The Nervous System

Question 1

What are the two major parts of the nervous system?

Answer 1

• Central Nervous System
  
  • brain
  • spinal cord
• Peripheral Nervous System
  
  • Somatic NS
  • Autonomic NS

Question 2

How are neurons classified?

Answer 2

• Sensory
  
  • Conduct impulses
  • From sensory receptors to CNS
• Motor
  
  • Conduct impulses
  • From CNS to muscles/glands
• Association
  
  • In the CNS
  • Integrate functions

Question 3

Describe the functions of the motor neurons in PNS

Answer 3

• Somatic motor neurons
  
  • Reflexes
  • Voluntary control of skeletal muscle
• Autonomic motor neurons
  
  • Involuntary control
  • Sympathetic/Parasympathetic

Question 4

Describe the structure of neurons

Answer 4

• Cell Body
  
  • CNS: Nuclei
  • PNS: Ganglia
• Dendrites
  
  • Receive signals
• Axon
  
  • Conducts impulses

Question 5

What are the four types of Glial cells in the CNS?

Answer 5

• Oligodendrocytes
  
  • Form myelin sheath around axons
• Microglia
  
  • Phagocytize foreign/degenerated material
• Ependymal cells
  
  • Line brain ventricles
  • Secrete CSF
• Astrocytes
  
  • Regulate external environment

Question 6

What are oligodendrocytes?

Answer 6

• Produces myelin sheath in CNS
• Sends extensions to several axons
• Myelin gives white color = White matter

Question 7

Describe the myelin sheath in the PNS

Answer 7

• Produced by Schwann cells
• Node of Ranvier left open

Question 8

What are the functions of astrocytes?

Answer 8

• Repair damaged neural tissue
• Maintain blood-brain barrier
• Control blood volume (vasodilation)
• Provide structural support
• Guide neural development
• Regulate ion, nutrient, dissolved gas concentrations
• Absorb and recycle neurotransmitters

Question 9

Ion gating in axons - What are the types of channels?

Answer 9

• Passive
  
  • Always open (not gated)
  • “Leaky”
• Voltage-gated
  
  • Opens when membrane potential reached
• Chemically activated
  
  • On dendrites and soma
  • Open when activated by chemical messenger
  • “Receptors”

Question 10

Describe the resting membrane potential of a neuron

Answer 10

• Resting potential of -70mV
• Sodium ions more concentrated outside the cell
• Potassium ions more concentrated inside the cell
• Sodium-potassium exchange pump brings 3 Na+ outside and 2 K+ inside to stabilize resting potential

Question 11

What are electrical synapses?

Answer 11

• Cytoplasm of adjacent cells is directly connected by gap junctions
• Occur in
  
  • Smooth muscle
  • Cardiac muscle
  • Some neurons in brain
  • Between glial cells

Question 12

What are chemical synapses?

Answer 12

1. Action potential reaches axon terminals
2. Voltage-gated calcium channels open
3. Ca2+ binds to sensor protein in cytoplasm
4. Ca2+-protein complex stimulates fusion and exocytosis of neurotransmitter
   (Release of neurotransmitter from presynaptic neuron)
5. Diffuse across synaptic cleft
6. Bind to receptors on plasma membrane of postsynaptic neuron

Question 13

Describe the action potential in a neuron

Answer 13

• Resting membrane potential: -70mV
• Depolarizes to reach threshold: -45mV
• Voltage-gated sodium channels open → Na+ move inside
• Voltage-gated sodium channels inactivate, potassium channels open → K+ move outside: +30mV
• Repolarizes as channels return to normal states: -70mV

Question 14

Describe the refractory periods during the action potential of a neuron

Answer 14

• When nerve cannot fire another action potential
• Absolute refractory period: Inactivated Na+ channels
• Relative refractory period: K+ channels still open → hyperpolarized, requires strong stimulus for Na+ channels to open

Question 15

What is saltatory conduction?

Answer 15

• Myelin sheath provides insulation
• Speeds up conductions
• Action potential signals “leap” from node to node

Question 16

What is acetylcholine (a neurotransmitter)?

Answer 16

• Directly opens ion channels when bound to receptors
• Excitatory or inhibitory
• Depolarization ends: Acetylcholinesterase (AChE) breaks down ACh into acetate and choline
• Choline reabsorbed by axon terminal for resynthesization

Question 16

What are graded potentials?

Answer 16

• Changes in the transmembrane potential
• Cannot spread far from site of stimulation
• Produced by any stimulus that opens a gated channel

Question 17

What is an excitatory postsynaptic potential (EPSP)?

Answer 17

• Open Na+ or Ca2+ channels
• Graded depolarization
• Brings membrane potential closer to the threshold
• May require EPSPs from several neurons to produce an action potential

Question 18

What is an inhibitory postsynaptic potential (IPSP)?

Answer 18

• Open K+ or Cl- channels
• Graded repolarization
• Brings membrane potential further from the threshold
• Can counter EPSPs from other neurons

Question 19

Features of the sympathetic nerve

Answer 19

• Emerge from spinal cord at thoracic and lumbar levels
• Preganglionic nerve: Short
• Postganglionic nerve: Long
• Cholinergic fibres (ACh) and adrenergic fibres (NE)
• Active in stressful situations

Question 20

Features of the parasympathetic nerve

Answer 20

• Emerge from spinal cord at sacral segments
• Preganglionic: Long
• Postganglionic: Short
• Only cholinergic fibres (ACh)
• “Vegetative” activites eg digestion

Question 21

Receptors of ACh

Answer 21

• Sympathetic
  
  • Nicotinic receptors at the neuromuscular junction
  • Muscarinic receptors at sweat glands
• Parasympathetic
  
  • Muscarinic receptors

Question 22

Receptors of NE

Answer 22

• Sympathetic
  
  • Adrenergic receptors
  • Heart muscle: Faster heartbeat

Question 23

Effects of ACh in the parasympathetic nerve

Answer 23

• Bind to muscarinic receptors
• Heart
  
  • Opens K+ channels
  • Creates ISPS: Hyperpolarization
  • Slows heart rate
• Smooth stomach muscle
  
  • Closes K+ channels
  • Creates ESPS: Depolarization
  • Muscles contract

Question 24

Clinical applications