Lecture 6.1: The Organisation of the Nervous System

Question 1

Four structural classes of neurones

Answer 1

Multipolar Neuron
Bipolar Neuron
Psuedo-Unipolar Neuron
Unipolar Neuron

Question 2

Multipolar Neuron Structure

Answer 2

Most commonly, neurones are multi-polar with one axon and multiple dendrites

Question 3

Bipolar Neuron Structure

Answer 3

One dendrite and one axon
Include sensory neurones of the retina

Question 4

Unipolar Neuron Structure

Answer 4

Have one axon and no dendrites
All sensory neurones (except bipolar retinal) are unipolar (or pseudo-unipolar)

Question 5

Three functional classes of neurones

Answer 5

Motor
Sensory
Interneurons

Question 6

The Reflex Arc: the “simple” neurone circuit

Answer 6

Sensory neurones transmit information towards the CNS. Their axons form afferent fibres

Processing of information takes place in the interneurons

Motor neurones transmit impulses towards effector organs. Their axons form efferent fibres

In the simplest circuits, sensory neurones synapse directly with motor neurones

Question 7

Interneurons

Answer 7

• They are the smallest neurones
• They have a multi-polar morphology
• They make up at least 95% of the neurones of the nervous system
• Contained mostly within brain or spinal cord, also in enteric nervous system
• They act as information processors between sensory and motor networks

Question 8

Motor Neurons (Efferents)

Answer 8

Motor neurones are multipolar neurones that conduct impulses from the CNS to their effector organs/muscles

Hence are known as efferents

They are amongst the largest neurones in the body

They integrate a large array of inputs to synthesise a single output.

Question 9

The Anatomy of an Axon

Answer 9

Axons are supported by neurofilaments

Microtubules allow transport of molecules to (anterograde) and from (retrograde) axon terminals

Myelin is an insulator

Nodes of Ranvier facilitate saltatory conduction

Question 10

Anterograde

Answer 10

Towards axon terminal

Question 11

Retrograde

Answer 11

Away from axon terminal

Question 12

Resting Potential

Answer 12

3 Na+ pumped out for every 2 K+ pumped in

In unstimulated neurones, intracellular [K+] =160mM, extracellular [K+] =4.5 mM

Extracellular [Na+]= 145mM, intracellular [Na+]=10mM

This produces a potential difference of -70mV

Question 13

Action Potentials

Answer 13

Upon stimulation, nerve impulses (action potentials) are propagated as waves of depolarisation (+40mV)

Voltage-gated Na+ channels open to allow the entry of sodium ions down a concentration gradient

After depolarisation, voltage-gated Na+ channels close and K+ channels open, allowing potassium ions efflux to restore the resting potential

Normal resting [Na+] and [K+] restored by the action of the sodium/potassium ATPase

Question 14

Dendrites: Function

Answer 14

Dendrites are specialisations of the cell body that provide a large surface area for signal reception and processing

Question 15

Dendrites: Structure

Answer 15

Their tree-like structure (arborisation) means that one neurone can integrate signals from a large number of upstream neurones

Most synapses occur at dendritic spines, which are important in neural plasticity

Question 16

Dendrites: Downs Syndrome

Answer 16

People with Down syndrome have fewer dendritic spines, which may explain associated learning difficulties

Question 17

Neural Plasticity

Answer 17

It is the ability of neural networks in the brain to change through growth and reorganisation

Question 18

Temporal Summation

Answer 18

Inputs are summated with respect to their time of arrival at the dendrite

Converts a rapid series of weak pulses from a single source into one large signal

Question 19

Spatial Summation

Answer 19

Inputs are summated with respect to their relative spatial location on the cell body or dendrites

Several weak signals from different locations are converted into a single larger one

Question 20

Multiple Sclerosis

Answer 20

Unknown aetiology
Is a disorder of myelination
Most common cause of non-traumatic neurological impairment
Disease results from the autoimmune destruction of myelin in the CNS

Question 21

Multiple Sclerosis: Treatments

Answer 21

No effective treatments exist for progressive forms of the disease
But β-interferon can effect improvements in relapsing-remitting disease

Question 22

Guillain–Barré Syndrome

Answer 22

Rapid onset muscle weakness in Guillain–Barré syndrome results from auto-immune damage to myelin in the PNS

Question 23

Classification of Nerve Fibres (3)

Answer 23

Group A, B and C fibres

Question 24

Classification of Nerve Fibres: Group A

Answer 24

Mainly somatic sensory and motor fibres
Most heavily myelinated
Largest diameter
A-alpha fibres conduct impulses at up to 120m/s
A-beta fibres carry sharp, localised pain at 3-15 m/s

Question 25

Classification of Nerve Fibres: Group B

Answer 25

Lightly-myelinated include pre-ganglionic autonomic fibres. Speeds of 12-30m/s

Question 26

Classification of Nerve Fibres: Group C

Answer 26

Non-myelinated Include post-ganglionic autonomic fibres Include somatic sensory fibres relaying diffuse pain Speeds up 0.5-2.3m/s

Question 27

Communication between Neurones (2)

Answer 27

Electrical Synapses Chemical Synapses

Question 28

Electrical Synapses

Answer 28

Allow for the fastest conduction of nerve impulses Although there is no capacity to increase the “gain” of the signal They are quite rare, being found (for example) in the retina

Question 29

Chemical Synapses

Answer 29

Unidirectional Can be excitatory or inhibitory, Thus can modulate the signal received from the presynaptic neurone

Question 30

Chemical Synapses: How it works?

