Neurons

Question 1

Neurons

Answer 1

Neurons, Glia and mechanisms of communication within the neuron

• junction between cell body and axon called axon hillock

Question 2

Multiple Sclerosis (MS)

Answer 2

• Acquired neurological disorder that attacks the myelin that surrounds the axons of neurons
• Begins with visual problems, numbness, weakness of the limbs
• Ultimately leads to paraplegia (paralysis of leg and lower body), slurred speech, problems with vision and eye movements
• Characterised by occasional ‘attacks’ (on average every couple of years), in which symptoms worsen, followed by quiescence (state/period of inactivity/dormancy) or remissions [symptoms disappear or are less severe]
• Autoimmune disorder that affects the insulation covering nerve cells (myelin)
• Cause is unknown, possibly believed to be due to a virus contracted around the time of birth/early childhood
• More common in females, typically appears in people in their late 20s or 30s

Question 3

Withdrawal Reflex

Answer 3

• Sense: dendrites of sensory neuron responds to noxious stimulus in the environment >> signal sent back along axon to the terminal buttons (axon terminal), located in the spinal cord
• Terminal buttons release neurotransmitter into the synapse, this stimulates the interneuron (resides in the spinal cord)
• Interneuron sends message down its axon, releasing a neurotransmitter to excite the motor neuron
• Axon of motor neuron joins a nerve (bundle of motor neurons) and travels to muscle in the arm (at motor end plate, forms neuromuscular junction), causing muscle to contract and pulling the hand away from the hot surface

Question 4

The role of Inhibition

Answer 4

• Withdrawal reflex cause excitatory effects in the synaptic connections
• This excitation can be counteracted by inhibition arising from another source: the brain
• Brain contains complex circuits of neurons that represent the consequences of dropping the dish e.g. embarassment, loss of meal, mother’s anger etc.
• These circuits send information to spinal cord to prevent you from dropping the dish
• Relevant neuron in the brain sends message along its axon to the spinal cord. Here, it excites an inhibitory interneuron, which releases an inhibitory neurotransmitter. This decreases activity of the motor neuron, thus blocking the withdrawal reflex
• Excitatory and inhibitory effects compete against one other to achieve desired result - illustrate principles of neural communication

Question 5

Neuronal structure reflects the function (to some degree)

Answer 5

• The shape and size fo a neuron is related to its function
• Neurons of similar architecture tend to be clustered together in CNS, reflecting the functioning of that particular region

Question 6

Neuron - what is it?

Answer 6

• Basic information-processing and information receiving unit of the nervous system.
• Neurons from complex networkds within the nervous system but are not directing connected with one another, they are separated by tiny gaps called synapses, across which chemicals called neurotransmitters are passed
• Neurons come in many different shapes and sizes. Almost all have four basic structures/regions
• Neurons may receive information from the terminal buttons of many other neurons and many themselves send information to many other neurons via their own terminal buttons. Terminal buttons may form synapses with the cell body or the dendrites of other neurons.

Question 7

Cell Body

Answer 7

• Contains the nucleus (genetic material) and internal organelles necessary for cell maintenance and proper functioning of the cell
• The cytoplasm is a jelly-like substance inside the cell. Within the cytoplasm are the mitochondria, which use glucose to produce energy. The mitochondria produce a chemical called ATP which can be used throughout the cell as an energy source
• The nucleus contains the cell’s chromosomes, which are composed of DNA; the genes that make up the chorosomes provide the reciples for making proteins. These proteins are needed to build the cell, and also to form enzymes that create and break down molecules

Question 8

Dendrites

Answer 8

• Tree-like brances that allow neurons to communicate with one another
• Dendrites receive information from other neurons

Question 9

Axon

Answer 9

• a long slender fibre that carries signals from the cell body
• the signal carried by an axon is an action potential, which as we shall see later is a wave of electrical potential that begins at the cell body an travels down the axon to the terminal buttons (axon terminal)

Question 10

Terminal Buttons (axon terminal)

Answer 10

• small knobs at the ends of the many branches of the axons
• these structures play a critical role in transmitting information from one neuron to another, by secreting a chemical called a neurotransmitter. This chemical passes across the synaptic gap and can either excite or inhibit the next neuron in the chain

Question 11

Supporting cells

Answer 11

Neurons constitute only half the volume of the CNS, the other half is made up of various other cells, collectively known as glial cells (or glia)

• Glial cells:
• provide support
• assist with chemical transport to and from neurons
• provide insulation
• destroy and remove neurons that have died from injury or old age (phagocytosis)

Question 12

Astrocytes

Answer 12

• provides physical support for neurons and also do housekeeping jobs (cleaning up waste, providing nutrients to neurons, maintaining the correct chemical composition of the extracellular fluid that surrounds neurons).
• Some astrocytes literally crawl around the CNS, cleaning away the debris from dead neurons, a process called phagocytosis.
• After removal of dead neurons, other astrocytes will take their place, thus maintaining a supportive structure for nearby cells.

