Neuronal communication Flashcards
(121 cards)
1
Q
Homeostasis -
A
Function of organs must be coordinated, in order to maintain a relatively constant internal environment, seen in large multicellular animals, with different organs having different functions in the body.
2
Q
Cell signalling -
A
Nervous and hormonal systems coordinate the activities of whole organisms. the communication is required on a cellular level. Occurs through one cell releasing a chemical which has an effect on another cell (target cell) Can be done locally or across large distances.
3
Q
Cell signalling - Short distances
A
For example between neurons at synapses, signal used from neurotransmitters.
4
Q
Cell signalling - longer distances
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Transfer signals across large distances, from the use of hormones. E.G, the cells of the pituitary gland secrete ADH which acts on the kidneys to maintain the water balance in the body.
5
Q
Coordination in plants -
A
Plants do not have a nervous system, but still respond to their external and internal influences, E.G plant stems grow towards a light source, to maximise the rate of photosynthesis, this achieved from plant hormones.
6
Q
Neurons -
A
NS is made up of billions of specialised nerve cells called neurons, which transmit electrical impulses rapidly around the body, so the organism can rapidly respond to changes to its internal and external environment, several within a mammal that are different, work together to carry out information detected by sensory receptors to the effector, which in turn carries out the appropriate response.
7
Q
Structure of a mammalian neuron - Cell body
A
Cell body - contains the nucleus surrounded by the cytoplasm and contain large amounts of mitochondria and ER. Involved in the production of neurotransmitters.
8
Q
What are neurotransmitters?
A
Chemicals which are used to pass signals from one neuron to the next.
9
Q
Structure of a mammalian neuron - Dendrons
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Short extensions which come from the cell body. these divide into smaller and smaller branches known as dendrites, responsible for transmitting electrical impulses towards the cell body.
10
Q
Structure of a mammalian neuron - Axon
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Axons - They are singular elongated, nerve fibres that can be very long, for example those that transmit impulses from the tips of the toes and fingers to the spinal cord, the fibre is cylindrical in its shape and is surrounded by a plasma membrane
11
Q
Types of neuron - Sensory neurons
A
Sensory neurons transmit impulses from a receptor cells to a relay neuron, motor neuron or the brain, have one dendron which carries the impulse to the cell body and one axon which carries the impulse away from the cell body.
12
Q
Types of neuron - Relay neuron
A
Relay neurons transmit impulses between neurons, for example between sensory and motor neurons, have short axons and dendrons.
13
Q
Types of neuron - Motor neuron
A
Motor neurons transmit impulses from a relay or sensory neuron to an effector, such as a muscle or gland, have one long axon and many short dendrites.
14
Q
How does the nervous response of electrical impulse usually follow in a pathway.
A
Receptor-sensory neuron-relay neuron-motor neuron-effector cell.
15
Q
The use of myelinated neurons -
A
The axons on some neurons are covered in a myelin sheath made up of layers of plasma membrane, these are produced from special cells called Schwann cells and these membranes produced grow around the axon many times, each time causes a double layer of a phospholipid bilayer to be laid down. Acts as an insulating layer and allows the electrical impulse to be conducted at much faster speeds compared to unmyelinated neurons.
16
Q
Nodes of Ranvier -
A
Between each adjacent Schwann cell there is a small gap known as a node of Ranvier, this creates small gaps in the myelin sheath, the myelin sheath acts as electrical insulating layer, in these myelinated neurons the electrical impulse jumps from one node to the next node as it travels along the neuron. This allows the impulse to be transmitted much further. In non-myelinated neurons the impulse does not jump, it transmits continuously along the nerve fibre so its much slower.
17
Q
Features of sensory receptors -
A
Specific to a single type of stimulus and are transducer.
18
Q
What is a transducer?
A
Convert stimulus into a nerve impulse (electrical energy)
19
Q
Example of a sensory receptor (photoreceptors)
A
Stimulus - light
Receptor - cone cell detect differences in light wavelengths
Sense organ - eye
20
Q
The role of a transducer -
A
The receptor converts the stimulus into a nervous impulse, which is a generator potential. E.G a rod cell (in the eye) responds to lights and produces a generator potential.
