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neurones better
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- 1What is a stimulusa detectable change in the internal or external environment of an organism that produces a response in that organism.
In mammals, responses to many external and internal stimuli involve the reception of information and its transfer from a receptor to an effector via the nervous system or as hormones via the blood. - 2Neurones descriptionSpecialized cells adapted to rapidly carry nervous impulses from one body part to another- 3 types of neurones. Categorised into 3 main groups- sensory relay and motor
- 3Common features of neuronsAll 3 have these common features:
Cell body- contains organelles found in typical animal cell, Proteins and neurotransmitter chemicals made here
Axons-Thin fibre carrying impulses away from cell body, a cell body only has one axon.
Dendrons-Thin fibre carrying impulses towards the cell body
-carries action potentials to surrounding cells - 4Common features pt2- schwann cells/myelin sheathSchwann cells surround and support nerve fibres. In vertebrate embryos, they wrap around the developing axons many times and withdraw their cytoplasm, leaving a multi latyered phospholipid myelin sheath- lipid- doesn’t allow charged ions to pass through it- electrical insulator- speeds up transmission of impulses
- 5Common features pt 3- nodes of ranvier1 um gaps in myelin sheath, where adjacent Schwann cells meet and where the axon membrane is exposed- allows impulses to be transferred rapidly
- 6synaptic knob functionThe synaptic knob is a swelling which contains synaptic vesicles. The synaptic knob is the location where the nerve impulse is transmitted across the synpatic cleft. There are also lots of mitochondria in the synaptic knob. This is because lots of energy is needed to synthesise neurotransmitters.
- 7Axon ending/terminal function/ other partsBranched endings of axon that approach muscle fibre
Secretes neurotransmitter which transmits impulse to adjacent neurone
Cell body-contains a nucleus and granular cytoplasm
Cytoplasm-Granular-- contains many ribosomes
Nucleus- Holds DNA
Nissl granules-Cytoplasmic granules comprising ribosomes grouped on ER - 8Sensory neuronecarries impulses from sense receptors or organs into CNS- long dendron- carries impulse from sensory receptor to cell body of neurone then an axon to carry impulse to next neurone
(look at diagram to understand) - 9Relay neuroneRelay, connector or association- receive impulses from sensory neurones or other relay neurones and transmit them to motor or other relay neurones- multiple short axons and dendrons
- 10Sensory receptorSensory receptors give an organism its senses. There are specialised sensory cells eg pressure sensors in the skin and in complex sense organs eg ear and eye.
- 11Sensory receptor-morewhen a sensory receptor detects a stimulus- produces a nervous impulse- electrical impulses pass down neurone to CNS- does this by converting one form of energy into a nervous impulse- called a transducer
- 12The nervous system has 2 partsThe human nervous system consists of the:
Central nervous system (CNS) – the brain and the spinal cord
Peripheral nervous system (PNS) – Pairs of nerves that originate from CNS and carry impulses into and out of the CNS
It allows us to make sense of our surroundings and respond to them and to coordinate and regulate body functions - 13- part 1- CNSThe central nervous system= brain and spinal cord
CNS processes information provided by a stimulus. Both the brain and spinal cord are surrounded by tough membranes-collectively called meninges - 14CNS- spinal cord descriptionwhite matter contains nerve fibres surrounded by myelin which is fatty- makes white matter look white
Grey matter has much less myelin- is largely nerve fibres of relay neurones and cell bodies of relay and motor neurones - 15Basic pattern of spinal nerves- dorsal and ventral rootSpinal nerves are an integral part of the peripheral nervous system (PNS). They are the structures through which the central nervous system (CNS)- receives sensory information from the periphery, and through which the activity of the trunk and the limbs is regulated- Also transmit the motor commands from CNS to muscles of the periphery- composed of both motor, sensory fibres and autonomic fibres
They begin as nerve roots that emerge from a segment of the spinal cord at a specific level. Each spinal cord segment has four roots: an anterior(ventral) and posterior (dorsal) root on both right and left sides. - 16VentralThe ventral root contains efferent nerve fibres-carry stimuli away from the CNS towards their target structures. The cell bodies of the ventral root neurons are located in the central grey matter of the spinal cord. Motor neurons controlling skeletal muscle, as well as preganglionic autonomic neurons are located in the anterior roots.
