Perpiheral NS and NMJ Flashcards
How are axons packaged into nerves?
Spinal nerves contain both sensory and motor neurones (and therefore afferent and efferent axons).
Individual axons are wrapped in myelin (though not all e.g. nociceptive (pain) neurones) and endoneurium (connective tissue around the myelin sheath).
These nerves are bundled into fascicles surrounded by perineurium.
The whole nerve is in a tough epineurium capsule, containing blood vessels also.
When the nerve reaches the spinal cord, it splits off to a dorsal and ventral root.
What is a neuromuscular junction (NMJ)?
A specialised synapse between a motor neurone and a muscle fibre.
Area where nerve impulse is transmitted into a muscle and stimulate contraction.
What does a neuromuscular junction look like?
The whole structure is called a motor-end plate: the pre-synaptic knob is enlarged, and the post-synaptic membrane is has a large SA. This increased SA maximises transmission of impulse across to muscle fibre.
Presynaptic knob is embedded in the muscle fibre. Because it is embedded, the conduction velocity of the impulse is much faster, as it increases SA of the post-synaptic membrane.
The synaptic cleft is the space between the motor neurone and muscle fibres.
The post-synaptic membrane contains acetylcholine receptors. Acetylcholine is the neurotransmitter for voluntary striated muscle.
Acetylcholinesterase are located near to the receptors, embedded in junctional folds. This increases SA and increases effectiveness of depolarisation and transmission.
What are the two types of motor neurone?
Motor neurones organised into upper and lower motor neurones. Upper belong in the brain. They originate either in the motor region of the cerebral cortex or in the brain stem and carry motor information down to the lower motor neurons. Lower belong to the brainstem and spinal cord. They control all voluntary movement.
What are the roots and horns of the spinal cord?
Two branches. POSTERIOR (OR DORSAL) ROOT: Afferent and brings in sensory neurones and inputs. The cell bodies of the sensory neurones cluster in the spinal dorsal root ganglion. ANTERIOR (OR VENTRAL) ROOT: Efferent and sends motor neurones from the spinal cord. Dorsal and ventral horns are small bulges of grey matter in the spinal cord that extend out into roots.
What is a motor unit? Properties in relation to muscle and contraction size?
Motor unit: Composed of one motor neurone, and all the muscle fibres it innervates. Each motor neurone branches out for larger muscular coverage. ONE FIBRE is stimulated by ONE MOTOR UNIT (i.e. a muscle fibre is innervated by just one NMJ). Muscles are supplied by multiple motor units (can be 2, 3, 4……). When one motor unit is excited, only the muscle fibres associated with that motor unit are stimulated to contract. When a larger contraction is needed, more motor units are stimulated, and so more muscle fibres are stimulated for contraction.
How does contraction occur?
PLEASE LOOK AT NOTES FROM MCD – I NEED TO HAVE SOME UNDERSTANDING OF MUSCULAR CONTRACTION! MCD – TISSUES – SPECIALISED CELLS. LOOK AT IT! Including excitation, sarcoplasmic reticulum and tropomyosin…
What happens to membrane potential at rest in muscle?
Membrane potential of muscle fibres fluctuates at rest. Not enough though to cause contraction. At rest, individual vesicles release Ach at a very low rate causing miniature end-plate potentials (MEPPs).
What is the structure of a muscle? – size stages?
Epimysium is connective tissue which surrounds muscle. Muscle is made up of bundles of fascicles, each surrounded by perimysium. Fascicles contain collections of muscle fibres, surrounded by endomysium. Endomysium overlays the sarcolemma (which is the specialised cell membrane of each muscle fibre). The endomysium contains capillaries and nerves. Muscle fibres (aka myofibers) are muscle cells which contain t-tubules which allow propagation of action potentials into the cell. Sarcoplasm of muscle fibres contain sarcoplasmic reticulum and myofibrils.
What are myofibrils?
They are organelles containing actin and myosin (myofilaments). Extend along the length of muscle fibres. They contain sarcomeres (between each Z-line) which are the functional units of muscles.
What is the arrangement of myofilaments? (x5 things to describe)
Dense protein Z-lines separate sarcomeres. Dark bands – A-bands (thick myosin). The lightened area in the middle of the A-band = the H-zone (the non-overlapping region of the myosin). Light bands – I-bands (thin actin). Small line in middle of H-zone is called the M-line – area that connects myosin filaments.