21
Q
Pacinian corpuscle - (as a sensory receptor)
A
These are specific sensory receptors, that detect mechanical pressure and are found deep inside the skin and are most common in the fingers and the soles of the feet, found in joints to enable you to know which joints change direction.
22
Q
Pacinian corpuscle anatomy -
A
Within the membrane of the neuron there are sodium ion channels. These are responsible for transporting sodium ion channels across the membrane.
23
Q
Stretch-mediated sodium channel -
A
Found in the ending neuron in a Pacinian corpuscle, consist of special type of sodium channel, when these channels change shape, their permeability to sodium ions also changes.
24
Q
How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 1 - Resting state
A
Normal state is known as the resting state, the specific sodium channel inside the Pacinian corpuscle are too narrow and not stretched to allow sodium ions through, the neuron of the Pacinian corpsucle is at resting state. (resting potential)
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How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 2 -
When mechanical pressure is applied to the Pacinian corpuscle, the corpuscle changes shape , causing the membrane around it to stretch.
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How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 3 -
When the membrane stretches, the sodium ion channel widens, so sodium ions can now diffuses into the neuron.
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How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 4 -
The influx of positive sodium ion channels change the potential of the membrane and it becomes depolarised, resulting in a generator potential.
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How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 5 -
The generator potential creates an action potential (nerve impulse) that can pass along the sensory neuron. The action potential will then be transmitted along the neurons until it reaches the CNS.
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Resting potential -
When a neuron is not transmitting an impulse, the potential difference across the membrane is known as the resting potential. In this state the outside of the membrane is more positively charged than the inside of the axon. Said to be polarised.
30
Sodium and potassium ions across the axon membrane -
Prevented by the phospholipid bilayer prevents the ions diffusing across the membrane and therefore have to be transported by via channel protein. Some are gated - must be opened to allow specific ions to pass through. Other channels remain open all the time allowing sodium and potassium ions to simply diffuse through.
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Events of a resting potential - movement of sodium and potassium ions -
Sodium ions - are actively transported out of the axon.
Potassium ions - are actively transported into the axon by a specific intrinsic protein. The sodium/potassium pump. Movement is not equal. For every 3 sodium ions pumped out there are 2 potassium ions pumped in.
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What does the sodium/potassium pump show?
As a result there are more sodium ions on the outside of the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the axon cytoplasm compared to the outside the axon. Therefore the sodium ions diffuse back into the axons as a result of it moving down the electrochemical gradient. whereas the potassium ions move out of the axon.
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What does having most of the sodium ion channels closed do?
Prevents the movement of sodium ions, whereas most of the potassium ion channels are opened, therefore there are more positively charged ions outside the axon than inside the cell, this creates the resting potential across the membrane of -70mV, which is negative relative to the outside of the cell.
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Action potential -
When a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reserves the charges on the axon membrane, this results potential difference across the membrane to rapidly to change and becomes positively charged at approximately +40mV. This is known as depolarisation.
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Depolarisation -
A change in potential difference from negative to positive.
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Repolarisation -
As the impulse passes repolarisation occurs a change from positive to negative, so it returns to resting potential.
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How does an action potential occur?
An action potential occurs occurs when the protein channels change shape, this results in opening or closing (voltage gated ions channels)
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Action potential - 1, neuron at resting potential
- Not transmitting a impulse
- Some potassium ion channels are open
- Sodium voltage - gated ion channels are closed
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Action potential - 2, triggered stimulus
- Energy of the stimulus triggers some sodium voltage ions to open.
- Membrane becomes more permeable to sodium ions, they then diffuse into the membrane down a electrochemical gradient, this makes the neuron less negative (positive), the charge causes more sodium ion channels to open allowing more sodium ions to diffuse into the axon, example of positive feedback.
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Positive feedback - 3
The change in charge has called more sodium ions to open, allowing more sodium ions to diffuse into the axon
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The potential difference finally reaches +40mV - 4
The voltage gated sodium ion channels close and voltage gated ion channels and voltage gated potassium ion channels open, the membrane is more permeable to potassium ions but Sodium ions can no longer enter.
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Movement of potassium ions - 5
Potassium ions diffuse out of the axon down their electrochemical gradient, reduces the charge resulting in the axon to be more negative than the outside. (Beginning of repolarisation)
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Hyperpolarisation - 6
Initially, lot's of potassium diffuse out of the axon, the axon becomes more negative to the outside, this is it's normal resting state. This forms hyperpolarisation. The voltage gated potassium ion channels now close. The sodium potassium pump allow it to return to it's resting potential where sodium ions move out of the cell an potassium ions to move inside the cell. returning to it's resting potential and is now repolarised.