- 17DorsalThe dorsal root contains afferent nerve fibres, which return sensory information from the trunk and limbs to the CNS. The cell bodies of dorsal root neurons are located in structure called the spinal/dorsal root ganglion. The anterior and posterior roots join to form the spinal nerve proper, containing a mixture of sensory, motor, and autonomic fibers.
- 18PNScomprises:
-somatic nervous system eg pairs of nerves that originate in the brain or the spinal cord and their branches - 19PNS pt2These nerves contain the fibres of sensory neurones which carry impulses of receptors to the CNS and motor neurones which carries impulses away from CNS to effectors
Also has autonomic nervous system provides unconscious control of the functions of internal organs eg heartbeat, digestion - 20Nervesnformation is sent through the nervous system as nerve impulses – electrical signals that pass along nerve cells known as neurones
A bundle of neurones is known as a nerve
Neurones coordinate the activities of sensory receptors (eg. those in the eye), decision-making centres in the central nervous system, and effectors such as muscles and glands - 21hh
- 22Stimulus= visible lightSensory receptor location: retina
Sense: sight - 23Stimulus: soundSensory receptor location: inner ear
Sense: hearing - 24stimulus: pressureSensory receptor location: dermis of the skin
Sense: touch - 25Stimulus: heavy pressureSensory receptor location: deeper in dermis of the skin
Sense: pain - 26Stimulus: chemical pt1Sensory receptor location: nose
Sense: smell - 27Stimulus: chemical pt2Sensory receptor location: tongue
Sense: taste - 28Stimulus: temperatureSensory receptor location: dermis of skin
Sense: temperature - 29Stimulus: gravitySensory receptor location: middle ear
Sense: balance - 30Reflex ActionRapid, automatic response from nervous impulses initiated by a stimulus. Decision making areas of brain not involved/ involuntary. Reflex action= generally protective.
- 31Reflex ArcSimplest type of nervous response to stimulus= reflex arc
Neural pathway taken by nervous impulse of reflex action
eg withdrawal effect-instantly withdraw hand - 32Elements of reflex arcThe transmission of a
nerve impulse along a three-neurone reflex arc involves
the following: stimulus ---> receptor ----> sensory neurone ----> relay neurone (in CNS) -----> motor neurone -----> effector-----> response. - 33Reflex arc- tempstimulus- heat
sensory receptor- temp and pain receptors in the skin
sensory neurone-sends impulse up the arm to the spinal cord
CNS-relay neurone in spinal cord transmits an impulse from sensory neurone to a motor neurone
Motor neurone- sends impulse to an effector- in this case a muscle
Response- arm muscles contract and hand is removed from heat source - 34Reflex arc- example pupil reflexstimulus- light
sensory receptor- photosensitive cells in retina
sensory neurone- optic nerve
CNS-brain
Motor neurone- carries impulse to muscles of iris
response- iris muscles relax/ contract- altering pupil diameter - 35nervous impulse- resting potentialNeurone= an excitable cell- means it can change its resting potential
resting potential- When a neuron is not conducting an impulse there’s a difference between electrical charge (potential difference) on the inside and outside of the neurone (across the membrane) known as resting potential - 36nervous impulse- potential differencePotential difference across a cell membrane = 70 mv- membrane is more negative inside so resting potential is -70 mv- potential difference across cell membrane means it's polarised
- 37reason for resting potential 1to maintain the resting potential, sodium-potassium pumps actively transport sodium ions out of the neurone and potassium ions into the neurone
(3Na+ out for each 2K+ in, per ATP hydrolysed);
• voltage-gated Na+ channels are closed but some K+ channels allow K+ to ‘leak’ out of the axon
• large protein anions and organic phosphates (e.g. ATP4-) remain in the cytoplasm thus producing a
negative potential difference across the membrane at around -70mV relative to the exterior of the axon; - 38What are sodium- potassium exchange pumpsTrans membrane proteins with ATPase activity transport K+ and Na+ ions against conc gradient by active transport- maintains conc and uneven distribution of ions across membranes.