What happens to the arrangement of myofilaments in muscular contraction? (x4)
I bands and H zone shortens as the myofilaments overlap more. A-band stays the same. Z-lines get closer (sarcomere shortens).
What are the dimensions of myofibrils?
1-2um.
What, generally, is an NMJ disorder?
Pathological process that interferes with NMJ function and causes MUSCLE WEAKNESS.
List examples of NMJ disorders. (x3)
Botulism Myasthenia gravis (MG) Lambert-Eaton myasthenic syndrome (LEMS)
What is botulism?
Caused by a toxin produced by bacteria called Botulinum. Produces an irreversible disruption in stimulation-induced acetylcholine release by the presynaptic nerve terminal. When action potential is being delivered into the muscle, the botulinum stops the release of the acetylcholine into the synaptic cleft – so there’s no excitation in the muscle = muscle paralysis.
What is Myasthenia Gravis?
Autoimmune disorder. Antibodies are directed to the acetylcholine receptor and destroy or block it. SO, acetylcholine gets to the receptor, but binding produces no response. As a result, you have muscle paralysis. The bulbar part of the nervous system is particularly affected. Bulbar = relates to signals that affect muscles in the face and eyes – so patient’s typically present initially with facial muscle weakness.
What is Lambert-Eaton myasthenic syndrome?
An autoimmune disease caused by antibodies directed to attack the voltage-gated calcium channels in the presynaptic terminals. So there’s no movement of acetylcholine into the cleft because calcium cannot move into the neurone.
What is an EMG?
Electromyography: records action potentials in skeletal muscle fibres. It is an extra-cellular way of recording electric potential.
What is an EEG?
Electroencephalogram: records action potential from the brain by placing electrodes extracellularly on the scalp. 4
How is intracellular recording of electric potential in a neurone measured?
An electrode filled with saline inside an axon. Place another electrode extracellularly. This is reference electrode. When connected to a very sensitive voltmeter, you can measure the membrane potential of the neurone.
How is electrical potential measured extracellularly?
Both electrodes are placed outside the muscle fibres. Voltage is measured between two sites outside the fibres.
How do extracellular electrodes recognise an action potential being transmitted in a muscle fibre? What would it record at rest?
At rest, the electrical potential would be 0mV because two areas of the skin are equal electrically [first photo]. At [second photo], the area of positive charge in the muscle fibres indicates an area of the muscle that has just received an action potential. The muscle fibre membrane becomes depolarised, and it is more positive inside the muscle fibre than outside. The electrode concerned with this area of the body detects this because there is now an electrical potential between the two areas of skin with electrodes. In the [third photo], the action potential moves down the muscle fibre, and electrical potential returns to zero. This is because no region of the muscle concerned with the electrodes are depolarised. In [photo 4], the action potential passes the region covered by the second electrode. The membrane depolarises, and the second electrode is more negative than the first.
Explain why the mV in the EMG goes up, and why it goes down. Not what happens in the muscle, but what is the electrical difference between the two electrodes?
THE FIRST ELECTRODE IS THE REFERENCE ELECTRODE.
When there is depolarisation at the reference electrode, the second electrode is more positive than the reference electrode, so electrical potential is POSTITVE.
When there is depolarisation at the second electrode, the second electrode is more negative compared to the reference electrode, so electrical potential is NEGATIVE.
Why does the EMG create an mV graph different to what we are familiar with when we see an action potential?
Because an EMG measures electrical potential extracellularly. i.e. it is not measuring membrane potential (electrical potential between inside and outside the muscle fibre); instead, it is measuring the electric potential between two areas of skin, and therefore two different regions of muscle.
How does twitch force change with an increase in the size of the electrical stimulus applied to a muscle (in an experiment – so impulse was supplied extracellularly)? Explain why this is.
On the EMG, the large upwards and downwards spike indicates a single action potential passing through the muscle that the electrodes cover.
Large enough stimulus indices a twitch force.
Large the action potential, larger the twitch force.
Larger twitch force because more muscle fibres are stimulated and recruited to exert the twitch force.
What happens to force as you increase the frequency of the same size electrical stimulation? (x3)
Increased frequency means larger force and a change in the type of contraction.
Low frequency = single TWITCHES. There is no summation/increase in the size of contractual force because contraction dies off before the next electrical stimulation.