44
What is it called after the action potential where it cannot form again?
Refractory period, where voltage gated sodium ion channels remain closed. It prevents the propagation of an action potential backwards as well as forward. It makes the action potential unidirectional, non-overlapping and occur as discrete impulses.
45
Saltatory conduction - (myelinated axons)
These axons transfer the electrical impulses at much higher rates than non-myelinated, as the depolarisation of the axon membrane can only occur at the nodes of Ranvier where no myelin is present. Here only the sodium ions can pass through the protein channels in the membranes.
46
The speed of an action potential can also be influenced by two other factors other than the neuron being myelinated -
Axon diameter - the bigger the faster, less resistance of flow of ions in the cytoplasm compared with a smaller axon.
Temperature - higher temperature the faster as the ions diffuse at higher temperatures occurs up to 40 degrees as higher temperatures would denature the sodium-potassium pump.
47
All or nothing principle -
Nerve impulses are said to be all or nothing. A certain level of stimulus (threshold) will always trigger a response and if reached will result in an action potential. No matter how large the stimulus it will always trigger the same sized action potential. If the threshold value is not reached no action potential.
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Synapses -
The junction between two neurons, impulses are transmitted across the synapse using chemicals called neurotransmitters.
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Synapse structure - (synaptic cleft)
The gap which separates the gap which separates the axon of one neuron from one dendrite to the next neuron (20-30nm across)
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Synapse structure - (presynaptic neuron)
Neuron along which the impulse has arrived from
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Synaptic structure - (postsynaptic neuron)
Neuron that receives the neurotransmitter.
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Synapse structure - (synaptic knob)
Swollen end of the presynaptic neuron, contains high amount of ER and mitochondria and manufactures neurotransmitters (in most cases)
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Synapse structure - (synaptic vesicles)
Vesicles containing neurotransmitters, fuse with the presynaptic membrane and release into the synaptic cleft by exocytosis
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Synaptic structure - (neurotransmitter receptors)
Receptor molecules which the neurotransmitter binds to on the postsynaptic membrane.
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How many groups of neurotransmitters -
2 (excitatory and inhibitory)
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Excitatory neurotransmitters -
Result in the depolarisation of the postsynaptic neuron. If the threshold value is reached then an action potential is triggered for example acetylcholine.
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Inhibitory neurotransmitters -
Result in the hyperolarisation of the postsynaptic neuron, preventing the action potential to be triggered for example GABA as a neurotransmitter.
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Transmission of impulses across the synapse - and results from
- Action potential reaches the end of the presynaptic neuron
- Depolarisation of the presynaptic neuron results in the opening of calcium ions to diffuse into the presynaptic knob.
- Causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane and are released into the synaptic cleft by exocytosis.
- These neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane.
- Causes sodium ion channels to open and sodium ions to diffuse into the postsynaptic neuron
- Triggers an action potential and the impulse is propagated along the the postsynaptic neuron.
59
What should happen to a neurotransmitter after an action potential has been released?
It is important it is removed to not maintain the stimulus and so another stimulus can arrive and affect the synapse. For example acetylcholine is broken down by enzymes which also releases them from receptors on the postsynaptic membrane. It allows the neurotransmitter to be recycled.
60
Transmission across cholinergic synapses -
Use the excitatory neurotransmitter acetylcholine and are common in the CNS. Where a motor neuron and effector (muscle cell) meet. If the neurotransmitters meet the receptors on the muscle it contracts. Acetylcholine is released from the vesicles in the presynaptic knob, binds to high specific receptors in the postsynaptic membrane. Triggers an action potential on muscle cell or post neuron, once triggered acetylcholine is hydrolysed by acetylcholinesterase, situated on the postsynaptic membrane, the broken products are taken back to the Knob to be reformed into acetylcholine, the postsynaptic membrane is ready to receive another impulse.
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Role of synapses - Unidirectional
Ensure the impulses are unidirectional as the receptors are present on the postsynaptic membrane, therefore impulses are only to travel in one direction.