- 39Action Potential(The rapid rise and fall of electrical potential across a nerve cell membrane as a nervous impulse passes)
The change from -70 to +40 to -70 again at one node- means that node is polarised-depolarised-repolarised-hyperpolarised-polarised- when you get to depolarised bit (reaches +40mv- triggers next bit of axon to open gates)- causes nervous impulse - 40Difference between action potential and nervous impulseAP- the process of the voltage change at one node- nervous impulse- the transmission of voltage change along entirety of axon
- 41Oscilloscope functionA membrane potential is the difference in charge between one side of a membrane and the other, sometimes described as the potential difference,
An oscilloscope is a type of electronic test instrument that graphically displays voltage across membrane changes with time. Measures magnitude and speed of impulse transmission and helps us analyse patterns of impulses generated in different parts of the nervous system in different situations
The display produced is like a graph with time in milliseconds on the x-axis and the membrane potential in millivolts on the y-axis - 42conclusions drawn using oscilloscopeNeurones transmit electrical impulsesalong cell surface membrane surrounding axon- put microelectrode inside axon and one in bathing solution to find:
a change in the voltage across the axon membrane (a stimulus) opens the Na+ channels so that Na+ flood in and depolarises the axon to about +40mV
– this is the action potential - 43conclusions drawn using oscilloscope pt 2repolarisation occurs as the Na+ channels close and K+ channels open resulting in K+ flooding out of the axon and reducing the potential difference across the axon membrane – an overshoot results in the membrane being hyperpolarised;
during the refractory period the concentrations of K+ and Na+ ions are restored to that of the resting potential – during this time the axon cannot transmit another action potential and that this ensures that transmission is in one direction only - 44Depolarisation definitionA temporary reversal of potential across the membrane of a neurone so the inside becomes less negative than the outside as an action potential is transmitted
- 45Absolute Refractory period- sodium channels- are inactivated- can't open again until resting potential has been re-established- so new action potential can be initiated this is the absolute refractory period-lasts around 1ms-ensures nervous impulse travels in 1 direction
- 46How action potential travels along axon+40- sodium channels- are inactivated- can't open again until resting potential has been re-established- but that+40 triggers gates closest- so second site opens- triggers either side- but previous site has gates closed- can't be retriggered till resting potential is restored (time section can't be triggered= absolute refractory period)- travels one way down axon
- 47Relative refractory periodfor next 3-4ms during hyperpolarisation if an impulse is strong enough a new action potential may pass - occurs while sodium and potassium pumps are restoring resting potential.
- 48Properties of nerves/impulses- 'All or nothing'An action potential is a rapid sequence of changes in the voltage across a membrane. Any stimulus that brings depolarization to -55mv will peak at same max voltage- bigger stimulus causes greater frequency of action potentials= the change of voltage from -70mv to + 40mv across the membrane
no energy lost in transmission- Allows the action potential to act as a filter, prevents minor stimuli from setting up nervous impulses- so brain isn't overloaded with info - 49Factors affecting speed of conduction of nerve impulse- tempTemp- ions move faster at high temperatures than lower temps- more kinetic energy- birds/ animals (warm blooded) transmit nervous impulses more quickly- faster responses
- 50Factors affecting speed of conduction of nerve impulse-Diameter of axonGreater diameter of axon- greater volume in relation to the area of the membrane- more sodium ions can flow through the axon, so impulses travel faster-
- 51Factors affecting speed of conduction of nerve impulse-Diameter of axon pt 2human non-myelinated axons= 0.2-1.5 um diameter- such dimensions= very slow action potential transmission especially at low temperatures- some marine habitats evolved in very low temps so to compensate squid has giant axons up to 1mm diameter- earthworm- even though warm blooded has large axons for evolved escape responses
- 52Factors affecting speed of conduction of nerve impulse- myelinationSpeeds up rate of rate of transmission- sodium ions flow through the axon- but myelinated nerve fibre only depolarises where resistance is low (at nodes of ranvier)-whole process of depolarization, repolarization, hyperpolarization and refractory period happens at each node goes all along a motor neurones axon until a synapse is reached-unmyelinated axons have voltage-gated sodium channels along the entire length of the membrane. In contrast, myelinated axons have voltage-gated sodium channels only in the nodal spaces.
