The CNS and normal neuromuscular and musculoskeletal function week 3 Flashcards
Anatomy of brainstem
Week 2 of CNS and Normal musculature
Link to neuroanatomy resource
Anatomy of brainstem
Posterior brainstem anatomy
Hypothalamus
What is a reflex ?
A reflex is defined as an involuntary, unlearned, repeatable, automatic reaction to a specific stimulus which does not require input from the brain.
Where do the cell bodies of the sensory neurons and motor neurons live ?
- The cell bodies of the sensory neurons are found in the dorsal root ganglia.
- The cell bodies of the motor neurons are found in the grey matter of the ventral horn
What are alpha motor neurons ?
- Are lower motor neurons of the brainstem and spinal cord.
- Within the spinal cord there neurons originate in laminae VIII and IX of the ventral horn and are somatotopically organised.
- That is to say, neurons which innervate distal musculature are located lateral to those which innervate axial muscles, and neurons which innervate extensors are ventral to those which innervate flexors.
- Multipolar neurons: is a type of neuron that possesses a single axon and many dendrites
- They innervate extrafusal muscle fibers of skeletal
muscle and are directly responsible for initiating their contraction - The function of α-motor neurons is to cause contraction of the muscle fibres they innervate. It has been described as ‘the final common pathway’, as α-motor neurons are essential for muscle contraction.
This can either been under voluntary control, through the action of UMNs, or through eliciting the myotatic stretch reflex, as α-motor neurons form the efferent portion of the reflex arc. Therefore, there can be no coordinated muscle contraction if the α-motor neurons are not functioning
Difference between upper motor neurons (UMN) and lower motor neurons (LMN) ?
- For upper motor neurons the axons remain in the spinal cord or brain.
- For lower motor neurons the axons leave the spinal cord and brain.
Inputs to the spinal alpha motor neurons ?
Alpha motor neurons receive input from:
- muscle spindles (Ia afferents)
- Golgi tendon organs (Ib afferents)
- Cutaneous receptors
- Spinal Interneurons
- Upper motor neurons
Motor neuron disease
Describe the Stretch (myotatic reflex) ?
Stretch reflex:
- The stretch reflex begins at the sensory neuron then synapses with the motor neuron. No intermediate neuron is needed.
Muscle spindles: are STRETCH RECEPTORS located within the muscles.
Muscle spindle is made up of Intrafusal and extrafusal muscle fibers. Wrapped around the muscle spindle is a sensory neuron which will send stimulus proprioception sensory information via a Ia afferent nerve.
Within the muscle spindle we have gamma (y) motor neurons.
y motor neurons only control the intraspinal muscle fibres.
a motor nerones control the extrafusal muscle fibres.
- Blue sensory neuron wrapped around the muscle spindle is activated when the muscle changes length and sends information through the Ia afferent into the dorsal horn.
- The Ia afferent neuron synapses directly with the a-motor neuron which controls the extrafusal muscle fibers causing contraction of the muscle.
- When we add weight to the muscle it stretches and elongates. This causes an increase in the number of action potentials they send to the motor neuron. The motor neuron fires more action potentials which is going to cause contraction of the muscle.
- When the spindle fiber is elongated it becomes floppy. To accommodate the length of the extrafusal fibers, the y neurons adjust the length of the spindle by contracting the intrafusal fibers.
Co activation of the y and alpha motor neurons ?
y motor neurons are coactivated with alpha motor neurons so the muscle spindle can adjust to whatever length the muscle is at any given moment.
Give an example of a stretch (myostatic) reflex?
- Knee Jerk reflex
- measure this clinically using the patellar tendon tap
- All spinal reflexes involve activation of one muscle and inhibition of another muscle. This is because all muscles work in pairs. Agonists and antagonists
Describe the inverse stretch reflex pathway ? (aka the Golgi tendon reflex pathway)
- This reflex involves the activation of the Golgi tendon organ.
- The Golgi tendon is not located in the muscle itself but instead is found in the tendon.
- the golgi tendon sends sensory signals via a 1b afferent that enters the dorsal horn. This synapses with an intermediatory neuron and causes INHIBITION of the tendon.
- The purpose pf the inverse stretch reflex pathway is to protect the muscles from being damaged.
- The golgi tendon reflex has a high reflex which means it requires a lot of force before it sends an action potential or is activated.
Function of the inverse stretch pathway: encodes and regulates muscle tension - protects the muscles and tendons from harm.
