Lecture 6.1: The Organisation of the Nervous System Flashcards
Four structural classes of neurones
Multipolar Neuron
Bipolar Neuron
Psuedo-Unipolar Neuron
Unipolar Neuron
Multipolar Neuron Structure
Most commonly, neurones are multi-polar with one axon and multiple dendrites
Bipolar Neuron Structure
One dendrite and one axon
Include sensory neurones of the retina
Unipolar Neuron Structure
Have one axon and no dendrites
All sensory neurones (except bipolar retinal) are unipolar (or pseudo-unipolar)
Three functional classes of neurones
Motor
Sensory
Interneurons
The Reflex Arc: the “simple” neurone circuit
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
Interneurons
• 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
Motor Neurons (Efferents)
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.
The Anatomy of an Axon
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
Anterograde
Towards axon terminal
Retrograde
Away from axon terminal
Resting Potential
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
Action Potentials
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
Dendrites: Function
Dendrites are specialisations of the cell body that provide a large surface area for signal reception and processing
Dendrites: Structure
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
Dendrites: Downs Syndrome
People with Down syndrome have fewer dendritic spines, which may explain associated learning difficulties
Neural Plasticity
It is the ability of neural networks in the brain to change through growth and reorganisation
Temporal Summation
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
Spatial Summation
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
Multiple Sclerosis
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
Multiple Sclerosis: Treatments
No effective treatments exist for progressive forms of the disease
But β-interferon can effect improvements in relapsing-remitting disease
Guillain–Barré Syndrome
Rapid onset muscle weakness in Guillain–Barré syndrome results from auto-immune damage to myelin in the PNS
Classification of Nerve Fibres (3)
Group A, B and C fibres
Classification of Nerve Fibres: Group A
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
Classification of Nerve Fibres: Group B
Lightly-myelinated
include pre-ganglionic autonomic fibres.
Speeds of 12-30m/s
Classification of Nerve Fibres: Group C
Non-myelinated
Include post-ganglionic autonomic fibres
Include somatic sensory fibres relaying diffuse pain
Speeds up 0.5-2.3m/s
Communication between Neurones (2)
Electrical Synapses
Chemical Synapses
Electrical Synapses
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
Chemical Synapses
Unidirectional
Can be excitatory or inhibitory,
Thus can modulate the signal received from the presynaptic neurone
Chemical Synapses: How it works?
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
Somatic Nervous System
Voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles
Autonomic Nervous System
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
Sympathetic NS
The sympathetic nervous system directs the body’s rapid involuntary response to dangerous or stressful situations, is excitatory
Parasympathetic NS
The parasympathetic nervous system leads to decreased reaction, is inhibitory
Enteric NS
Acetylcholine
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)
Noradrenaline
A common neurotransmitter in both the somatic & autonomic nervous systems
Most sympathetic post-ganglionic fibres release noradrenaline (hence are adrenergic)
Dopamine
Is involved in reward pathways and motor control
It also regulates kidney function
Parkinson’s Disease
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)
Glutamate
In the CNS, glutamate opens Na+ channels
Excitatory
Glycine
In the CNS, openS Cl- channels
Inhibitory
The PNS: cranial and spinal nerves
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
Collections of Axons in the NS
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
Organisation of Peripheral Nerves
Nerves are supported by three connective tissue layers:
• Endoneurium surrounds individual axons [smallest/innermost]
• Perineurium surrounds fascicles (bundles)
• Epineurium surrounds peripheral nerves) [largest/outermost]
Meninges
Layers of connective tissue that protect the brain and spinal cord
[look up image]
The Subarachnoid Space
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
The Subarachnoid Space: Injuries
Bleeds (e.g. following trauma) are confined to this region, and can put pressure on the brain
Cerebrospinal Fluid (CSF)
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
Cerebrospinal Fluid (CSF): Functions
CSF nourishes the brain and remove metabolites
It also provides physical protection by floating the brain
Cerebrospinal Fluid (CSF): where is made?
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
What is the name given to the inflammation of a peripheral nerve?
Neuritis
What factors affect speed of nerve conduction? (3)
Diameter of Axon
Myelination
Fibre Density
Neuropathic Pain
Shooting or burning pain
Usually goes away on its own
Is often chronic
What is the role of the endoneurium in nerve repair?
Provides a channel for regrowth of damaged nerves
