Neuronal Communication

  • Created by: LBCW0502
  • Created on: 03-01-17 11:47
Why is coordination needed?
To operate effectively. Few body systems can work in isolation (except the heart) e.g. respiring cells dependent on erythrocytes with oxygen in order to function. Plants need to coordinate with seasons (pollination)
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What homeostasis?
Maintenance of relatively constant internal environment
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Describe some of the functions of cells during cell signalling
Transfer signals locally e.g. at synapses using neurotransmitters. Transfer signals across large distances e.g. ADH
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How does coordination take place in plants?
Plants do not have a nervous system like animals. To survive, plants must respond to internal and external changes to their environment e.g. plant stems grow towards light source to maximise rate of photosynthesis (using plant hormones)
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What is a stimulus?
A change in the internal or external environment
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What is the nervous system responsible for?
Detecting changes in the internal and external environment. Information then needs to be processed and an appropriate response is triggered
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What is the nervous system made up of?
Billions of specialised nerve cells called neurones
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What is the role of neurones?
To transmit electrical impulses rapidly around the body so the organism can respond to changes in its internal or external environment. They work together to carry information
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Describe the structure of the cell body
Contains the nucleus surrounded by cytoplasm. The cytoplasm contains large amounts of ER and mitochondria (involved in the production of neurotransmitters).
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Describe the structure of dendrons
Short extensions which come from the cell body. These extensions divide into smaller branches called dendrites. They are responsible for transmitting electrical impulses towards the cell body
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Describe the structure of axons
Singular, elongated nerve fibres that transmit impulses away from the cell body. Fibres are very long and cylindrical in shape (consisting of a narrow region of cytoplasm surrounded by a plasma membrane)
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Describe the function and structure of a sensory neurone
To transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain. They have one dendron, which carries the impulse to the cell body and one axon, which carries the impulses away from the cell body
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Describe the function and structure of a relay neurone
To transmit impulses between neurones (e.g. sensory and motor neurones). They have many short axons and dendrons
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Describe the function and structure of a motor neurone
Transmit impulses from a relay neurone or sensory neurone to an effector (e.g. muscle or gland). They have one long axon and many short dendrites
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Describe the structure of myelinated neurones
Axons covered in myelin sheath (made of many layers of plasma membrane). Schwann cells produce these layers of membrane by growing around the axon many times. Each time they grow around the axon, a layer of phospholipid bilayer is laid down
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Describe the function of the myelin sheath in myelinated neurones
Myelin sheath acts as an insulating layer and allows myelinated neurones to conduct an electrical impulse at a much faster speed than an unmyelinated neurone
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What are the gaps between adjacent Schwann cells called?
Nodes of Ranvier
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How do the gaps affect the transmission of an electrical impulse across a myelinated neurone?
The electrical impulse jumps from one node to the next and travels along the axon (longer localised circuits are formed)
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What are the two main features of sensory receptors?
They are specific to a single type of stimulus and they act as a transducer (convert a stimulus into a nerve impulse called a generator potential/convert one form of energy into another)
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What is a mechanoreceptor?
A sensory receptor located in the skin which detects pressure and movement (e.g. Pacinian corpuscle)
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What is a chemoreceptor?
A sensory receptor located in the nose which detects chemicals (e.g. olfactory receptor)
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What is a thermoreceptor?
A sensory receptor on the tongue which detects heat (e.g. end-bulbs or Krause)
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What is a photoreceptor?
A sensory receptor located in the retina (eye) which detects light (e.g. cone cell - detects different wavelengths of light)
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Describe the structure of the Pacinian corpuscle
The end of the sensory neurone is found within the centre of the corpuscle, surrounded by layers of connective tissue. Each layer of tissue is separate by a layer of gel
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What is located within the membrane of the neurone?