When the frequency of stimulation is so high that the contraction event doesn’t have time to completely die off before the next electrical stimulation, you get SUMMATION OF A TWITCH.
FUSED/TETANIC CONTRATION – this is not pathological. It describes when you cannot see individual twitches because contraction and electrical stimulation are so close together. Although, a tetanic contraction can occur from tetanus.
What is the difference between EMG in voluntary force and external electrical stimulation?
EMG for voluntary movement looks messy – LOTS of different impulses sent down the axon creates a smooth contraction. Not the same in external stimulation. Hence the differences in force as well.
What is the peripheral nervous system split into?
Sensory (afferent) division (conducts impulses from receptors to CNS), and motor (efferent) division.
What is the motor nervous system split into?
Autonomic and somatic nervous system.
What is the autonomic nervous system split into?
Sympathetic and parasympathetic nervous system.
What do sensory ganglions contain?
Sensory ganglion contains the cell bodies of the sensory neurons.
They are essentially nuclei, but in the PNS.
What is the structure of the somatic nervous system (motor division)?
Each muscle that is innervated has two neurons involved. An upper motor neuron that extends down the CNS. And a lower motor neuron which is exclusively part of the PNS and extends efferently from the spinal cord via DORSAL AND VENTRAL RAMI.
SOMATIC MOTOR NEURONS HAVE THEIR CELL BODIES IN THE CNS.
The basic route of nerve signals begins in the primary motor cortex. Nerves terminate at an NMJ.
What is the structure of the autonomic nervous system (motor division)?
Composed of three neurons.
First neurone extends from the hypothalamic nuclei to brain stem nuclei (for parasympathetic nervous system), spinal cord (for sympathetic nervous system), OR sacral region (for parasympathetic nervous system).
Second neurone is a preganglionic neuron that extends from brain stem nuclei/spinal cord, to the AUTONOMIC GANGLIA. They are called presynaptic neurones if they extend from the vagal nerve or sacral parasympathetic nerves because there’s no ganglia associated with these nerves.
Third neurone is a postganglionic neuron. Extends from autonomic ganglia to visceral effectors. They are called post-synaptic neurones if associated with vagus nerves or sacral parasympathetic nerves, because there’s no ganglia associated with these nerves.
How is the sensory nervous system structured?
Have ganglion in the middle – between the CNS and periphery.
Cell bodies collect in these ganglia.
What are the properties of pre-ganglionic and pre-synaptic neurons? (x3)
Cell bodies are multipolar, and axon is myelinated. One Schwann cell for every axon.
What are the properties of post-ganglionic and post-synaptic neurons? (x3)
Cell bodies are multipolar, and axons are unmyelinated (although it is still wrapped by ONE layer of Schwann cell – this is called a NEURILEMMA). One Schwann cell for MULTIPLE axons.
What do A and C-fibre axons look like histologically?
Loot at photo.
What are nerve plexuses?
Networks of successive ventral rami that exchange fibres (criss-cross and redistribute). Mixing up means that by the time you reach the periphery, peripheral nerves will contain bundles of axons originating from more than one spinal nerve.
In the photo, plexuses can be seen in the cervical and lumbar regions.
What is a dermatome?
Areas of skin innervated by single sensory spinal nerve roots.
When we study dermatomes, we only look at dermatomes from the dorsal ROOTS of the spinal nerve (sensory neurones).
How does an EMG differentiate between the two outlined causes of muscle weakness?
In neurogenic disorders e.g. one neurone cannot innervate its muscle fibres, other motor units will try to compensate, and produce a large signal for contraction. There will also be fibrillation where the muscle contract by themselves.
In a myopathy, you will have smaller polyphasic units, fibrillation but with less frequency, and less effective contraction, but still spontaneous.
How is a nerve conduction study done?
APB muscle in hand is stimulated (there is some contraction) inerve externally. The median nerve runs down the length of the arm and is stimulated using an electrode with a positive and negative terminal.
Electrode placed distal (further away from centre of the body) and nerve innervated, and action potential measured in hand muscle using an EMG.
If you stimulate more proximally, nerve impulse will travel to the muscle, and action potential will be later because distance between electrode and hand is farther. Difference in times is used to measure conduction velocity.
What would EMGs looks like in a healthy patient, someone with a myelination issue, and someone with axonal issues?
Ignore fourth box.
