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HL Biology Notes for Nerves, Muscles & Movement The notes in this document cover IB topics 11.2 and option E.1, E.2, E.4 and E.5. Organization of the Nervous System The nervous system is divided into the peripheral nervous system (PNS) and central nervous system (CNS). The PNS consists of sensory neurons running from stimulus receptors that inform the CNS of the stimuli motor neurons running from the CNS to the effectors (muscles and glands) that respond to the stimuli The CNS consists of the spinal cord the brain The Peripheral Nervous System Sensory Neurons Sensory Neurons Internal Environment Autonomic NS CNS SensorySomatic NS External Environment Motor Neurons Motor Neurons The PNS is subdivided into the sensory‐somatic nervous system which connects the external environment and the CNS there are 12 pairs of cranial nerves, which connect directly to the brain (e.g. the optic nerve), and they may be sensory, motor, or mixed nerves there are 31 pairs of spinal nerves, all of which are mixed all our conscious awareness of the external environment and all our motor activity to cope with it operate through the sensory‐somatic division of the PNS actions of the sensory‐somatic nervous system are largely voluntary – skeletal muscle is controlled by this system the autonomic nervous system (ANS) which connects the internal environment and the CNS consists of sensory neurons and motor neurons that run between the CNS and various internal organs it is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them actions of the autonomic nervous system are largely involuntary ‐ cardiac muscle (heart), blood vessels, digestive system, smooth muscle, and glands are controlled by this system uses two groups of motor neurons to stimulate the effectors A. De Jong/TFSS 2007 1 of 21 HL Biology Notes for Nerves, Muscles & Movement preganglionic neurons arise in the CNS and run to a ganglion in the body ■ postganglionic neurons run to the effector organ (synapse occurs in the ganglion further subdivided into the sympathetic and parasympathetic nervous systems, which are largely antagonistic to each other: ■ Image from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/autonomic.gif The Sympathetic Nervous System The neurotransmitter of the preganglionic neurons is acetylcholine (Ach). It stimulates action potentials in the postganglionic neurons. The neurotransmitter of the postganglionic neurons is noradrenaline. The action of noradrenaline on a particular gland or muscle may be excitatory or inhibitory. Stimulation of the sympathetic branch of the ANS prepares the body for emergencies: “fight or flight”. The Parasympathetic Nervous System The main nerves of the parasympathetic nervous system are the vagus nerves, which originate in the medulla oblongata. Acetylcholine is the neuro‐transmitter at all pre‐ and many postganglionic neurons. Some postganglionic neurons release nitric oxide as their neurotransmitter. The parasympathetic nervous system returns the body to normal after they have been altered by sympathetic stimulation: “rest and digest”. A. De Jong/TFSS 2007 2 of 21 HL Biology Notes for Nerves, Muscles & Movement Although the ANS is considered to be involuntary, this is not entirely true. A certain amount of conscious control can be exerted over it as has long been demonstrated by practitioners of yoga and Zen Buddhism. During their periods of meditation, these people are able to alter a number of autonomic functions including heart rate and the rate of oxygen consumption. These changes are not simply a reflection of decreased physical activity because they are lower than levels found during sleep or hypnosis. Another example of conscious control of the ANS is the control of emptying of the bladder and bowels. The Central Nervous System The spinal cord conducts sensory information from the PNS to the brain, and conducts motor information from the brain to the effectors, including skeletal, smooth and cardiac muscle, and glands. It also serves as a minor reflex centre. The brain receives sensory input from the spinal cord and its own nerves. It devotes most of its computational power to processing its various sensory inputs and initiating appropriate and coordinated motor outputs. Both the spinal cord and brain consist of white matter (bundles of axons coated with myelin sheaths) and grey matter (cell bodies & dendrites, covered in synapses). They are also covered with connective tissue called the meninges. An extracellular fluid that differs in its composition from the ECF in the rest of the body surrounds the cells of the CNS. Cerebrospinal