Alevel Biology 2nd Year - The Nervous System Notes PDF

Alevel Biology 2nd year- Adela De Giorgio Nervous system THE NERVOUS SYSTEM 1 Alevel Biology 2nd year- Adela De Giorgio Nervous system As animals inc...

Alevel Biology 2nd year- Adela De Giorgio Nervous system THE NERVOUS SYSTEM 1 Alevel Biology 2nd year- Adela De Giorgio Nervous system As animals increased in size to multicellular organisms, they required more complex systems for coordinating and controlling the various organs and systems of the body. Animals, unlike plants have 2 different but related systems of coordination; the nervous system and the endocrine system. Irritability or sensitivity is one of the seven vital functions which all living organisms exhibit. It refers to their ability to detect and respond to a stimulus. The stimulus is received by a receptor, transmitted by means of nerves or hormones and an effector brings about a response. The nervous system has 3 overlapping functions: SENSORY INPUT: the system receives information from its environment through SENSORY RECEPTORS e.g. light-detecting cells in the eyes. INTEGRATION: The information received is INTERPRETED and ASSOCIATED with appropriate RESPONSES of the body. MOTOR OUTPUT: Signals generated on integration are transmitted to EFFECTOR CELLS (muscle or gland cells), which carry out the body’s responses to stimuli. Integration is carried out in the CENTRAL NERVOUS SYSTEM (CNS) i.e. the BRAIN and the SPINAL CORD. Sensory input and motor output are brought about by the PERIPHERAL NERVOUS SYSTEM (PNS), i.e. NERVES. 2 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Neuron Nerves are made up of bundles of cells called neurons, surrounded by connective tissue. The functions of the neurons are to: 1. Receive stimuli from the environment. 2. Convert the stimuli into the form of electrical impulses, a process called transduction. 3. Transmit them, often over considerable distances to the CNS. 4. Other neurons then transmit electrical impulses from the CNS to the effector cells. The Structure of the Neuron All neurons consist of:  Dendrites- these are thin, highly branched extensions that receive incoming stimulation and conduct electrical impulses towards the cell body. Dendrites increase the surface area of the neuron in order to receive impulses from several neurons.  A Cell Body (soma) - this is the central portion of the cell (neuron). This region contains the nucleus and other cell organelles and is where most of the neuron’s metabolic work is carried out. The cell body receives the information from the dendrites, integrates the information received and produces an output signal which is sent to the axon. 3 Alevel Biology 2nd year- Adela De Giorgio Nervous system  Axon (nerve fibre)- This is a single extension of cytoplasm that conducts impulses away from the cell body towards the next neuron. The cell body integrates the information received and produces an appropriate output signal. The axon is generally longer and thicker than a dendrite. Usually ONE axon is present on the neuron. Some axons can be quite long e.g. the cell bodies of neurons that control the muscles in your feet lie in the spinal cord and their axons may extend over a metre to your feet.  Neuroglia- these are supporting cells which are much smaller and more numerous than neurons and serve a number of functions: - they supply the neurons with nutrients. - They remove wastes from neurons. - They provide immune functions. Two important types of neuroglia cells in vertebrates are: Schwann cells and oligodendrocytes. These cells are important for providing insulation around the neurons. The axons of some neurons are encased in white, fatty myelin sheaths. Axons that have myelin sheaths are said to be myelinated axons and those that do not are unmyelinated axons. In the CNS, myelin is formed from the oligodendrocytes. In the PNS, myelin is formed from Schwann cells. During development of neurons, these cells wrap themselves around each axon several times to form the myelin sheath.  Myelin sheath- an insulating covering consisting of multiple layers of compacted membrane. This sheath permits a greater flow of impulses and thus speeds up the rate of transmission. 4 Alevel Biology 2nd year- Adela De Giorgio Nervous system  NODES OF RANVIER- are unmylenated regions found at 1 to 2 micrometres intervals along the myelin sheath.  SYNAPTIC TERMINAL/ KNOB: Specialized swellings at the branched endings of the axon. These transmit signals to other nerve cells, muscles or glands by releasing Neurotransmitters.  SYNAPSE: A specialised connection which allows the transmission of the nerve impulse between one neuron and another, or between one neuron and a muscle or gland cell Types of Neurons  SENSORY (AFFARENT) NEURON: Transmits impulses TOWARDS the central nervous system.  MOTOR (EFFERENT) NEURON: Transmits impulses AWAY from the CNS towards muscle and gland cells.  