Module 5 Nervous Communication PDF

# Neuronal Communication ## 13.1 Coordination **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the need for communication systems in multicellular organisms - the communication between cells by cell signalling. **Specification reference:** 5.1.1 When ch...

# Neuronal Communication ## 13.1 Coordination **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the need for communication systems in multicellular organisms - the communication between cells by cell signalling. **Specification reference:** 5.1.1 When changes occur in an organism's internal or external environment, the organism must respond to these changes in order to survive. Examples of these changes are shown in Table 1. | Internal environment | External environment | | ----------------------- | ------------------------ | | blood glucose concentration | humidity | | internal temperature | external temperature | | water potential | light intensity | | cell pH | new or sudden sound | Animals and plants respond to these changes in a variety of ways. Animals react through electrical responses (via neurones), and through chemical responses (via hormones). Plant responses are based on a number of chemical communication systems including plant hormones. These communication systems must be coordinated to produce the required response in an organism. ### Why coordination is needed As species have evolved, cells within organisms have become specialised to perform specific functions. As a result organisms need to coordinate the function of different cells and systems to operate effectively. Few body systems can work in isolation (apart from a few exceptions, for example, a heart can continue to beat if placed in the right bathing solution). For example, red blood cells transport oxygen effectively, but have no nucleus. This means that these cells are not able to replicate - a constant supply of red blood cells to the body is maintained by haematopoietic stem cells. In order to contract, muscle cells must constantly respire, and thus require a consistent oxygen supply. As these cells cannot transport oxygen, they are dependent on red blood cells for this function. In plants flowering needs to coordinate with the seasons, and pollinators must coordinate with the plants In temperate climates light-sensitive chemicals enable plants to coordinate the development of flower buds with the lengthening days that signal the approach of spring and summer. ## 13.2 Neurones **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the structure and functions of sensory, relay, and motor neurones. **Specification reference:** 5.1.3 The nervous system is responsible for detecting changes in the internal and external environment. These changes are known as a stimulus. This information then needs to be processed and an appropriate response triggered. Both the nervous system and hormonal system play a role in reacting to stimuli, but they do so in very different ways. In this chapter you will focus on neuronal communication. This is generally a much faster and more targeted response than that produced by hormonal communication. ### Neurones You have already learnt that the nervous system is made up billions of specialised nerve cells called neurones. The role of neurones is to transmit electrical impulses rapidly around the body so that the organism can respond to changes in its internal and external environment. There are several different types of neurone found within a mammal. They work together to carry information detected by a sensory receptor to the effector, which in turn carries out the appropriate response. ### Structure of a neurone Mammalian neurones have several key features: - **Cell body** - this contains the nucleus surrounded by cytoplasm. Within the cytoplasm there are also large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters. These are chemicals which are used to pass signals from one neurone to the next. You will find out more about the important role of neurotransmitters in the nervous system in Topic 13.5, Synapses. - **Dendrons** - these are short extensions which come from the cell body. These extensions divide into smaller and smaller branches known as dendrites. They are responsible for transmitting electrical impulses towards the cell body. - **Axons** - these are singular, elongated nerve fibres that transmit impulses away from the cell body. These fibres can be very long, for example, those that transmit impulses from the tips of toes and fingers to the spinal cord. The fibre is cylindrical in shape consisting of a very narrow region of cytoplasm (in most cases approximately 1 µm) surrounded by a plasma membrane. ### Types of neurone Neurones can be divided into three groups according to their function. As a result they have slightly different structures: - **Sensory neurones** - these neurones transmit impulses from a sensory receptor