Inverse stretch reflex pathway
If you pick up a heavy weight, the golgi tendon is activated. The muscle is inhibited and you drop the weight. This autonomic reflex stops the muscle from being damaged. You have no control over it. It just happens.
Because muscles work in pairs when the flexor muscle is contracting the exensor muscle relaxes.
Describe the flexor withdawal pathway ?
- this is a polysynaptic neuron
Describe central pattern generators ?
- are automated things we do that are not reflexes eg walking and running
- Central pattern generators: Neural networks that produce oscillatory (rhythmic) patterned outputs without sensory feedback. This system does not require any sensory feedback.
- Central pattern generators do not need to be learned.
What are the 3 types of muscle ?
Skeletal – striated muscle that is under voluntary control from the somatic nervous system. Identifying features are cylindrical cells and multiple peripheral nuclei.
Cardiac – striated muscle that is found only in the heart. Identifying features are single nuclei and the presence of intercalated discs between the cells.
Smooth – non-striated muscle that is controlled involuntarily by the autonomic nervous system. The identifying feature is the presence of one spindle-shaped central nucleus per cell.
Composition of skeletal muscle ?
Composition of Skeletal Muscle
A muscle cell is very specialised for its purpose. A single cell forms one muscle fibre, and its cell surface membrane is known as the sarcolemma.
T tubules are unique to muscle cells. These are invaginations of the sarcolemma that conduct charge when the cell is depolarised.
Muscle cells also have a specialised endoplasmic reticulum – this is known as the sarcoplasmic reticulum and contains a large store of calcium ions.
Muscles also have an intricate support structure of connective tissue. Each muscle fibre is surrounded by a thin layer of connective tissue known as endomysium. These fibres are then grouped into bundles known as fascicles, which are surrounded by a layer of connective tissue known as perimysium. Many fascicles make up a muscle, which in turn is surrounded by a thick layer of connective tissue known as the epimysium.
Skeletal muscle sarcomeres ?
The striated appearance of skeletal muscle fibres is due to the organisation of two contractile proteins: actin (thin filament) and myosin (thick filament).
The functional unit of contraction in a skeletal muscle fibre is the sarcomere, which runs from Z line to Z line. A sarcomere is broken down into a number of sections:
Z line – where the actin filaments are anchored.
M line – where the myosin filaments are anchored.
I band – contains only actin filaments.
A band – the length of a myosin filament, may contain overlapping actin filaments.
H zone – contains only myosin filaments.
A useful acronym is MHAZI – the M line is inside the H zone which is inside the A band, whilst the Z line is inside the I band.
When the muscle contracts:
- the sarcomere gets shorter
- A band does not change
- H band (contains only mysoin) and I band ( which contains only actin) shorten.
Structure and function skeletal muscle fiber ?
Muscle fibres are surrounded by supportive layers of connective tissue:
Endomysium – surrounds individual muscle fibres
Perimysium – surrounds a bundle of muscle fibres, forming a fascicle (functional unit)
Epimysium – surrounds the entire muscle
What are the three main types of skeletal muscle fibers ?
Skeletal muscle fibres differ in the speed at which they contract, the amount of force that they generate, how they produce ATP to meet their energy requirements and their susceptibility to fatigue.
There are three main types of skeletal muscle fibre. Most muscles are composed of a mixture of all three of varying proportions.
Type 1 (slow oxidative)
Type 11a ( fast oxidative)
Type 11x (fast glycolic)
What is a motor unit ?
Skeletal muscle is innervated by α-motor neurons, which stimulate its fibres to contract. The cell bodies of α-motor neurons are located in either the ventral horn of the spinal cord (for limbs and trunk muscles) or in the motor nuclei of the brainstem (for head and face muscles).
A motor unit is defined as an α-motor neuron and the group of individual muscle fibres that it innervates. A single muscle fibre is only innervated by one α-motor neuron, but each α-motor neuron can innervate a variable number of muscle fibres, depending on the muscle type.
Skeletal muscle contraction ?
The neuromuscular junction is a specialised synapse connecting an α-motor neuron and a skeletal muscle fibre.
Skeletal muscle contraction is triggered by an action potential arriving at the neuromuscular junction, causing opening of voltage-gated calcium ion channels.
The resulting increase in intracellular Ca2+ causes vesicles containing acetylcholine (ACh) to release their contents into the synaptic cleft.
ACh activates nicotinic ACh receptors (a type of ligand gated ion channel) in the muscle fibres’ plasma membrane, resulting in an influx of sodium ions and depolarisation of the muscle fibre membrane potential.