INTERNEURON: Connect sensory neurons with motor neurons WITHIN the CNS to INTEGRATE INFORMATION from many sources and co-ordinate responses. Often, motor neurons have a cell body on one end, a long axon in the middle and dendrites the axon terminal on the other end. Sensory neurons have receptors on one end, a long axon with a cell body in the middle and the axon terminal on the other end 5 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Resting Membrane Potential A potential difference exists across the plasma membrane of every cell. The inside of the cell is negatively charged with respect to the outside which is positively charged. The plasma membrane is thus polarised. The potential difference measured between these two poles (positive and negative poles) is called the membrane potential. When a neuron is not being stimulated, the membrane potential is called the resting membrane potential. The resting membrane potential of many vertebrate neurons ranges from -40 to -90 millivolts (mV). The average resting membrane potential is taken to be -70 mV. The negative sign indicates that the inside of the cell is negative with respect to the outside. 6 Alevel Biology 2nd year- Adela De Giorgio Nervous system Generating the Resting Membrane Potential 7 Alevel Biology 2nd year- Adela De Giorgio Nervous system The inside of the cell (axoplasm- cytoplasm inside the axon) is more negative with respect to the outside due to three main factors: 1. A higher concentration of molecules such as proteins, carbohydrates and nucleic acids inside the cell. Molecules such as proteins, carbohydrates and nucleic acids carry net negative charges. Because these molecules are too large to diffuse out, they are called fixed anions and they contribute to the increased negative potential inside the axon. 2. A higher concentration of Na+ ions outside the axon and higher concentration of K+ ions inside the axon due to: a. The sodium- potassium (Na+/K+) pump brings 2 potassium ions into the cell for every 3 sodium ions it pumps out. This helps to keep:  A high K+ and low Na+ concentration inside the cell.  A high Na+ and low K+ concentration outside the cell. b. Selective Permeability due to potassium leakage proteins The plasma membrane is more permeable to K+ ions than to Na+ ions. This is due to the presence of K+ leakage (channel) proteins in the membrane. Since the concentration of K+ ions is greater inside the cell (due to the Na+/K+ pumps), K+ ions leak out through these proteins (facilitated diffusion). Such leakage proteins are absent for Na+ ions. 8 Alevel Biology 2nd year- Adela De Giorgio Nervous system Another type of channel protein in the membrane exists. These are voltage gated channel proteins. These ion channels are gated i.e. they can open and close. They are voltage gated because they open and close in response to a change in voltage.  The sodium channels in the membrane are all voltage gated.  In the case of the potassium channels, there are two channel types. One type lack gates, and are always open (leakage proteins) while the others are voltage gated. The plasma membrane is thus more permeable to K+ ions than to Na+ ions. Thus since the K+ concentration is higher inside the cell than outside, K+ ions tend to diffuse out of the cells (through the potassium leakage proteins) and as a result, the concentration of positive ions outside the plasma membrane is always higher than that inside the cells. Measuring the resting potential An OSCILLOSCOPE measures the difference in electrical potential between two electrodes. When one electrode is placed inside an axon at rest and one is placed outside, the electrical potential across the cell surface is about -70mV. Graded Potentials All cells have a resting membrane potential which is maintained at about -70mV. The uniqueness of neurons in comparison to other cells is that the resting membrane potential is regularly and temporarily disrupted suddenly in response to stimuli in order to bring about conduction of nerve impulses through the neuron. These sudden disruptions are called graded potentials and are caused by the activation of the gated ion channels. The changes in permeability of the plasma membrane in response to stimuli can be measured as depolarisations or hyperpolarisations on the oscilloscope. 9 Alevel Biology 2nd year- Adela De Giorgio Nervous system A depolarisation makes the membrane potential less negative (more positive) e.g. a permeability change which causes the resting membrane potential to go from -70 mV to -65 mV. The oscilloscope will show a small upward deflection of the line which soon returns back to the resting membrane potential. A hyperpolarisation makes the membrane potential more negative e.g. a permeability change which causes the resting membrane potential to go from -70 mV to -75 mV. The oscilloscope will show a small downward deflection of the line which soon returns back to the resting membrane potential. These small changes in the plasma membrane permeability are called graded potentials because their amplitudes (size) depend on the strength of the stimulus. Depolarising and hyperpolarising potentials can add together to amplify or reduce their effects. The ability of graded potentials to combine together is called summation and is important in the generation of an action potential. The Action Potential This is a rapid change from a negative to a positive electrical potential in a nerve cell. This signal travels along an axon without a change in intensity. The action potential lasts for 2 - 4 milli seconds. 