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Role of synapses -
They allow an impulse from one neuron to be transmitted to a number of neurons, so one stimulus can create a number of simultaneous responses.
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Summation and control -
Each stimulus from a presynaptic neuron causes the release the same amount of release of neurotransmitters. However the amount of neurotransmitter released from a single impulse may not be enough to trigger an action potential in the postsynaptic membrane. Summation is if the amount of neurotransmitter builds up sufficiently to reach the threshold then this will trigger an action potential. Summation can occur in two ways.
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Spatial summation -
Occurs when a number of presynaptic neurons connect to one postsynaptic neuron. Each release neurotransmitters which builds up high enough in the synapse to trigger an action potential in the single postsynaptic neuron.
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Temporal summation -
When a single presynaptic neuron releases a neurotransmitter as a result of an action potential several times over a short period. This keeps building up until it is sufficient enough to trigger an action potential.
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Effects of drugs on synapses - Stimulate synapses
Drugs that stimulate the NS create more action potentials in the postsynaptic neurons, these drugs may work by mimicking the shape of the neurotransmitters E.G nicotine is the same shape as acetylcholine so can bind to this excitatory neurotransmitters receptors and trigger action potentials. Drugs can simulate the release of more neurotransmitters. Can inhibit the enzyme responsible for the breaking down the neurotransmitter so that they wont break down and not stimulate an action potential on the postsynaptic neuron.
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Effects of drugs on synapses - Inhibiting synapses
These are known to create fewer action potentials in the postsynaptic neuron, these drugs may work by blocking receptors so the neurotransmitter can no longer bind and activate the receptor on the membrane. E.G curare blocks acetylcholine receptors as neuromuscular junctions therefore muscles (effectors) cannot be contracted. May suffer from paralysis. Some may bind to the receptors on the postsynaptic membrane of some neurons and changing the shape of the receptor so neurotransmitters cannot stimulate an action potential. Alcohol binding to GABA receptors.
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Structural organisation of the NS -
Organised into two systems the central nervous system and the peripheral nervous system.
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CNS -
Consists of the brain and spinal cord
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Peripheral nervous system -
Consists of all the neurons that connect the CNS to the rest of the body, these are sensory neurons that carry nerve impulses to the CNS and the motor neurons which carry nerve impulses away from the CNS to effectors.
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Functional organisation of the peripheral nervous system - Somatic nervous system
This is linked under conscious control, used to understand when you want to voluntarily do something. When moving an arm the SNS carries the impulses to the body's muscle.
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Functional organisation of the peripheral nervous system - Autonomic nervous system
This system works under subconscious control and works when the body does something automatically without you deciding to do it (involuntarily). For example the cause of a heart beat or digestion. The ANS carries nerve impulses to the glands and smooth muscle.
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Functional organisation of the Autonomic nervous system - Sympathetic nervous system
Generally if the outcome increases the activity it involves the sympathetic nervous system for example increase in heart rate or stimulate production of glucose in the blood. This nervous system prepares the body for fight or flight.
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Functional organisation of the Autonomic nervous system - Parasympathetic nervous system
If the outcome decreases the activity it involves the parasympathetic nervous system for it example it results in the decrease of heart rate and glucose is stored, this can be known as rest and digest.
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Gross structure of the brain -
- Brain is protected by the skull
- Surrounded by protective membranes called meninges
- Consist of 5 main areas (Cerebrum, Cerebellum, medulla oblongata, hypothalamus, pituitary gland)
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Cerebrum -
Control voluntary movement, such as learning and conscious thought.
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Cerebellum -
Controls unconscious functions such as posture, balance and non-voluntarily movements.
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Medulla oblongata -
Used for autonomic control, for example controlling the heart rate and breathing rate.
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Hypothalamus -
Regulatory centre for temperature and water balance.
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Pituitary gland -
Stores and releases hormones that regulate bodily functions.
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Cerebrum in detail -
- Receives sensory information, sends impulses along motor neurons to effectors to produce an appropriate response coordinates the bodily functions consciously.
- Cerebrum is highly convoluted, which increases its surface area considerably, divided into two cerebral hemispheres where each hemisphere controls one half of the body and discrete areas perform different functions.