This local depolarisation activates voltage-sensitive sodium channels, resulting in generation of an action potential in the skeletal muscle fibre.
-ACh is then rapidly broken down (hydrolysed) in the synaptic cleft by the enzyme acetylcholinesterase to terminate signal transmission and allow membrane repolarisation.
Mysoin and Actin
- Myosin is made up of contractile proteins and regulatory proteins.
- The contractile proteins are myosin and actin.
- The regulatory proteins are troponin and tropomyosin
Actin filaments are made up of small proteins called G actin. The mysoin head binds to g actin.
- A long row of g actin like in the image is called f actin
The sliding filament model describes the mechanism of skeletal muscle contraction
Actin and Myosin
Muscle fibres are formed from two contractile proteins – actin and myosin.
Myosin filaments have many heads, which can bind to sites on the actin filament. Actin filaments are associated with two other regulatory proteins, troponin and tropomyosin. Tropomyosin is a long protein that runs along the actin filament and blocks the myosin head binding sites.
Troponin is a small protein that binds the tropomyosin to the actin. It is made up of three parts:
Troponin I – binds to the actin filament (f - actin)
Troponin T – binds to tropomyosin.
Troponin C – can bind calcium ions.
Describe excitation and contraction coupling ?
The unique structure of troponin is the basis of excitation-contraction coupling:
When depolarisation occurs at a neuromuscular junction, this is conducted down the t-tubules, causing a huge influx of calcium ions into the sarcoplasm from the sarcoplasmic reticulum.
This calcium binds to troponin C, causing a change in conformation that moves tropomyosin away from the myosin head binding sites of the actin filaments.
This allows the myosin head to bind to the actin, forming a cross-link. The power stroke then occurs as the myosin heads pivots in a ‘rowing motion’, moving the actin past the myosin towards the M line.
ATP then binds to the myosin head, causing it to uncouple from the actin and allowing the process to repeat.
Hence in contraction, the length of the filaments does not change. However, the length of the sarcomere decreases due to the actin filaments sliding over the myosin. The H zone and I band shorten, whilst the A band stays the same length. This brings the Z lines closer together and causes overall length of the sarcomere to decrease.
Describe excitation contraction coupling ?
Excitation-contraction coupling describes the process whereby an action potential triggers a skeletal muscle fibre to contract.
1.Action potentials generated at the neuromuscular junction travel along the sarcolemma and down into the transverse tubule (T-tubule) system to depolarise the cell membrane.
2.Depolarisation of the sarcolemma triggers opening of voltage-gated L-type Ca2+ channels (also known as dihydropyridine receptors), allowing calcium to enter into the cell.
3.Calcium influx leads to activation of ryanodine receptors located in the sarcoplasmic reticulum (intracellular calcium store), which allows calcium to flow from the sarcoplasmic reticulum into the cytoplasm and further increases intracellular calcium concentration.
4.Calcium binds to troponin-c, inducing a conformational change which reveals a binding site on actin for the myosin head.
5.This binding results in ATP hydrolysis, providing energy for the actin and myosin filaments to slide past each other and shorten the sarcomere length, thereby initiating muscle contraction.
What are satellite cells ?
- Satellite cells (SCs) are skeletal muscle stem cells that grow, maintain, and repair muscle tissue. They account for between 3% and 11% of skeletal muscle tissue and are found between the sarcolemma and basement membrane of the muscle fibers.
- Satellite cells are usually quiescent (inactive) which means they are not actively undergoing division.
- They are activated by an injury to the muscle.
- The process of maintaining and repairing muscle cells is known as MYOGENESIS.
- Injuries to the muscle tissue stimulate the proliferation of SCs, which then differentiate to form myoblasts. The myoblasts then become myocytes; long, tubular cells that fuse together to form skeletal muscle fibers (called myofibers).
Structure of a sarcomere ?
A sarcomere is the basic contractile unit of muscle fiber. Each sarcomere is composed of two main protein filaments—actin and myosin—which are the active structures responsible for muscular contraction
Calcium induced calcium release ?
Calcium-Induced Calcium Release
Calcium can be released from the sarcoplasmic reticulum (SR) in 2 ways:
Via IP3
Gq-protein coupling facilitates the release of Ca2+ from the sarcoplasmic reticulum. This is particularly important in muscle types that require calcium-induced calcium release (e.g. in cardiac muscle).
