Week 1 - nerve and muscle Flashcards
1
Q
What are nerves and muscle cells known as and what does this mean?
A
- They are known as excitable cells meaning they can undergo transient, rapid fluctuations in their membrane potentials which serve as electrical signals
- These signals are used to recieve, process, initiate and transmit messages
2
Q
What does the nervous system consist of at the highest level?
A
- The central nervous system (CNS) and the peripheral nervous system (PNS)
3
Q
At the second, level what does the nervous system consist of?
A
- CNS:
- Brain
- Spinal chord
- PNS:
- Numerous nerves in the body
- Special sense organs required for:
- Sight
- Hearing
- Taste
- Smell
- Touch
4
Q
What are the subdivisions of the PNS and what do they do?
A
- The autonomic nervous system encompasses the structures involved in regulating involuntary functions, e.g. heart rate & contractions of the stomach and intestine
- The somatic nervous system is under voluntary control
5
Q
Describe the structure of nerve cells
A
Neurones, or nerve cells, are specialised cells and consist of three main parts:
- Soma (cell body) - contains the nucleus and other cellular organelles
- Dendrites - extend from the cell body and receive signals from other neurones
- Axon - long extensions that transmit signals away from the cell body
6
Q
What is myelin and what does it do?
A
- Myelin is a lipid produced by Schwann cells which forms a layer around the axon called the myelin sheath
- The myelin sheath insulates the axon to allow electrical impulses to travel rapidly along it
- Myelinate neurons conduct impulses more quickly than unmyelinated neurons
- At regular intervals in the myelin sheath there are Nodes of Ranvier which are areas enriched with voltage-gated sodium channels and they potentiate the impulse along the axon
- The axon branches at the end to form axon terminals - sites wherer signal is passed onto other neurons or effector cells
7
Q
How are neurons classified and what are the types of neurons?
A
- Neurons are classified according to the direction they transmit electrical impulses
- Sensory (afferent) neurons - transmit impulses from all areas of the body to the CNS
- Moter (effent) neurons - transmit impulses away from the CNS to muscle and glandular epithelial tissues
- Interneurons - transmit impulses from sensory to motor neurons (sometimes called connecting neurons)
8
Q
What are glial cells?
A
- They are dervived from the Greek word glia meaning “glue”
- One of the functions of glial cells is to hold neurons together and protect them
- Generally they function to support the nervous system and assist in repair and maintainance
- Different types of glial cells differ in shape and size
9
Q
Name three types of glial cells
A
- Astrocytes
- Microglia
- Oligodendrocytes
10
Q
Outline astrocytes
A
- Astrocytes are a type of glial cell
- They are larger cells with threadlike extensions that attach neurons to blood vessels
- These branches form a two layer structure called the blood-brain barrier (BBB)
11
Q
Outline microglia
A
- They are a type of glial cell
- Extremely small cells of the CNS, their primary function is to remove cellular waste and protect against microorganisms
- They are usually stationary but when the CNS is inflammed or degenerating, they enlarge, move around and draw microorganisms into their cytoplasm by phagocytosis and digst them
12
Q
Outline oligodendrocytes
A
- They are a type of glial cell
- Produce the lipid myelin that surrounds the axons of neurons in the CNS
- They differ from the Schwann cells which only produce myelin around the neurons of the PNS
- Oligodendrocytes form part of the myelin sheath around several nerve fibres at once wheareas Schwann cells wrap themselves entirely around one fibre
13
Q
Outline multiple sclerosis and how it relates to oligodendrocytes
A
- The most common primary disease of the CNS
- It is an autoimmune condition where the body mistakenly attacks healthy myelin covering the axons
- Loss and destruction of myelin is accompanied by oligodendrocyte cell injury or death
- Loss of myelin in CNS impairs nerve conduction & can lead to impaired vision and speach and muscle weakness and incoordination
- Occurs in both men and women but more common in women ages 20-40
- Women with MS are more likely to have gene which produces high levels of protein interferon gamma which promotes inflammation and tissue damage - aggrefvates MS
- No cure & treatments are aimed at relieving symptoms
14
Q
Describe the complete / greater structure of neurons
A
- Nerve is a group of nerve fibres (axons) bundled together
- Myelin is which and neurons in the PNS are usually myelinated - these nerve fibre bundles often look white
- In CNS there are both myelinated and unmyelinated nerve fibres, myelinated nerves for white matter of brain and spinal chord, unmyelinated forms grey matter
- Each individual axon is surrounded by endoneurium - thin layer of fibrous connective tissue
- Groups of these axons form fascicles
- Each fascicle is surrounded by another thin layer of fibrous tissue called the perineurium
- The entire nerve is covered by a tough fibrous sheath called the epineurium
15
Q
What is an action potential?
