neuronal communication Flashcards
1
Q
Why is coordination needed?
A
Organisms need to coordinate the
function of different cells and
systems to operate effectively
2
Q
What is homeostasis?
A
The maintenance of a stable equilibrium in the conditions inside the body e.g. digestive organs such as exocrine pancreas, duodenum, and ileum along with the endocrine pancreas and the liver work together to maintain a constant blood glucose concentration
3
Q
Through the process of cell
signalling, nervous and
hormonal systems can…
A
• Transfer signals locally, e.g. between neurones and synapses. Here the signal is used for a neurotransmitter • Transfer signals across large distances, using hormones, e.g. the cells of the pituitary glad secrete ADH, which acts on cells in the kidneys to maintain water balance in the body
4
Q
Describe coordination in plants
A
Unlike animals, plants do not have a nervous system However, they still respond to internal and external changes in their environment in order to survive
5
Q
What is a neurone?
A
A specialised cell which transmits
impulses in the form of action
potentials
6
Q
Describe the structure of a
neurone
A
• Cell body - contains the nucleus surrounded by the cytoplasm. The cytoplasm contains large amounts and endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters • Dendrons - short extensions which come from the cell body. Divide into smaller branches called dendrites. Transmit electrical impulses towards the cell body • Axons - singular, elongated nerve fibres that transmit impulses away from the cell body. Cylindrical in shape consisting of a narrow region of cytoplasm surrounded by a cell membrane
7
Q
Describe the 3 different types
of neurone
A
• Sensory neurones - transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain. One dendron and one axon • Relay neurones - transmit impulses between e.g. sensory and motor neurones. Many short axons and dendrons • Motor neurones - these neurone transmit impulses from a relay or sensory neurone to an effector. One long axon and many short dendrites
8
Q
What are myelinated
neurones?
A
A neurone where the axon is covered in a myelin sheath (made up of many layers of plasma membrane) • Schwann cells produce these layers of membrane by growing around the axon many times • Myelinated neurones transmit impulses much faster than nonmyelinated neurones
9
Q
What are sensory receptors?
A
Cells/sensory nerve endings that respond to a stimulus in the internal or external environment of an organism and can create action potentials. • Most are energy transducers that convert one form of entry to another
10
Q
What are Pacinian corpuscles?
A
A pressure sensor found in the skin • Oval-shaped structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell • When pressure on the skin changes, this deforms the rings of connective tissue, which push against the nerve ending • The corpuscle is sensitive only to changes in pressure that deform the rings connective tissue • Therefore when pressure is constant, they stop responding
11
Q
How is mechanical pressure
converted into an impulse?
A
1. Normal state (resting state) the stretch-mediated sodium channels are too narrow to allow sodium ions through 2. Pressure applied the Pacinian corpuscle changes shape (membrane around neurone stretches) 3. This causes the stretchmediated channels to widen and sodium ions diffuse in 4. This influx of positive ions causes a change in the potential difference across the membrane and it depolarise, resulting in a generator potential 5. The generator potential creates an action potential (an impulse) which is passed along the sensory neurone into the CNS
12
Q
What is resting potential?
A
The potential difference across the membrane while the neurone is at rest • The outside is more positively charged than the inside of the axon (-70mV) • The membrane is said to be polarised as there is a potential difference
13
Q
How are resting potentials
maintained?
A
• Na+ ions actively transported out of the axon, and K+ actively transported into the axon • 3 Na+ ions pumped out for every 2K+ pumped in • Gated Na+ ions channels are kept closed, but some of the K+ channels are open, so membrane is more permeable to K+ • K+ diffuses out of cell • Cell cytoplasm also contains large organic anions • Therefore inside of the cell has negative potential compared to outside • The cell membrane is said to be polarised • The potential difference is about -70mV
14
Q
What is action potential?
A
A brief reversal of the potential
across the membrane of a neurone
causing a peak of +40mV compared
to the resting potential of -60mV
15
Q
What happens when a stimulus
is detected by a sensory
receptor?
A
• The energy of the stimulus temporarily reverses the charges on the axon membrane • As a result, the potential difference across the membrane rapidly changes and becomes positively charged at approximately +40mV • This is known as depolarisation - a change in potential difference from negative to positive • The neurone returns to its resting potential
16
Q
When does an action potential
occur?
A
• When protein channels in the axon membrane change shape as a result of the change voltage across its membrane • The change in protein shape results in the channel opening or closing • These channels are known as voltage-gated ion channels
17
Q
What are the stages of an
action potential?