fluid (CSF) contains less protein than ECF, and is found within the cerebrospinal canal of the spinal cord and within the four ventricles of the brain. The Spinal Cord Image from http://neuro.wehealny.org/images/14_01.jpg There are 31 pairs of spinal nerves. These are all classed as mixed nerves because they contain both sensory and motor axons. sensory axons pass into the dorsal root ganglion where their cell bodies are located and then on into the spinal cord itself motor axons pass into pass into the ventral roots before uniting with the sensory axons to form the mixed nerves A. De Jong/TFSS 2007 3 of 21 HL Biology Notes for Nerves, Muscles & Movement The primary functions of the spinal cord: it connects a large part of the PNS to the brain it is a minor coordinating centre responsible for some simple reflexes such as the withdrawal reflex The Brain Medulla Oblongata controls involuntary and visceral activities Cerebellum controls body balance, muscular coordination and equilibrium. Hypothalamus maintains the internal environment regulates body temperature, thirst, hunger, metabolism, pleasure, pain, etc. Thalamus sorts incoming and outgoing impulses and sends to the appropriate centre Cerebral Cortex centre of all voluntary muscle control and mental activity analysis coding info. storage recognition memory understanding intelligence sense integration Image from http://www.emc.maricopa.edu/faculty/farabee/BIOBK/brain_3.gif The pituitary gland, located at the base of the brain, is approximately the size of a pea, and is composed of two lobes. anterior lobe: stimulated by the hypothalamus of the brain to secrete several hormones thyroid stimulating hormone (TSH) follicle‐stimulating hormone (FSH) luteinizing hormone (LH) prolactin growth hormone adrenocorticotropic hormone (ACTH) posterior lobe: releases two hormones, synthesized by the hypothalamus, into the bloodstream antidiuretic hormone (ADH) oxytocin A. De Jong/TFSS 2007 4 of 21 HL Biology Notes for Nerves, Muscles & Movement How do we know what the brain does? Investigating brain‐damaged patients o For example, the experiences of soldiers surviving bullet‐wounds to the rear of the skull led to the discovery of the role of the visual cortex on the rear of the cerebral hemispheres. o Patients who are not immediately killed by a stroke often experience paralysis or loss of a specific body function – post‐mortem analysis identifies the particular part of the brain affected by the stroke. Animal experiments o We have learned a lot about brain function by studying mammals and other vertebrates, removing parts of a healthy brain or severing connections between neurons. o In one investigation using cats, severing the fibres that cross over in the centre of the brain below the two halves of the cerebral hemispheres gave clues to the interaction of left and right halves of the brain. fMRI o Functional magnetic resonance imaging is an advanced form of MRI that detects the parts of the brain that are active when the body performs specific tasks. There is always a demand for oxygen and glucose (food energy) in the brain, but there are local increases in demand when a particular area of the brain is in use. fMRI detects increases in red blood cell oxygenation at the site of neural activity. A. De Jong/TFSS 2007 5 of 21 HL Biology Notes for Nerves, Muscles & Movement Neurons All neurons are specialized cells that carry an electrochemical impulse called an action potential. Sensory neurons run from the stimulus receptors (e.g. for touch, vision, sound, odour and taste) to the CNS. Interneurons, found only in the CNS, and are stimulated by sensory neurons, other interneurons, or both. The brain is estimated to contain 100 billion interneurons averaging 1000 synapses each. Motor neurons such as the one pictured can have an axon that is up to one metre in length. They transmit impulses from the brain to the effectors – muscles and glands. Image from http://www.gonzaga.k12.nf.ca/academics/science/sci_page/biology/neuron1.gif Nerve Impulse Transmission Neurons send messages electrochemically, which means that chemicals cause an electric signal. Ions have either a positive (+) or negative (‐) charge. Important ions for nerve impulse transmission are: sodium (Na+) potassium (K+) calcium (Ca++) chloride (Cl‐) Some definitions: Membrane potential: the electrical potential difference (voltage) across a cell's membrane. Action potential: a wave of electrical discharge that travels along the membrane of a cell. Action potentials are used by the nervous system to transmit information between neurons, and between neurons and effectors. Resting