10 Alevel Biology 2nd year- Adela De Giorgio Nervous system Three phases in the action potential 1. DEPOLARISATION A stimulus (e.g. a change in pH, a touch, a signal from another neuron) causes a change in voltage on the neuron. This causes some Na+ gated channels to open completely for 0.5ms. This leads to a sudden and rapid inward rush of Na+ ions down an electrochemical gradient. As the Na+ ions flow into the neuron, the inside of the membrane becomes less negative. Hence it is DEPOLARISED (i.e. it loses its charge). These proteins only remain open for a limited amount of time since the strength of the stimulus is limited. Thus, soon after the gated channel proteins close, the sodium ions which entered, leave via the Na+/K+ pumps. If a stronger stimulus is received, the membrane potential will become even more positive since more channels are opened and more sodium enters (graded potentials). If the strength of the stimulus is strong enough to reach the threshold limit (about -55 mV), all Na+ gated channels in that area of the membrane open and the charge of the axon reaches about + 40 mV. At this stage the action potential has been produced. Threshold level/ limit is the level of depolarization needed to produce an action potential. 11 Alevel Biology 2nd year- Adela De Giorgio Nervous system 2. REPOLARISATION At this peak of the action potential, a) Na+ gated channels close. b) K+ gated channels open up for 2-3 milliseconds. K+ ions move out of the cell interior as:  K+ concentration is higher inside than outside so potassium ions diffuse out of the cell.  They are repelled from the now positively charged interior. The inside of the membrane is more positively charged than outside the cell due to the higher concentration of Na+ ions. Thus the axoplasm becomes negatively charged due to this outrush of K+ ions. The original negative charge inside the membrane is restored, but the resting potential is exceeded (i.e. the membrane potential will be more -ve than resting potential). This occurs as the outflow of K+ ions is larger than the inflow of Na+ ions as the K+ gates stay open slightly longer than the Na+ gates. 12 Alevel Biology 2nd year- Adela De Giorgio Nervous system 3. HYPERPOLARISATION This slight overshoot into a more negative potential (i.e. resulting in an undershoot) than the original resting potential is called hyperpolarisation. It is due to the slight delay in closing the potassium gates. As potassium gates close, the ion concentrations return to normal as the sodium-potassium pumps work to create the original situation and reset the membrane potential to the resting potential. Characteristics of Action Potentials 1. Action potentials are ‘all or none’ Stimuli generate action potentials. If the stimulus is below threshold level, only small local electrical changes will be detected in the neuron. No action potential results. If the stimulus is large enough to reach or surpass the threshold level, an action potential will be transmitted. The level of positive charge reached by the action potential is fixed and no matter what the strength of the stimulus is, the response will always be of the same size (all sodium channels in that region are open so the effect cannot be made greater). Action potentials thus always have a constant amplitude and are described as ALL OR NONE EVENTS. This means that the magnitude of the action potential is independent of the strength of the stimulus that produced it. 13 Alevel Biology 2nd year- Adela De Giorgio Nervous system 2. Propagation of Action Potentials An action potential is a LOCALIZED ELECTRICAL EVENT i.e. depolarisation of the neuron at a specific point. How does it ‘travel’ along the neuron? The action potential does not travel along the neuron, but it PROPAGATES (regenerates) itself along the axon. Sodium ions diffuse down the axon and cause the opening of gated channels further down the axon. Therefore, the action potential propagates itself through repeated depolarisations of immediately adjacent regions of the membrane.  As one region of the membrane becomes depolarized, the positive charge in this region acts as a depolarization stimulus for the adjacent region of the membrane to become depolarized. 14 Alevel Biology 2nd year- Adela De Giorgio Nervous system  Meanwhile the previous region of membrane repolarises back to the resting membrane potential. As propagation involves a self-regenerating event, the AMPLITUDE of the action potential remains CONSTANT. Thus, once an action potential is created, unlike a wave in water, its intensity cannot increase or decrease along the neuron. Action potentials are thus said to move with non- decremental conduction (decremental conduction occurs whereby the intensity of the wave produced from a stimulus decreases the further away from the stimulus one goes). Frequency code As the action potential is an all-or-none event, how can the nervous system distinguish strong stimuli from weaker ones? The FREQUENCY OF NERVE IMPULSES (i.e. number of nerve impulses per second) is DIRECTLY RELATED to the STIMULUS STRENGTH. Therefore, the stronger the stimulus, the greater the frequency of action potentials. This means that if a stimulus is intense, the neuron will fire more action potentials per second. The amplitude of the action potential is unaffected by the strength of the stimulus. 15 Alevel Biology 2nd year- Adela De Giorgio Nervous system The refractory period The REFRACTORY PERIOD is a period of INEXCITABILITY that follows the action potential. This means that during this time interval, the neuron cannot be depolarised. The refractory period includes two phases: 1. The absolute refractory period is the interval during which a second action potential absolutely cannot be initiated, no matter how large a stimulus is applied.  