A
- Impulses and action potentials are terms that are use interchangeably
- An action potential is a self-propagating wave of membrane depolarisation, i.e a change in membrane potential to more +ve (less -ve)
- Cells that generate and transmit action potentials are known as excitable cells
- In excitable cells the rapid and transient change in membrane potential serves as the basis of information transfer
16
Q
How is the membrane potential of a nerve cell determined?
A
- By the relative permeability of the membrane to specific ions such as Na+, K+ and Cl-
- The concentration gradient of those ions across the membrane
17
Q
Define the four terms associated with a voltage time graph of an action potential
A
-
Resting potential is the term given to describe the membrane potential of a nerve cell when unstimulated
- For nerve cells this is typically -70 mV because there is usually an excess of Na+ ions outside the cell
- Depolarisation is when the inside of the cell becomes less negative with respect to the outside of the cell, i.e. becomes closer to 0 mV
- Repolarisation is when the inside of the cell becomes more negative, i.e. further away from 0 mV
- Hyperpolarisation is when the membrane repolarises but beyond the initial state
18
Q
Describe what is happening at each stage of the voltage / time graph of an action potential
A
- Impulses are not continuously transmitted by neurons, they must be initiated by a stimulus e.g. temperature
- Stimulus initiates change in membrane potential by causing sodium channels to open allowing Na+ to flow across that area of the membrane into the cell - causing membrane depolarisation
- If the magnitude of depolarisation is sufficient and reaches the threshold potential an action potential is initiated - this is an all-or-nothing event
- When the membrane potential reaches the threshold potential, votlage-gated (v. g.) Na channels are opened causing a large influx of Na+ and a rapid and substantial depolarisation of the membrane
- Depolarisation rapidly recovers as the v. g. Na channels shut and the v. g. K channels open (allowing K+ out of the cell) causing repolarisation
- Depolarisation has already stimulated Na channels in the neighbouring section of membrane to open and action potential travels along the membrane
- After depolarisation the membrane repolarised beyond the initial resting state - hyperpolarisation
- This is due to the v. g. K+ channels not closing as quickly as the v. g. Na+ channels & threshold potential is increased momentarily
- This period is the refractory period in which a second action potential cannot be initiated in this time
19
Q
How does the nervous system detect changes in both the internal and external environment?
A
- It is able to detect stimuli by means of receptors that are sensitive to chemical and physical stimuli
20
Q
With regards the nervous system, what does the term “integration” mean?
A
- The process by which information from internal and external sensory receptors is coordinated to initiate appropriate responses is called integration
21
Q
Why are synapses necessary?
A
- In order for stimuli to be translated into an appropriate response, information needs to be passed along the neuron pathway, for this to happen the neurons need to communicate and this is done via synapses
22
Q
What is a synapse?
A
- It is the site where nerve impulses are transmitted from one neuron, called the presynaptic neuron, to another, called the postsynaptic neuron
23
Q
What does a synapse consist of ?
A
- The synaptic knob - a bulge at the end of an axon terminal branch of the presynaptic neuron
- The plasma membrane of the postsynaptic neuron
- A synaptic cleft - the gap between the presynaptic knob and the postsynaptic plasma membrane
24
Q
What is the purpose of the synaptic vesicles in the presynaptic knob?
