How Nerves Work

Question 1

Describe the anatomical organisation of the nervous system

Answer 1

CNS - brain, spinal chord

PNS - nerves

Question 2

Describe the structure of the brain

Answer 2

meninges (protective membrane) - dura, arachnoid + pia mater

cerebellum

cerebrum - frontal, temporal, pariental, occipital lobes

diencephalon - thalamus, hypothalamus

brainstem - midbrain, pons, medulla oblongata

Question 3

Describe the various spinal nerves

Answer 3

31 pairs of spinal nerves

8 cervical - neck, shoulders, arms

12 thoracic - chest, abdomen

5 lumbar - genitalia, gastrointestinal tract

1 coccygeal

Question 4

describe the structure of the spinal chord

Answer 4

white matter - myelinated axons

grey matter - neuronal cell bodies

dorsal horn, ventral horn

dorsal root afferent (sensory)

ventral root efferent (motor)

Question 5

Describe the general structure of Neurons

Answer 5

cell body (soma) - contains the nucleus

dendrites - receive information

initial segment (axon hillock) - triggers action potential

axon - sends action potential

axon (presynaptic) terminals - release transmitter

Question 6

describe the main cells in the nervous system

Answer 6

neurons - sensory (sensory receptor), inter (CNS), motor (acts on muscles, glands or neuron)

glia - 90% of cells in CNS
astrocytes (maintain the cell external environment for neuron, surrounds blood vessels & produce blood brain barrier)
obligondendrocytes (form myelin sheaths in the CNS - schwann cells in the PNS)
microglia (phagocytic hoovers mopping up infection)
Ependymal cells (produce the cerebrospinal fluid)

Question 7

Describe the ionic basis of the resting membrane potential

Answer 7

Phospholipid bilayer impermeable to water and ions, equal conc. of NaCl and KCl inside and outside cell = no membrane potential

now Na/K pump has evolved, uses ATP to pump K ions into cell and Na ions out 1:1, charge still balanced = no membrane potential

now leaky K ion channels leak out some K ions down conc. gradient, builds up electrical gradient, an equilibrium is reached when the electrical gradient is equal and opposite to conc. gradient = resting membrane potential

Question 8

Describe the properties of graded potentials

Answer 8

generator potentials - at sensory receptor

Postsynaptic potential - at synapse

Endplate potentials - at neuromuscular junction

Pacemaker potentials - in pacemaker tissues

small stimulus will trigger small response and open few channels

large stimulus will trigger large response and open many channels

only useful for short distance as axon is “very leaky” as signal gets smaller as travel along membrane

Question 9

Describe the function of graded potentials

Answer 9

to depolarise the cell to threshold and make it fire an action potential

Question 10

Describe the ionic basis of action potentials

Answer 10

K ions leak out of cell

voltage gated Na ion channels open - Na ions rush into cell

voltage gated channels close cell repolarises and voltage gated K ion channels open

Question 11

Describe the properties of action potentials

Answer 11

have a threshold

are all or nothing - action potential produced or not

stimulus intensity in their firing frequency, not amplitude

have a refractory period - always goes forward

self-propagating - grows action potentials as it goes along membrane

travels slowly

Question 12

How to increase conduction velocity

Answer 12

large axons - electrical current flows more easily down large axon than small axon (lower axial resistance), allows the Na channels to be wider spaced along the membrane - depolarisation spreads further

myelination - schwann cells in PNS and oligodendrytes in CNS, wrap myelin and sections of axon, increases membrane resistance and reduces membrane capacitance - less current wasted, action potential spread passively from node to node (saltatory conduction)

Question 13

Describe the consequences of demyelination

Answer 13

multiple sclerosis in CNS & Guillain - Barre syndrome in PNS

attack myelin sheath

membrane resistance is decreased and capacitance is increased

more current lost between nodes

conduction fails

Question 14

Describe the structure of the neuromuscular junction

Answer 14

synapse between motor neuron and skeletal muscle

contains: voltage gated Na channel, voltage gated Ca channel, Ach receptor, acetylcholinesterase enzyme, vesicle containing Ach (mEPPs and quanta)

Question 15

Describe the process of the neuromuscular junction

Answer 15

1. action potential in motor neuron
2. opens voltage gated Ca channels in presynaptic terminal
3. ACh binds to ACh (nicotrinic) receptors
4. Opens ligand-gatedNa/K channels
5. Evokes graded (local) potential (end plate potential) - always depolarises adjacent membrane to threshold
6. Opens voltage gated Na channels - evokes new AP
7. ACh removed by acetylcholinesterase

Question 16

Describe the classification of nerve fibre types

Answer 16

classed into three types:

group A nerve fibers, group B nerve fibers, and group C nerve fibers

Groups A and B are myelinated, and group C are unmyelinated.

These groups include both sensory fibers and motor fibers.

Question 17

Describe the ultrastructure of synapses between the neurons

Answer 17

1) the presynaptic membrane which is formed by the terminal button of an axon
2) the postsynaptic membrane which is composed of a segment of dendrite or cell body
3) the space between these two structures which is called the synaptic cleft.

Question 18

Describe the function of synapses between the neurons

Answer 18

allows a signal to pass from one neuron to the next

Question 19

Describe the process of synaptic transmission in the CNS

Answer 19

Range of neurotransmitters- acetylcholine, noradrenaline, dopamine, serotonin, histamine, glutamate, GABA, glycine etc

range of postsynaptic potentials, small postsynaptic potentials

several receptors - fast EPSPs, slow EPSPs, fast IPSPs, slow IPSPs - generally small and enables complex synaptic integration

synaptic connectivity - convergence, divergence, feedback inhibition, monosynaptic & polysynaptic pathways

Question 20

Explain the role of synapses in integration of neuronal function

Answer 20

initial segment crucial to determining if cell reaches threshold (fire action potential)

axo-axonic synapse - manipulates the effects of a postsynaptic neuron’s firing on the neurons further downstream in the network = pre synaptic inhibition

axo-somatic synapse -formed by one neuron projecting its axon terminals onto another neuron’s axon = post synaptic inhibition

axo-dendritic synapse -synapses that one neuron makes onto the dendrite of another neuron = spatial summation

Question 21

describe common excitatory and inhibitory neurotransmitters

Answer 21

Excitatory - epinephrine and norepinephrine

inhibitory - serotonin and gamma-aminobutyric acid (GABA)

Question 22

Explain the basic properties of signal transduction in neurons

Answer 22

Neurons are connected by synapses in biological neural networks

When presynaptic neuron emits a spike, then neurotransmitters are released from the synapses and bind to receptors located in the postsynaptic cell to excite or inhibit the postsynaptic neurons

This process facilitates the transmission of information

Question 23

What is meant by EPSP and IPSP

Answer 23

EPSP - excitatory postsynaptic potential (opening Na/K channels or closing leaky K ion channels)

IPSP - inhibitory postsynaptic potential ( opening Cl ion channels or opening K ion channels)