A
1. The membrane starts in its resting state - polarised with the inside of the cell being -70mV compared to the outside. There is a higher concentration of Na+ ions outside than inside, and a higher concentration of K+inside than outside 2. Na+ ions channels open and some Na+ ions diffuse into the cell 3. The membrane depolarises - it becomes less negative with respect to the outside and reaches the threshold value of -50mV 4. Positive feedback causes nearby voltage-gated Na+ ions channels open, and many Na+ ions flood in. As more Na+ ions enter, the cell becomes positively charged inside compared with outside 5. The p.d. across the plasma membrane reaches +40mV. The inside of the cell is positive compared with the outside 6. The Na+ ions channels close and potassium channels open 7. K+ ions diffuse out of the cell, the p.d. inside the cell goes back to negative compared with the outside - this is called repolarisation 8. The p.d. overshoots slightly, making the cell hyperpolarised 9. The original p.d. is restored so that the cell returns to its resting state
18
Q
What are the stages in the
propagation of action
potentials?
A
1. When an action potential occurs, the Na+ ions channels open at that point in the neurone 2. Localised increase in concentration of Na+ ions inside the neurone - action potential 3. Na+ ions diffuse sideways along the neurone, away from the increased region of concentration. The movement of charged particles is a current called a local current 4. The local current causes a slight depolarisation further along the neurone which affects voltagegated Na+ ion channels 5. Na+ ion gate, which was initially closed will now open because of the movement of sodium ions, allowing the action potential to move along the neurone as more Na+ ions enter and set up another action potential 6. The region of the membrane which has been depolarised as the action potential passed along now undergoes repolarisation and to return to its resting potential
19
Q
What is the refractory period?
A
A short period of time when the axon cannot be excited again • Voltage-gated Na+ remain closed, preventing the movement of Na+ ions into the axon • No
20
Q
Why is the refractory period
important?
A
• It prevents the propagation of an action potential backwards along the axon • Makes sure action potentials are unidirectional • Ensure that action potential do not overlap and occur as discrete impulses
21
Q
What does saltatory
conduction mean?
A
• Na+ and K+ cannot diffuse through the fatty layer of the myelin sheath • The ionic movements that create an action potential can only occur at the nodes of Ranvier • The local currents are elongated and Na+ ions diffuse along the neurone from one node of Ranvier to the next • This means that the action potential appears to jump from one node to the next • This is called saltatory conduction • Therefore a myelinated neurone can conduct action potentials more quickly than non-myelinated neurones
22
Q
Give two other factors that
affect the speed at which
action potential travels
A
Axon diameter • The bigger the diameter, the faster the impulse is transmitted • Because there is less resistance to the flow of ions in the cytoplasm, compared with those in smaller axon Temperature • The higher the temperature, the faster the nerve impulse • Because ions diffuse faster at higher temperatures
23
Q
What is the all-or-nothing
principle?
A
Nerve impulses are all-or-nothing responses • If a stimulus reaches the threshold value it will always trigger a response and an action potential • No matter how large the stimulus, the same sized action potential will always be triggered • The larger the stimulus, the more frequently the action potentials will be generated
24
Q
What is a neurotransmitter?
A
A chemical involved in communication across a synapse between adjacent neurones, or a neurone and a muscle cell • Used as a signalling molecule between two neurones in a synapse
25
Q
What is a synapse?
A
The junction (small gap) between two neurones, or a neurone and an effector • A cholinergic synapse uses acetylcholine as its neurotransmitter
26
Q
What are the key features in
the pre-synaptic bulb?
A
The pre-synaptic neurone ends in a swelling called the pre-synaptic bulb • many mitochondria - indicating that an active process needing ATP is involved • A large amount of smooth endoplasmic reticulum, which packages the neurotransmitter into vesicles • Large numbers of vesicles containing molecules of a chemical called acetylcholine, the transmitter that will diffuse across the synaptic cleft • A number of voltage-gated calcium ion channels on the cell surface membrane
27
Q
Describe the post-synaptic
membrane
A
• Contains specialised sodium ion channels that can respond to the neurotransmitter • These channels consist of 5 polypeptide molecules. 2 of these have a special receptor site that is specific to acetylcholine • The receptor sites have a complementary shape to that of the acetylcholine molecule • When acetylcholine is present in the synaptic cleft, it binds to the 2 receptor sites, and causes the sodium ion channel to open
28
Q
Describe the transmission of a
signal across the synaptic clef
A
1. An action potential arrives at the synaptic bulb 2. The voltage-gated calcium ion channels open 3. Calcium ions diffuse into the synaptic bulb 4. The calcium ions cause the synaptic vesicles to move to, and fuse with, the pre-synaptic membrane 5. Acetylcholine is released by exocytosis, and the molecules diffuse across the cleft 6. Acetylcholine molecules bind to the receptor sites on the sodium ion channels in the post-synaptic membrane 7. The sodium ion channels open, and sodium ions diffuse across the post-synaptic membrane into the post-synaptic neurone 8. A generator potential or excitatory post-synaptic potential (EPSP) is created 9. If sufficient generator potentials combine, then the potential across the post-synaptic membrane reaches the threshold potential 10. A new action potential is created in the post-synaptic neurone
29
Q
What is acetylcholinesterase?
