Nervous System: Nerves

Question 1

Nerve Cells

Answer 1

• 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.

Question 2

Nervous Tissue

Answer 2

• 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.

Question 3

Synapses

Answer 3

• 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.

Question 4

Types of Neurons: Multipolar

Answer 4

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

Question 5

Types of Neurons: Bipolar

Answer 5

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

Question 6

Types of Neurons: Unipolar

Answer 6

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

Question 7

Types of Neurons: Pseudo-Unipolar

Answer 7

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

Question 8

Recap of Electricity

Answer 8

• 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)

Question 9

Membrane Potential

Answer 9

• 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

Question 10

Action Potential: Overview

Answer 10

• 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

Question 11

Action Potential: Step 1

Answer 11

• 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.

Question 12

Action Potential: Step 2

Answer 12

• 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

Question 13

Action Potential: Step 3

Answer 13

• 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

Question 14

Action Potential Step 4:

Answer 14

• 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

Question 15

After Action Potential

Answer 15

• 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

Question 16

Conduction along unmyelinated fibres

Answer 16

• 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.

Question 17

Conduction of Nerve Impulses

Answer 17

• 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

Question 18

Transmission across a synapse

Answer 18

• 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

Question 19

Effects of chemicals on the transmission of nerve impulses

Answer 19

• 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.