A
- The vesicles contain neurotransmitters which, when a action potential reaches the presynaptic terminal, are stimulated to release the neurotransmitter into the synaptic cleft
25
Q
What is the function of receptors in the postsynaptic plasma membrane?
A
- When the released neurotransmitters bind to these specific receptors a nerve impulse in the postsynaptic neuron is initiated
26
Q
In the synapse, how is neurotransmitter activity terminated?
Why is this important?
A
- It is important to terminate neurotransmitter activity to prevent contstant firing of nerve impulses
- The termination is acheived in one or both of two ways:
- Released neurotransmitters are transported back into the presynaptic knob (neurotransmitter reuptake)
- Specific enzyme metabolise the neurotransmitter into inactive compounds
27
Q
What are the different ways in which neurotransmitters work?
A
- Several different compounds have been identified as neurotransmitters but they work in different ways.
- Neurotransmitters can inhibt or stimulate postsynaptic neurons
- Examples:
- Acetylcholine (ACh) is a neurotransmitter that is released from neurons in the spinal chord and at neuromuscular junctions and stimulates the muscle fibres to contact
- Endorphins are neurotransmitters that are released at various spinal chord and brain synapses, they play a dominant role in inhibiting the conduction of pain-related nerve impulses - natural pain killers
- They are also released during heavy exercise
28
Q
Describe the two ways in which stimuli can be detected by sensory neurons
A
- For stimuli such as pressure an touch, nerve endings directly detect the stimulus, there are different types of nerve ending depending on the stimulus (see diagram below)
- For some stimuli such as taste and visual the stimulus is detected by specialised epithelial cells
- These epithelial cells are in close proximity to a sensory neuron
- Upon stimulation there is a change in ion permeability which results in neurotransmitters being released from the epithelial cell at the synapse of the sensory neuron & an action potential is triggered
29
Q
Name four sense organs
A
- Eyes
- Tongue
- Ear
- Nose
30
Q
Describe the eye as a sense organ
A
- The eye is the organ that senses light
- Light passes through the cornea - clear protective outer layer of the eye
- Light enters the pupil and passes through the lens
- Ciliary muscles can alter the shape of the lens so that light is focused on the retina
- The retina contains light receptors that trigger nerve impulses in the optic nerve which relays the signal to the brain
31
Q
Describe the ear a sense organ
A
- The ear senses sound waves that enter the ear canal and trtavel to the ear drum causing it to vibrate
- These vibrations are transmitted to the cochlea which is a spiral shaped, fluid filled cavity - the fluid is called perilymph
- These vibrations cause the fluid to move
- The movement of the fluid is detected by the bending of cilia on thousands of hair cells
- This mechanical stimulus triggers a nerve impulse in the auditory nerve which relays the signal to the brain
32
Q
Describe the tongue as a sense organ
A
- Taste is a chemical sense
- On the tongue there are riges called papillae and these contain taste buds
- A taste bud is a cluster of cells which assemble to form a taste pore
- Taste receptors can be stimulated by a large variety of solute moleculesm, but they are roughly grouped into the four primary flavours: sweet, salty, sour and bitter.
- Taste cells will respond strongly to one of the primary flavours and weakly to the others
33
Q
Describe how the tongue detects different substances
A
- Different substances (or flavours) differ in how they affect the ion permeability of the taste cell membrane
- Sour substances contain H+ which block channels in the membrane whereas salty substances alter the flow of Na+ across the membrane
- These alteration is membrane permeability can trigger nerve impulses
- The nerve stimulated depends on where the receptors that are stimulated are located on the tongue
34
Q
What are the different nerves that are stimulated on the tongue and what locations do they correspond to?
A
- Chorda tympani nerve detects stimuli from the front and sides of the tongue
- The glosso-pharyngeal nerve detects stimuli from the back of the tongue
- The vagus nerve detects stimuli from the mouth and larynx
35
Q
Describe the nose as a sense organ
A
- Smell is also a chemical sense
- The system used to sense smell is the olfactory system
- Supporting cells of the olfactory epithelium secrete mucus which covers all the surfaces of the nasal cavity
- Odour molecules travel to the nasal cavity where they dissolve in the mucus
- The odour molecules bind to the olfactory receptors present on the cilia of olfactory neurons
- Binding of an odour molecule causes a conformational change in the memrane resulting in altered ion permeability that triggers a nerve impulse in the olfactory neuron
- This impulse is transmitted along the neuron which synapses with a glomerulus in the olfactory bulb of the brain
- Olfactory glomeruli are thought to bevital for odourant signal detection
36
Q
How are we able to smell different odours?