Sodium ion channels - responsible for transporting sodium ions across the membrane. The neurone ending in a Pacinian corpuscle has stretch-mediated sodium channels. When these channels change shape (e.g. stretch), their permeability to sodium changes
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Explain how a Pacinian corpuscle converts mechanical pressure into a nervous impulse (1)
In a normal/resting state, the stretch-mediated sodium ion channels in the sensory neurone's membrane are too narrow to allow sodium ions to pass through them. The neurone of the Pacinian corpuscle has a resting potential
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Explain how a Pacinian corpuscle converts mechanical pressure into a nervous impulse (2)
When pressure is applied to the Pacinian corpuscle, the corpuscle changes shape. This causes the membrane surrounding it neurone to stretch
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Explain how a Pacinian corpuscle converts mechanical pressure into a nervous impulse (3)
When the membrane stretches, the sodium ion channels present widen. Sodium ions can diffuse into the neurone
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Explain how a Pacinian corpuscle converts mechanical pressure into a nervous impulse (4)
The influx of positive sodium ions changes the potential of the membrane - it becomes depolarised. This results in a generator potential
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Explain how a Pacinian corpuscle converts mechanical pressure into a nervous impulse (5)
In turn, the generator potential creates an action potential (a nerve impulse) that passes along the sensory neurone. The action potential will then be transmitted along neurones to the CNS
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Describe and explain the transmission of an action potential (1)
The neurone has a resting potential (-70mV). Some potassium ions channels are open but voltage gated sodium ion channels are closed.
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Describe and explain the transmission of an action potential (2)
Concentration of sodium ions outside the axon membrane is higher compared to the inside. Concentration of potassium ions are higher inside the axon membrane compared to the outside. The axon membrane is polarised
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Describe and explain the transmission of an action potential (3)
Energy of stimulus triggers some voltage gated sodium ion channels to open, making the membrane more permeable to sodium ions. Sodium ions diffuse into axon down electrochemical gradient
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Describe and explain the transmission of an action potential (4)
The action potential generated causes the axon membrane to become depolarised
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Describe and explain the transmission of an action potential (5)
This change in charge (localised electrical circuits) causes more voltage gated sodium ions channels to open, allowing more sodium ions to diffuse into the axon (e.g. of positive feedback)
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Describe and explain the transmission of an action potential (6)
Behind the region of depolarisation, the voltage gated sodium ion channels close and the voltage gated potassium ion channels open. Potassium ions leave axon down electrochemical gradient
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Describe and explain the transmission of an action potential (7)
When the potential difference reaches +40mV the voltage gated sodium ion channels close and voltage gated potassium ion channels open. Sodium ions can no longer enter the axon but the membrane is more permeable to potassium ions.
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Describe and explain the transmission of an action potential (8)
Potassium ions diffuse down their electrochemical gradient, which reduces the charge, resulting in the inside of the axon becoming more negative than the outside
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Describe and explain the transmission of an action potential (9)
Lots of potassium ions diffuse out of axon, resulting in the inside of the axon becoming more negative than its normal resting state. Voltage gated potassium ion channels close. Sodium-potassium ion pump causes 3Na+ to move out and 2K+ move in
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Describe and explain the transmission of an action potential (10)
Axon returns to resting potential and is now repolarised
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What is hyperpolarisation?
The inside of the axon membrane becomes more negative than the resting potential
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How is an action potential propagated in one direction? (1)
The action potential (depolarisation) is propagated further along the axon. The outward movement of potassium ions continues to the extent that the axon membrane behind the action potential has returned to its original state (repolarised)
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How is an action potential propagated in one direction? (2)
Following repolarisation, the axon membrane returns to its resting potential in readiness for a new stimulus if it comes
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What is the refractory period?
After an action potential there is a short period of time when the axon cannot be excited again. During this time, the voltage gated sodium ion channels remain closed, preventing the movement of sodium ions into the axon membrane.
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Why is the refractory period important?
It prevents the propagation of an action potential backwards along the axon as well as forwards. It makes sure action potentials are unidirectional and ensures action potentials do not overlap and occur as discrete impulses
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What is saltatory conduction?