cell to a relay neurone, motor neurone, or the brain. They have one dendron, which carries the impulse to the cell body, and one axon, which carries the impulse away from the cell body. - **Relay neurones** - these neurones transmit impulses between neurones. For example, between sensory neurones and motor neurones. They have many short axons and dendrons. - **Motor neurones** - these neurones transmit impulses from a relay neurone or sensory neurone to an effector, such as a muscle or a gland. They have one long axon and many short dendrites. In most nervous responses the electrical impulse follows the pathway: Receptor → sensory neurone → relay neurone → motor neurone → effector cell ### Myelinated neurones The axons of some neurones are covered in a myelin sheath, made of many layers of plasma membrane. Special cells, called Schwann cells, produce these layers of membrane by growing around the axon many times. Each time they grow around the axon, a double layer of phospholipid bilayer is laid down. When the Schwann cell stops growing there may be more than 20 layers of membrane. The myelin sheath acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a much faster speed than unmyelinated neurones. Myelinated neurones can transmit impulses at up to 100 metres per second. In comparison, non-myelinated neurones can only conduct impulses at approximately 1 metre per second. ## 13.3 Sensory receptors **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the roles of mammalian sensory receptors in converting different types of stimuli into nerve impulses. **Specification reference:** 5.1.3 The body is able to detect changes in its environment using groups of specialised cells known as sensory receptors. These are often located in the sense organs, such as the ear and eye. Sensory receptors convert the stimulus they detect into a nerve impulse. The information is then passed through the nervous system and on into the central nervous system (CNS) – normally to the brain. The brain coordinates the required response and sends an impulse to an effector (normally a muscle or gland) to result in the desired response. ### Features of sensory receptors All sensory receptors have two main features: - They are specific to a single type of stimulus.  - They act as a transducer - they convert a stimulus into a nerve impulse. There are four main types of sensory receptor present in an animal, shown in Table 1. | Type of sensory receptor | Stimulus | Example of receptor | Example of sense organ | | -------------------------- | -------- | ---------------------- | ------------------------ | | mechanoreceptor | pressure and movement | Pacinian corpuscle (detects pressure) | skin | | chemoreceptor | chemicals | olfactory receptor (detects smells) | nose | | thermoreceptor | heat | end-bulbs of Krause | tongue | | photoreceptors | light | cone cells (detects different light wavelengths) | eye | ### Role as a transducer Sensory receptors detect a range of different stimuli including light, heat, sound, or pressure. The receptor converts the stimulus into a nervous impulse, called a generator potential. For example, a rod cell (found in your eye) responds to light and produces a generator potential. ### Pacinian corpuscle Pacinian corpuscles are specific sensory receptors that detect mechanical pressure. They are located deep within your skin and are most abundant in the fingers and the soles of the feet. They are also found within joints, enabling you to know which joints are changing direction. ## 13.4 Nervous transmission **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the generation and transmission of nerve impulses in mammals. **Specification reference:** 5.1.3 After the sensory receptor has detected a change in the environment, an impulse is sent along the neurone by temporarily changing the voltage (potential difference) across the axon's membrane. As a result, the axon membrane switches between two states - a resting potential and an action potential. You will find out how the impulse travels between neurones in Topic 13.5, Synapses. ### Resting potential When a neurone is not transmitting an impulse, the potential difference across its membrane (difference in charge between the inside and outside of the axon) is known as a resting potential. In this state, the outside of the membrane is more positively charged than the inside of the axon. The membrane is said to be polarised as there is a potential difference across it. It is normally about -70 mV. The resting potential occurs as a result of the movement of sodium and potassium ions across the axon membrane. The phospholipid bilayer prevents these ions from diffusing across the membrane and, therefore, they have to be transported via channel proteins. Some of these channels are gated - they must be opened to allow specific ions to pass through them. Other channels remain open all of the time allowing sodium