Nervous System: Nerves Flashcards

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1
Q

Nerve Cells

A
  • Neuron = nerve cells (Use neuron in answers)
  • Neurons are the basic functional units
  • They vary in size and shape, but all have a cell body and two types of extensions: dendrites and the axon.
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2
Q

Nervous Tissue

A
  • Neurons outside the central nervous system have axons with a myelin sheath created by Schwann cells, which wrap around the axon.
  • The outermost coil of the Schwann cell is called the neurilemma, which helps in the repair of injured fibres
  • There are gaps in the myelin sheath at intervals along the axon called nodes of Ranvier
  • The axons and dendrites of neurons are known collectively as nerve fibres
  • Outside the CNS, nerve fibres are arranged into bundles called nerves
  • White matter is referred to as an axon covered in a myelin sheath and is white as the myelin sheath is made up of a phospholipid bilayer which is fatty and because fat is white, the axon is described as white while dendrites which doesn’t have the myelin sheath is referred to as grey matter.
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3
Q

Synapses

A
  • Nerve impulses are passed from neuron to neuron across a synapse
  • At the synapse the neurons do not actually join – there is a very small gap between them
  • Messages have to be carried across the synapse
  • A similar synapse exists where an axon meets a skeletal muscle cell
  • This tiny gap is called the neuromuscular junction.
  • The neuron transmitting through the axon terminal to the dendrites of the receiving neuron is called the pre-synaptic neuron and the receiving neuron is called the Post-synaptic neuron.
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4
Q

Types of Neurons: Multipolar

A
  • One axon
  • Many dendrites from the cell body
  • Motor Neurons
  • Interneurons
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5
Q

Types of Neurons: Bipolar

A
  • One axon
  • One dendrite from the cell body
  • Sensory: eye, ear and nose
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6
Q

Types of Neurons: Unipolar

A
  • One axon
  • No dendrites connected to the cell body
  • Not in humans
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7
Q

Types of Neurons: Pseudo-Unipolar

A
  • One axon that divides in two
  • No dendrites connected to the cell body
  • Sensory neurons
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8
Q

Recap of Electricity

A
  • Elements that have a different number of protons to electrons are called IONS
  • There are two types of electrical charges of ions, positive and negative
  • For example:
  • Sodium exists as Na+
  • Chlorine exists as Cl-
  • Like charges repel
  • Opposite charges attract
  • The force that pulls unlike charges together can be measured and its strength increases as the charges get closer
  • If a group of positive and negative charges are separated, they have the potential to come together and release energy
  • The potential, or potential difference, between two places can be measure
  • It is called the voltage and is measure in volts (V) or millivolts (mV)
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9
Q

Membrane Potential

A
  • If there is a difference between the concentration of ions inside and outside a cell, there would be a potential between the inside and the outside of the cell membrane
  • This occurs in all body cells: there is a difference in the ion concentration on either side of the cell membrane
  • The nerve fibre (inside of an axon) (intracellular fluid) is more negatively charged than the extracellular fluid by a difference of about 70 millivolts
  • The resting membrane potential of neurons is due mainly to differences in the distribution of potassium ions (K+) and sodium ions (Na+) on either side of the cell membrane
  • The potential difference created is called the membrane potential and it is particularly large in nerve and muscle cells.
  • The membrane potential of unstimulated nerve cells, known as the resting membrane potential, can be measured and is about -70 mV
  • This means that the potential of the inside of the membrane is 70 mV less than that of the outside
  • The cell membrane maintains this potential difference in two ways
  • It actively moves ions across the membrane: This actively is described as a sodium-potassium pump that transports sodium ions out of the cell and potassium ions in.
  • The cell membrane is not equally permeable to all ions
  • There are large number of negatively charged ions trapped inside the cell
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10
Q

Action Potential: Overview

A
  • If a sufficiently strong stimulus is applied to a nerve fibre, the membrane becomes more permeable to sodium ions
  • Sodium channels open a little so that sodium ions diffuse across the membrane and into the cell
  • This inward movement is too great to be balanced by and outward movement of potassium ions and the membrane becomes depolarised
  • Depolarisation occurs only if the level of stimulation exceeds a certain threshold
  • If the stimulus is strong enough to cause a change of about 15 mV (-55mV), then then movement of sodium ions proceeds independently of the stimulus; that is , the size of the response is not related to the strength of the stimulus
  • This is known as an all-or-none response
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11
Q

