Organisation & Classification of Nerves Flashcards
What is the function of the nervous system?
to gather and process information
produce responses to stimuli
coordinate the workings of different cells
What are the 2 components of the nervous system?
Peripheral Nervous System
Central Nervous System
What is the CNS and what is its role?
CNS consists of the brain and spinal cord
It receives, processes, interprets and stores information
What is the PNS and what is its role?
Consists of peripheral nerves that transmit information to and from the CNS via sensory and motor nerves
What are the roles of sensory and motor neurones?
Sensory neurones carry information to the CNS
Motor neurones carry information from the CNS
How is the PNS divided?
It is divided into the somatic and autonomic nervous system
What are the roles of the somatic and autonomic nervous systems?
Somatic is involved with voluntary action
Autonomic is involved with involuntary action
How is the autonomic nervous system divided?
Divided into the sympathetic and parasympathetic branches
What are the roles of the sympathetic and parasympathetic nervous systems?
Sympathetic is excitatory and controls ‘fight or flight’ response
Parasympathetic is inhibitory and controls the ‘rest and digest’ response
What are the key similarities between motor neurones?
They have similar structural characteristics and produce acetylcholine as a neurotrasmitter
Where are neurones with a similar structure/function found?
They are aggregated together in specific areas of the CNS
They may act together as a functional unit
Why do neurones need a constant supply of energy to survive?
They have a very high metabolic rate
What are dendrites, the axon and the synaptic terminals?
Dendrites are signal inputs that receive information from other neurones
The axon is involved in signal conduction after information is processed in the cell body
Synaptic terminals are involved in signal output and will signal to other neurones
What is the role of sensory neurones?
What is their other name?
They convert a stimulus, via sensory receptors, into an action potential
This is sensory transduction
They are afferent neurones
They bring sensory impulses from sensory organs to the CNS
What is the role of motor neurones?
What is their other name?
They carry impulses from the CNS to a muscle or gland
They are efferent neurones as they carry impulses from the CNS to specific effectors
What are interneurones?
They transmit impulses between other neurones
They are often involved in reflex arcs
What is a multipolar neurone?
Where are they found?
what shape are they?
They have a single long axon with many dendrites emerging from the cell body
Found in the CNS and autonomic ganglia
Can be pyramidal or stellate in shape
What is a unipolar neurone?
It has only one protoplasmic process extending from its cell body
This is a neurite
Where are pseudounipolar neurones found and what is their role?
Found in sensory ganglia
They can transduce a physical stimulus into electrical activity
What is a bipolar neurone?
It has two extensions - one axon and one dendrite
It is a specialised sensory neurone
What is a neurite?
A projection out from the neurone cell body
Dendrites and axons are types of neurite
How do dendrites compare to axons?
Dendrites are shorter and thicker
They produce smaller spines that can spread over a large volume of tissue to produce a “dendritic tree”
What is the purpose of the “dendritic tree”?
Increases surface area for the neurone to receive inputs from other neurones
What is found in the grey matter in the CNS?
Cell bodies of sensory and motor neurones
What is found in the white matter in the CNS?
Axons of neurones found within the CNS
What is the role of the dorsal and ventral root?
The dorsal root takes sensory information into the spinal cord
Ventral root leaves the spinal cord
What stain can be used to observe white and grey matter?
Weigart’s stain
It is specific to myelin
The white matter stains much darker as it consists mainly of myelinated axons
What is a nucleus within the CNS?
It is a group of nerve cells
What are the 4 main nuclei within the grey matter?
Somatosensory area
Viscerosensory area
Visceromotor area
Somatomotor area
What is found in the somatosensory and somatomotor areas in grey matter?
Somatosensory comprises interneurones responding to inputs from the body’s external tissues
Somatomotor comprises motor neurones supplying external tissues
What is found in the viscerosensory and visceromotor areas?
Viscerosensory comprises interneurones responding to inputs from internal organs
Visceromotor comprises motor neurones supplying internal organs
What is a tract in the CNS?
What are the 2 types?
A tract is a bundle of axons in the white matter
Ascending tracts carry sensory information to the brain
Descending tracts will carry commands to the motor neurones
Within the PNS, what is a ganglion and a nerve?
A ganglion is a group of nerve cells within the PNS
A nerve is a bundle of axons within the PNS
What is found within the ganglia?
Cell bodies of sensory fibres, along with many myelinated and non-myelinated axons passing through
What is found within a nerve?
Bundles of myelinated and non-myelinated neurones of different types, blood vessels and Schwann cells
What is the definition of ganglia?
Ganglia are nodular masses of neuronal cell bodies and the supporting neuroglia
What 2 types of cell make up the ganglia?
Ganglion cells are the neuronal cell bodies
They are less dendritic and more rounded in shape
Satellite cells are the supporting neuroglia
What are sensory ganglia and what is their role?
They contain the cell bodies of sensory neurones
They receive signals from the periphery and send them to the CNS
What are autonomic ganglia and what is their role?