Myelinated axons form longer localised circuits which allows action potentials to jump from one node to another (myelin sheath is impermeable to Na+ and K+ so depolarisation can only take place at the nodes of Ranvier).
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Why is saltatory conduction more energy efficient?
Repolarisation uses ATP in the Na-K ion pump so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction of impulses more efficient
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How does axon diameter affect the speed at which an action potential travels?
The bigger the axon diameter, the faster the impulse is transmitted. This is because there is less resistance to the flow of ions in the cytoplasm compared with those in a smaller axon
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How does temperature affect the speed at which an action potential travels?
The higher the temperature, the faster the nerve impulse. This is because the ions diffuse at higher temperatures. Above 40 degrees Celsius, the proteins become denatured
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What is the all-or-nothing principle? (1)
A certain level of stimulus (threshold value, -55mV) always triggers a response. If this threshold is reached, an action potential will always be created. No matter how large the stimulus is, the same sized action potential will always be triggered.
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What is the all-or-nothing principle? (2)
If the threshold is not reached, no action potential will be triggered
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How does the size of the stimulus affect the number of action potentials generated in a given time?
The larger the stimulus the more frequently the action potentials are generated
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What is a synapse?
The junction between two neurones (or a neurone and effector)
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What is the synaptic cleft?
The gap which separates the axon of one neurone from the dendrite of the next neurone (20-30 micrometres across)
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What is the presynaptic neurone?
Neurone along which the impulse has arrived
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What is the postsynatic neurone?
Neurone that receives the neurotransmitter
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What is the synaptic knob?
The swollen end of the presynatic neurone. It contains many mitochondria and large amounts of ER to enable it to manufacture neurotransmitters (in most cases)
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What are synaptic vesicles?
Vesicles containing neurotransmitters. The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft
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What are neurotransmitter receptors?
Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane
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What is an excitatory neurotransmitter?
These neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane an action potential is triggered (e.g. acetylcholine)
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What is an inhibitory neurotransmitter?
These neurotransmitters result in the hyperpolarisation of the postsynaptic membrane. This prevents an action potential being generated (e.g. Gamma-aminobutyric acid (GABA) - found in some synapses in the brain)
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Describe the transmission of impulses across cholinergic synapses (1)
The arrival of an action potential at the end of the presynaptic neurone causes voltage gated calcium ion channels to open and calcium ions enter the synaptic knob
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Describe the transmission of impulses across cholinergic synapses (2)
The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylcholine into the synaptic cleft (via exocytosis and diffusion)
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Describe the transmission of impulses across cholinergic synapses (3)
Acetylcholine molecules fuse with receptor sites on the sodium ion channels in the membrane of the post synaptic neurone. This causes sodium ion channels to open, allowing sodium ions to diffuse rapidly along a concentration gradient
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Describe the transmission of impulses across cholinergic synapses (4)
The influx of sodium ions generates a new action potential in the postsynaptic neurone
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Describe the transmission of impulses across cholinergic synapses (5)
Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl) which diffuse back across the synaptic cleft into the presynaptic neurone (recycling).
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Describe the transmission of impulses across cholinergic synapses (6)
Breakdown of acetylcholine prevents it from continuously generating a new action potential in the postsynaptic neurone
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Describe the transmission of impulses across cholinergic synapses (7)
ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in the synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites
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What are the roles of synapses? (1)
They ensure impulses are unidirectional. As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neurone to the postsynaptic neurone
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What are the roles of synapses? (2)
They can allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses. This results in a single stimulus creating a number of simultaneous responses
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What are the roles of synapses? (3)
A number of neurones may feed into the same synapse with a single postsynaptic neurone. This results in stimuli from different receptors interacting to produce a single results
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What is spatial summation?
This occurs when a number of presynaptic neurones connect to one postsynaptic neurone. Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone
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What is temporal summation?
This occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient to trigger an action potential
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Describe the effects of drugs on synapses (1)
Nicotine mimics the shape of acetylcholine (triggers action potentials in postsynaptic neurone), drugs stimulate the release of amphetamines or inhibit the enzyme responsible for the breakdown of neurotransmitters (loss of muscle control).
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Describe the effects of drugs on synapses (2)
Some drugs block receptors (person suffers from paralysis) or bind to specific receptors on the postsynaptic neurone (increases activity e.g. alcohol binding to GABAa receptors)
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What does the central nervous system consist of?
The brain and spinal cord
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What does the peripheral nervous system consist of?
All the neurones that connect the CNS to the rest of the body. These are sensory neurones which carry nerve impulses from the receptors to the CNS and the motor neurones which carry nerve impulses away from the CNS to the effectors
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What is the peripheral system divided into?
Somatic and autonomic
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Describe the somatic nervous system
System is under conscious control and is used when voluntarily deciding to do something e.g. move arm. This system carries impulses to the body's muscles
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Describe the autonomic system
System works constantly. It is under subconscious control and is used when the body does something automatically without you deciding to do it (involuntary) e.g. heart beat. System carries impulses nerve impulses to glands, smooth/cardiac muscle
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What is the autonomic system divided into?
Sympathetic and parasympathetic
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Describe the sympathetic motor system
Fight or flight response which uses noradrenaline as a neurotransmitter
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Describe a single neurone from CNS to effector organs in the sympathetic motor system
Shorter/myelinated preganglionic axons, longer unmyelinated postganglionic axons (ACh used as neurotransmitter but NA is used for glands)
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Give examples of sympathetic stimulation
Saliva production reduced, bronchial muscle relaxed, decreased urine secretion, peristalsis reduced in stomach and small intestine
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Describe the parasympathetic motor system
Relaxing responses which uses ACh as a neurotransmitter
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Describe a single neurone from CNS to effector organs in the parasympathetic motor system
Longer myelinated preganglionic axon, shorter unmyelinated postganglionic axon with ACh as a neurotransmitter
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What is the name of the protective membranes which surround the brain?
Meninges
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State the functions of the cerebrum
Controls voluntary actions, such as learning, memory, personality and conscious thought
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State the functions of the cerebellum
Controls unconscious functions such as posture, balance and non-voluntary movement
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What are the functions of the medulla oblongata?
Used in autonomic control e.g. control heart rate and breathing rate
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What is the function of the hypothalamus?
Regulatory centre for temperature and water balance
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What is the function of the pituitary gland?
Stores and releases hormones that regulate many body functions
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Describe how the brain processes inputs from the eye
Inputs from the eye pass to the visual area in the occipital lobe. Impulses from the right of the field of vision in each eye are sent to the visual cortex in the left hemisphere, whereas impulses from the left side of field of vision are sent right
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Describe the pathway of an impulse in a reflex arc
Stimulus - receptor- sensory neurone - relay neurone - motor neurone - effector - response
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Describe the knee-jerk reflex
Leg tapped below patella. Stretches patellar tendon (stimulus). Stimulus initiates reflex arc. Extensor muscle contracts. Relay neurone inhibits motor neurone of flexor muscle (relaxes). After tap, leg relaxes
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What does the absence of the knee-jerk reflex indicate?
Nervous problems and multiple oscillation of leg may be a sign of cerebellar disease (reflex helps to maintain posture and balance)
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Describe the blinking reflex
Cornea stimulated, impulse along sensory neurone (5th cranial nerve), relay neurone, motor neurone (7th cranial nerve), response (blink)
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What does the absence of the blinking reflex indicate?
Used to examine unconscious patients. Used to determine whether or not a patient is brain-dead
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What are the advantages of reflexes?
Involuntary response, not learnt, extremely fast (and considered to be everyday actions e.g. control of digestion, stand upright)
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What are the three types of muscle in the body?
Skeletal, cardiac and involuntary (smooth muscle)
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