- Each sensory area within the cerebral hemispheres receives information from the receptor cells located in the sense organ, the information is then passed onto other areas of the brain known as the association areas. Impulses come into motor areas where motor areas send out impulses for example to move skeletal muscles, the size of the motor area allocated is proportion to the relative number of motor endings in it.
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Cerebellum in detail -
Concerned with the control of muscular movement, body posture and balance. Doesn't initiate movement but coordinates it, uncoordinated movement results in people suffering from a jerky if the cerebellum is damaged, receives information from organs of balance in the ears and about the tone of muscles and tendons. Relays this information to the areas of the cerebral cortex that are involved in motor control.
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Medulla oblongata in detail -
Contains many important regulatory centers of the autonomic nervous system, controls reflex activities such as ventilation and heart rate, controls other activities of coughing and swallowing.
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Hypothalamus in detail -
Main controlling regions of the autonomic nervous system, has two centres one for the parasympathetic and one for the sympathetic nervous system. Number of functions:
- Controlling complex patterns of behaviour, sleeping, feeding, aggression
- Monitoring the composition of blood plasma, concentration of blood plasma and blood glucose
- Producing hormones - it is an endocrine gland, producing some hormones.
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Pituitary gland in detail -
Found at the base of the hypothalamus, size of the pea but controls most glands in the body, divided into two sections posterior and anterior.
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Anterior pituitary (front) -
Produces 6 hormones, including FSH involved in reproduction and growth.
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Posterior pituitary (back) -
Stores and releases hormones produced by the hypothalamus, such as ADH in urine production.
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Reflex arc -
The pathway of neurons involved in reflex action is known as reflex arc.
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Reflex arc processes -
Receptor - detects stimulus and creates an action potential in the sensory neuron
Sensory neuron - carries the electrical impulse to the spinal cord
Relay neuron - carries the sensory neuron to the motor neuron within the spinal cord or brain
Motor neuron - carries impulse to the effector to carry out appropriate response
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Spinal cord -
Is a column of nervous tissues running up the back, spine protects it.
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Knee-jerk reflex -
It is a spinal reflex, meaning that the neural circuit only goes up the spinal cord not the brain.
When the leg is tapped just below the patella (kneecap), it creates a reflex arc, causing the extensor muscles on top of the thigh to contract. At the same time, a relay neuron inhibits the motor neuron of the flexor muscle causing it to relax. This contraction is coordinated with the relaxation of the antagonistic flexor hamstring muscle, causes the leg to kick.
After the tap of the hammer, the leg is normally extended once.
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Blinking reflex -
The blinking reflex is an involuntary blinking of the eyelids. Occurs when the cornea is stimulated. Known as the corneal reflex to help it become safe from damage. A blink reflex also occur from loud sounds 40-60dB or due to very bright light blinking over bright light is known as the cranial reflex occurs in the brain and not the spinal cord. When the cornea of the eye is irritated it triggers an impulse along sensory neurons. The impulse through a relay neuron in the lower brain stem, then sent along the motor neuron to initiate a motor response to close the eyelids. Creates a consensual response where both eyelids are shut.
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Why do doctors test blinking reflex?
When examining unconscious patients, if the reflex is present, it indicates the lower brain stem is functioning. The procedure is therefore used as a part of an assessment to determine whether or not the patient is brain dead.,
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Reflexes and there survival importance -
Essential for the body as they stop the body being harmed, reflexes also improve your chances of survival by:
- Being involuntary, where regions of the brain is not involved, therefore the brain is able to deal with more complex responses.
-Present at birth, therefore provide immediate protection
-Reflex arc is very short, It normally only involves one or two synapses which are which are the slowest part of transmission.
-Many reflexes are what we consider everyday actions.
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Types of muscle -
3 types of skeletal, cardiac and involuntary (smooth) muscle.
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Skeletal muscle -
Make up the bulk of the body muscle tissue. Cells responsible for movement. Biceps and triceps.
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Cardiac muscle -
Are only found in the heart, myogenic meaning they contracts without the need for a nervous stimulus causes the heart to beat in a regular rhythm.
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Involuntary (smooth) muscle -
In the walls of hollow organs such as the stomach and bladder. Found in walls of the blood vessels and the digestive system.
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Skeletal muscle characteristics -
Striated, conscious (voluntary), regularly arranged so muscle contracts in one direction. Length of contraction is short.