A
- Thousands of different receptors and neurons that are sensitive to different odours
- How the sense of smell is encoded by the brain is not fully understood
37
Q
What is important about the spatial organisation of olfactory glomeruli?
A
- Olfactory receptor neurons that are stimulated by the same or similar odour molecules converge their axons to one or a few distinct glomeruli
- This suggests that smell is organised spatially in the olfactory bulb, i.e. neurons that detect similar smells are grouped together
38
Q
What happens once an action potential is triggered?
A
- It is relayed to the CNS for integration
- Integration is the process by which information from internal and external receports is coordinated to initiate an appropriate response
39
Q
What is the route called that an impulse travels to complete the “detection-integration-response” process?
A
- The route that an nerve impulse travels to complete the “detection-integration-response” process is called the neuron pathway
40
Q
What is a basic type of neuron pathway?
A
- A basic type of neuron pathway is a reflex arc
41
Q
Outline the simplest type of reflex arc and the neurons that are involved
A
- The knee-jerk reflex uses just two of the three types of neuron: sensory and motor neurons
- Sensory receptors located inside the quadricep muscle are stimulated when the muslce is stretched in response to a tap on the patella ligament
- The nerve impulse that is generated by the sensory neuron travels along the axon to the cell body which is located in the dorsal root ganglion
- The impulse travels from the cell body and ends near the dendrites of the motor neuron located in the grey matter of the spinal chord
- The sensory and motor neuron communicate via a synpase allowing the impulse to continue along the dendrites, cell body and axon of the motor neuron
- The motor neuron synapses with the effector, the quadricep muscle, resulting in the contraction of the muscle
42
Q
What is the dorsal root ganglion?
A
- The dorsal root ganglion is a cluster of nerve cell bodies near the spinal chord
43
Q
What is the function of the knee-jerk reflex?
A
- It serves to protect the body from harmful stimuli
44
Q
After the knee-jerk reflex, what does the next simplest reflex arc involve?
Outline three neuron reflexes
A
- It involves all three of the types of neuron:
- Sensory neurons
- Motor neurons
- Interneurons
- In three-neuron reflexes the sensory neuron synapses with the interneuron
- Interneurons are situated entirely in the grey matter of the CNS
- Interneuron integrates the information from the sensory neuron and determines the magnitude of response before passing the information to the motor neuron
*
45
Q
What is a characteristic of reflex arcs that contain interneurons?
A
- Reflex arcs that contain interneurons are typically under greater control as the interneuron can control and modify the response to a stimuli
46
Q
What does the CNS contain and where are they located?
How are the components of the CNS protected?
A
- The CNS contains the brain and spinal chord, as the name suggests they are centrally located
- Both brain and spinal chord are especially protected, the brain is encased in the cranial cavity of the skull and the spinal chrod is surrounded by the vertebral column
47
Q
What are the subdivisions of the brain?