Question 13

Oligodendrocytes

Answer 13

• provide physical support to neurons, but most importantly they provide the insulating myelin sheath that surrounds the axon. This prevents unwanted cross-talk between neighbouring axons. Most, but not all, axons are myelinated
• CNS
• This tube is made up of a series of segments of myelin, each roughly 1mm long and with a small gap of uncoated axon between them. These gaps are called Nodes of Ranvier (after their discoverer)

Question 14

Microglia

Answer 14

• the smallest glial cells
• act as phagocytes, like some astrocytes; thet also act as the brain’s immune system, attacking invading micro-organisms
• largely responsible for inflammation after brain damage

Question 15

Schwann Cells

Answer 15

• Perform the same function as oligodendrocytes, only they do this in the peripheral nervous system (PNS)
• They create myelin sheath around the axons of neurons in the PNS
• the tube is made of segments, and each segment consists of one entire schwann cell
• Also possess a special function not shared by oligodendrocytes: when there is damage to an axon, they digest the remaning portion of the fibre and then aligh themselves into a hollow cylinder, to act as a guide for an axonal stump that resprouts after damage. This process helps reconnect axons with the muscles and sense organs with which they were originally connected

Question 16

Neuron’s cell membrane

Answer 16

• cell membrane of a neuron is composed of a double layer of lipid (fat) molecules, and contains complex protein molecules that regulate the entrance and exit of chemicals from the neuron
• keeps the fluid outside the cell (extracellular fluid) separated from that inside the cell (intracellular fluid)
• the cell membrane is critical for the transmission along the axon

Question 17

Resting membrane potential

Answer 17

• an electrical process which involves movement across the axon membrane of electrically charged molecules called ions
• by inserting a very fine microelectrode (less than 1/1000th of a mm in diameter) into a neuron and recording the difference in electrical potential between the intracellular and extracellular fluid, it has been found that the inside of an axon is more negatively charged than the outside
• The difference is known as the resting membrane potential = about -70 millivolts (mV, a thousandth of a volt)
• the message that is conducted (action potential) down an axon involves a brief change in the membrane potential that sweeps rapidly along the axon from the cell body toward the terminal buttons

Question 18

Depolarisation by injecting current

Answer 18

• it is possible to disturb the resting potential of a neuron by passing a current into it, via another electrode placed into the cell body
• the inside of the neuron is negatively charged so adding a positive electrical current through the electrode causes depolarisation
• a very rapid charge reversal (depolarisation) of the membrane potential of an axon is called an action potential
• the action potential constitues the basic message that is transmitted down an axon from the cell body to the terminal buttons

Question 19

Membrane potential - a balance of two forces

Answer 19

• this is determined by the balance of positively and negatively charged ions inside and outside the neuron
• there are two forces that are relevant:

1. Diffusion
2. Electrostatic pressure

Question 20

Diffusion

Answer 20

• movement of solute from an area of high solute concentration to an area of low solute concentraton until equilibrium is established

Question 21

Electrostatic pressure

Answer 21

• when some substances are added to water, they split into two parts (ions), each with an oppositing electrical charge
• Two types of ions:

1. anion: negatively charged (gained an electron)
2. cation: positively charged (lost an electron)

Question 22

Ions movement across the axon membrane

Answer 22

• forces of diffustion and electrostatic pressure determine the resting membrane potential of a neuron
• four ions that are critical to the resting membrane potential; two cartion and two anions
• anions: chloride, organic ions (which are proteins A-)
• cations: sodium, potassium
• organic anions only found inside the neuron, the other three ions are found both inside and outside
• sodium and chloride higher conc. ouside the neuron, potassium and organic anions are in higher concentration inside
• extracellular fluid is salty (NaCl) like seawater, ancient ancestors of our cells lived in the ocean so the sea water wouldve literally been their extracellular fluid
• organic anions A- cannot pass through membrane of axon so remain within
• potassium ions are higher in conc. inside the axon so diffusion pushes them out but outside of axon is positively charged so the force of electrostatic pressure tends to drive them back in (+ repels +); two opposing forces are in balance
• Similar for chloride, higher conc. outside cell so diffusion pushes them into the ions, whereas electrostatic pressure pushes them out (- repels -); two opposing forces in balance
• different for sodium, higher conc. outside cell so diffusion pushes them in. Inside of cell is negatively charged and sodium is positively charged