coincides with nearly the entire duration of the action potential.  caused by the closure and inactivation of the Na+ channels that originally opened to depolarize the membrane.  These channels remain inactivated until the membrane repolarises, after which they regain their ability to open in response to stimulus  sodium channels are inactivated so will not open up and there cannot be another depolarisation during this time in the same part of the axon. 2. The relative refractory period is the interval immediately following during which initiation of a second action potential is inhibited but not impossible.  The relative refractory period immediately follows the absolute.  As voltage-gated potassium channels open to terminate the action potential by repolarising the membrane, the potassium conductance of the membrane increases dramatically.  This causes brief hyperpolarisation of the membrane, that is, the membrane potential becomes transiently more negative than the normal resting potential.  Until the potassium conductance returns to the resting value, a greater stimulus will be required to reach the initiation threshold for a second depolarization 16 Alevel Biology 2nd year- Adela De Giorgio Nervous system  i.e. the membrane potential becomes more negative than -70 mV therefore it is more difficult to depolarise again.  The return to the equilibrium resting potential marks the end of the relative refractory period. Important consequences of the refractory period: 1. It sets an upper limit to the frequency. 2. Ensures unidirectional conduction. 3. Prevents action potentials from fusing into each other. Speed of Nerve Impulses The speed of conduction of nerve impulses depends on: 1. The Axon Diameter The speed of the impulse is directly proportional to the diameter of the axon. This is because axons with a wide diameter offer less resistance to the flow of ions and so conduct impulses faster. 17 Alevel Biology 2nd year- Adela De Giorgio Nervous system E.g. diameter 0.1 mm conduction velocity 0.5 ms-1. diameter 1.0 mm conduction velocity 100 ms-1. (Axons of 1mm diameter are typical of annelids, arthropods and molluscs. These giant axons are ideal for transmitting information vital to survival). 2. The Electrical Resistance of the Membrane The axons of most neurons are covered by a myelin sheath. Myelin is a fatty material and due to its high electrical resistance, acts as an electrical insulator in the same way as the rubber and plastic wiring of electrical wires. At the nodes of Ranvier, there is a break in the myelin sheath. The impulse ‘jumps’ from node to node as the myelin insulation prevents ion flow across the membrane, but a small electrical current spreads instantly between nodes since ions can flow in the regions of the nodes of Ranvier. The velocity at which action potentials spread is thus increased since less time is wasted in producing many action potentials along the neuron. Instead, an action potential is created only in the regions of the Nodes of Ranvier. An action potential thus travels along a myelinated axon by ‘jumping’ from one node of Ranvier to another. This type of transmission is called SALTATORY CONDUCTION. Saltatory conduction jumps the nonconducting ‘gaps’ of myelinated axon and thus speeds up transmission of nerve impulses. E.g. A myelinated axon of 0.01 mm diameter conducts impulses at 120 ms-1. (i.e. an impulse travels from toe to spinal cord in less than 1/100 of a second). 3. Temperature  Velocity of conduction increases with an increase in temperature. (since a higher temperature gives ions energy and so the sodium ions will diffuse faster down the axon) E.g. frog (ectotherm) axon diameter 3.5µm velocity 30ms-1. E.g. cat (endotherm) axon diameter 3.5µm velocity 90ms-1. 18 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Synapse A synapse is a specialised link between one neuron and another. It is a functional but not a physical contact between 2 neurons for the purpose of transferring information. The small gap between one neuron and the next is called the synaptic cleft. There are two main types of synapses:  CHEMICAL SYNAPSES  ELECTRICAL SYNAPSES NEUROMUSCULAR JUNCTIONS are specialized types of synapses which link motor neurons with skeletal muscle fibres. Chemical Synapse A chemical synapse involves the axon of 1 neuron and the dendrite of another neuron. SYNAPTIC (TERMINAL) KNOB: Swelling at the end of a nerve fibre, lying close to the membrane of a dendrite. Cytoplasm of knob typically contains a lot of mitochondria and synaptic vesicles. SYNAPTIC VESICLES: small sacs that hold neurotransmitter molecules. PRE-SYNAPTIC MEMBRANE: thickened membrane of synaptic knob belonging to the neuron transmitting the message. This membrane is modified for the attachment of synaptic vesicles and the release of neurotransmitter into the synaptic cleft. SYNAPTIC CLEFT: a narrow gap between the presynaptic membrane and the post-synaptic membrane. POST-SYNAPTIC MEMBRANE: thickened membrane of the dendrite found on the neuron receiving the neurotransmitter molecules. 19 Alevel Biology 2nd year- Adela De Giorgio Nervous system 20 Alevel Biology 2nd year- Adela De Giorgio Nervous system Synaptic transmission 1. The action potential moves down the axon and arrives at the synaptic knob, whose structure creates more space to accommodate vesicles. 