Action Potential: Step 1

A
  • A stimulus causes sodium to leak into the neuron
  • Sodium channels open a little so that sodium ions diffuse across the membrane and into the cell (-55 mV)
  • Not action potential, leading to have an action potential
  • Threshold:
  • If enough sodium diffuses in to reach threshold (-55mV), the sodium voltage-gated channels open.
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12
Q

Action Potential: Step 2

A
  • Sodium floods into the neuron causing the original polarity of the membrane to reverse and the inside become positive relative to the outside
  • This is called depolarisation
  • Na+ gate open
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13
Q

Action Potential: Step 3

A
  • At +15mV, the sodium voltage-gated channels close, and the potassium voltage-gated channels open
  • Causing potassium to rush out of the cell
  • This restores the membrane to negatively charged on the inside, so this is called repolarisation
  • K+ gate open
  • Na+ gate close
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14
Q

Action Potential Step 4:

A
  • At -70mV the potassium voltage-gated channels close but a little more potassium leaks out than necessary causing the cell to become slightly more negative: hyper-polarisation
  • Running in the background is the sodium potassium pumps (3 Na+ out and 2 K+ in), returning the sodium and potassium to their original positions, ready to fire again when needed
  • Almost as quickly, the membrane is restored to its original condition
  • This rapid repolarisation – repolarisation of the membrane is called an action potential
  • K+ gate close
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15
Q

After Action Potential

A
  • The movement of the action potential along a nerve fibre is the nerve impulse
  • During an action potential, and for a very brief time afterwards, the part of the nerve fibre cannot be stimulated to respond again
  • This is called the refractory period
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16
Q

Conduction along unmyelinated fibres

A
  • In an unmyelinated nerve fibre, depolarisation of one area of the membrane causes a local current flow between neighbouring areas on the membrane
  • This current flow causes depolarisation immediately adjacent to the site of the original stimulus
  • The process repeats itself along the whole length of the membrane so that the action potential moves along the membrane away from the point of stimulation.
  • The myelin sheath insulates the fibre from the extracellular fluid so that ions cannot flow between the inside and outside of the membrane, and an action potential cannot form
  • In this case, the action potential jumps from one node of Ranvier to the next because the myelin sheath is absent from the nodes.
  • Because of this jumping conduction, known as saltatory conduction, the nerve impulse travels much faster along myelinated fibres.
17
Q

Conduction of Nerve Impulses

A
  • The same size of a nerve impulse that travels along a fibre is always the same
  • A weak stimulus, provided it exceeds the threshold, produces the same action potential as a strong one, an all-or-none response – a stimulus is either strong enough to trigger off an impulse or it is not.
  • The magnitude of the impulse is always the same
  • There are two things that enable us to determine the strength of a stimulus:
  • A strong stimulus causes depolarisation of more nerve fibres than a weak stimulus
  • A strong stimulus produces more nerve impulses in a given time than a weak stimulus
18
Q

Transmission across a synapse

A
  • As the action potential reaches the axon terminals, depolarization triggers threshold of calcium voltage-gated channels.
  • Calcium rushes into the cell and binds with vesicles (containing neurotransmitters) in the axon terminal
  • This causes the vesicles to migrate to the edge of the cell and attach to the cell membrane
  • Exocytosis occurs, releasing the neurotransmitters into the synaptic cleft.
  • The neurotransmitters diffuse across the cleft/gap and attach to receptors on the membrane of the next neuron
  • (Usually) initiating a new action potential/impulse in the next neuron
  • For example: acetylcholine, adrenaline, dopamine and histamine
  • The transmission of nerve impulses across a synapse will only occur in one direction - from axon to dendrite or from axon to cell body
19
Q

Effects of chemicals on the transmission of nerve impulses

A
  • There are many chemicals, both natural and synthetic, that influence the transmission of nerve impulses.
  • Most of these work by affecting transmission at the synapse or at the neuromuscular junction.
  • Stimulants such as caffeine and Benzedrine stimulate transmission at the synapse.
  • Other drugs, such as anaesthetics or hypnotics, depress the transmission.
  • Venom from certain species of snakes and spiders also affects the neuromuscular junction.