Ch. 3 Nerve Cells and Nerve Impulses

Question 1

What are the two divisions of the nervous system and what do they consists of

Answer 1

1. Central NS (CNS)
   - Brain and Spinal chord
2. Peripheral NS (PNS)
   - Nerves connecting CNS to receptors, muscles + glands

Question 2

What parts of the nerve cell carry impulses into and out of the cell

Answer 2

into= dendrites

out= axon

Question 3

Explain myelinated fibers

Answer 3

myelin sheath

Question 4

how is the myelin sheath made and what are its three functions

Answer 4

• formed by Schwann cells
  1. Protects axon from damage
  2. Speeds up movement of nerve impulses along axon
  3. Insulates

Question 5

what is the gap between each sheath called

Answer 5

nodes of ranvier

Question 6

difference between grey and white matter

Answer 6

Grey Matter
- Consists of nerve cell bodies + unmyelinated fibres
White Matter
- Consists of myelinated fibres

Question 7

What are the three functional types of nerve cells and explain

Answer 7

¬ Sensory (receptor)
- Carry messages from receptors in sense organs, or skin, to the CNS

¬ Motor (effector)
- Carry messages from CNS to muscles + glands (the effectors)

¬ Interneurons (connector/relay/association)

• Located in the CNS
• Link b/w sensory + motor neurons

Question 8

what are the three structural types of nerve cells and explain

Answer 8

¬ Multipolar

• One axon + multiple dendrites
• Most interneurons in CNS
• Motor neurons (to muscles)

¬ Bipolar

• One axon + one dendrite (many branches at each end)
• Eye, ear, nose= take impulses from receptor cells to other neurons

¬ Unipolar

• One extension (axon)
• Cell body to one side of axon
• Most sensory neurons (to spinal)

Question 9

What is the synapse

Answer 9

• Junction b/w branches of adjacent neuron where nerve impulses passed from one neuron to next (synaptic cleft)

Question 10

define neuromuscular junction

Answer 10

Synapse at axon + skeletal muscle cell

Question 11

why is a nerve impulse called a electrochemical change

Answer 11

o called as it involves change in electrical voltage brought about by changes in concentration of ions inside + outside cell membrane of neuron

Question 12

what are the two conditions that cause variation in speed of transmission of nerve impulses

Answer 12

1. Type fibre (un/myelinated)
   - Unmyelinated
   o Travels steadily (2m/s)
   - Myelinated
   o Salutatory conduction (jump from one node to another) (140m/s)
2. Diameter of Fibre

Question 13

What are the 6 steps of a nerve impulse

Answer 13

1. polarisation of membrane
2. resting potential
3. action potential
4. repolarisation
5. hyper polarisation
6. refractory period

Question 14

Nerve impulse Step 1: Polarisation of the Membrane, what is membrane potential

Answer 14

• Inside membrane has negative electrical charge compared to outside
  o Extracellular Fluid: high concentration of NaCl // Na + and Cl-
  o Intracellular Fluid: high concentration of K+ and negative ions
• = Membrane potential

Question 15

Nerve impulse Step 1: Polarisation of the Membrane, what are the two ways membrane potential is maintained

Answer 15

1. Actively moves ions across membrane (sodium potassium pump)
   o Transports Na+ out of cell and K+ inside
2. Cell membrane not equally permeable to all ions
   o // large number of neg ions trapped inside
   o although Na+ and K+ both +ve, not enough to counteract effect of large –ve ions

Question 16

Nerve impulse Step 2. Resting potential, explain

Answer 16

• Neuron inactive and polarized= resting potential

- Remains this way until stimulus comes along

Question 17

Nerve impulse Step 3. Action potential, what occurs

Answer 17

• When sodium ions move inside membrane

- Stimulus reaches neuron > membrane becomes more permeable to Na+

Question 18

Nerve impulse Step 3. Action potential, explain threshold

Answer 18

• Depolarisation occurs when threshold exceeded (15Mv)
  o inward movement= too great to be balanced by outward movement of K+ // membrane > depolarised
  o then movement of Na+ proceeds independently of stimulus (size of response not related to strength of stimulus (all-or-none response)
• Size of nerve impulse= same (regardless of size of stimuli)
• Weak stimulus (if exceeds threshold) = same action potential as strong (all or nothing)
• impulse magnitude= constant

Question 19

What is intensity of a nerve impulse caused by

Answer 19

• Strength determined by two things
  1. Strong= depolarisation of more nerve fibres
  2. Strong= produces more nerve impulses in a given time

Question 20

Nerve impulse Step 4. Repolarisation, what occurs

Answer 20

• Where K+ ions move outside, and Na+ ions stay inside the membrane
• Ion channels inside membrane open= K+ move outside > restores electrical balance
  o Opposite of initial polarization with K and Na on opposite sides
• After K gates open, Na closes otherwise could not repolarise

Question 21

Nerve impulse Step 5. Hypoerpolarisation, explain

Answer 21

• When K gates close, more K ions are on outside than Na on inside
  o // membrane potential drops lower than resting potential
  o greater potential

Question 22

Nerve impulse Step 6. Refractory period, explain

Answer 22

• When Na and K are returned to original sides
• That part of fibre cannot be stimulated
• Neuron back to polarized state

Question 23

How are nerve impulses conducted along unmyelinated fibres

Answer 23

• action potential generates another a.p. in front of it // a.p. does not travel, the message, nerve impulse does
• impulse prevented from backwards movement by refractory period

Question 24

How are nerve impulses conducted along myelinated fibres

Answer 24

saltatory conduction

• insulated from extracellular fluid except at nodes // ions cannot flow b/w inside/outside of membrane + action potential cannot form
  o // action potential jumps from node to node (not covered in sheath)

Question 25

What are the 8 steps of transmission across synapse

Answer 25

1. Nerve impulse/action potential arrive at axon terminal
2. Ca+ channels open and enter cell (calcium ions flow into presynaptic knob) > signal vesicles to move and fuse to membrane of presynaptic cell
3. Vesicles release neurotransmitters by exocytosis
4. Neurotransmitter diffuse across cleft to membrane and bind to receptors of postsynaptic cell (> triggers action potential)
5. Sodium ion channels in the post synaptic membrane open and sodium ions diffuse into the post synaptic neuron
6. Excitatory response produced causing the depolarization of the post synaptic membrane
7. Neurotransmitters are destroyed after impulse gone at reception site (// cannot go backwards)
   The neurotransmitter is broken down by a specific enzyme and released from the receptor proteins, sodium channels lose so no further sodium ions can diffuse into post synaptic neuron

Question 26

What effect do chemicals have on nerve impulses

Answer 26

• Prevent neurotransmitter fomr being broken down
  o // builds remains in synaptic cleft and builds up
  o Nerve impulses more likely
• Can result in all muscles in the body trying to contract
  o Cramping, vomiting
  o Loss of consciousness or coma
  o Sweating or nausea/ diarrhea