Answer 30

1) Action potential reaches the axon terminal 2) Causes an influx of calcium ions 3) This stimulates fusion of neurotransmitter vesicles with the cell membrane 4) Contents are released into the synaptic cleft via exocytosis 5) Here they can interact with receptors on postsynaptic membranes

Question 31

Somatic Nervous System

Answer 31

Voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles

Question 32

Autonomic Nervous System

Answer 32

Component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric

Question 33

Sympathetic NS

Answer 33

The sympathetic nervous system directs the body's rapid involuntary response to dangerous or stressful situations, is excitatory

Question 34

Parasympathetic NS

Answer 34

The parasympathetic nervous system leads to decreased reaction, is inhibitory

Question 35

Enteric NS

Question 36

Acetylcholine

Answer 36

A common neurotransmitter in both the somatic & autonomic nervous systems Preganglionic fibres in the ANS release ACh, as do parasympathetic post ganglionic fibres (hence are cholinergic)

Question 37

Noradrenaline

Answer 37

A common neurotransmitter in both the somatic & autonomic nervous systems Most sympathetic post-ganglionic fibres release noradrenaline (hence are adrenergic)

Question 38

Dopamine

Answer 38

Is involved in reward pathways and motor control It also regulates kidney function

Question 39

Parkinson’s Disease

Answer 39

Results from the loss of dopamine-secreting neurones in the substantia nigra (a region in the midbrain that is considered part of the basal ganglia)

Question 40

Glutamate

Answer 40

In the CNS, glutamate opens Na+ channels Excitatory

Question 41

Glycine

Answer 41

In the CNS, openS Cl- channels Inhibitory

Question 42

The PNS: cranial and spinal nerves

Answer 42

Ten pairs of cranial nerves emerge from the brain stem, and two emerge from the brain These mainly provide motor and sensory innervation to structures within the head There are 31 pairs of segmental nerves emerging from the spinal chord Most peripheral nerves are termed mixed nerves as they carry both afferent and efferent signals

Question 43

Collections of Axons in the NS

Answer 43

Bundles of nerve axons (or fibres) with a common origin and destination are bound together Collections of axons in the peripheral nervous system make up a nerve Collections of axons in the central nervous system make up fibre tracts

Question 44

Organisation of Peripheral Nerves

Answer 44

Nerves are supported by three connective tissue layers: • Endoneurium surrounds individual axons [smallest/innermost] • Perineurium surrounds fascicles (bundles) • Epineurium surrounds peripheral nerves) [largest/outermost]

Question 45

Meninges

Answer 45

Layers of connective tissue that protect the brain and spinal cord [look up image]

Question 46

The Subarachnoid Space

Answer 46

The space between the arachnoid and pia maters is known as the subarachnoid space The subarachnoid space is filled with cerebrospinal fluid (CSF), and blood vessels supplying the brain travel in this plane

Question 47

The Subarachnoid Space: Injuries

Answer 47

Bleeds (e.g. following trauma) are confined to this region, and can put pressure on the brain

Question 48

Cerebrospinal Fluid (CSF)

Answer 48

The blood: brain barrier tightly controls the substances that reach the cells of the CNS Its ionic composition is close to that of plasma It circulates through the ventricles and enters the subarachnoid space before returning to the bloodstream

Question 49

Cerebrospinal Fluid (CSF): Functions

Answer 49

CSF nourishes the brain and remove metabolites It also provides physical protection by floating the brain

Question 50

Cerebrospinal Fluid (CSF): where is made?

Answer 50

CSF is is formed by ependymal cells of the choroid plexuses Found in the ventricles of the brain. the ≈125 ml of CSF is renewed ≈4 times a day

Question 51

What is the name given to the inflammation of a peripheral nerve?

Answer 51

Neuritis

Question 52

What factors affect speed of nerve conduction? (3)

Answer 52

Diameter of Axon Myelination Fibre Density

Question 53

Neuropathic Pain

Answer 53

Shooting or burning pain Usually goes away on its own Is often chronic

Question 54

What is the role of the endoneurium in nerve repair?

Answer 54

Provides a channel for regrowth of damaged nerves