- 53What you see on oscilloscopeInstead of repolarisation causing the membrane potential to return immediately to the normal resting potential of -70 mv, the trace often shows a short period of hyperpolarisation
This is when the membrane potential briefly becomes more negative than resting potential - 54What you see on an oscilloscope pt 2Instead of repolarisation causing the membrane potential to return immediately to the normal resting potential of -70 mv, the trace often shows a short period of hyperpolarisation
This is when the membrane potential briefly becomes more negative than resting potential - 55Role of synapsea very small gap, known as the synaptic cleft, separates them
The ends of the two neurones, along with the synaptic cleft, form a synapse
This stimulates the postsynaptic neurone to generate an electrical impulse that then travels down the axon of the postsynaptic neurone- A cholinergic synapse is one that uses the neurotransmitter acetylcholine. - 56Synaptic transmission pt 1Action potential arrives. When an action potential reaches the synaptic knob, the membrane of the pre-synaptic neurone changes permeability by opening voltage-dependent calcium channels.
Calcium ions rush into the pre-synaptic neurone. Calcium ions diffuse into the synaptic knob down the concentration gradient.
The synaptic vesicles fuse with the pre-synaptic membrane. The excess of calcium ions cause the vesicles to move towards and then fuse with the membrane, releasing acetylcholine (the contents of the vesicles) into the synaptic cleft. This process is exocytosis. - 57hSynaptic transmission pt2Acetylcholine diffuses across the synaptic cleft. The neurotransmitters diffuse down the concentration gradient.
Acetylcholine binds with receptors in the post-synaptic membrane. These receptors change shape, opening a channel to allow sodium ions to diffuse in. As the receptors are only present on the post-synaptic membrane, the impulse cannot travel back, so it is unidirectional.
Sodium ions rush into the post-synaptic neurone. If enough sodium ions diffuse in, the action potential threshold is reached and this triggers depolarisation of the neurone.
An action potential in the post-synaptic neurone is generated. - 58Impact of drugs pt1Many drugs impact the nervous system by altering the events that occur at synapses
Drugs can increase transmission of impulses at a synapse and so increase frequency of APs at post synaptic (stimulants) by:
Causing more neurotransmitter to be produced in the synaptic knob
Causing more neurotransmitter to be released at the presynaptic membrane
Imitating the effect of a neurotransmitter by binding to and activating receptors on the postsynaptic membrane
Preventing the breakdown of neurotransmitters by enzymes
Preventing the reuptake of neurotransmitters by the presynaptic cell - 59Impact of drugs pt2Drugs can decrease transmission of impulses at a synapse (sedatives) by
Preventing production of neurotransmitter in the presynaptic knob
Preventing the release of neurotransmitter at the presynaptic membrane
Enabling neurotransmitter to gradually leak out of the presynaptic knob so there is little left when an action potential arrives
The neurotransmitter that leaks out of the cell is destroyed by enzymes
Binding to receptors on the postsynaptic membrane and so preventing neurotransmitters from binding - 60NicotineNicotine is the addictive chemical found in tobacco
Nicotine affects synapses in more than one way
It mimics acetylcholine
Nicotine binds to a type of acetylcholine receptor on the postsynaptic neurone known as a nicotinic receptor
The binding of nicotine to nicotinic receptors initiates an action potential in the postsynaptic neurone
After stimulation by nicotine these receptors become unresponsive to other stimulation
While it is normal for receptors to be briefly unresponsive to further stimulation after being activated, nicotine causes a prolonged period of unresponsiveness - 61Nicotine how you become addictedIt stimulates release of dopamine