2. The action potential stimulates the opening of voltage- gated Ca2+ channels and thus, Ca2+ ions rush into the neuron, flooding the synaptic knob. 3. This rise in concentration of Ca2+ leads to fusion of the synaptic vesicles with the pre-synaptic membrane. 4. Neurotransmitter molecules are thus released into the synaptic cleft by exocytosis. 5. Neurotransmitter molecules diffuse across the synaptic cleft (which is narrow so diffusion distance is short) until they reach the post-synaptic membrane. 6. Neurotransmitter molecules bind to receptor proteins on post-synaptic membrane. There are different types of neurotransmitters which act in different ways, producing different effects in the neuron receiving it. Transmission across synapses introduces a delay of 0.5m sec. 21 Alevel Biology 2nd year- Adela De Giorgio Nervous system Types of Chemical Synaptic transmission Channel proteins in the post-synaptic membrane determine the type of responses produced by the post-synaptic neuron. This is due to ion selectivity (i.e. ability to allow only some ions to pass through). Excitatory Synapse At an excitatory synapse, ion-specific channels OPEN up allowing Na+ ions to enter and K+ ions to leave through the post-synaptic membrane. This leads to depolarisation (of the post-synaptic neuron) and is known as an EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP).  An EPSP is a graded response which moves with decremental conduction.  A single EPSP is generally too small to reach the threshold required to propagate an action potential.  However EPSPs can add up to each other- summation. Inhibitory synaptic transmission At an inhibitory synapse, ion-specific channels open up allowing Cl- ions to enter and K+ ions to leave through the post-synaptic membrane. This leads to hyperpolarisation (of the post-synaptic neuron) and is known as the INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP) Thus, the inside of the neuron becomes more negative (-90V), DECREASING the chance of propagating an action potential. An IPSP is also a graded response which moves with decremental conduction. Once a response has been set up in the dendrite (EPSP or IPSP), the neurotransmitter molecules in the synaptic cleft are inactivated or destroyed. 22 Alevel Biology 2nd year- Adela De Giorgio Nervous system In summary: EXCITATORY SYNAPSES INHIBITORY SYNAPSES Response Depolarisation Hyperpolarisation Name of response Excitatory Post synaptic potential Inhibitory post synaptic potential (I.P.S.P.) (E.P.S.P.) Permeability Changes Na+ and K+ channels open Cl- and K+ channels open Outcome Increased excitability Decreased excitability Nervous/ Synaptic Integration The activity of a post- synaptic neuron in the brain and spinal cord is influenced by different types of input (EPSPs and IPSPs) coming from a number of presynaptic neurons i.e. a single postsynaptic neuron may receive impulses from a number of excitatory and inhibitory presynaptic neurons. This is known as convergence of neurons. The EPSPs and IPSPs from these synapses interact with eachother when they reach the cell body of the neuron. The postsynaptic neuron is able to summate the stimuli from all the presynaptic neurons supplying it. 23 Alevel Biology 2nd year- Adela De Giorgio Nervous system Summation occurs at the axon hillock i.e. the region of the cell body at the base of the axon.  EPSPs and IPSPs from the dendrites and cell body spread to the axon hillock.  These are ADDED UP ALGEBRAICALLY and if the threshold is reached, an action potential results.  Small EPSPs add together to bring the membrane closer to the threshold while IPSPs subtract from the depolarizing effect of the EPSPs, deterring the membrane potential from reaching threshold. 24 Alevel Biology 2nd year- Adela De Giorgio Nervous system There are two types of summation (or synaptic integration) processes: 1. SPATIAL SUMMATION: Several EPSPs and IPSPs arriving from different presynaptic neurons at the same time are summated. 2. TEMPORAL SUMMATION: Two or more EPSPs/ IPSPs arriving in rapid succession from the same presynaptic neuron are summated. This is the rapid, repeated release of neurotransmitter from several synaptic vesicles by the same synaptic knob. 25 Alevel Biology 2nd year- Adela De Giorgio Nervous system Neurotransmitters The PNS uses only 2 neurotransmitters: Acetylcholine (Ach) and Noradrenaline  Acetylcholine - neurons releasing acetylcholine are described as cholinergic neurons. - neurotransmitter released by parasympathetic pathway - Is the only neurotransmitter used at a neuromuscular junction  Noradrenaline - neurons releasing noradrenaline are described as adrenergic neurons. - used as a neurotransmitter by sympathetic ganglia- causing increases in heart rate, blood pressure, triggers release of glucose from energy stores etc. 26 Alevel Biology 2nd year- Adela De Giorgio Nervous system The CNS uses about 50 neurotransmitters. All these different neurotransmitters have different effects on the brain, and thus affect behaviour or on different parts of the body. Rapid inactivation or destruction of the neurotransmitter is important to ensure that : 1. Its effect on the post-synaptic cell is brief and precise. 