Chapter 03: Nerve Cells And Nerve Impulses

Question 1

• What is the nervous system?
• Explain the difference between the CNS and the PNS.
• Explain the difference between a neurone, a nerve fibre and a nerve.
• What are dendrites?
• What are axons?
• What is myelin?
• What is grey matter?
• What is white matter?
• How is myelin sheath formed?

Answer 1

• The body system involved with control and coordination of the body. It consists of a network of nerves and cells carrying messages to and from the CNS to various parts of the body.
• CNS is the N.S’s control centre consisting of brain and spinal cord. Whereas the PNS is the nerves that connect and carry messages to and from the CNS with various parts of the body.
• A Neurone is the basic structural and functional units of the N.S. A nerve fibre is a long extension of the nerve cell body - axon. A nerve is a bundle of nerve fibres held together by connective tissue.
• Extensions of the neurone cell body carrying nerve impulses to the cell body.
• Extensions of the neurone cell body carrying nerve impulses away from the cell body.
• White, fatty material surrounding some nerve fibres, increasing impulse speed.
• Part of the CNS made up of nerve cell bodies and unmyelinated fibres
• Part of the CNS made up of myelinated fibres.
• Schwann cells wrap around the axon and deposit layers of myelin between each coil.

Question 2

• What are nodes of ranveir?
• What are 3 important functions of the myelin sheath?
• What is the neurilemma?
• Depending on their functions there are 3 different types of neurones. Name the 3 types and describe the function of each.
• Explain the difference between multipolar, bipolar and unipolar neurones and which part of the N.S each type of neurone is found?
• What is a synapse?
• The speed at which an impulse travels can depend on whatelse other than myelination?

Answer 2

• A gap in the myelin sheath of a nerve fibre.
• Insulate, protect and speeds up nerve impulse along the axon.
• Outermost layer of myelin sheath that helps repair the injured fibres.
  1. Sensory neurones: carries messages from receptors to CNS.
  2. Motor neurones: carries messages from CNS effectors.
  3. Interneurones: Carries messages between sensory and motor neurones and are located in the CNS.
  1. Multipolar: Have 1 axon and many dendrites extending from the cell body. Makes up most interneurones of CNS and motor neurones that carry messages to the skeletal muscles.
  2. Bipolar: Have 2 extensions from the cell body. Occurs in the eye, ear and nose.
  3. Unipolar: Has 1 extension from the cell body. Makes up sensory neurones carrying messages to the spinal cord.
• Gap between successive neurones allowing the creation of a specific nerve pathway.
• Diameter of the fibre.

Question 3

• What is a neuromuscular junction?
• Why is a nerve impulse described as electrochemical?
• Explain how nerve impulses travel faster in myelinated fibres.
• Explain the process of how a neurone sends an impulse to another neurone in 6 steps.
• When a neurone is not stimulated, it’s membrane is polarised. What does this mean?

Answer 3

• A synapse of a axon and a skeletal muscle cell.
• It involves a change in electrical voltage, through changes in concentrations of ions inside and outside the neurone cell membrane.
• Action potential jumps from one node of ranveir to the next, allowing the nerve impulse to travel much faster along myelinated fibres.
  1. Nerve impulse arrives at axon terminal signalling calcium ions to enter the terminal.
  2. The entering calcium ions causes vesicles containing acetylcholine to rupture and release it into the synapse.
  3. Protein receptors on the dendrite of the next neurone are then stimulated by the acetylcholine and the nerve impulse continues.
  4. The synapse of the 2 neurones must be recreated so the acetylcholine must be destroyed.
  5. The enzyme acetylcholinesterase breaks the acetylcholine into acetate + choline and reabsorbed back into the axon terminal
  6. Energy in the mitochondria, joins the acetate + choline back into acetylcholine and repackages it into vesicles for the next impulse.
• Electrical charge outside the membrane is + while - inside.

Question 4

• What is an ion?
• Outside of the cell contains +ive sodium ions (NA+) and the inside of the cell membrane contains +ive potassium ions (K+). How can the charge inside the cell be -ive if the cell contains positive ions?
• What 2 ways maintains the nerve cell membrane resting potential?
• What is the resting membrane potential?
• What is special about -55mV
• Fully explain the process of an action potential.

Answer 4

• Atoms of an element with a positive or negative charge.
• In addition to K+, negatively charged organic protein molecules also inhabit the cell, so the inside is -ive compared to the outside.
  1. Na + P pumps on membrane maintains the NA+ ions out and the K+ ions in by pumping 3 NA+ ions out and 2 K+ ions in.
  2. Not enough +ive charged potassium ions inside cell to balance the effect of the large negatively charged organic ions trapped inside, thus the inside of the membrane is less +ive.
• -70mV, so the inside of the neurone is 70mV less than the outside.
• -55mV is the threshold. If the neurone does not reach this, no action potential will fire and will fire if is reached.
• Stimulus causes cell body to release Na ions into axon raising the resting potential to -55mV. Na channels open and diffuses the Na ions in and raises resting potential to +30mV, depolarising the cell. The Na channels then close and K channels opens, diffusing the K ions out. While the +ive K ions escape, inside the membrane becomes less +ive, re-polarising the cell so rapidly that the cell hyper-polarises. Eventually the Na K pump returns the ions back to a level of -70mV. This cycle of channels opening and closing continues along the whole length of the axon.

Question 5

• Define ‘all-or-none response’.
• What is the ‘refractory period’ of an action potential?
• Explain why the message can only cross a synapse in one direction.
• What are 2 differences of a nerve cell to other cells?
• Hyperkalaemia is a higher than normal level of potassium in the blood and therefore in the extracellular fluid. What effect would hyperkalaemia have on the resting membrane potential of nerve cells.
• Lightly press a pencil onto your palm. Gradually increase the force which you are pushing the pencil. How are you able to distinguish different intensities of the same stimulus?

Answer 5

• Size of response is not related to the strength of stimulus.
• A period during the action potential where the nerve fibre cannot be stimulated to respond again.
• Neurotransmitter receptors only occur on dendrite membranes.
• Nerve cells have extensions of the cytoplasm and can be stimulated to generate an action potential along its membrane.
• Potassium ions cannot be pushed out of the cell and the normal membrane potential cannot be maintained so no action potential.
• As you push hard more pain receptors are stimulated and more impulses are sent to the brain raising the feeling of pain.

Question 6

• What is the function of the nerve cell body?

- What is the function of the motor end plate?

Answer 6

• Contains the nucleus and other major organelles which determine the activities of the rest of the cell.
• Part of the sarcolemma of a muscle cell in close contact with an axon. Any nerve impulse that cross the synapse causes contraction of the muscle fibre.