Gq activates the effector: Phospholipase C enzyme
Phospholipase C breaks down PIP2 (phosphatidylinositol 4,5-biphosphate), into IP3 + DAG
IP3 then binds to the IP3 receptor on the SR
DAG acts on various proteins (e.g PKC)
Ca2+ channels on the SR open and Ca2+ is released.
Action is then terminated by IP3 phosphatase (IP3 is cleaved to IP2)
Via Ryanodine receptors (RyR): these a family of Ca2+ releasing channels found on intracellular organelles that store or release Ca2+.
Membrane depolarisation opens voltage-operated calcium channels (VOCCs) in the T-tubule system and calcium is released.
Calcium binds to RyR on the sarcoplasmic reticulum. This induces conformational changes in a Ca2+ channel which is closely associated with RyR.
RyR is activated and opens to release Ca2+ from the SR stores. This is known as the ‘calcium spark’.
Released calcium leads to a calcium spike. Calcium binds to troponin C and thus activating the cross-bridge cycling mechanism for contraction.
What is rigor mortis ?
Myasthenia gravis ?
Slow twitch vs fast twitch muscles
Isotonic and isometric contraction ?
Factors affecting the contration of a force ?
Factors affecting the contration of a force ?
Anatomy of the cerebellum ?
- gyri are raised areas
- sulci are depressions
- the central sulcus separates the frontal and parietal lobe.
- within the frontal lobe we have the precentral sulcus
- within the parietal lobe we have teh post central sulcus
- the precentral gyrus: is found between the central sulcus and precentral sulcus
- post central gyrus is found between the central sulcus and post central sulcus.
Coronal view of the cerebral cortex
- The right and left hemispheres of the brain is divided by the longitudal fissure. Separating the right and left side of teh brain us the corpus callosum.
-found within the white matter of the brain are the basal ganglia (nuclei).
- between the basal ganglia we have the internal capsule.
- Running in between these basal ganglia are more white matter afferent and efferent axon tracts, the most notable being the internal capsule. The internal capsule is a collection of densely packed white matter, or axons, which divides the corpus striatum and acts like a highway for information flow between the cerebral cortex and the brainstem and spinal cord.
Label the basal ganglia ?
The basal ganglia consist of the caudate and putamen, the globus pallidus, the subthalamic nucleus and the substantia nigra. Note that the term striatum refers to the caudate and putamen; the term lentiform nuclei, or lenticular nuclei, refers to the putamen and globus pallidus; and the term corpus striatum refers to all three structures, the caudate, putamen and the globus pallidus. All of the structures of the basal ganglia have their own unique functions and pathways.
Which divides the four main lobes ?
- central sulcus
- Lateral fissure
- Parieto-occipital fissure
What are broadmann areas ?
The cerebral cortex can also be divided into histologically similar regions called Brodmann’s areas, which provides a map of the primary cortices and functional regions.
What are the main anatomical regions of the frontal lobes ?
The main anatomical regions of the frontal lobe are: the precentral sulcus, precentral gyrus, and the superior and inferior frontal sulci, which delineate the superior, middle, and inferior frontal gyri.
- These form the superior, middle and inferior frontal gyrus
What are the five functional regions of the frontal lobes ?
The frontal lobe has five functional regions: primary motor cortex, premotor cortex, the frontal eye field, Broca’s area, and the prefrontal cortex.
Primary motor cortex: 4
Premotor cortex: 6
Frontal eye field: 8
Broca’s area is also known as the motor speech area. It is near the motor cortex and utilized in speech production, located in the inferior frontal gyrus
What is the motor homunculus ?
The motor homunculus is a visual representation of the areas of primary motor cortex dedicated to the voluntary motor control of body parts.
- the bigger the area the more motor supply it receives
The motor homunculus is a topographic representation of the body parts and its correspondents along the precentral gyrus of the frontal lobe. While the sensory homunculus is a topographic representation of the body parts along the postcentral gyrus of the parietal lobe.
What is found in the parietal lobe ?
The parietal lobe contains the postcentral sulcus and intraparietal sulcus that border the postcentral gyrus, superior parietal lobule, and inferior parietal lobule.
What are the functional regions of the parietal lobe ?
Its functional regions are the primary somatosensory cortex, the somatosensory association cortex, and the supramarginal and angular gyri that both contribute to Wernicke’s area.
Wernicke area, region of the brain that contains motor neurons involved in the comprehension of speech
What is the sensory homonculus ?