They contain the cell bodies of motor neurones from the autonomic NS
They receive signals from the CNS and send them to the periphery
Where are satellite cells found?
They cover the surface of nerve cell bodies in sensory, sympathetic and parasympathetic ganglia
What are the three layers of connective tissue found in peripheral nerves?
The epineurium covers the whole nerve
The perineurium covers an individual fascicle
The endoneurium covers individual nerve axons
What is a fascicle?
A small bundle of nerve fibres
What are neuroglia?
Glia is the connective tissue of the nervous system
It consists of several different types of glial cells associated with neurones
How do glial cells help to regulate ionic concentration?
They pump ions, such as K+ , into capillaries
How do astrocytes play a role in glucose metabolism?
They take up glucose from the blood
They convert it to lactate and shuttle it into neurones as an energy supply
How are glial cells involved in neurotransmitter metabolism and neuronal development?
They can take up, inactivate and recycle several essential neurotransmitters
They can produce growth factors that influence neuronal growth and development
How do glial cells protect the brain?
They restrict the blood-brain barrier
They restrict the access of some substances into the brain’s intracellular space
What are 3 important functions of glial cells?
Regulate metabolism of neurone
Involved in repair and recovery after injury
Protect against bacterial infection by destroying pathogens and dead neurones
What is the function of astrocytes?
They are involved in metabolic exchange between neurones and the blood
They take nutrients from the blood and remove waste from the neurone into the blood
How do astrocytes form the blood-brain barrier and what is its function?
They form tight junctions between endothelial cells of capillaries
It is an exchange site for nutrients and waste and a protection layer around the brain and spinal cord
What are the 5 roles of the blood-brain barrier?
Structural roles as it provides a supportive framework
Metabolic roles as it provides neurones with nutrients in the form of lactate
Growth and promotion of synapse formation
Potassium clearance as it absorbs K+ released from the neurone at the synapse
Transmitter uptake as it expresses membrane transporters for several neurotransmitters
What does an astrocyte look like?
It is a star-shaped glial cell
What is the role of microglia?
The are involved in immune defence
They attack invading bacteria and work like a phagocyte
What is the role of ependyma and where are they found?
They line the cells of the ventricles in the brain and the central canal of the spinal cord
They produce cerebrospinal fluid
The cilia waft the CSF
What is the role of oligodendrocytes?
They myelinate the axon
One single oligodendrocyte can myelinate several different axons at the same time
What is multiple sclerosis and how is it caused?
It is a disease of myelin caused by failure of myelin producing cells, or by destruction of these cells by the immune system
How does damage to myelin cause symptoms of MS?
Damage to myelin disrupts messages travelling along nerve fibres
They may be slowed down, become distorted or not get through at all
What are the symptoms of MS?
Symptoms depend on the part of the CNS affected or the role of the damaged nerve
Fatigue, problems with vision, muscle spasms, weakness, depression and anxiety, speech and swallowing problems
What is adrenoleukodystrophy (ALD) and how is it caused?
It is an X-linked recessive rare axon disorder
It is caused by the accumulation of very long chain fatty acids in the adrenal gland and the brain
This is due to a defect of peroxisomes which are needed for the breakdown of fatty acid cells
What is the consequence of VLCFAs in the brain?
They cannot be processed so will accumulate and destroy the myelin sheath
What happens in ALD and what are the symptoms?
Breakdown or loss of myelin and progressive dysfunction of the adrenal gland
It leads to loss of reasoning, speech and mobility
It leads to adrenal insufficiency which causes weakness, tiredness, vomiting and muscle pains
What is amyotrophic lateral sclerosis (ALS) and how is it caused?
It is motor neurone disease
Leads to death of neurones controlling voluntary muscles
Toxic substances can cause motor neurone dysfunction
What are the symptoms of ALS?
Stiff muscles, muscle twitching and gradually worsening weakness as muscles decrease in size
Eventually leads to difficulty speaking, swallowing and breathing
What are the 2 main types of neuroglia in the PNS?
Schwann cells and Satellite cells
What is the role of Schwann cells?
They provide myelination to the axons in the PNS, forming the myelin sheath
They also have phagocytic activity
They clear cellular debris allowing for regrowth of PNS neurones
What is the role of Satellite cells?
They help to regulate the external chemical environment
What is the difference in the way oligodendrocytes and Schwann cells myelinate axons?
Oligodendrocytes are in the CNS and can form myelin sheaths around several axons
Schwann cells are in the PNS and can only myelinate one segment of a single axon
How do oligodendrocytes and Schwann cells myelinate axons?
They wrap the axon in a spiral of concentric layers of fatty myelinated membrane
What is the mesaxon?
It is a pair of parallel membranes of a Schwann cell, marking the point of edge-to-edge contact by the Schwann cell encircling the axon
What is the node of Ranvier and what does it permit?