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Cardiac muscle characteristics -
Specialised striated, involuntary, cells are branched and interconnected resulting in simultaneous contraction, intermediate contraction speed.
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Involuntary muscle characteristics -
Non-striated, involuntary, no regular arrangement cells can contract in different directions. Can remain contracted for a very long time.
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Structure of skeletal muscle -
Made up of bundles of muscle fibres enclosed in within a plasma membrane known as the sarcolemma.
Contain a number of nuclei, formed as a result of embryonic muscle cells fusing together. Makes the muscle stronger, as the junction between adjacent cells would act as a point of weakness.
Part of the sarcolemma folds inwards to help spread electrical impulses throughout the sarcoplasm so that the whole of the fibre receives the impulse to contact at the same time.
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Muscle fibres organelles -
Lots of mitochondria to provide ATP that is needed for muscle contraction.
Modified version of ER known as sarcoplasmic reticulum, extends throughout the muscle fibre and contains calcium ions required for muscle contraction.
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Myofibrils -
Each muscle fibre contains many myofibrils, long cylindered organelles made up of protein and specialised for contraction, lined up in parallel to provide maximum force when contracted together. Myofibrils are made up of two types of protein filament. Myofibrils have alternating light and dark bands that result in their striped appearance.
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Actin -
The thinner filament, consists of two strands of actin twisted around each other.
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Myosin -
The thicker filament, consists of long-rod shaped fibres with bulbous heads projecting to one side.
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Light bands -
These appear light, where acting and myosin do not overlap, can be known as I bands.
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Dark bands -
These appear darker due to the presence of thick myosin filaments, they are quite dark as the myosin is overlapped with the actin, can be known as A bands.
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Z line -
Found at the centre of each light band, the distance between the adjacent Z lines is called the sarcomere. When a muscle contracts the sarcomere shortens.
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H zone -
The lighter coloured region found in centre of each dark band. Where there is only myosin filaments present at this point. When the muscle contracts the H zone decreases.
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Histology of the skeletal muscle -
Should be able to observe these stained features.
- Individual muscle fibres - long and thin multinucleated fibres that are crossed with a regular pattern
- Structured arrangement of sarcomeres which appear as dark A bands and light I bands.
- Streaks of connective tissue and capillaries running in between the fibres.
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Muscle contraction is usually defined as the
Sliding filament theory
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The sliding filament in detail -
Contraction of myosin filaments pull the actin filaments inwards towards the center of the sarcomere results in the light bands becoming narrower, the Z lines move closer together, shortening the sarcomere, the H zone becomes narrower.
The myosin filaments remain the same length as the myosin filaments themselves have not shortened.
The simultaneous contraction of sarcomeres means the myofibrils and muscle fibers contract to cause movement, when returned to their original length the muscle relaxes.
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Structure of myosin -
Myosin filaments have globular heads allowing them to move backwards and forwards, head contains a binding site for ATP, the tails of several hundred myosin molecules are aligned together to form the myosin filament.
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Structure of actin -
Actin filaments have binding sites for myosin heads these are called actin-myosin binding sites.
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Structure of actin when relaxed -
When they are in the resting state they have tropomyosin which are held in place by the protein troponin and block the actin-myosin binding sites.
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Structure of actin when contracting -
The myosin heads form bonds with the actin filaments, known as actin-myosin cross bridges, the myosin head flexes in unison pulling the actin filament along the myosin filament, once done the myosin detaches from the actin and head returns to its original angle using ATP.
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How muscle contraction occurs?
Action potential arrives as the neuromuscular junction, (motor neuron and skeletal muscle meet), many neuromuscular junctions so action potentials can reach them and the muscle fibres react simultaneously. If only one existed the contraction would not be as powerful and be slower.
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Motor unit -
All the muscle fibres supplied by a single motor neuron are known as a motor unit. Act as a single unit.
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Calcium ions - in neuromuscular junctions
When an action potential reaches the neuromuscular junction it stimulates the release of calcium ions, diffuse from the synapse to the synaptic knob causing presynaptic to fuse with the presynaptic membrane. This causes acetylcholine to be released in the synaptic cleft by exocytosis and diffuses across the synapse binds to the receptors on the postsynaptic membrane opening the sodium ion channels results in depolarisation.
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What happens to acetylcholine when it passes to the postsynaptic membrane