The sensory homunculus is a visual representation of the proportion of sensory axons the primary somatosensory cortex allocates to a particular part of the body.
What is found underneath the lateral fissure ?
Furthermore, the area of cortex called the insula, or insular cortex, lies at the bottom of the lateral fissure, hidden from the external surface of the brain, so we need to open up the folds of the lateral fissure or dissect them in order to see it. The insula has the central sulcus of the insula running through it, forming both short gyri rostral to the sulcus, and long gyri caudal to the sulcus.
What supplies the anterior part of the cerebellar circulatory system ?
What are the terminal branches of the internal carotid artery ?
- Internal carotid arteries
- The terminal branches of the ICA are the anterior and middle cerebral arteries.
- The anterior cerebral arteries are connected via the anterior communicating artery.
What supplies the posterior aspect of the cerebellar circulatory system ?
The posterior aspect of the cerebral circulatory system arises from the vertebral arteries ( branch of the subclavian) which merge to form the basilar artery.
Ultimately, the basilar artery divides into a pair of terminal branches called the posterior cerebral arteries, which in turn give off the left and right posterior communicating arteries.
The circle of willis is formed by the anterior cerebral arteries, that are connected by the anterior communicating artery, and the posterior cerebral arteries and the internal carotid arteries, which are connected by the posterior communicating arteries.
Anterior cerebral artery mainly supplies the lower limbs, so its damage would affect the lower limbs more than the upper limbs.
The middle cerebral artery supplies the upper limb and the face. So if there is ischemic damage to this cerebral artery it will affect the upper limb more the the lower limb.
Describe the venous drainage of the brain ?
The venous drainage of the brain, which starts as a series of small superficial veins.
These veins drain into dural venous sinuses, which are venous channels located between the two layers of dura mater: the periosteal layer and meningeal layer.
The superior sagittal sinus runs along the longitudinal fissure, where it absorbs cerebrospinal fluid from the meninges, it empties into the transverse sinuses.
The transverse sinuses drain to the sigmoid sinuses, which then drain to the internal jugular veins, ultimately returning to the heart.
Understand how a motor cortical lesion produces a contralateral deficit
- The right cerebral cortex supplies motor innervation to the left side of the body.
- The corticospinal tract supplies the trunk and limb. There are two pathways anterior and lateral corticospinal.
- The lateral corticospinal (90%) tract decussates in the medulla.
- The anterior corticospinal tract (10%) decussates in the spinal cord
- If a lesion occurs after decussation point it will produce a ipsilateral functional loss.
- If a lesion occurs before decussation it will produce a contralateral functional loss.
- So a ischemic damage or hemorrhage on the primary motor cortex of the right hemisphere will lead to functional loss of the muscles of the left side of the body.
Describe how a sensory cortical lesion produces a sensory defect
Any lesion in the primary sensory area leads to loss of sensation on the opposite side of the body.
How is the cerebellum connected to the brainstem ?
- Connected to the brainstem by three cerebellar peduncles which contain afferent (sensory) and efferent ( motor) fibers.
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Describe the anatomy of the medulla ?
- Dividing the two hemispheres of the cerebellum is the vermis.
The medulla has three lobes:
- Anterior lobe
- Posterior lobe
- Flocculondular lobe.
- dividing the lobes are the primary fissure and posterolateral fissure.
What are the basal ganglia ?
- collection of subcortical nuclei
Made up of:
- consist of the globus pallidus (external and internal)
- striatum ( caudate nucleus, putaman
There are associated nuclei:
- ventral anterior, ventral lateral ( located in the thalamus)
- substantia nigra ( of the midbrain)
Describe the direct (excitatory) pathway in scientific terms ?
-Striatum inhibits the internal segment of the globus pallidus, pars reticulata of substantia nigra.
- substantia nigra - inhibitory input to the thalamus
- thalamus - excitatory input to the motor cortex.
- overall input is excitatory
Describe the indirect (inhibitory) pathway in scientific terms ?
Striatum inhibits the external segment of the globus pallidus - inhibits subthalamic nucleus
- Subthalamic nucleus projects excitatory input to the internal segment of the globus pallidus - internal segment of globus palliduus, pars reticulata of substantia nigra - inhibits - thalamus
- thalamus - excitatory input to the motor cortex
- the overall input of the indirect pathway is inhibitory
Bells palsy
Bell’s palsy is temporary weakness or lack of movement affecting 1 side of the face.
Labelled basal ganglia
Corpus callosum