potential: the membrane potential that would be maintained if there were no action potentials, synaptic potentials or other active changes in the membrane potential. For most cells, this is a negative number. The resting potential of a neuron is usually ‐ 70 mV. At rest, K+ can easily cross through the membrane, while Cl‐ and Na+ have more trouble crossing. The negatively‐charged protein molecules (A‐) cannot cross the membrane. In addition, the sodium‐potassium ion pump is actively pumping three Na+ out for every two K+ it puts in. Image from http://faculty.washington.edu/chudler/gif/ioncon.gif A. De Jong/TFSS 2007 6 of 21 HL Biology Notes for Nerves, Muscles & Movement An action potential occurs when a neuron sends an impulse down an axon, away from the cell body. It is an explosion of electrical activity that is created by a depolarizing current. (This means that a stimulus has caused the resting potential to move toward 0 mV.) When the depolarization reaches about ‐55 mV, a neuron will fire an action potential. This value is called the threshold. If this value is not reached, the action potential will not fire. * The action potential for any given neuron is always the same. There is no “big” or “small” action potential for a neuron. Action potentials are caused by an exchange of ions across the membrane of a neuron: A stimulus causes sodium channels to open, allowing sodium ions to enter the cell. This causes depolarization, because the sodium ions are positively charged. The potassium channels open after depolarization begins, which causes potassium to leave the cell, reversing the depolarization. Around this time, sodium channels begin to close, which causes a repolarisation, as the action potential goes back toward ‐70 mV. The action potential actually goes past ‐70 mV (a hyperpolarisation) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to ‐70 mV. Image from http://faculty.washington.edu/chudler/ap3.gif Synapses Nerve impulses are transmitted along an individual neuron by means of an action potential. Since these signals must be transmitted not only along a single neuron, but from one neuron to another, or from a neuron to an effector, there must be a means of passing the signal from one neuron to another. A junction between two neurons is called a synapse. For information to pass between neurons, it must cross the synapse. Invertebrates, and some fish have electrical synapses, in which the action potential in the pre‐synaptic neuron can trigger an action potential in the post‐synaptic neuron because there is a physical connection between the two neurons. Electrical synapses are faster than chemical synapses. Most nerves are connected by chemical synapses, which consist of: a pre‐synaptic ending that contains neurotransmitters, mitochondria and other cell organelles a neurotransmitter is a substance (such as norepinephrine or acetylcholine) that transmits nerve impulses across a synapse a post‐synaptic ending that contains receptor sites for neurotransmitters A. De Jong/TFSS 2007 7 of 21 HL Biology Notes for Nerves, Muscles & Movement a synaptic cleft or space between the pre‐synaptic and post‐synaptic endings For communication between neurons to occur, an electrical impulse must travel down an axon to the synaptic terminal. Action of Neurotransmitters: 1. At the pre‐synaptic terminal, an electrical impulse (action potential) causes a change in membrane permeability to Ca++, which allows Ca++ to flow into the synaptic knob. Image from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Synapse.gif 2. Presence of Ca++ will trigger the migration of vesicles containing neurotransmitters toward the pre‐synaptic membrane. 3. The vesicle membrane will fuse with the pre‐synaptic membrane, releasing neurotransmitters into the synaptic cleft. (an example of exocytosis) 4. Neurotransmitter molecules diffuse across the synaptic cleft where they can bind with receptor sites on the post‐synaptic ending to influence the electrical response in the post‐synaptic neuron. When a neurotransmitter binds to a post‐synaptic receptor, it changes the post‐synaptic cell's excitability, making it either more or less likely to fire an action potential. If the number of excitatory post‐synaptic events is large enough, they will add to cause an action potential in the post‐synaptic cell and a continuation of the “message”. Many psychoactive drugs and neurotoxins can change the properties of neurotransmitter release, neurotransmitter reuptake and the availability of receptor binding sites. Neurotransmitters and Synapses Synapses of the PNS are classified according to the neurotransmitter