A
- The brainstem
- Medulla oblongata
- Pons
- Midbrain
- The cerebellum
- The diencephalon
- Hypothalamus
- Thalamus
- The cerebrum
48
Q
Describe the brainstem region of the brain
A
- The medulla oblongata is an extension of the spinal chord & coordinates internal organs, e.g. cardiovascular centre and respiratory centre
- The brainstem links the brain to the spinal chord & functions as a two way conduction path
- Sensory nerve fibres conduct impulses up from the spinal chord to the brain and motor nerve fibres conduct impulses down from the brain to the spinal chord and rest of the body
- The pons (latin for “bridge”) coordinates and communicates information between the two hemispheres of the brain & connects the cerebral cortex with the medulla
- The midbrain (aka the mensencephalon) contains nerves responsible for eye movement, lens shape & pupil diameter
49
Q
Describe the cerebellum region of the brain
A
- 2nd largest part of the brain
- Folded grey matter forms the outer layer (large surface area for neuron connection and nerve impulse processing)
- Interior consists mainly of white matter
- Plays a large role in the production of normal movement such as walking as well as controlling balance and coordination
50
Q
Describe the diencephalon region of the brain
(hypothalamus)
A
- Contains the hypothalamus, one of the most important brain structures
- One key function of the hypothalamus is to translate a nerve impulse to a endocrine response, i.e. hormone secretion, and the hypothalamus exerts a certain amount of control over every internal organ
- Hypothalamus is also involved in:
- regulating heartbeat
- constriction and dilation of blood vessels
- Contraction of stomach and intestines
- Communicates with pituitary gland to secrete hormones sect and anti-diuretic horomes (ADH) into the blood
- Plays crucial role in controlling body temperature, appetite, sleep cycle and emotions such as pain, pleasure, fear & hunger
51
Q
Describe the diencephalon region of the brain
(thalamus)
A
- Thalamus is a section of grey matter just above the hypothalamus that also plays an important role in sensations
- Nerve impulses originating from the sense organs ofthe body are relayed to the sensory areas of the cerebrum via neurons that extend upwards from the thalamus
52
Q
Describe the division of the cerebrum region of the brain
A
- Largest part of the brain
- Surface composed of may gyri (ridges) and sulci (grooves)
- Deepest sulci are called fissures, there is a large longitudinal fissure that divides the cerebrum into right and left hemisphere
- Each hemisphere is subdivided into four major lobes names according to the bone that covers them
- Frontal, parietal, temporal and occupital lobes
53
Q
Describe the surface & interior of the cerebrum region of the brain
A
- A thin layer of grey matter called the cerebral cortex forms the surface of the cerebrum
- The interior is primarily composed of white matter but there are islands of grey matter which serve to control automatic movements and postures
54
Q
What is the function of the cerebrum?
A
- Neurons in the cerebrum work with many other neurons in other parts of the brain and the spinal chord and nerve impulses are being continuously transmitted to and from the cerebrum
- The cerebrum is where conscious thoughts are processed and managed
55
Q
What functions are associated with the cerebrum?
A
- The following are associated with cerebral function:
- Determining intelligence
- Determining personality
- Thinking
- Perceiving
- Producing and understanding language
- Interpretation of sensory impulses
- Motor function
- Planning and organisation
- Touch sensation
56
Q
What can be the effect of damaging the cerebrum?
A
- Neurons in the cerebrum can be damaged or destroyed due to inujury or illness and this can affect one or more of the functions of the cerebrum (mentioned in flashcard #55)
- E.g. a hemorrhage in the motor area of the cerebrum (which is in the frontal lobe) can result in not being able to voluntarily move parts of the body
57
Q
Outline the spinal chord
A
- It is approximately 18 inches long and functions as a communiation system between the brain and the body
- Nerves branch off from the spinal chord through openings in the vertebrae to all areas of the body in order to send and recieve signals
- Spinal nerve routes branch off the spinal chord in pairs - one going to each side of the body
- Spinal chord is delicate and needs protection, two types of connective tissue surround and protect the spinal chord:
- meninges and vertebrae
58
Q
Describe the meninges
A
- Meninges are three layers that surround the spinal chord and brain
- The outer layer is called the dura mater and is a thick layer of connective tissue
- The middle layer is called the arachnoid mater and is much thinner
- Inner layer is called the pia mater and is a very thin and delicate tissue layer
- The denticulate ligaments are thickenings of the pia mater. They are present on both sides of the spinal chord and attach the pia mater to the other meninges to provide stability
- The subarachnoid space (between arachnoid mata and pia mater) contains the cerebrospinal fluid which is a clear, colourless liquid that acts as an additional barrier, a shock absorber and a lubricant
59
Q
What is inflammation of the meninges called?