A
It’s an enzyme found in the synaptic
cleft
40
Q
Describe the structure of the
brain
A
• Cerebrum - controls voluntary actions e.g. learning, memory, personality, and conscious thought • Cerebellum - controls unconscious functions e.g. posture, balance and nonvoluntary movement • Medulla oblongata - used in autonomic control, e.g. controls heart rate and breathing rate • Hypothalamus - regulatory centre for temperature and water balance • Pituitary gland - stores and releases hormones that regulate many body functions
48
Q
Describe the spinal cord
A
A column of nervous tissues running up the back • It is surrounded by the spine for protection • At intervals along the spinal cord, pairs of neurones emerge
52
Q
Describe skeletal muscle
A
• Fibre appearance: Striated • Control: Conscious (voluntary) • Arrangement: Regularly arranged so muscle contracts in one direction • Contraction speed; Rapid • Length of contraction: Short • Structure: Muscles showing cross striations are known as striated or striped muscles. Fibres are tubular and multinucleated • Fatigues quickly
54
Q
Describe muscle fibres
A
Contain a number of nuclei and are much longer than normal cells, as they are formed as are result of many individual embryonic muscle cells fusing together • This makes the muscle stronger, as the junction between adjacent cells would be a point of weakness • The shared cytoplasm within a muscle fibre is known as sarcoplasm • Parts of the sarcolemma fold inwards (aka transverse or T tubules) to help spread electrical impulses through the sarcoplasm • This ensures the whole fibre receives the impulse to contract at the same time • Lots of mitochondria to provide the ATP needed for muscle contraction • Modified version of the endoplasmic reticulum known as the Sarcoplasmic reticulum. This extends throughout the muscle fibre and contains calcium ions required for muscle contraction
57
Q
What causes the striped
appearance of myofibrils?
A
Light bands (aka isotopic or Ibands): Appear light as they are the region where actin and myosin filament don’t overlap • Dark bands (aka anisotropic or Abands): Appear dark because of the presence of thick myosin filaments. Edges are especially dark as the myosin is overlapped with actin • Z-line: Line found at the centre of each light band. The distance between adjacent Z-lines is a sarcomere. Sarcomere is the functional unit of the myofibril. When a muscle contracts, the sarcomere contracts • H-zone: Lighter coloured region at the centre of each dark band. Only myosin present. When muscle contracts, H-zone decreases.
58
Q
Describe cardiac muscle
A
• Fibre appearance: Specialised striated • Control: Involuntary • Arrangement: Cells branch and interconnect resulting in simultaneous contraction • Contraction speed: Intermediate • Length of contraction: Intermediate • Structure: Shows striations, but they are much fainter than those in skeletal muscle. Fibres are branched and uninucleated • Doesn’t fatigue easily
60
Q
Describe involuntary muscle
A
Fibre appearance: Non-striated Control: Involuntary arrangement: No regular arrangement - different cells can contract in different directions Control: Slow Contraction speed: Can remain contracted for a relatively long time Structure: Muscles showing no cross striations are called nonstriated or unstriped muscles. Fibres are spindle shaped and uninucleated
62
Q
How is a contraction
stimulated at the
neuromuscular junction?
A
1. Action potentials arriving at the end of the axon open calcium ion channels in the membrane. Calcium ions flood into the end of the axon 2. Vesicles of acetylcholine move towards and fuse with the end of the membrane 3. Acetylcholine molecues diffuse across the gap and fuse with receptors in the sarcolemma 4. This opens sodium ion channels, which allow sodium ions to enter the muscle fibre, causing depolarisation of the sarcolemma 5. A wave of depolarisation spreads along the sarcolemma and down transverse tubules (ttubules) into the muscl
64
Q
Describe the structure of
myosin
A
Myosin filaments have globular heads that are hinged, which allows them to move back and forth • On the head is a binding site for each of actin and ATP • The tails of several hundred myosin molecules are aligned together to form the myosin filament
65
Q
Describe the structure of actin
A
• Actin filaments have binding sites for myosin heads (actin-myosin binding sites) • These binding sites are often blocked by tropomyosin, which is held in place by troponin
69
Q
Describe the mechanism of
contraction
A
• The sliding action is caused by the movement of the myosin heads • When the muscle is stimulated, the tropomyosin is moved aside, exposing the binding sites on the actin • The myosin heads attach to the actin and move, causing the actin to slide past the myosin