2. The next action potential arriving at the synapse will be transmitted. Neurotransmitters are removed from the synaptic left in 3 possible ways:  Hydrolysed by enzymes e.g. acetylcholine is degraded by acetylcholinesterase.  Reabsorption by the presynaptic membrane.  Diffusion out of the synaptic cleft. Effect of drugs on neurotransmitters Drugs are chemicals and some of these may alter the effects of neurotransmitters, leading to excessive stimulation or inhibition of stimulation.  Some drugs mimic neurotransmitters and bind to their receptors; e.g. Nicotine mimics the effect of acetylcholine on some of the receptors which acetylcholine normally binds to. These receptors are called nicotinic receptors. They are found in both the sympathetic and parasympathetic nervous systems. This leads to: o Strong sympathetic vasoconstriction in abdominal organs e.g. gut, and in the limbs. o Parasympathetic effects- e.g. increased gastrointestinal activity and slowing of the heart. 27 Alevel Biology 2nd year- Adela De Giorgio Nervous system Nicotinic receptors are also found at neuromuscular junction and thus: o Makes muscles contract.  Amphetamines are stimulants that are primarily used to treat narcolepsy (a neurological condition most characterized by Excessive Daytime Sleepiness (EDS) and ADHD (Attention Deficit Hyperactivity Disorder). It is also used recreationally as a club drug and as a performance enhancer. Amphetamines exert their effects by increasing extracellular levels of the neurotransmitters dopamine (controls body movements) and norepinephrine (noradrenaline) (released by synapses of sympathetic nervous system) in the synaptic cleft. e.g. Amphetamines increase level of noradrenaline by: - inhibiting the enzyme that removes noradrenaline from synapses. These drugs enhance the normal effects of noradrenaline e.g. increasing alertness, help to maintain the state of arousal. - raising levels of noradrenaline at sympathetic nerve endings. Functions of Chemical Synapses Chemical synapses slow down nerve impulses by about 0.5 ms per synapse. This may seem as a disadvantage but the advantages of the synapse far outweigh this disadvantage: 1. UNIDIRECTIONAL MOVEMENT- this is ensured since neurotransmitter is only released at the presynaptic membrane and it is only the postsynaptic membrane which contains receptors for binding the neurotransmitter. 2. ADAPTATION- Following constant stimulation, the amount of neurotransmitter released decreases until the supply is exhausted and the synapse is said to be fatigued. Further passage of information along this pathway is inhibited for a period of time (recovery). This prevents damage to an effector due to overstimulation. 3. DISCRIMINATION- Due to summation, weak background stimuli can be filtered out before they reach the brain e.g. background noise. 28 Alevel Biology 2nd year- Adela De Giorgio Nervous system 4. INTEGRATION- Spatial summation of several EPSPs and IPSPs enables the synapse to integrate stimuli from several sources, and produce a coordinated response. 5. FACILITATION- If 2 action potentials arriving at the presynaptic knob are close together (in time) the 2 nd EPSP produced in the dendrite of the next neuron will be larger than expected. This is because some neurotransmitter would be left over in the synaptic left from the first stimulation and thus, a greater EPSP will be generated. This occurs at some synapses and involves each stimulus leaving the synapse more responsive to the next stimulus. 6. INHIBITION – Transmission may be prevented in response to neurotransmitters or drugs. Inhibition at synapses enables nerve terminals to give a variable response. Electrical synapses Nerve impulses flow directly from pre-synaptic to post-synaptic neurones via gap junctions. These synapses involve connexion proteins which form intercellular channels. These allow the local ion currents of an action potential to flow between neurons. Features of electrical synapses:  Short delay in transmission- v. fast conduction.  They are always excitatory.  Not affected by drugs.  Allow bidirectional transmission (not at the same time).  Bridge 2 neurons. Electrical synapses do not allow modulation of the response and occur where speed of conduction is imperative. They are very common in primitive organisms such as in invertebrates. They are also present between muscle cells such as cardiac muscle and smooth muscle cells. 29 Alevel Biology 2nd year- Adela De Giorgio Nervous system Electrical and chemical synapses differ fundamentally in their transmission mechanisms. (A) At electrical synapses, gap junctions between pre- and postsynaptic membranes permit current to flow passively through intercellular channels (see blowup). This current flow changes the postsynaptic membrane potential, initiating (or in some instances inhibiting) the generation of postsynaptic action potentials. (B) At chemical synapses, there is no intercellular continuity, and thus no direct flow of current from pre- to postsynaptic cell. Synaptic current flows across the postsynaptic membrane only in response to the secretion of neurotransmitters which open or close postsynaptic ion channels after binding to receptor molecules 30 Alevel Biology 2nd year- Adela De Giorgio Nervous system Neuromuscular junctions This is a specialised synapse found between a motor neuron and a skeletal muscle fibre. It is the synapse which leads to stimulation of skeletal muscle fibre contraction. 