used. Each synapse uses only one neuro‐transmitter. Most synapses in the parasympathetic nervous system are cholinergic synapses, and use acetylcholine. Neuromuscular junctions are also cholinergic. Most synapses in the sympathetic nervous system are adrenergic synapses, and use noradrenaline. Synapses of the brain use a much wider range of neurotransmitters, including dopamine and enkephalins. A. De Jong/TFSS 2007 8 of 21 HL Biology Notes for Nerves, Muscles & Movement Neurotransmitters bind to receptors on the postsynaptic membrane, causing temporary changes in its permeability. Some neurotransmitters cause Na+ or other positive ions to enter the post‐synaptic neuron, helping to depolarize it and cause an action potential. Other neurotransmitters cause Cl‐ to move into the post‐synaptic neuron – this causes hyperpolarisation. Hyperpolarisation makes it more difficult to create an action potential (further from threshold). These are called excitatory synapses. These are called inhibitory synapses. Most post‐synaptic neurons have synapses with more than one pre‐synaptic neuron – these may be a mix of excitatory and inhibitory synapses, and whether an action potential is initiated in the post‐synaptic neuron is determined by the sum of all neurotransmitter messages. Parkinson's Disease is caused by the death of neurons in a part of the brain called the substantia nigra. These neurons release the neurotransmitter dopamine at inhibitory synapses with neurons that help to control muscle contractions. Without dopamine, muscle contractions cannot be properly controlled – this causes the symptoms of Parkinson's: early symptoms include feeling tired and shaky, and a loss of concentration eventually, the body becomes stiff because antagonistic muscles cannot relax uncontrollable shaking affects the hands and other body parts and movements become very slow Pain receptors are found in the skin and other organs. They consist of free nerve endings, which perceive mechanical, chemical or thermal stimuli. Pain signals are sent from these nerve endings to the spinal cord via nerve fibres, which carry them up to the thalamus or brain stem. From here, pain signals may be passed on to sensory areas of the cerebral cortex, giving conscious recognition of pain. Since there are both fast and slow nerve endings, a painful stimulus causes an initial sharp pain sensation, followed by a slow, burning pain. The sensation of pain is necessary to tell the body when it is being damaged – this allows the pain withdrawal reflex or other reactions to occur. Sometimes pain interferes with the ability to concentrate. In these situations, pain control systems in the brain and spinal cord can be used to reduce or prevent feelings of pain. This involves two natural painkillers: enkephalins released by the brain block calcium channels in the membrane of the pre‐synaptic neurons, blocking synaptic transmission so that pain signals do not reach the brain endorphins produced by the pituitary gland are carried to the brain and other organs by the blood, and bind to receptors in the membranes of neurons that send pain signals to the brain – endorphins are secreted during stressful times, after injuries, and sometimes during physical exercise such as running Psychoactive Drugs Psychoactive drugs affect the brain and personality. They either increase or decrease synaptic transmission: they can bind to the receptor site on post‐synaptic membranes, mimicking the neurotransmitter or blocking the binding of the neurotransmitter A. De Jong/TFSS 2007 9 of 21 HL Biology Notes for Nerves, Muscles & Movement they can also reduce the effect of the enzyme which normally breaks down the neurotransmitter, which causes an increase in the effect of the neurotransmitter nicotine mimics acetylcholine, while curare blocks acetylcholine Excitatory Psychoactive Drugs increase the activity of the nervous system, and may have different effects on behaviour: Nicotine stimulates synaptic transmission at cholinergic synapses in many parts of the brain, and causes release of adrenaline from the adrenal gland. This results in increased blood pressure and cardiac frequency. It affects mood, acting like a stimulant and causing euphoria. Cocaine blocks the reabsorption of dopamine and noradrenaline at synapses in the brain, causing increased energy, alertness and talkativeness. It gives an intense feeling of euphoria. Physical effects include increased cardiac frequency and body temperature, and dilation of the pupils. Amphetamines stimulate transmission at adrenergic synapses and have similar effects to cocaine. Users experience increased alertness and