Unit 4 - Nervous System

Question 1

Describe the parts of the neuron (nerve cell), starting from the dendrites to the synaptic knob (12)

Answer 1

• Dendrites (arms)
  
  • Contain receptor proteins that receive messages
  • Connects to another synaptic knob
    
    • Synaptic knob contains synaptic vesicles with release neurotransmitter chemicals
• Nucleus
• Cell body (like cytoplasm)
• Axon hillock (beginning of axon with accessory cells:)
  
  • Nodes of Ranvier
  • Myelin sheath (inner insulater)
  • Schwann cell (cells that compose the axon)
  • Schwann cell nucleus
  • Axoplasm (cytoplasm within axon)
• Axon terminals (end pieces of axon)
• Synaptic knob (tip of axon)

Question 2

State the 5 functions of the neuron and attrubute them to the specific parts

Answer 2

1. Receive information from the external environment (dendrites)
2. Integrate the information and produce a relevant signal (cell body)
3. Conduct the signal (axon)
4. Transmit the signal to other nerves/glands/muscles (synaptic knob)
5. Coordinate metabolic activities of nerve cell (nucleus)

Question 3

State the exterior parts of a nerve (6)

Answer 3

• Epineurium: membranous outermost covering tissue of the nerve; surrounds all the separate bundles of neurons called nerve fibres
• Perineurium: membranous covering tissue which surrounds a single bundle of neurons
• Endoneurium: connective material inside the bundle of neurons; located between the separate neurons; provides support for neurons and blood vessels
• Small artery and vein
• Muscle fibers
• Neuromuscular junction: location where neuron meets muscle

Question 4

State the two specific types of nervous systems

Answer 4

The central nervous system (deals with brain and spinal chord) and the peripheral nervous system (divided into somatic nervous system and autonomic nervous system)

Question 5

What is the somatic nervous system? Autonomic?

Answer 5

The somatic nervous system covers two pathways: the sensory and motor pathways

The autonomic nervous system is divided into the sympathetic nervous system (fight of flight) and the parasympathetic nervous system (rest and digest)

Question 6

Explain fight or flight and rest and digest

Answer 6

“Fight or Flight”: heart rate, blood pressure and respiratory rate increase; digestion decreases, liver breaks down glycogen, kidneys activate adrenal gland (instead, energy is focused on critical components of the body) (ex., pupils expand)

“Rest and Digest”: heart rate, blood pressure and respiratory rate decrease; digestion increases (ex., pupils dilate)

Question 7

Define homoestasis and explain how the body maintains it (homoestasis feedback control)

Answer 7

Homeostasis: maintenance of “steady state”; i.e. maintaining body temperature of blood pH.

1. The body has many different types of receptors (ex., pain, chemo, thermo) that detect changes in different things (pain, smell and taste, temperature) triggered by a stimulus (too hot!).
2. The receptors register these fluctuations and convey the information via the afferent pathway (a.k.a. sensory pathway) to the integrating centre (brain).
3. The brain determines the best action to be performed, sending the message via the efferent pathway (a.k.a. motor pathway) to the effector unit (ex., muscle, gland, vessel).
4. This feedback control typically reverses the effects of the stimulus through a “response”. When this happens, it is called negative feedback.

Question 8

Explain what is meant by negative feedback and positive feedback

Answer 8

Negative: a response that reverses a changed condition

Positive: a response that intensifies the original condition.

Question 9

Describe a sample feedback control scenario

Answer 9

Low body temperature –> thermoreceptor –> brain –> blood vessels and muscle –> constrict vessels to reduce heat loss, shiver to produce heat (stimulus reversed)

Question 10

Describe a sample reflex arc scenario

Answer 10

Dog bites hand –> pain receptor –> sensory pathway –> interneuron (association neuron) found in spinal cord –> motor pathway –> muscles

Response: pull hand away

Question 11

Electrical activity in the brain can be monitored by:

Answer 11

Electroencephalography

Question 12

State and define the 3 terms used to describe a change in membrane voltage

Answer 12

Depolarisation: a shift to a more positive value than resting potential (ex., -30 mV)

Repolarisation: a shift back to resting potential

Hyperpolarisation: a shift to a more negative value (ex., -90 mV)

Question 13

Describe resting potential. Why is the inside always more negative than the outside of the membrane?

Answer 13

The resting membrane potential of a neuron is about -70 mV - this means that the inside of the neuron is 70 mV less than the outside.

Along the axon exists sodium and potassium pumps (3 Na⁺ out, 2 K⁺ in). The reason the outside of the axon is more positive than the inside is because:

1. The pump is more efficient with Na⁺ than K⁺ (more positives out than in)
2. The membrane is more permeable to K⁺ (any positives brought in tend to leak out)

Question 14

Describe graded potential

Answer 14

When the neuron is stimulated, a little bit of Na⁺ spills into the neuron through leak channels, depolarising the neuron. If enough Na⁺ enters (lots of simulation) and the “threshold potential” is breached, action potential begins. If not, this is graded potential.