Question 23

Sodium-Potassium Pump

Answer 23

• actively pushes sodium ion out of the cell
• consists millions of protein molecules (sodium-potassium transporters) embedded in the cell membrane
• uses energy, in the form of ATP, provided by the cell’s mitochondria to drive out sodium ions in exchange for potassium in a ratio of 3:2
• 3 sodium ions out and 2 potassium ions in
• sodium potassium transporters are present in neurons and glia (and most other cells of the body), they consume about 40% of a neuron’s metablic resources

Question 24

The action potential - ion exchange across the axon

Answer 24

• normally the cell membrane is not very permeable to sodim but if the membrane were suddently to become permeable to sodium, the forces of diffusion and electrostatic pressure would allow sodium ions to rush into the cell, causing a sudden increase in the conc. of positively charged ions and changing the membrane potential
• This change in membrane permeablity is precisely what causes an action potential
• certain protein molecules in the cell membrane, known as ion channels, provide an opening through which ions can rapidly enter or leave the cell
• When ion channels for sodium open, sodium ions rush into the cell
• shortly after, ions channels specific to potassium open, allowing potassium ions to rush out of the cell

Question 25

Action potential - change in membrane potential over time

Answer 25

• An action potential can be described as the following sequence of events:

1. Once a neuron’s threshold (-50mV) for excitation is reached, sodium channels in the cell membrane open and there is a rapid influx of positively charged sodium ions. This produces a sudden change in membrane potential, from -70mV to +40mV (depolarisation)
2. Shortly afterwards (less than 1 millisecond), the potassium channels also open, allowing positively allowing positively charged potassium ions to leave the axon (repolarisation)
3. At the peak of the action potential (about 1 millisecond), the sodium channels close and cannot re-open until the membrane reaches its resting potential again (refractory period)
4. As potassium ions are moved out of the axon, the membrane slightly overshoots its resting values to -90mV (hyperpolarisation) before returning to its resting level (-70mV)

Question 26

Action Potential movement along the axon

Answer 26

An action potential is triggered when excitatory input is passed from the terminal buttos of a presynaptic neuron and received by the dendrites of the postsynaptic neuron. This excitatory signal is passed passively towards the axon of the postsynaptic neuron, where is stimulates the depolarisation fo the mmebrane if it reaches the threshold for excitation (-50mV)

Question 27

Saltatory Conduction

Answer 27

• Rather than moving as a single continuous wave down the axon, action potentials ‘jump’ into the gaps (Nodes of Ranvier) between segments of myelin that are wrapped around the axon (oligodendrocytes and schwann cells)
• Because an action potential is generated by the rapid influx of sodium ions into the cell, this process can only occur where the axon membrane is in direct contact with the extracellular fluid (i.e. at the Nodes of Ranvier). Within the myelinated portion of the axon, the elecrtrical signal is conducted possively (like electricity down a wire) until it gets to the next Node, at which point another action potential is generated. Although the strength of the electrical potential decreases as it moves along the myelinated portions of the axon, it is still large enough to trigger a new action potential at the next Node.
• This jumping of the action potential along myelinated axons has two advantages:

1. It saves energy because sodium-potassium pumps only have to work within the Nodes
2. It increases the speed of neural signalling, and thus the speed with which we perceive, react and think

Question 28

All-or-none law of the action potential

Answer 28

• Neurons have a threshold for excitation (-50mV), above which an action potential will reliably be triggered
• The action potential either occurs or it does not occur at all
• Once triggered, an action potential remains at the same amplitude (its membrane reahces the same level of depolarisation) and travels down the axon to its end

Question 29

Rate law of the Action Potential

Answer 29

• We all know that muscle contractions can be weak or strong and that sensory stimuli can be weak or intense
• If action potentials are all-or-none in nature, and their amplitude remains constant along the length of the axon, how can they represent information that can vary continuously from weak to strong?
• Variable information is signalled by the number of action potentials produced by a neuron (neuron’s rate of firing). A strong muscle contraction is caused by a high rate of firing of a motor neuron; similarly, a loud sound is represented by a high rate of firing of an auditory nerve fibre
• Thus the basic unit of information carried by axons is their rateof firing (known as the rate law)

Question 30

Effects of myelin damage in Mutiple Sclerosis (MS)

Answer 30

• the symptoms of MS are due to disruption of the normal process of transmission of the action potential along the axon. The usual saltatory conduction between the nodes of ranvier is disrupted
• Symptoms: sensory loss and weakness
• There is no cure for MS. The only form of treatment, a drug called interferon beta, modulates the responsiveness of the immune system and reduces the frequency and severity of attacks