1. Axon branches repeatedly to form a MOTOR END PLATE. The branches end in synaptic knobs. 2. The fine branches of the motor end plate lie in shallow infolded depressions of the cell surface membrane of the muscle fibre (called the sarcolemma). 3. On stimulation, the synaptic knobs release acetylcholine which binds to receptors on the folds of the sarcolemma, producing a depolarization of the sarcolemma due to increased permeability to Na+ and K+ ions. This depolarization is known as an END PLATE POTENTIAL (EPP). 4. The action potential in the sacrolemma, spreads through the muscle fibre bringing about muscle contraction. 31 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Nervous System Organisation The nervous system is divided into 2 main parts:  The Central Nervous System (CNS)- Brain and Spinal cord  The Peripheral Nervous System (PNS). The PNS is divided into: Somatic nervous system- under voluntary control from the brain. Autonomic nervous system- operates automatically. The Autonomic Nervous system is divided into: Sympathetic nervous system (SNS)- mainly excitatory effect on the body. Parasympathetic nervous system (PNS)- mainly calming effect on the body. Acts antagonistically to (has opposite effect on) the SNS. 32 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Peripheral Nervous System The PNS consists of all the nerves of the body together. All these nerves are connected to the CNS and thus either enter (in the case of sensory neurons) or leave (in the case of motor neurons) the brain or spinal cord. Spinal nerves: - Arise from the spinal cord between the vertebrae, along most of the length of the spinal cord. - All carry both sensory and motor neurons and are therefore mixed nerves. Cranial nerves: - Arise from the brain. They are attached to the brain and are largely concerned with the head, neck and facial regions of the body. They may be: - Sensory only e.g. olfactory, optic & auditory nerves. - Motor only e.g. fibres controlling speech & swallowing. - Mixed nerves e.g. VAGUS NERVE which branches to the heart (decreases heart rate), gut (stimulates peristalsis) and part of respiratory tract (concerned with speech and swallowing). 33 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Somatic System The somatic nervous system is that part of the peripheral nervous system associated with the voluntary control of body movements. This includes all nerves that serve the MUSCULO-SKELETAL SYSTEM and the exterior sense organs, including those in the skin. The exterior sense organs are the RECEPTORS that detect ENVIRONMENTAL STIMULI and then start nerve impulses e.g. pain receptors. MUSCLE FIBERS and GLANDS are EFFECTORS, which bring about a reaction to a stimulus. 34 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Autonomic Nervous System This part of the nervous system controls activities inside the body that are normally involuntary e.g. heart rate, peristalsis, breathing, sweating etc. The autonomic nervous system consists of sensory neurons and motor neurons that run between the central nervous system (especially the hypothalamus and medulla oblongata) and various internal organs. The motor neurons pass to the smooth muscle of these internal organs and bring about appropriate responses (smooth muscles are involuntary muscles). 35 Alevel Biology 2nd year- Adela De Giorgio Nervous system The autonomic nervous system is thus responsible for monitoring conditions in the internal environment and bringing about appropriate changes. The A.N.S. comprises two, ANTAGONISTIC sub- divisions. These are the:  SYMPATHETIC (thoraco-lumbar) division  PARASYMPATHETIC (cranio-sacral) division. BOTH these systems:  Function automatically and subconsciously.  Innervate internal organs.  Utilize TWO MOTOR NEURONS and ONE GANGLION for each impulse. A ganglion is a swelling consisting of an accumulation of cell bodies. Unlike the somatic system, the autonomic system uses two groups of motor neurons to stimulate the effectors instead of one.  The first, the preganglionic neurons, arise in the CNS and run to a ganglion in the body. Here they synapse with  postganglionic neurons, which run to the effector organ (cardiac muscle, smooth muscle, or a gland). The preganglionic neuron consists of a cell body in the CNS and a myelinated preganglionic fibre. The postganglionic neuron has the cell body in the ganglion and a non- myelinated postganglionic fibre. 36 Alevel Biology 2nd year- Adela De Giorgio Nervous system Sympathetic and Parasympathetic Nervous Systems The SYMPATHETIC SYSTEM brings about those responses associated with ‘FIGHT OR FLIGHT’, whilst the PARASYMPATHETIC SYSTEM brings about the responses associated with a RELAXED STATE- REST and DIGEST. CHARACTERISTIC SYMPATHETIC PARASYMPATHETIC Origin of preganglionic Middle portion of spinal cord Brain and lower portion of spinal neuron chord. Position of ganglion Close to spinal cord Close to effector Length of fibres Short preganglionic fibres Long preganglionic fibres Long postganglionic fibres Short postganglionic fibres Transmitter released at Noradrenaline Acetylcholine effector Condition when active Organism is under stress Organism is relaxed General effects Prepares organism for vigorous Promotes normal metabolic activity processes. 