and potassium ions to simply diffuse through them. The following events result in the creation of a resting potential: - **Sodium ions (Na+)** are actively transported out of the axon whereas **potassium ions (K+)** are actively transported into the axon by a specific intrinsic protein known as the sodium-potassium pump. However, their movement is not equal. For every three sodium ions that are pumped out, two potassium ions are pumped in. - As a result there are more sodium ions outside the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the cytoplasm than outside the axon. Therefore, sodium ions diffuse back into the axon down its electrochemical gradient (this is the name given to a concentration gradient of ions), whereas potassium ions diffuse out of the axon. - However, most of the 'gated' sodium ion channels are closed, preventing the movement of sodium ions, whereas many potassium ion channels are open, thus allowing potassium ions to diffuse out of the axon. Therefore, there are more positively charged ions outside the axon than inside the cell. This creates the resting potential across the membrane of -70 mV, with the inside negative relative to the outside. ### Action potential When a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reverses the charges on the axon membrane. As a result the potential difference across the membrane rapidly changes and becomes positively charged at approximately +40 mV. This is known as depolarisation – a change in potential difference from negative to positive. As the impulse passes repolarisation then occurs a change in potential difference from positive back to negative. The neurone returns to its resting potential. An action potential occurs when protein channels in the axon membrane change shape as a result of the change of voltage across its membrane. The change in protein shape results in the channel opening or closing. These channels are known as voltage-gated ion channels. Figure 2 shows the changes in potential difference which occur across the axon membrane during an action potential. The numbers on the graph correspond to the sequence of events that take place during an action potential: 1. The neurone has a resting potential – it is not transmitting an impulse. Some potassium ion channels are open (mainly those that are not voltage-gated) but sodium voltage-gated ion channels are closed. 2. The energy of the stimulus triggers some sodium voltage-gated ion channels to open, making the membrane more permeable to sodium ions. Sodium ions therefore diffuse into the axon down their electrochemical gradient. This makes the inside of the neurone less negative. 3. This change in charge causes more sodium ion channels to open, allowing more sodium ions to diffuse into the axon. This is an example of positive feedback. 4. When the potential difference reaches approximately +40mV the voltage-gated sodium ion channels close and voltage-gated potassium ion channels open. Sodium ions can no longer enter the axon, but the membrane is now more permeable to potassium ions. 5. Potassium ions diffuse out of the axon down their electrochemical gradient. This reduces the charge, resulting in the inside of the axon becoming more negative than the outside. 6. Initially, lots of potassium ions diffuse out of the axon, resulting in the inside of the axon becoming more negative (relative to the outside) than in its normal resting state. This is known as hyperpolarisation. The voltage-gated potassium channels now close. The sodium-potassium pump causes sodium ions to move out of the cell, and potassium ions to move in. The axon returns to its resting potential - it is now repolarised. ### Propagation of action potentials A nerve impulse is an action potential that starts at one end of the neurone and is propagated along the axon to the other end of the neurone. The initial stimulus causes a change in the sensory receptor which triggers an action potential in the sensory receptor, so the first region of the axon membrane is depolarised. This acts as a stimulus for the depolarisation of the next region of the membrane. The process continues along the length of the axon forming a wave of depolarisation. Once sodium ions are inside the axon, they are attracted by the negative charge ahead and the concentration gradient to diffuse further along inside the axon, triggering the depolarisation of the next section. ### Saltatory conduction Myelinated axons transfer electrical impulses much faster than non-myelinated axons. This is because depolarisation of the axon membrane can only occur at the nodes of Ranvier where no myelin is present. Here the sodium ions can pass through the protein channels in the membrane. Longer localised circuits therefore arise between adjacent nodes. The action potential then 'jumps' from one node to another in a process known as saltatory conduction. This is much faster than a wave of