Chapter 3 - Nerve Cells And Nerve Impulses

Question 1

In a one sentence summary, what are nerve cells?

Answer 1

Nerve cells, or neurons, are the basic structural and functional units of the whole nervous system.

Question 2

What are are the differences between dendrites and axons?

Answer 2

Dendrites -

• carry nerve impulses to the cell body
• often highly branched.

Axon -

• a long, single extension of the cytoplasm.
• carry messages away from the cell body.

Question 3

Describe what a myelin sheath is and the differences between myelinated fibres and unmyelinated fibres.

Answer 3

Myelin sheaths are layers of fatty material that insulate the axon. In between each section of myelin are small spaces called nodes of ranvier. Schwann cells make up the myelin sheath.

Question 4

What is grey and white matter?

Answer 4

Grey matter consists of nerve cell bodies and unmyelinated fibres.
White matter consists of myelinated fibres and lipid.

Question 5

What is a neurilemma?

Answer 5

Around the myelin sheath the outmost coli of the Schwann cell forms a structure called the neurilemma, which helps in the repair of injured fibres.

Question 6

What are the 3 main functional types of neurons?

Answer 6

• Sensory (receptor) neurons carry messages from receptors in the sense organs or skin to the central nervous system.
• Motor (effector) neurons carry messages from the central nervous system to the muscles and glands - the effectors.
• Interneurons (association, connector, relay) neurons are located in the central nervous system and are the link between sensory and motor neurons.

Question 7

What are the 3 main structural types of neurons?

Answer 7

• multipolar neurons have one axon and multiple dendrites. Found in the brain, spinal cord, motor neurons to the skeletal muscles.
• bipolar neurons have one axon and one dendrite. Found in the eye, ear, nose and where they take impulses from sensory cells to other neurons.
• unipolar neurons have just one extension an axon where the cell body is located to one side. Found in the sensory neurons that carry messages to the spinal cord.

Question 8

Define what a synapse is and it’s function.

Answer 8

A synapse is the small space between the end branches of an axon of one neuron and a dendrite or cell body of another neuron. Messages are carried across the synapse.

Question 9

What is a neuromuscular junction?

Answer 9

The synapse that exists where an axon meets a skeletal muscle cell.

Question 10

What is a nerve impulse? (hint - electro)

Answer 10

A message that travels along a nerve fibre. Also known as a electrochemical change because it involves a change in electrical voltage brought about by changes in the concentration of ions inside and outside the cell membrane of a nerve cell.

Question 11

What is Salvatore conduction?

Answer 11

When a nerve impulse leaps from node of ranvier to node of ranvier of myelinated fibres.

Question 12

What is the concentration differences when a nerve cell is at resting membrane potential? (hint - what ions?)

Answer 12

Extracellular fluid - positive
HIGH in sodium chloride NA+
LOW in potassium K+

Intracellular fluid - negative
LOW in NA+
HIGH in K+

Question 13

How many mV are present at resting membrane potential?

Answer 13

-70 mV

Question 14

What is the difference between the central nervous system and the peripheral nervous system?

Answer 14

The CNS consists of the brain and the spinal cord. The control centre for the whole nervous system.

The PNS consists of the nerves that connect the central nervous systems to associated receptors, muscles and glands.

Question 15

What are Schwann cells?

Answer 15

Schwann cells are specialised cells which wrap around the axon and form the myelin sheath.

Question 16

What are the three main important functions of the myelin sheath?

Answer 16

Myelin sheath have three important functions ;

• acts as an insulator
• protects the axon from damage
• speeds up the movement of never impulses along the axon.

Question 17

Label a nerve cell.

Answer 17

• cell body
• nucleus
• dendrites
• axon
• myelin sheath
• Schwann cell
• neurilemma
• node of ranvier
• axon terminals
• synapse

Question 18

What are the maximum speeds a nerve impulse can travel along a myelinated and non myelinated axon?

Answer 18

In unmyelinated fibres, the maximum speed a nerve impulse can travel is 2m/s (7km/h).

In myelinated fibres, the maximum speed ranges between 18m/s (65/h) up to 140m/s (500km/s).

Question 19

What are nerve cells permeable to ?

Answer 19

The cell membrane is highly permeable to potassium and chloride ions,
Only slightly permeable to sodium ions
And impermeable to the large negatively charged organic ions.

Question 20

Explain the process of depolarisation.

Answer 20

Depolarisation occurs only if the level of the stimulation exceeds a threshold of 15mV. If the nerve impulse exceeds this threshold, sodium channels open ; sodium ions flood into the cell ; membrane becomes depolarised ; the membrane voltage rises to a maximum of 130mV. After this, the sodium channels close and the membrane becomes repolarised. For a very brief time, that part of the nerve fibre undergoes the refractory period until it returns to its normal resting membrane potential .

Question 21

What are neuro-transmitters?

Answer 21

At the synapse, special chemicals are released from the axon and diffuse across the gap and attach to receptors of the next neuron. Substances that diffuse across the synaps in this way are called neuro transmitters.

Question 22

What are some examples of neuro-mitters?

Answer 22

Acetylcholine, adrenaline, dopamine and histamine.