A
- Meningitis
60
Q
Describe the vertebral body
A
- The vertebral body is a dense cortical bone, with a thinner centre of cancellous (spongy) bone
- Facet joints are the joints between the vertebrae and are what allow the spine to twist, bend and turn
- The vertebrae are grouped into sections according to their location
61
Q
Outline the different groups of vertebrae
A
- 7 vertebrae in the neck are called the cervical vertebrae and are numbered from top to bottom, from the head to the tail bone (C1-7)
- The next 12 vertebrae are called the thoracic vertebrae (T1-12)
- The next 5 vertebrae are called the lumbar vertebrae (L1-5)
- At the base of the spine there are five fused vertebrae that forms a large traigular bone that sits between the two hip bones
- Extending from here are several sacral nerves (S1-5)
- At the very bottom of the spine is the coccyx (aka tailbone) which is made of three loosely fused bones
- Extending from here is the coccygeal nerve which innervates the skin around the coccyx
62
Q
What can be the result of injury to the spinal chord?
A
- Can lead to the loss of sensation and movement
- Generally speaking the higher the injury on the spinal chord the more dysfunction can occur
63
Q
Describe the intervertebral disks
A
- The intervertebral disks lie between adjacent vertebrae
- The disks are pads of fibrocartilage that consists of an outer annulus fibrosus and an inner nucleus pulposus
- Individual lamellae of the annulus fibrosus consist mainly of collagen type 1 fibres
- The nucleus pulposus consists of proteoglycans (mainly aggrecan) and water, held together by a network of collagen type II and elastin fibres
- The high aggrecan content of the disk means the disk is resistant to compression; ensuring that the load is spread evenly over the vertebral body
64
Q
What is the peripheral nervous system?
A
- PNS consists of the nerves that connect the brain and spinal chord (CNS) to the rest of the body
- There are 12 pairs of cranial nerves that attach to the brain and extend through the brainstem
- In addition to the cranial nerves there are 31 pairs of spinal nerves :
- 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal
65
Q
Describe the how the PNS interacts with the spinal chord
A
- H shaped core of the spinal cord consists of grey matter and is composed of neuron cell bodies and dendrites. The white matter consists of bundles, known as tracts, of myelinated axons that run outside the central grey matter
- Sensory neurons have their cell body in the dorsal root ganglion and project into the spinal chord
- Sensory neurons either synapse directly with motor neurons or interneurons
- Motor neurons leave the spinal cord via the ventral root
- The sensory and motor neurons then join to form a mixed spinal nerve (a mixture of sensory and motor neurons that work together for a certain function)
- After the dorsal and ventral roots have joined to form the spinal nerve, they then split apart again and are now called rami (ramus for singular)
66
Q
Give two examples of rami
A
- Dorsal rami innervate the skin and muscles of the back
- Ventral rami form plexus and innervate the anterior trunk and limbs
67
Q
What is a nerve plexus?
A
- Several nerve fibres branch off the ventral rami. These nerve fibres will intersect with nerve fibres from other ventral rami to form a network of nerve fibres (known as a nerve plexus) that all serve a particular area of the body
68
Q
What are the four nerve plexuses?
A
- Cervical - innervates muscles of the neck and areas of skin on the head, neck and chest.
- Brachial - innervates all areas of the shoulder and upper limb
- Lumbar - innervate the anterior (front) aspects of the lower limb i.e. the quadricep and adductor muscles and overlying skin, and some of the abdominal wall
- Sacral - innervates the buttocks, pelvis, perineum and lower limb (except for the thigh)
69
Q
Describe the innervation of the skin the neurons of the PNS
A
- All skin is innervated by nerves of the spinal chord, except for skin on the face, which is supplied by cranial nerve V.
- All spinal nerves, except C1, innervate a specific segment of skin
- Each segment is called a dermatome
- Dermatome maps, like the one below are useful for identifying damaged or malfunctioning spinal nerves.
70
Q
Summarise the PNS
A
- PNS consists of sensory neurons that inform the CNS of a stimuli
- Motor neurons that carry processed information from the CNS to muscles and glands (the effectors) in the rest of the body, that take action and produce an effect.