It is a 0.5 micrometer gap between adjacent Schwann cells
Ions can freely move in and out of the axon
The action potential can “jump” along the axon by saltatory conduction
Where are non-myelinated axons found?
They lie within invaginations of a mesaxon
They are surrounded by myelinated axons
How are non-myelinated axons associated with Schwann cells?
Non-myelinated axons have a supporting Schwann cell
A single Schwann cell will support several axons and supply them with nutrients
Why do nerves contain a specialised basement membrane?
Nerves contain blood vessels
The membrane prevents blood leaking out into the neurone
What stain is used to make myelinated neurones stand out?
Osmium
This is a heavy metal
What is the the role of the somatic nervous system and which neurones does it consist of?
It innervates and controls voluntary, striated muscles
It consists of somatic motor neurones and sensory neurones
How are signals carried in the somatic nervous system?
Where are the cell bodies found?
Sensory neurones carry impulses to the CNS from the target tissue
The cell body is in the ganglion
Motor neurones carry and impulse from the CNS back to target tissue
The cell body is in the CNS
What is the autonomic nervous system?
It controls unconscious actions and supplies the body’s visceral organs (involuntary muscles)
It is divided into the sympathetic and parasympathetic branches
Describe the motor pathway in the autonomic nervous system
The motor component consists of 2 motor neurones
There is a 2-neurone link from the CNS to the target tissue
Describe the ganglia and transmitters of the parasympathetic nervous system
Preganglionic fibres are long and myelinated
Postganglionic fibres are very short with few branches
The ganglia are near or in the target tissue
The pre- and post-ganglionic transmitter is acetylcholine
Describe the ganglia and transmitters of the sympathetic nervous system
Preganglionic fibres are short and myelinated
Postganglionic fibres are long with many branches
The ganglia are close to the spinal cord
The preganglionic transmitter is acetylcholine but the postganglionic transmitter is noradrenaline
How are motor fibres and some non-sensory muscle fibres classified?
There are categories A, B and C
A = 120 m/s B = 18 m/s C = 0.5 - 2 m/s
How are nerve fibres classified?
Classification systems are based on the fibre diameter and conduction velocity
How are sensory fibres from muscle classified?
There are categories I, II, III, IV
I = 75 - 120 m/s II = 35 - 75 m/s III = 5 - 35 m/s IV = 0.5 - 2 m/s
What is the difference in conduction velocity in myelinated and non-myelinated fibres?
Myelinated fibres are thick and have much greater conduction velocities
What 2 factors influence conduction velocity?
Fibre diameter and extent of myelination
What is a nerve conduction test and how is it conducted?
It measures the speed at which impulses travel along a nerve
Nerves are stimulated by electrodes placed on the skin
The time for the impulse to travel to a surface electrode on the skin is recorded
Why is diagnosis from a nerve conduction test hard?
It does not only consider velocity, but also latency and shape of response
Why is there a range of normal NCVs?
This is due to mixed fibre types
Each nerve is made from a mixture of myelinated and non-myelinated axons of varying diameters
What will influence NCV and what will decrease it?
Depends on the point on the limb recorded, temperature and age
Decreased with nerve damage through trauma, inflammation, diabetes, alcohol abuse and demyelination (MS)
What are sensory receptors and what is their role?
They are modified nerve endings of sensory neurones
They pick up sensory signals and conduct them to the CNS
What are the 3 types of sensory receptor classified by location in the body?
Exteroreceptors are found on the external surface
Interoreceptors are found in internal organs and inside blood vessels
Proprioreceptors are internal but are concerned with the position of muscle, tendons and joints
What are the 5 types of sensory receptor classified by the type of stimulus they detect?
Mechanoreceptors detect touch, pressure, vibration stretch
Thermoreceptors detect temperature change
Photoreceptors detect light
Chemoreceptors detect chemicals
Nociceptors detect pain (usually chemicals)
Where are muscle spindles found and what do they detect?
Skeletal muscles
Detect stretch
They are non-adapting
What do free unencapsulated endings detect and where are they found?
Connective tissue, joints, skin
Detect pain, touch, light pressure
they are slow or fast adapting
Where are Pacinian corpuscles found and what do they detect?
They are found in the deep dermis, tendons, joints, genitalia
They detect vibration and deep pressure
They are fast adapting
Where are Meissner’s or Krause’s bulbs found and what do they detect?
Oral mucosa, lips, genitalia, fingertips
Detect touch, vibration, light pressure
They are rapid adapting
Where are Ruffini organs found and what do they detect?
Deep dermis, ligaments, joint capsules
Detect stretch, deep pressure
Very slow adapting
Describe the spinal cord reflex pathway
Receptor terminals of sensory fibre detect the stimulus and transmit signals along sensory axons in the spinal nerve
Interneurones in the dorsal horn project axons down to the motor neurone cell bodies located in the ventral horn
The motor axons leave via the ventral root and enter the spinal nerve
They will supply motor endings in the skeletal muscles in the limb