reduced appetite. “Ecstasy” is a derivative of amphetamines. It causes feelings of empathy, openness and caring, lowering aggression and increasing sexual behaviour. Caffeine increases heart rate and urine production. It causes some mood elevation and increases alertness. Inhibitory Psychoactive Drugs decrease the activity of the nervous system. Benzodiazepines such as Valium® relax muscles, decrease circulation, respiration and blood pressure. They reduce anxiety and elevate mood. In high doses they cause drowsiness, slurred speech and loss of muscle control. Doctors prescribe them for use as tranquillizers. Cannabis contains many chemicals, including THC, which binds to cannabinoid receptors in the rain, blocking synaptic transmission. Its users claim it increases the intensity of sensory perception, gives a feeling of emotional well‐being and allows clear thinking about complex ideas. There is strong evidence, though, that the ability to concentrate, control muscle contractions and judge times and distances is diminished. Alcohol acts as an inhibitor in at least two ways (enhances GABA, an inhibitory neurotransmitter, and by decreasing the activity of glutamate, an excitatory neurotransmitter.) In small quantities, alcohol reduces inhibitions, making people more confident and talkative. It also reduces reaction times and fine muscle coordination. In larger quantities it causes memory loss, slurred speech, loss of balance and poor muscle coordination, and may cause violent behaviour. Addiction is a state of taking a mood‐altering drug habitually and being unable to give it up without experiencing unpleasant side effects. It has many causes: THC interferes with dopamine metabolism – this produces a state of dependence, with more & more of the drug being required to produce its effect. Genetic predisposition may be a factor with some people – insufficient levels of the enzymes required to break down the drug, for example, or a personality type that is inclined towards unnecessary risk‐taking. Social factors: poor diet, high unemployment & limited access to education & training that could lead to rewarding employment, combined with little opportunity for self‐fulfilment can generate a sense of hopelessness that could lead to seeing drugs as an escape mechanism. A. De Jong/TFSS 2007 10 of 21 HL Biology Notes for Nerves, Muscles & Movement Perception of Stimuli Sensory receptors act as energy transducers. This means that they convert a non‐electrical signal (e.g. light or sound) to an electrical one. This results in an action potential in a sensory neuron because gated ion channels (for Na+) are opened. Types of Sensory Receptors chemoreceptors have membrane proteins which bind a particular substance binding results in depolarization of the membrane action potential brings message to the brain e.g. scent, taste, pH of blood mechanoreceptors are sensitive to movement in humans, semi‐circular canals in the inner ear associated with a system of hair cells a change in speed or direction (of the body) moves fluid in the canals, which bends the hairs this causes action potentials to the brain thermoreceptors are sensitive to temperature cold receptors in the skin send an action potential when the temperature drops warm receptors (deeper in the skin than cold receptors) send an action potential when temperature increases the temperature centre in the hypothalamus also contains thermoreceptors, which monitor the temperature of the blood (body) photoreceptors are sensitive to light rods and cones in the eye contain photopigments which break down when exposed to light this causes an action potential to the brain ■ rods contain rhodopsin and are sensitive to light intensity ■ cones contain iodopsins (red, green or blue) and are responsible for colour vision Reflexes are a fast response to a stimulus. Spinal Reflexes involve the spinal cord and not the brain. They are part of innate behaviour, and involve only two or three nerve cells. Knee Jerk Reflex the knee is tapped; this stretches the tendon stretch receptor in the muscle sends an action potential to the spinal cord the action potential is passed to a motor neuron, which makes the muscle contract the lower leg moves Pain Withdrawal Reflex you prick your finger (or stub your toe) a pain receptor neuron sends and action potential to the spinal cord an association neuron passes the action potential to a motor neuron this causes the biceps to contract, moving your finger away from the source of the pain Cranial Reflexes involve the nerves of the brain: Pupil Reflex when bright light is perceived, the iris will immediately contract this