71
Q
What % body mass is muscle tissue?
What are the different types of muscle?
A
- Muscle accounts for 40-50% body mass of an adult
- There are three different types of muscle
- Skeletal - attached to bone via tendons & allow for movement about joints
- Smooth - major component of internal muscular organs such as arteries, veins, the urinary bladder and the gastrointestinal tract
- Cardiac - composes the bulk of the heart and is a highly specialised muscle
72
Q
A
73
Q
Briefly outline muscle cells and how they are formed
A
- Muscle cells, referred to as muscle fibres, are the largest cells in the body, often running the entire length of the muscle, ranging from 10 to 100 microns in diameter.
- Muscle cells are formed from spindle-shaped precursor cells called myoblasts. Myoblasts can fuse together to form an elongated, multinucleated muscle fibre
74
Q
Describe the structure of muscles
A
- Each multinucleate muscle fibre is surrounded by a sheath of connective tissue called endomysium
- Several muscle fibres bundle together to form muscle fascicles, which are surrounded by a sheath of perimysium.
- A muscle is composed of several muscle fascicles, and each muscle is surrounded by a connective tissue sheath called the epimysium.
- The cell membrane of muscle fibres is called the sarcolemma.
- In each muscle fibre, there are long, parallel myofibrils, which are made up of repeating subunits called sarcomeres
75
Q
What are sarcomeres?
A
- Sarcomeres are the contractile units of a muscle cell.
- Composed of overlapping thin and thick filaments containing the contractile proteins actin and myosin respectively.
- Stacked within the fibres to form the striated appearance.
76
Q
Describe the appearance of sarcomeres and how the muscle contracts
A
- Striated in appearance
- H zone is the section of thick filament not overlapped by thin filament
- A band is the entire region that is occupied by thick filaments
- I bands are the regions of thin filament either side of the A band that are not overlapped by thick filament
- Adjacent sarcomeres are separated from each other by Z line to which thin filaments are attached
- Muscle contraction causes the thick and thin filaments to slide across one another, shortening the sarcomere and thus the entire muscle. This is called the sliding filament model.
77
Q
Outline the sliding filament theory of muscle contraction
A
- Individual myofilaments do not change length but overlap to shorten the sarcomere
- At rest, myosin has both ADP + Pi attached to the individual myosin heads
- Ca2+ released from the sarcoplasmic reticulum
- Ca2+ bind to troponin molecules causing tropomyosin molecules to mvoe and expose the myosin attachment site on the actin myofilaments
- The Pi on the myosin head is released as the myosin head moves to connect with the attachment site on actin
- This forms a cross-bridge
- ADP is expended in the “powerstroke” when the myosin head pull the actin inwards
- Once ADP released, ATP binds to the myosin heads triggering the release of myosin heads from the actin attachment sites
- ATP is immediately broken down to ADP + Pi as the head smove back to the resting position
- Cycle repeats until all Ca2+ transported back into the sarcoplamsic reticulum
78
Q
What are the key points for the sliding filament theory of muscle contraction?
A
- Ca2+ necessary for muscle contraction
- Calcium ions are stored in the smooth endoplasmic reticulum within muscle cells
- A nerve signal can stimulate calcium ion release from these stores
- Calcium ions bind to actin thin filaments, permitted actin to react with myosin
- Myosin heads form cross bridges with actin, which pulls the thin filaments towards the middle of the sarcomere, thus shortening it
79
Q
What is necessary for a muscle to contract?
A
- It must be stimulated by a nerve impulse
- Muscles are stimulated by motor neurons and the point of contact between the motor neuron and the muscle fibre is called the neuromusclular junction (NMJ)
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80
Q
What does the term excitable cell mean regarding muscle cells?