depolarisation along the whole length of the axon membrane. Every time channels open and ions move it takes time, so reducing the number of places where this happens speeds up the action potential transmission. Long-term, saltatory conduction is also more energy efficient. Repolarisation uses ATP in the sodium pump, so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction of impulses more efficient. ## 13.5 Synapses **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the structure and roles of synapses in neurotransmission. **Specification reference:** 5.1.3 In Topic 13.4, Nervous transmission, you learnt about how impulses travel along each neurone in the form of an action potential. However, to reach the CNS or an effector, the impulse often needs to be passed between several neurones. The junction between two neurones (or a neurone and effector) is called a synapse. Impulses are transmitted across the synapse using chemicals called neurotransmitters. ### Synapse structure All synapses have a number of key features: - **Synaptic cleft** - the gap which separates the axon of one neurone from the dendrite of the next neurone. It is approximately 20-30 nm across. - **Presynaptic neurone** – neurone along which the impulse has arrived. - **Postsynaptic neurone** - neurone that receives the neurotransmitter. - **Synaptic knob** – the swollen end of the presynaptic neurone. It contains many mitochondria and large amounts of endoplasmic reticulum to enable it to manufacture neurotransmitters (in most cases). - **Synaptic vesicles** – vesicles containing neurotransmitters. The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft. - **Neurotransmitter receptors** - receptor molecules which the neurotransmitter binds to in the postsynaptic membrane. ### Types of neurotransmitter Neurotransmitters can grouped into two categories: * **Excitatory** – these neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane an action potential is triggered. Acetylcholine is an example of an excitatory neurotransmitter. * **Inhibitory** – these neurotransmitters result in the hyperpolarisation of the postsynaptic membrane. This prevents an action potential being triggered. Gamma-aminobutyric acid (GABA) is an example of an inhibitory neurotransmitter that is found in some synapses in the brain. ### Transmission of impulses across synapses Synaptic transmission occurs as a result of the following:  * the action potential reaches the end of the presynaptic neurone  * depolarisation of the presynaptic membrane causes calcium ion channels to open * calcium ions diffuse into the presynaptic knob * this causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis * neurotransmitter diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane * this causes sodium ion channels to open * sodium ions diffuse into the postsynaptic neurone * this triggers an action potential and the impulse is propagated along the postsynaptic neurone. Once a neurotransmitter has triggered an action potential in the postsynaptic neurone, it is important that it is removed so the stimulus is not maintained, and so another stimulus can arrive at and affect the synapse Any neurotransmitter left in the synaptic cleft is removed. Acetycholine is broken down by enzymes, which also releases them from the receptors on the postsynaptic membrane. The products are taken back into the presynaptic knob. Removing the neurotransmitter from the synaptic cleft prevents the response from happening again and allows the neurotransmitter to be recycled. ### Cholinergic synapses Cholinergic synapses use the neurotransmitter acetylcholine. They are common in the CNS of vertebrates and at neuromuscular junctions - where a motor neurone and a muscle cell (an effector) meet. If the neurotransmitter reaches the receptors on a muscle cell, it will cause the muscle to contract. Acetylcholine is released from the vesicles in the presynaptic knob (Figure 2). It then diffuses across the synaptic cleft where it binds with specific receptors in the postsynaptic membrane. This triggers an action potential in the postsynaptic neurone or muscle cell Once an action potential has been triggered, acetylcholine is hydrolysed by a specific enzyme - acetylcholinesterase. This enzyme is also situated on the postsynaptic membrane. Acetylcholine is hydrolysed to give choline and ethanoic acid One molecule of acetylcholinesterase can break down around 25000 molecules of acetylcholine per minute. The breakdown products are taken back into the presynaptic knob to be reformed into acetylcholine, and the postsynaptic membrane is ready to receive another impulse. ## 13.6 Organisation of the nervous system **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the organisation of the mammalian nervous system. **Specification reference:** 5.1.5 In the last few topics, you have looked in detail at how nervous impulses are transmitted around the