Question 15

Describe action potential and outline the full process

Answer 15

1. When the threshold potential is breached voltage-gated Na⁺ channels open, allowing a large influx of sodium into the neuron, increasing positivity creating an upstroke (when threshold breached).
2. After a millisecond of being open, the sodium gates close and become temporarily deactivated (refractory period).
3. When the Na⁺ gates close, they trigger the opening of K⁺ gates, which allow potassium to flow out of the neuron, creating a downstroke.
4. When the downstroke dips below the threshold potential, the Na⁺/K⁺ pump kicks in and restores the membrane potential, completing the action potential.
5. This is the nerve impulse that travels along the neurons, jumping from nerve cell to nerve cell and travelling around the body. These action potentials occur many times along the length of the axon

Question 16

Compare graded potential vs. action potential

Answer 16

Question 17

What is saltatory conduction? Why is it a thing and why does it work well? How do invertebrates overcome this?

Answer 17

Saltatory Conduction: (vertebrates) the propagation of action potentials along myelinated axons. It allows for faster A.P. transmission with less metabolic energy.

Fast propagation is difficult without myelin sheaths because the ion gates are very slow moving. So, A.P. only occurs at the nodes of Ranvier.

Myelin sheath create a greater threshold between nodes and increase permeability between nodes.

Invertebrates overcome this slowness instead by increasing axon diameter. If humans did this, we would weight 10x as much!

Question 18

How does the neuron detect the intensity of an action potential?

Answer 18

Frequency; the greater the frequency, the stronger the stimulus

Question 19

Describe how the A.P. is transferred from one nerve to the next. What happens to NTs after?

Answer 19

Synaptic transmission occurs between the synaptic knob of the presynaptic neuron and a dendrite of the postsynaptic neuron.

1. When an A.P. arrives at the presynaptic cell membrane, it causes voltage-sensitive gated channels for Ca²⁺ to open
2. When Ca²⁺ flows into the cell, they cause the synaptic vesicles to fuse with the plasma membrane
3. The neurotransmitters are released into the clef (this is exocytosis)
4. Diffusion carries the NTs to the postsynaptic neuron where they briefly bind to the membrane receptor proteins on the synaptic membrane
5. After the transmitters exert their effects, they are either:
   
   1. Reabsorbed by the original synaptic knob
   2. Broken down an enzyme
   3. Diffused away

Question 20

Describe the ion channels that NTs can trigger

Answer 20

The NTs cause 1 of 3 ion channels to open which allow for specific ions to diffuse across the membrane in the dendrite:

1. Na⁺ - excitatory
2. K⁺ - inhibitory
3. Cl^- - inhibitory

Question 21

What is meant by excitatory or inhibitory?

Answer 21

Excitatory: any stimulus that either causes a nerve cell to fire, or simply makes it more likely to fire (aka more likely to send a signal).

Inhibitory: any stimulus that makes a nerve cell less likely to fire (aka less likely to send a signal).

For example, excitatory can refer to something that would encourage the nervous system to send more pain signals, more frequently. One nerve cell can have an excitatory effect on another, making that second cell want to fire.

On the other hand, some nerve cells communicate with other nerve cells and tell them not to fire. This is what is referred to as an inhibitory effect.

Question 22

A neurotransmitter may excite or inhibit depending on:

Answer 22

• The type of NT
• The concentration of the NT in the synapse/synaptic cleft (space between presynaptic neuron knob and postsynaptic neuron dendrite)
• Type of receptor
• Type of gated channel

Question 23

Define pharmacology

Answer 23

Pharmacology: the study of effects of chemicals on the body, both endogenous (made in body) and exogenous (made outside body)

Question 24

How do neurotransmitters differ from hormones?

Answer 24

Neurotransmitters act on adjacent cells and are rapidly degraded. Hormones are different, as they persist all around the body.

Question 25

Describe the importance of popular neurotransmitter ACh. How is it affected by tetanus?

Answer 25

Acetylcholine: ACh is released at the neuromuscular junction and causes the muscles to contract.

• ACh, when released, binds to the muscle receptors and cause the opening of depolarising ion channels, sending an action potential along the muscle and causing it to contract.
• In tetanus, ACh is not broken down by acetylcholinesterase (enzyme) after its use and so can cause things like lockjaw.

Question 26

Describe the function and location of secretion of the 4 main types of neurotransmitters

Answer 26

Question 27

What are neurotoxins? Provide an example and outline treatment

Answer 27

Neurotoxin: a poison that acts on the nervous system.

Curare: used as South American arrow poison in the past.

• Competes with ACh for the receptors on the muscle.
• When curare successfully binds to the receptors, ACh cannot contract the muscle (can’t breathe!)

Treatment: neostigmine blocks the enzyme action of acetylcholinesterase so that ACh can continue competing and growing in its numbers. In time, curare is destroyed.

Question 28

What are endorphins/enkephalins?

Answer 28

NTs that act as natural painkillers; they inhibit the release of “substance P”, an NT which simulates pain.