37 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Sympathetic and Parasympathetic Nervous Systems have opposing (antagonizing) effects on the organs they supply. This enables the body to make rapid and accurate adjustments of involuntary activities in order to maintain a steady state. E.g. if the heart rate is increased due to release of noradrenaline by sympathetic neurons, this is compensated for by the action of parasympathetic neurons which release acetylcholine. This prevents the heart rate from becoming excessive and will restore it to its normal level. 38 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Central Nervous System This comprises the brain and spinal cord, and is responsible for the co-ordination and control of the activity of the nervous system. The CNS is surrounded by 3 layers or membranes called meninges and is completely encased within the protective bones of the skull and vertebral column. The spaces between the meninges are filled with:  strands of connective tissue  blood vessels  cerebrospinal fluid (CSF) CSF is also present within the CENTRAL CANAL of the spinal cord and the 4 VENTRICLES (expanded cavities) within the brain. Thus the CSF is in contact with both the inside and the outside of the spinal cord and the brain. Blood vessels within it supply the brain with oxygen and nutrients whilst lymphocytes protect against infection. The CNS is composed of two types of matter: GREY MATTER- composed of nerve cell bodies, dendrites and synapses. WHITE MATTER- composed of nerve fibres whose fatty myelin sheaths give this type of matter its characteristic colour. 39 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Spinal Cord This is a CYLINDER of nervous tissue running from the base of the brain down the back. It is protected by the vertebrae and the meninges. The central area of the spinal cord consists of grey matter and is H- shaped. This grey matter surrounds a central canal which contains cerebrospinal fluid. Around the grey matter is an outer layer of white matter. The white matter consists of:  cables of sensory axons in the dorsal column region (ascending tracts).  cables of motor axons in the ventral column region (descending tracts). These nerve tracts relay information between the brain and the body, up and down the spinal cord. Ascending tracts relay information to the brain while descending tracts relay information to the body. 40 Alevel Biology 2nd year- Adela De Giorgio Nervous system Spinal nerves divide close to the spinal cord to form 2 branches: The dorsal root and the ventral root.  Sensory neurons enter the dorsal root and have their cell bodies located in a ganglion called the dorsal root ganglion, close to the spinal cord.  The sensory neurons then enter the dorsal horn of the grey matter where they synapse with interneurons (relays message from sensory neurone to motor neurone).  Interneurons synapse with motor neurons in the ventral horn.  In some cases, the sensory neuron synapses directly with a motor neuron and not via an interneuron e.g. knee- jerk reflex. Thus, the functions of the spinal cord are the following:  It acts as a minor co-ordinating centre for SIMPLE REFLEX RESPONSES, e.g. knee-jerk response, and AUTONOMIC REFLEXES e.g. contraction of bladder.  It provides a means of communication between the spinal nerves and the brain via nerve tracts. Reflex Actions and Reflex Arcs The simplest form of response in the nervous system is the reflex action. A reflex action is a rapid, automatic, involuntary and stereotyped (the same stimulus produces the same response every time) response to a stimulus. Reflexes may be:  Spinal reflexes- reflex actions which occur in regions below the brain (in the spine) e.g. withdrawing hand from a sharp object.  Cranial reflexes- reflex actions taking place in the brain. These concern the organs in the head e.g. blinking of eye. The nervous pathway taken by the reflex action is called the reflex arc. It is a neural pathway that links a sensory receptor to an effector such as a muscle. 41 Alevel Biology 2nd year- Adela De Giorgio Nervous system Reflex arcs can be: 1. MONOSYNAPTIC: (only one synapse) e.g. postural reflexes, knee- jerk reflex. 2. POLYSYNAPTIC: (several synapses) eg. Pain- withdrawal reflex. Monosynaptic reflexes:  Involve only 2 neurons, a sensory and motor neuron.  Are rapid and independent of the brain. Polysynaptic reflexes:  Possess interneurons which link sensory with motor neurons.  Reach the conscious level. Knee- jerk reflex (Monosynaptic reflex arc) The patellar reflex or knee jerk is a monosynaptic reflex. 1. Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps tendon. 2. This stimulates stretch sensory receptors that trigger an afferent impulse in a sensory nerve fibre leading the spinal cord. 3. There, the sensory neuron synapses directly with a motor neuron that conducts an efferent impulse to the quadriceps muscle, triggering contraction. This contraction causes the leg to kick. 42 Alevel Biology 2nd year- Adela De Giorgio Nervous system The Pain - Withdrawal Reflex (Polysynaptic reflex arc) 1. Fingertip detects a sharp thorn/ heat from hot object etc. due to PAIN-SENSITIVE receptor at the tip of a sensory neuron. 2. An AP is generated in the sensory neuron and transmitted to the spinal cord. 3. In the spinal cord the impulse is propagated to an interneuron within the grey matter. 4. The action potential is relayed to a motor neuron. 5. This causes contraction in the muscle, and thus pulling away of the hand from the thorn’ hot object. 43