will reduce the amount of light upon the retina so that it is not damaged the brain stem is responsible for this reflex ‐ absence of the pupil reflex can indicate damage to the brain stem (brain death) A. De Jong/TFSS 2007 11 of 21 HL Biology Notes for Nerves, Muscles & Movement Blink Reflex when an object comes close to the eye, you will blink or close your eye this helps prevent damage to the eye you can learn to control your blink reflex, for example, learning to put in contact lenses Reflex Arc Image from http://www.biotopics.co.uk/humans/refarc.gif Structure of the Eye fovea Image from http://www.ai.rug.nl/~lambert/projects/BCI/literature/misc/oog‐retina.gif A. De Jong/TFSS 2007 12 of 21 HL Biology Notes for Nerves, Muscles & Movement Structure of the Retina Image from http://www.rhsmpsychology.com/images/retina.jpg Processing Visual Stimuli The retina contains rods, cones, and nerve cells responsible for vision. Rods are responsible for detecting light intensity, while cones are responsible for colour vision. The fovea is a “yellow spot” on the retina which is entirely composed of cones – this is the site of most accurate vision. The fovea is found just above the blind spot, where the optic nerve connects at the back of the eye. White light hitting the fovea triggers action potentials in all cones and is perceived as white by the brain. Blue light hitting the fovea triggers action potentials in blue cones, and is perceived as blue by the brain. There is a certain amount of overlap in the absorption of colour, particularly between green and red – this means that red or green light could trigger action potentials in both red and green cones. Light entering the eye is refracted by the cornea and lens. It passes through the vitreous humour (clear) to reach the retina. light must also pass through ganglia and bipolar neurons to reach the rods and cones cones are mostly located in the fovea rods are found throughout the retina (except the fovea) Cones are linked individually to bipolar neurons. This makes them less sensitive to light but increases their accuracy. A. De Jong/TFSS 2007 13 of 21 HL Biology Notes for Nerves, Muscles & Movement Several rods are connected to a single bipolar neuron. This makes them more sensitive to light but reduces their accuracy. When light has caused an action potential in the rods or cones, it is passed on to the bipolar neurons. action potentials from other cells may inhibit or further excite a bipolar cell action potentials are passed from bipolar cells to ganglia and on to the optic nerve The optic nerve is composed of many nerve fibres, which are connected to different parts of the retina. some fibres connect in the optic chiasma, while others do not as a result a complete picture is transmitted to the brain Image from http://media‐2.web.britannica.com/eb‐media/48/63348‐004‐3D434AC1.gif Contralateral processing is due to the optic chiasma, where the right brain processes information from the left visual field, and vice versa, as illustrated above. Edge enhancement occurs within the retina, and is best demonstrated by the Hermann grid illusion (at right): • dark, grey blobs appear at the ‘crossroads’ where the white lines intersect – unless you are directly looking at that spot • this has to do with the receptive fields of the retina, which are smaller when looking directly at the intersection points (see left) Images from http://www.michaelbach.de/ot/lum_herGrid/index.html A. De Jong/TFSS 2007 14 of 21 HL Biology Notes for Nerves, Muscles & Movement Colour Blindness is a result of a deficiency in one or more types of cones. The most common type is a red‐ green deficiency, in which it is difficult to distinguish between certain shades of red and green. Those with red‐green colour‐blindness would be unable to see the number “15” in the image below: Image from http://www.biologie.uni‐hamburg.de/b‐online/library/falk/vision/colorblind.jpg Controlling How Light Enters the Eye Light enters the eye through the pupil, and opening in the centre in the iris (the coloured part of the eye). Pupil Size changes in response to brightness of light. In bright light, the circular muscles of the iris contract, and the pupil becomes smaller. This reduces the amount of light entering the eye to prevent retina damage. In dim light, these muscles relax, opening the pupil. This increases the amount of light that enters the eye. Image from http://www.schools.net.au/edu/lesson_ideas/optics/images/eye_contract.gif Lens Thickness changes in order to focus light on the retina. Light reflected off a distant object has parallel rays. Refraction through the lens focuses it