along the surface of the axon membrane. It is not, however, an electrical current, but a temporary reversal of the electrical potential difference across the axon membrane. This reversal is between a state called the resting potential and another called the action potential. ### Resting potential The movement of sodium ions (Na+) and potassium ions (K+) across the axon membrane is controlled in a number of ways: - The **phospholipid bilayer** of the axon cell surface membrane is impermeable to sodium ions (Na+) and potassium ions (K+). - The **channels** are specific for either sodium or potassium ions. One type of channel, termed a **voltage-gated channel**, can be opened to allow the specific ion across or closed to prevent movement of that ion. A different type of channel remains open and allows diffusion of the ion across. There are more of these channels, sometimes termed 'leak' channels, for potassium ions than for sodium ions. - Some **intrinsic proteins actively transport** potassium ions into the axon and sodium ions out of it. This is called the **sodium-potassium pump** (cation pump). As a result of these various controls, the inside of an axon is negatively charged relative to the outside. This is known as the **resting potential** and is in the range 50-90 millivolts (mV), but is usually 65mV. In this condition the axon is said to be **polarised**. To achieve this potential difference the following events occur: - **Sodium ions are actively transported out of the axon by sodium-potassium pumps** (specialised carrier proteins). - **Potassium ions are actively transported into the axon by sodium-potassium pumps.** - The **active transport of sodium ions is faster than that of potassium ions**, so that three sodium ions move out for every two potassium ions that move in. - Although both sodium and potassium ions are positively charged, the **outward movement of sodium ions is greater than the inward movement of potassium ions**. As a result, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, and more potassium ions in the cytoplasm than in the tissue fluid. For each ion, this creates a **chemical gradient**. - The **sodium ions begin to diffuse back naturally into the axon while the potassium ions begin to diffuse back out of the axon.** - As there are more of the **leak channel proteins** for potassium ions, the result is that the axon membrane is 100 times more permeable to potassium ions, which therefore diffuse back out of the axon faster than the sodium ones diffuse back in. This further increases the potential difference between the negative inside and the positive outside of the axon. - Apart from the **chemical gradient** that causes the movement of the potassium and sodium ions, there is also an **electrical gradient**. As more and more potassium ions diffuse out of the axon, so the outside becomes more and more positive. Further outward movement of potassium ions is therefore made difficult because, being positively charged, they are attracted back into the axon by its overall negative state and repelled from moving outwards by the overall positive state of the surrounding tissue fluid. - The presence of large, **negatively charged proteins** within the cytoplasm of the axon contributes to this overall negative state. - An **equilibrium is established** whereby there is no net movement of ions and which is a balance between the chemical and electrical gradients. These events are summarised in Figure 1. ### The action potential The energy conversion that occurs when a stimulus is received by a receptor leads to a temporary reversal of the charges on the axon membrane (Topic 15.2). As a result, the negative charge of -65 mV inside the membrane becomes a positive charge of around +40 mV. This is known as the **action potential**, and in this condition the membrane is said to be **depolarised**. This depolarisation involves the voltage-gated channels. The sequence of events is described below and the numbers relate to the stages shown in Figure 2. 1. At resting potential some potassium ion channels (leak channels) are open but the potassium voltage-gated and sodium voltage-gated channels are closed. 2. As a result of the stimulus some voltage-gated sodium channels in the axon membrane open and therefore sodium ions diffuse in through the channels along their electrochemical gradient. Being positively charged, they begin the reversal in the potential difference across the membrane and the membrane depolarises. Voltage-gated potassium channels remain closed. 3. As sodium ions enter, so more voltage-gated sodium channels open, causing an even greater influx of sodium ions. This is an example of positive feedback (Topic 14.1). An action potential will only occur if the membrane depolarises enough to allow the remaining voltage-gated sodium channels to open. This is known as the **threshold potential** (approximately -5 to-15 mV less negative than the resting potential). 4. Once the action potential of around +40 mV has been established (depolarisation has occurred), the voltage gates on sodium channels close (so further influx of sodium is prevented) and the voltage gates on the potassium channels begin to open. 5. With some voltage-gated potassium channels now open this causes the other voltage-gated channels to open and more potassium ions diffuse out, causing **repolarisation** of the axon membrane. 