nervous system – how are the billions of neurones in your nervous system organised? ### Structural organisation The mammalian nervous system is organised structurally into two systems: - **Central nervous system (CNS)** – this consists of your brain and spinal cord. - **Peripheral nervous system (PNS)** - this consists of all the neurones that connect the CNS to the rest of the body. These are the sensory neurones which carry nerve impulses from the receptors to the CNS, and the motor neurones which carry nerve impulses away from the CNS to the effectors. ### Functional organisation The nervous system is also functionally organised into two systems: - **Somatic nervous system** - this system is under conscious control - it is used when you voluntarily decide to do something. For example, when you decide to move a muscle to move your arm. The somatic nervous system carries impulses to the body's muscles. - **Autonomic nervous system** - this system works constantly. It is under subconscious control and is used when the body does something automatically without you deciding to do it - it is involuntary. For example, to cause the heart to beat, or to digest food. The autonomic nervous system carries nerve impulses to glands, smooth muscle (for example, in the walls of the intestine), and cardiac muscle. The autonomic nervous system is then further divided by function into the sympathetic and parasympathetic nervous system. Generally, if the outcome increases activity it involves the sympathetic nervous system - for example, an increase in heart rate. If the outcome decreases activity it involves the parasympathetic nervous system – for example, a decrease in heart or breathing rate after a period of exercise. | Structure | Sympathetic stimulation | Parasympathetic stimulation | | -------- | ------------------------- | --------------------------- | | salivary glands | saliva production reduced | saliva production increased | | lung | bronchial muscle relaxed | bronchial muscle contracted | | kidney | decreased urine secretion | increased urine secretion | | stomach | peristalsis reduced | gastric juice secreted | | small intestine | peristalsis reduced | digestion increased | ## 13.7 Structure and function of the brain **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the structure of the human brain and the functions of its parts. **Specification reference:** 5.1.5 An adult human brain contains approximately 86 billion neurones. The brain is responsible for processing all the information collected by receptor cells about changes in the internal and external environment. It also receives and processes information from the hormonal system through molecules in the blood. It must then produce a coordinated response.  The advantage of having a central control centre for the whole body is that communication between the billions of neurones involved is much faster than if control centres for different functions were distributed around the body. With the exception of reflex actions, all other nervous reactions are processed by the brain. You will find out about reflex actions in Topic 13.8, Reflexes. ### Gross structure The brain is protected by the skull. It is also surrounded by protective membranes (called meninges). The human brain is extremely complex, but the structures you need to know about are shown in Figure 1. There are five main areas. They are distinguishable by their shape, colour, or microscopic structure:  - **Cerebrum** - controls voluntary actions, such as learning, memory, personality, and conscious thought. - **Cerebellum** - controls unconscious functions such as posture, balance, and non-voluntary movement. - **Medulla oblongata** - used in autonomic control, for example, it controls heart rate and breathing rate. - **Hypothalamus** - regulatory centre for temperature and water balance. - **Pituitary gland** - stores and releases hormones that regulate many body functions ### Different images of the brain Many different techniques are used to study the brain in order to understand its function. Figures 2, 3, and 4 all show a cross-section through the brain. Can you identify the main structures? ### Cerebrum The cerebrum receives sensory information, interprets it with respect to that stored from previous experiences, and then sends impulses along motor neurones to effectors to produce an appropriate response. It is responsible for coordinating all of the body's voluntary responses as well as some involuntary ones. The cerebrum is highly convoluted, which increases its surface area considerably and therefore its capacity for complex activity. It is split into left and right halves known as the cerebral hemispheres. Each hemisphere controls one half of the body, and has discrete areas which perform specific functions - these areas are mirrored in each hemisphere. The outer layer of the cerebral hemispheres is known as the cerebral