Dopamine is released from the pleasure centres in the brain in response to nicotine
The release of dopamine is thought to reinforce rewarding behaviours, in this case smoking
Unlike acetylcholine- nicotine isn’t removed by hydrolysis- continues to initiate impulses- may become habituated- nervous system only functions normally when nicotine is present- as drug tolerance is developed if you don’t take nicotine impulses aren’t transmitted normally- unpleasant withdrawal symptoms are experienced - 62OrganophosphateOrganophosphate
Drugs may inhibit breakdown of neurotransmitters- OPs inhibit acetylcholinesterase so acetylcholine isn’t hydrolysed- remains in synaptic cleft- causes repeated firing of postsynaptic neurone- OPs are esters of phosphoric acid- A chemical substance made when an acid and an alcohol combine and water is removed. - alcohol added to acid to make ester- can be inhaled/ absorbed/ ingested - 63Organophosphates- effectswhen long term health can be damaged when OPs are used
insecticides eg malathion, dichlorvos, herbicides eg glyphosate, nerve gases eg sarin
Nerve gases inhibit acetylcholinesterase- at neuromuscular function junction generating repeated uncontrollable contraction of muscles- occurs in antagonistic muscles where it can break bones
These chemicals stop a key enzyme in the nervous system called cholinesterase from working, - 64Nerve nets-evolutionanimals in the phyla that appear early on fossil record dont have nervous systems
Those that appeared later have radical symmetry/ nervous system=nerve net eg phylum cnidaria-includes jellyfish
those that appeared even later have bilateral symmetry and have CNS eg chordates - 65Nerve net-descriptionNerve net is the simplest type of nervous system- it's a diffused (distributed) network of cells that groups into a ganglia but doesn't form a brain
- 66nerve net- cell typesGanglion cells- provide connections in several directions
Sensory cells- detect stimuli eg light, sound, touch, temperature - 67Hydra-nerve netHydra is in phylum cnidaria
nerve net-
has a simple pattern,is easy to manipulate in exps, regenerates rapidly eg when replacing lost tentacle
so- model organism for studying nerve nets - 68hydra- nerve net- ectodermHydras nerve net is in its ectoderm-(outer 2 layers of body wall)- Nerve net allows hydra to sense light- physical contact and chemicals- so it can contract, perform locomotion, hunt and feed. Without a brain has complex movement/ behaviour- larger stimulus stimulates more cells-causes larger response
- 69Compare hydra to humannervous system type- nerve net/ CNS
NO. cell types in nervous system- 2/many
Regeneration of neurones- Rapid/ slow if at all
Myelin Sheath-Absent/present
onduction speed- Slow approx 5m/s to Fast up to 120m/s
In simple organisms, such as Hydra, the sense receptors respond to a limited number of stimuli and so the number of effectors is small. Their nerve net system consists of simple
nerve cells with short extensions joined to each other and branching in a number of different directions. - 70If sheath= unmylejated what happens to O2 consumptionRate of consumption increases- saltatory conduction wouldn’t occur- membrane= not depolarised- AP’s along neurone- more Na+ and K+ pumps working- more active transort- more ATP needed- more aerobic respiration
- 71Symptoms of you myelination gets damagedFeel weak/ lack of feeling, slower reaction times
- 729 mark planCompare 7.1 to 7.2- describe both- takes impulse through spinal cord not brain- takes through white matter for sensory then grey for relay- motor takes impulse to muscle not pain centre in brain
Anaesthetic A- prevents passage of sodium ions into neurons- prevents initial ap being set up- won’t depolarise other gates- pass impulse
Anaesthetic B- stops calcium ions going in- no vescicles containing neurotransmitters move down- not released by exocytosis- doesn’t set up so in postsynaptic