6. The outward movement of these potassium ions and the slight delay in the closing of the gates causes the temporary **overshoot** of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual. This is called **hyperpolarisation**. The gates on the potassium channels now close and the activities of the sodium-potassium (cation) pumps cause sodium ions to be pumped out and potassium ions in, once again. The axon membrane returns to a resting potential and the axon is said to be **repolarised**. The terms action potential and resting potential can be misleading, because the movement of sodium ions inwards during the action potential is purely due to diffusion - a passive process - and the resting potential is maintained by active transport - an active process. The term action potential simply means that the axon membrane is transmitting a nerve impulse, whereas resting potential means that it is not. ## 15.5 Transmission of impulses along a neurone On these pages you will learn to: - Describe and explain the transmission of an action potential in a myelinated neurone As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon. This reversal of electrical charge is reproduced and action potentials are generated along each small region of the axon membrane. As one action potential triggers the next, the previous region of the membrane returns to its resting potential, i.e. it is repolarised. The size of the action potential remains the same from one end of the axon to the other. Strictly speaking, nothing physically 'moves' from place to place along the axon of the neurone, but rather the reversal of electrical charge is reproduced at different points along the axon membrane. The process can be likened to the 'Mexican wave' that frequently takes place in a crowded stadium during a sporting event. Although the wave of people standing up and raising their hands (action potential) moves around the stadium, the people themselves do not move from seat to seat with the wave. They do not physically pass around the stadium until they return to their original seat. Rather, their individual action of standing and raising their hands is reproduced by the person to one side of them, in the same way that they were stimulated to stand and wave by the person on the other side of them. ### Transmission of the nerve impulse in an unmyelinated neurone It is easier to understand how a nerve impulse is transmitted in a myelinated nerve if we first look at how it is transmitted in an unmyelinated one. The process is described and illustrated in Figure 2. This shows how local circuit currents are set up. The movement in of sodium ions during an action potential will lead to some passive movement of ions (current) within the axon. This is enough to begin depolarisation of the adjacent section of membrane. The positively charged ions have a tendency to repel each other and move towards the less positively charged region. A similar situation happens when the ions have moved out of the axon into the surrounding fluid. ### Transmission of a nerve impulse in a myelinated neurone In myelinated neurones, the fatty sheath of **myelin** around the axon acts as an electrical insulator, preventing action potentials from forming. At intervals of 1-3 mm there are breaks in this insulatory myelin, called **nodes of Ranvier**, where there is a high concentration of voltage-gated ion channels and sodium-potassium pumps (Topic 15.1). Action potentials can only occur at these points. The localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials in effect 'jump' from node to node in a process known as **saltatory conduction** (Latin 'saltare' = to jump) (Figure 1). This results in an action potential passing along a myelinated neurone faster than an unmyelinated one (Topic 15.6). In our Mexican wave analogy, this is equivalent to a whole block of spectators leaping up simultaneously, followed by the next block and so on. Instead of the wave passing around the stadium in hundreds of small stages, it passes around in 20 or so large ones and so is more rapid. ## 15.6 Speed of nerve impulse transmission On these pages you will learn to: - Explain the importance of the myelin sheath (saltatory conduction) in determining the speed of nerve impulses and the refractory period in determining their frequency Once an action potential has been set up, it is propagated from one end of the neurone to the other without any decrease in amplitude (size). In other words, the final action potential at the end of the axon is as large as the first action potential. A number of factors, however, affect the speed at which the action potential passes along the axon. Depending upon these factors, an action potential may travel as little as 0.5 m in a second or as much as 100m in the same time. Table 1 gives some examples of transmission speeds in different axons. | Axon | Myelin | Axon diameter / μm | Transmission speed / ms-1 | | :-------------------------------------------------------------------------- | :---- | :------------------ | :------------------------ | | Human motor axon to leg muscle | Yes | 20 | 100 | | Human sensory axon from skin pressure receptor | Yes | 10 | 50 | | Squid giant axon | No | 500 | 25 | | Human motor axon to internal organ | No | 1 | 2 | ### Factors affecting the transmission of action potentials - **The myelin sheath** - we saw in Topic 15.5 that the myelin sheath acts as an electrical insulator, preventing an action potential