cortex. It is 2-4mm thick. The most sophisticated processes such as reasoning and decision-making occur in the frontal and prefrontal lobe of the cerebral cortex. Each sensory area within the cerebral hemispheres receives information from receptor cells located in sense organs. The size of the sensory area allocated is in proportion to the relative number of receptor cells present in the body part. The information is then passed on to other areas of the brain, known as association areas, to be analysed and acted upon. Impulses come into the motor areas where motor neurones send out impulses, for example, to move skeletal muscles. The size of the motor area allocated is in proportion to the relative number of motor endings in it. The main region which controls movement is the primary motor cortex located at the back of the frontal lobe. In the base of the brain, impulses from each side of the body cross - therefore the left hemisphere receives impulses from the right-hand side of the body, and the right hemisphere receives impulses from the left-hand side of the body. For example, inputs from the eye pass to the visual area in the occipital lobe. Impulses from the right side of the field of vision in each eye are sent to the visual cortex in the left hemisphere, whereas impulses from the left side of the field of vision are sent to the right hemisphere. Through the integration of these inputs the brain is able to judge distance and perspective. ### Cerebellum This area of the brain is concerned with the control of muscular movement, body posture, and balance – it does not initiate movement, but coordinates it. Therefore, if this area of the brain is damaged, a person suffers from jerky and uncoordinated movement. The cerebellum receives information from the organs of balance in the ears and information about the tone of muscles and tendons. It then relays this information to the areas of the cerebral cortex that are involved in motor control. ### Medulla oblongata The medulla oblongata contains many important regulatory centres of the autonomic nervous system. These control reflex activities such as ventilation (breathing rate) and heart rate. It also controls activities such as swallowing, peristalsis, an coughing. ### Hypothalamus This is the main controlling region for the autonomic nervous system. It has two centres – one for the parasympathetic and one for the sympathetic nervous system. It has a number of functions, which include: - controlling complex patterns of behaviour, such as feeding, sleeping, and aggression - monitoring the composition of blood plasma, such as the concentration of water and blood glucose - therefore it has a very rich blood supply - producing hormones – it is an endocrine gland, that is, it produces hormones. ### Pituitary gland This is found at the base of the hypothalamus. It is approximately the size of a pea but it controls most of the glands in the body. It is divided into two sections: - **Anterior pituitary (front section)** – produces six hormones including follicle-stimulating hormone (FSH), which is involved in reproduction and growth hormones. - **Posterior pituitary (back section)** - stores and releases hormones produced by the hypothalamus, such as ADH involved in urine production. ## **13.8 Reflexes** **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the reflex actions. **Specification reference:** 5.1.5 When the body is in danger, it can respond to situations without conscious thought. This causes a faster response, preventing or minimising damage to the body. This is known as a reflex action. A reflex is an involuntary response to a sensory stimulus. ### Reflex arc The pathway of neurones involved in a reflex action is known as a reflex arc. Most reflexes follow the same steps between the stimulus and the response: - **Receptor** - detects stimulus and creates an action potential in the sensory neurone. - **Sensory neurone** - carries impulse to spinal cord. - **Relay neurone** - connects the sensory neurone to the motor neurone within the spinal cord or brain. - **Motor neurone** - carries impulse to the effector to carry out the appropriate response. Figure 1 illustrates what happens when you touch a hot candle – this is known as a withdrawal reflex. Before your brain registers that your hand is hot, the muscles in your arm have already pulled your hand away from the danger, minimising damage to your hand. ### Spinal cord The spinal cord is a column of nervous tissues running up the back. It is surrounded by the spine for protection. At intervals along the spinal cord pairs of neurones emerge, as shown in Figure 2. ### Knee-jerk reflex The knee-jerk reflex is a reflex commonly tested by doctors. It is a spinal reflex - this means that the neural circuit only goes up to the spinal cord, not the brain. When the leg is tapped just below the kneecap (patella), it stretches the patellar tendon and acts as a stimulus. This stimulus initiates a reflex arc that causes the extensor muscle on top of the thigh to contract. At the same time, a relay neurone inhibits the motor neurone of the flexor muscle, causing it to relax. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle, causes the leg to kick. After the tap of a hammer, the leg is normally extended once and comes to rest. The absence of this reflex may indicate nervous problems and multiple oscillation of the leg may be a sign of a cerebellar disease. ### Blinking reflex The blinking reflex is an involuntary blinking of the eyelids (Figure 4). It occurs when the cornea is stimulated, for example, by being touched. Its purpose is to keep the cornea safe from damage due to foreign bodies such as dust or flying insects entering the eye - this type of response is known as the corneal reflex. A blink reflex also occurs when sounds greater than 40-60 dB are heard, or as a result of very bright light. Blinking as a reaction to over-bright light (to protect the lens and retina) is known as the optical reflex. The blinking reflex is a cranial reflex – it occurs in the brain, not the spinal cord. When the cornea of the eye is irritated by a foreign body, the stimulus triggers an impulse along a sensory neurone (the fifth cranial nerve). The impulse then passes through a relay neurone in the lower brain stem. Impulses are then sent along branches of the motor neurone (the seventh cranial nerve) to initiate a motor response to close the eyelids. The reflex initiates a consensual response - this means that both eyes are closed in response to the stimulus. The blinking reflex is very rapid - it occurs in around one tenth of a second. Doctors test for the blinking reflex when examining unconscious patients. If this reflex is present, it indicates that the lower brain stem is functioning. This procedure is therefore used as part of an assessment to determine whether or not a patient is brain-dead – if the corneal reflex is present the person cannot be diagnosed as brain-dead. ## 13.9 Voluntary and involuntary muscles **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the structure of mammalian muscle and the mechanism of muscular contraction - the examination of stained sections or photomicrographs of skeletal muscle. **Specification reference:** 5.1.5 There are around 650 muscles in the body, making up roughly half of the body's weight As the contraction of many muscle cells causes the body to move (Topic 13.10, Sliding filament theory). However, there are many muscle cells in the body whose contractions you are largely unaware of. ### Types of muscle There are three types of muscle in the body: - **Skeletal muscle** – skeletal muscles make up the bulk of body muscle tissue. These are the cells responsible for movement, for example, the biceps and triceps. - **Cardiac muscle** – cardiac muscle cells are found only in the heart. These cells are myogenic, meaning they contract without the need for a nervous stimulus, causing the heart to in a regular rhythm. - **Involuntary muscle (also known as smooth muscle)** – involuntary muscle cells are found in many parts of the body – for example, in the walls of hollow organs such as the stomach and bladder. They are also found in the walls of the blood vessels and the digestive tract, where through peristalsis they move food along the gut. | Type of muscle | Fibre appearance | Control | Arrangement | Contraction speed | Length of contraction | Structure | Involuntary | | -------------- | ----------------- | ------- | ------------ | ------------------ | ----------------------- | ---------- | ---------- | | Skeletal | striated | conscious (voluntary) | regularly arranged so muscle contracts in one direction | rapid | short | Muscles showing cross striations are known as striated or striped muscles. Fibres are tubular and multinucleated. | | | Cardiac | specialised striated | involuntary | cells branch and interconnect resulting in simultaneous contraction | intermediate | intermediate | Cardiac muscle does show striations but they are much fainter than those in skeletal muscle. Fibres are branched and uninucleated. | | | Involuntary | non-striated | involuntary | no regular arrangement - different cells can contract in different directions | slow | can remain contracted for a relatively long time | Muscles showing no cross striations are called non-striated or unstriped muscles. Fibres are spindle shaped and uninucleated. | | ### Structure of skeletal muscle Skeletal muscles are made up of bundles of muscle fibres. These are enclosed within a plasma membrane known as the sarcolemma.  The muscle fibres contain a number of nuclei and are much longer than normal cells, as they are formed as a result of many individual embryonic muscle cells fusing together. This makes the muscle stronger, as the junction between adjacent cells would act as a point of weakness. The shared cytoplasm within a muscle fibre is known as sarcoplasm.  