on the retina. Light reflected off a near object has divergent rays. Light has to be refracted more in order to focus properly on the retina, so the lens thickens. A. De Jong/TFSS 2007 15 of 21 HL Biology Notes for Nerves, Muscles & Movement Brain Death and the Pupil Reflex Brain death is defined as the irreversible cessation of all brain functions. Modern medical technology can keep a patient alive (heart beating, lungs ‘breathing’) long after the brain stops directing these functions – because of this, and the possibility of using brain‐dead patients’ organs for transplant surgery, it is necessary to have indicators of brain death. The agreed criteria for brain death (absence of all brain function) are: absence of pupil reflex absence of blink reflex eyes do not rotate in their sockets when the head is moved eyes do not move when iced water is placed in the outer ear canal no cough (or gagging) when a suction tube is placed deep into the trachea breathing does not commence when the patient is taken off the ventilator Structure of the Human Ear Image from http://www.perceptualentropy.com/wiki/images/7/7c/HumanEar.jpg The malleus (hammer), incus (anvil) and stapes (stirrups) are the ossicles (bones) of the inner ear. A. De Jong/TFSS 2007 16 of 21 HL Biology Notes for Nerves, Muscles & Movement Perception of Sound The ear consists of three basic parts ‐ the outer ear, the middle ear, and the inner ear. Each part of the ear serves a specific purpose in the task of detecting and interpreting sound: The outer ear (pinna and ear canal) serves to collect and channel sound to the middle ear. o Sound entering the ear canal is a pressure wave, with alternating high and low pressure regions: o When the sound reaches the eardrum (tympanic membrane), the energy causes it to vibrate. The middle ear serves to transform the energy of a sound wave into the internal vibrations of the bone structure of the middle ear and ultimately transform these vibrations into a compressional wave in the inner ear. o The middle ear is an air‐filled cavity. o Vibration of the tympanic membrane causes the interconnected ossicles to vibrate, transmitting the sound wave to the fluid of the inner ear. o The middle ear is connected to the mouth by the Eustachian tube, which allows for equalization of pressure within the middle ear. The inner ear serves to transform the energy of a compressional wave within the inner ear fluid into nerve impulses that can be transmitted to the brain. o The inner ear consists of the cochlea, semicircular canals and the auditory nerve. o The semicircular canals have no role in hearing – they act as accelerometers that assist with balance. o The cochlea is fluid‐filled and lined with hair‐like cells. When the ossicles vibrate, they transmit the energy of the vibration to the cochlea via the oval window. o Because each of the hair‐like nerve cells differs in length and sensitivity to the fluid’s motion, each responds to a different frequency. When stimulated by its natural frequency, the nerve cell will vibrate, triggering an action potential in the auditory nerve. A. De Jong/TFSS 2007 17 of 21 HL Biology Notes for Nerves, Muscles & Movement Muscles Humans have three types of muscle tissue: skeletal muscle is attached to bones via tendons called striated muscle because of its striped appearance under the microscope can contract quickly and powerfully but tires easily under voluntary control smooth muscle is not striated it is controlled automatically by the nervous system ■ therefore it is involuntary muscle found in the digestive tract and blood vessels takes longer to contract, but does not tire as easily as skeletal muscle cardiac muscle is found in the heart it is myogenic (beats of its own accord) and is under influence of the nervous system Images from http://biodidac.bio.uottawa.ca The Muscle Fibre Skeletal muscle is made up of thousands of cylindrical muscle fibres often running all the way from origin to insertion. The fibres are bound together by connective tissue through which run blood vessels and nerves. Each muscle fibre contains: an array of myofibrils that are stacked lengthwise and run the entire length of the fibre. mitochondria an extensive smooth endoplasmic reticulum (SER) many nuclei. The multiple nuclei arise from the fact that each muscle fibre develops from the fusion of many cells (called myoblasts). Because a muscle fibre is not a single cell, its parts are often given special names such as sarcolemma (plasma membrane), sarcoplasmic reticulum (endoplasmic reticulum), sarcosome (mitochondrion) and sarcoplasm (cytoplasm); although this tends to obscure the