forming in the part of the axon covered in myelin. It does, however, jump from node of Ranvier to node of Ranvier (**saltatory conduction**), speeding transmission from 0.5 ms-¹ in a unmyelinated neurone to 100ms in a similar myelinated one. - **The diameter of the axon** - the greater the diameter of an axon, the faster the speed of transmission. This is due to a combination of less leakage of ions from a large axon (leakage makes membrane potentials harder to maintain) and an increase in current flow within the axon. - **Temperature** - affects the rate of diffusion of ions and therefore the higher the temperature the faster the nerve impulse. Above a certain temperature, the cell surface membrane proteins are denatured and impulses fail to be conducted at all. Temperature is clearly an important factor in response times in **ectothermic animals**, in which body temperature varies with the environment. - **The refractory period** - we shall look at this in more detail next. ### The refractory period Once an action potential has been created in any region of an axon, there is a period afterwards when inward movement of sodium ions is prevented because the sodium voltage-gated channels are closed and temporarily inactivated. During this time it is not possible for a further action potential to be generated. This is known as the **refractory period**. The refractory period is made up of two portions: - The **absolute refractory period** lasts for about 1 ms, during which no new impulses can be passed, however intense the stimulus. On Figure 1 this can be seen as a neurone excitability of zero. - The **relative refractory period** lasts around 5 ms, during which a new impulse may be propagated provided the stimulus exceeds the normal threshold value. The degree to which it needs to exceed the threshold value becomes less over the period. On Figure 1 this is shown by an increase in neurone excitability. At normal resting excitability the voltage-gated channels are returned to their resting potential state. The refractory period serves two purposes: - The action potential cannot be propagated in the region that is refractory, i.e. it can only move in a forward direction. This prevents the action potential from spreading out in both directions, which it would otherwise do. - Because a new action potential cannot be formed immediately behind the first one, it ensures that action potentials are separated from one another and therefore limits the number of action potentials that can pass along an axon in a given time, i.e. it determines their frequency. ### All or nothing response Nerve impulses are described as **all or nothing responses**. There is a certain level of stimulus, called the **threshold value**, which triggers an impulse. Below the threshold value no impulse is generated; above the threshold value an impulse is generated. The action potential, however, is the same regardless of how much the stimulus is above the threshold value. How then can an organism determine the size of a stimulus? This is achieved in two ways: - by the number of impulses passing in a given time. This is known as **frequency coding**. The larger the stimulus, the more action potentials that are generated in a given time (Figure 3). - by having different neurones with different threshold values. The brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size. ## Multiple sclerosis Multiple sclerosis (MS) causes progressive damage to the nervous system. It usually affects young adults, causing weak limbs, 'pins and needles' (a pricking or tingling sensation), numbness and blurred vision. In some people the condition causes continuous problems, leading to severe physical disability. It affects around 2.5 million people worldwide. In multiple sclerosis there is gradual breakdown of the myelin sheath that surrounds most nerve axons. These areas of the axon, known as **plaques**, are approximately 2-10 mm in length and leave the axon demyelinated and unable to conduct impulses in the usual manner. Scar tissue develops in these areas - a process known as **sclerosis**, from which the disease gets its name. While the exact cause of MS is unknown, it is generally accepted that it is an autoimmune disease in which the body's own immune system reacts to self antigens as if they were non-self antigens. It is not clear what triggers this response, but several factors are involved, including gender (it is more common in females), genetics and environmental factors. ## Summary test 15.1 - The speed at which an action potential passes along an axon may vary from as little as (1) metress-¹ to as much as (2) metress-1. - Various factors influence this speed of transmission. For example, it is (3) if the diameter of the axon is smaller, and it is slower if the temperature is (4), because the rate of (5) of ions is slower. - The presence of a myelin sheath (6) the rate of transmission of an action potential because the action potential jumps from one node of (7) to the next in a process called (8). - Once an action potential has passed along an axon, the sodium (9) remain closed, preventing any inward movement of sodium ions. - There is hence a period of about 1 ms, called the (10) refractory period, during which no new action potential can be passed. - This is followed by a period called the (11) refractory period, during which a new action potential will pass only if the normal (12) is exceeded.