WEEK 3 - NERVOUS SYSTEM

Question 1

Parasympathetic Nervous System (PNS)

Answer 1

Controls homeostasis and the body at rest, is responsible for the body’s “rest and digest” function

Question 2

Sympathetic Nervous System (SNS)

Answer 2

Controls the body’s response to perceived threat and is responsible for the “fight or flight” response

Question 3

Describe and identify major components of a neuron

Answer 3

Cell body: biosynthetic centre of a neuron
Dendrites: main input region (receiving signals from other neurons)
- convey incoming messages towards cell body (NOT action potentials, they are short distance signals called GRADED POTENTIALS)

Axons - any long nerve fibre
- The conducting region of the neuron; generates nerve impulses and transmits them.
- Nerve impulse is generated at the junction of the axon hillock and axon
- It is conducted to the axon terminals (secretory region)

Myelin Sheath; nerve fibres are covered with a segmented myelin sheath
* Protects and electrically insulates fibres, which increases the transmission speed of nerve impulses
* Myelin sheath in PNS are formed by Schwann cells
* CNS contains both myelinated and nonmyelinated axons.
* Myelin sheath is composed of oligodendrocytes

Question 4

TRUE or FALSE
Sensory (or afferent) convey impulses TO CNS

Answer 4

TRUE

Question 5

TRUE or FALSE
Motor (or efferent) neurons convey impulses TO CNS

Answer 5

FALSE
Motor (or efferent) convey impulses FROM CNS to effector organs (muscles and glands)

Question 6

Describe the division of CNS

Answer 6

Central Nervous System (CNS); consists of the brain and spinal cord.
* Occupy dorsal body cavity

Question 7

Describe the division of PNS

Answer 7

Peripheral Nervous System (PNS); part of the nervous system outside of CNS

Question 8

Describe the division of Somatic Nervous System

Answer 8

Somatic/ Voluntary Nervous System; composed of somatic motor nerve fibres that conduct impulses from CNS to skeletal muscles

Question 9

Describe the division of ANS

Answer 9

Autonomic/ Involuntary nervous system (ANS); consists of visceral motor nerve fibres that regulate the activity of smooth muscles, cardiac muscles and glands

Question 10

Define resting membrane potential (RMP) and recognise the contribution that ion distribution across the membrane makes to RMP

Answer 10

RMP: voltage across the membrane is approximately -70mV.
* The minus sign indicates that the inside of the membrane is negatively charged relative to the outside.
* -70mV is the potential difference in a resting neuron.
* The membrane is said to be polarised.
* Is 25 times more permeable to potassium
* The K+ flowing out of the cell causes the inside to be more -ve
* Na+ trickles into the cell which makes it slightly more +ve
* Because some K+ is always leaking out of the cell, some Na+ is always leaking in * Sodium-potassium pump: ejects 3 Na+ from the cell and transports 2 K+ back into the cell

Question 11

When do ligand-gated channels open?

Answer 11

Chemically gated/ ligand-gated channels: open when the appropriate chemical (i.e. neurotransmitter) binds

Question 12

When do voltage-gated channels open?

Answer 12

Voltage-gated channels: open and close in response to changes in the membrane potential

Question 13

When do mechanically gated channels open?

Answer 13

Mechanically gated channels: open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure)

Question 14

Action potential

Answer 14

• Long-distance signal of axons. Aka nerve impulse
• Brief reversal of the membrane potential from -70mV to +30mV along the axon

Question 15

Repolarisation

Answer 15

• when membrane potential is returning to resting value

Question 16

Depolarisation

Answer 16

• when membrane potential is more positive than the RMP

Question 17

Hyperpolarisation

Answer 17

• when membrane potential is more negative than RMP

Question 18

RESTING STAGE

Answer 18

All gated Na+ and K+ channels are closed
* ONLY leakage channels are open (maintains RMP)

Question 19

DEPOLARISATION

Answer 19

Na+ channels open
* Na+ rushes into the cell
* Interior becomes less and less negative
* When depolarisation reaches threshold level, it becomes self-generating (positive feedback)

Question 20

REPOLARISATION

Answer 20

Na+ channels are inactivating, and K+ channels open
* Slow voltage-gated Ka+ channels open and K+ rushes out of the cell (following its electrochemical gradient)
* Na+ entry into the cell declines/ stops
* This restores the internal negativity of resting neuron

Question 21

HYPERPOLARISATION

Answer 21

Some K+ channels remain open and some Na+ channels reset
* The period of increased K+ permeability lasts longer than needed
* Sodium-potassium pump redistributes the ions

Question 22

Describe how neurons communicate with each other by chemical signalling molecules released at the synapse

Answer 22

Axon terminal: contains synaptic vesicles (containing neurotransmitters)

Question 23

Describe the relevance of graded potentials such as excitatory postsynaptic potential (EPSPs) and inhibitory postsynaptic potentials (IPSPs) to synaptic transmission

Answer 23

• Local changes in membrane potential of postsynaptic membrane
• Changes in membrane potential are graded (vary in size) * EPSP; Excitatory Postsynaptic potential * IPSP; Inhibitory Postsynaptic potential

Question 24

Why is a neurotransmitter that causes depolarisation of the postsynaptic membrane, excitatory?

Answer 24

It brings the membrane closer to threshold

Question 25

What is a depolarising postsynaptic potential called?

Answer 25

excitatory postsynaptic potential (EPSP)

Question 26

Does a single EPSP initiate a nerve impulse? Will it reach threshold?

Answer 26

No, but the postsynaptic cell does become more excitable and because it’s partially depolarised, it is most likely to reach threshold when the next EPSP occurs

Question 27

What is an inhibitory neurotransmitter?

Answer 27

A neurotransmitter that causes hyperpolarisation of the postsynaptic membrane

Question 28

Why is action potential more difficult than usual during hyperpolarisation?

Answer 28

The membrane potential becomes more negative and thus even farther from threshold than in its resting state

Question 29

A hyperpolarising postsynaptic potential

Answer 29

Is termed an inhibitory postsynaptic potential (IPSP)

Question 30

Explain why neurons have absolute and relative refractory periods and recognise why this means that action potential propagation can only be in one direction

Answer 30

• Absolute refractory period; ensures that a stimulus cannot produce a second action potential. Ensures each action potential is a separate event and enforces a one-way transmission of APs
• Ends when the voltage gated Na+ channels begin to transition from inactivated to resting/ closed state
• Relative refractory period: a second AP CAN be generated, but that’s only when the stimulus is exceptionally strong

Question 31

Explain the difference between saltatory and continuous action potential propagation, and explain the role of myelin sheath (nodes of Ranvier) in determining the speed of action potential propagation

Answer 31

In saltatory conduction, the nerve impulse will jump between the spaces between the nodes of Ranvier. This is faster than continuous conduction, where the nerve impulse travels down the whole unmyelinated neuron

Question 32

Stimulus intensity is encoded for within a single neuron by?

Answer 32

Action potential frequency and number of receptors stimulated.