essential similarity in structure and function of these structures and those found in other cells. nuclei and mitochondria are located just beneath the plasma membrane the endoplasmic reticulum extends between the myofibrils. A. De Jong/TFSS 2007 18 of 21 HL Biology Notes for Nerves, Muscles & Movement Seen from the side under the microscope, skeletal muscle fibres show a pattern of cross banding, which gives rise to the other name: striated muscle. The striated appearance of the muscle fibre is created by a pattern of alternating dark A bands and light I bands. o The A bands are bisected by the H zone o The I bands are bisected by the Z line. Each myofibril is made up of arrays of parallel filaments. The thick filaments have a diameter of about 15 nm. They are composed of the protein myosin. The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins: troponin and tropomyosin. Image from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/sarcomere.gif The anatomy of a sarcomere The thick filaments produce the dark A band. The thin filaments extend in each direction from the Z line. Where they do not overlap the thick filaments, they create the light I band. The H zone is that portion of the A band where the thick and thin filaments do not overlap. The entire array of thick and thin filaments between the Z lines is called a sarcomere. Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the muscle fibre of which it is a part. Image from http://www.mrothery.co.uk/images/ Imag109.gif A. De Jong/TFSS 2007 19 of 21 HL Biology Notes for Nerves, Muscles & Movement Neuromuscular Junctions A synapse between a motor neuron and a muscle is called a neuromuscular junction: Image from http://www.emc.maricopa.edu/faculty/farabee/BIOBK/synapse.gif Muscle Contraction & Sliding Filament Theory During muscle contraction the myofilaments myosin and actin slide toward each other and overlap. This shortens the sarcomeres and the entire muscle. Muscle cells are “shocked” by nerve impulses from motor neurons. The point of attachment of the nerve to the muscle is called the neuromuscular junction. A motor neuron and its muscle cells are referred to as a motor unit. The nerve impulse is carried from the motor neuron across the gap to the sarcolemma (membrane) of the muscle cell by a neurotransmitter called acetylcholine (ACh). After the impulse has passed, an enzyme called cholinesterase deactivates acetylcholine, readying the muscle for the next nerve impulse. Stimulation of the muscle cells causes Ca++ to be released into the cell. The Ca++ binds to the actin filaments causing them to expose active sites to the myosin cross‐bridges. The cross bridges bind to the active sites, forming a new molecular structure, which causes the cross‐bridge to bend toward the centre, pulling the actin filament with it. Energy from ATP is used to break the bond, straighten the cross bridge, and allow the cross bridge to form a new bond with another active site further down the actin filament. This cycle continues until the muscle contraction is complete. Then, ATP is used to cause active transport, moving the calcium ions out of the muscle fibre, resulting in relaxation of the muscle. The Nervous System & Movement Nerves stimulate muscle contraction. Each different muscle used in locomotion must contract at the correct time, so the movement is coordinated. Since muscles are connected to bones (by tendons), contraction causes the bones to move. The movement is usually reversed by another muscle on the opposite side of the bone – an antagonistic pair. Joints are places where bones meet, and are classified by the range of motion at the joint and type of connection: o fibrous: no movement (e.g. sutures between bones of the cranium) o cartilaginous: bones connected by cartilage; limited range of motion (e.g. between vertebrae) o synovial: fluid‐filled cavities between the bones allow greater range of motion elbow: a hinge joint allows for extension & retraction hip: a ball and socket joint with a wide range of motion A. De Jong/TFSS 2007 20 of 21 HL Biology Notes for Nerves, Muscles & Movement Antagonistic Pair: Images from www.saburchill.com/chapters/chap0009.html The elbow is a typical hinge joint involving bones, cartilage, ligaments, tendons and muscles: Image from http://www.botany.uwc.ac.za/sci_ed/grade10/manphys/images/man/hinge.gif bones support the body and allow for locomotion muscles move the bones nerves stimulate muscle contraction synovial fluid protects the bones and lubricates the joint and is contained within bursa ligaments connect bones to bones tendons connect muscles to bones A. De Jong/TFSS 2007 21 of 21