Parts of the sarcolemma fold inwards (known as transverse or T tubules) to help spread electrical impulses throughout the sarcoplasm. This ensures that the whole of the fibre receives the impulse to contract at the same time. Muscle fibres have lots of mitochondria to provide the ATP that is needed for muscle contraction. They also have a modified version of the endoplasmic reticulum, known as the sarcoplasmic reticulum. This extends throughout the muscle fibre and contains calcium ions required for muscle contraction. ### Myofibrils Each muscle fibre contains many myofibrils. These are long cylindrical organelles made of protein and specialised for contraction. On their own they provide almost no force but collectively they are very powerful. Myofibrils are lined up in parallel to provide maximum force when they all contract together. Myofibrils are made up of two types of protein filament: - **Actin** - the thinner filament. It consists of two strands twisted around each other. - **Myosin** - the thicker filament. It consists of long rod-shaped fibres with bulbous heads that project to one side. Myofibrils have alternating light and dark bands – these result in their striped appearance: - **Light bands** - these areas appear light as they are the region where the actin and myosin filaments do not overlap. (They are also known as isotopic bands or I-bands.) - **Dark bands** - these areas appear dark because of the presence of thick myosin filaments. The edges are particularly dark as the myosin is overlapped with actin. (They are also known as anisotropic bands or A-bands.) - **Z-line** - this is a line found at the centre of each light band. The distance between adjacent Z-lines is called a sarcomere. The sarcomere is the functional unit of the myofibril. When a muscle contracts the sarcomere shortens. - **H-zone** - this is a lighter coloured region found in the centre of each dark band. Only myosin filaments are present at this point. When the muscle contracts the H-zone decreases. ### Histology of skeletal muscle Figure 9 is a stained section of skeletal muscle viewed through a microscope. You should be able to identify the following features: * Individual muscle fibres – long and thin multinucleated fibres that are crossed with a regular pattern of fine red and white lines. * The highly structured arrangement of sarcomeres which appear as dark (A-bands) and light (I-bands) bands. * Streaks of connective and adipose tissue. * Capillaries running in between the fibres. ### Slow-twitch and fast-twitch muscles There are two types of muscle fibres found in your body. Different muscles in the body have different proportions of each fibre **Properties of slow-twitch fibres:** * fibres contract slowly * provide less powerful contractions but over a longer period * used for endurance activities as they do not tire easily * gain their energy from aerobic respiration  * rich in myoglobin, a bright red protein which stores oxygen - this makes the fibres appear red  * rich supply of blood vessels and mitochondria.  Slow-twitch fibres are found in large proportions in muscles which help to maintain posture such as those in the back and calf muscles which have to contract continuously to keep the body upright. **Properties of fast-twitch fibres:** * fibres contract very quickly * produce powerful contractions but only for short periods * used for short bursts of speed and power as they tire easily * gain their energy from anaerobic respiration * pale coloured as they have low levels of myoglobin and blood vessels * contain more, and thicker, myosin filaments  * store creatine phosphate - a molecule that can rapidly generate ATP from ADP in anaerobic conditions. Fast-twitch fibres are found in high proportions in muscles which need short bursts of intense activity, such as biceps and eyes.  ## 13.10 Sliding filament model **Learning outcomes** - Demonstrate knowledge, understanding, and application of: - the structure of mammalian muscle and the mechanism of muscular contraction - the examination of stained sections or photomicrographs of skeletal muscle. **Specification reference:** 5.1.5 In the previous topic, you looked in detail at the structure of skeletal muscle fibres In order to contract and cause movement, the actin and myosin filaments within the myofibrils have to slide past each other. Muscle contraction is usually described using the sliding filament model. ### Sliding filament model During contraction the myosin filaments pull the actin filaments inwards towards the centre of the sarcomere. This results in: - the light band becoming narrower - the Z lines moving closer together, shortening the sarcomere - the H-zone becoming narrower. The dark band remains the same width, as the myosin filaments themselves have not shortened, but now overlap the actin filaments by a greater amount.