A
- They contain voltage-gated channels which enable them to respond to and conduct electrical impulses
81
Q
Describe the NMJ
A
- Motor neuron synapses directly with a muscle fibre
- Fine branches extend from a signle axon into the perimysium and each branch ends at a presynaptic terminal (synaptic end bulb)
- Mitochondria and presynaptic vesicles containing acetylcholine (ACh) are found in the presynaptic terminal (PST)
- Surface of muscle cell with ACh receptors is called the motor end plate which contains several deep junctional folds
- Increases the area for ACh
82
Q
Describe the process of impulse transmission across the NMJ
A
- Action potential reaches the presynaptic terminal, ACh is released via exocytosis into the synaptic cleft.
- ACh molecules can alter the permeability of the sarcolemma to sodium ions by binding to ACh receptors.
- The synaptic cleft and sarcolemma contain molecules of acetylcholinesterase (AChE), which breaks down ACh.
- Sodium ion influx continues until all ACh molecules have been broken down and removed by AChE
- The rapid increase in sodium ions triggers an action potential in the sarcolemma, which travels along the length of the T-tubules
83
Q
What is a T tubule?
A
- Invaginations of the sarcolemma that are rich in L-type calcium channels
84
Q
What is the minimum level of stimulation required before musckle contraction called?
A
-
Threshold stimulus meaning muscle contraction is an all-or-nothing event
- Once the threshold stimulus is reached, the muscle will contract completely
- Each muscle fibre in a muscle is controlled by a different motor neuron and will have different threshold-stimulus levels
- Even though each muscle fibre responds in an all or nothing manner, the muscle as a whole does not
- This allows for an appropriate response e.g. picking up something light vs heavy
85
Q
What is the link between action potential in the sarcolemma and a muscle contraction called?
A
- Excitation-contraction coupling
86
Q
Describe the process of releasing Ca2+ required for muscle contraction
A
- Action potential reaches synaptic end bulb and v. g. Ca2+ channels open leading to influx of Ca2+
- Ca2+ stimulates exocytosis of synaptic vesicles
- ACh diffuses across the synaptic cleft
- ACh binds to ligand gated Na+ channels causing them to open
- Influx of Na+ raises the potential of the motor end plate (depolarising it) bringing it closer to the threshold potential
- If end plate potential great enough v. g. ion channels open allowing Na+ in and K+ out
- Muscle action potential is generated and travels along the sarcolemma and into T-tubules
- The muscle action potentials in T-tubules are close enough to the membrane of the sarcoplasmic reticulum terminal cistern to open v. g. Ca2+ channels
- Ca2+ are released into the sarcoplasm
87
Q
What is the autonomous nervous system for?
A
- ANS controls involuntary functions such as heartbeat, contractions of the stomach and intestines and secretion from glands
88
Q
What does the ANS consist of?
A
- Cardiac muscle tissue
- Smooth muscle tissue
- Glandular epithelial tissue
89
Q
What are the systems under autonomic control known as?
A
- Visceral effectors
90
Q
What neurons are autonomic neurons? What is the significance of this?
A
- Autonomic neurons are motor neurons and originate in the grey matter of the brain or spinal chord
- Autonomic motor pathways to visceral organs are composed of two neurons: the preganglionic and postganglionic neurons
- Cell bodies of preganglionic neurons are in the CNS and their axons extend out and synapse with postganglionic neurons whose cell bodies are in the autonomic ganglia (outside the CNS) and their axons extend and synapse with visceral effectors
91
Q
What are the divisions of the ANS?
A
- The sympathetic and parasympathetic nervous systems
92
Q
Outline the sympathetic nervous system
A
- Automomic ganglia for sympathetic neurons are located just outside the spinal chord on either side
- Sympathetic preganglionic neurons have cell bodies in the thoracic and upper lumbar segmetns of the spinal chord - known as the thoracolumnar system
- Axons of these neurons leave the spinal chord via the ventral root and enter the spinal nerve before extending through the sympathetic ganglion and synapsing with postganglionic neurons in the collateral ganglion
- A signle sympathetic preganglionic neuron branches and synapses with several postganglionic neurons
- Results in sympathetic nerve impulse affecting several organs at once
- A signle sympathetic preganglionic neuron branches and synapses with several postganglionic neurons
93
Q
What is the function of the sympathetic nervous system?
A
- Prepare the body for emergency situations, otherwise known as fight-or-flight reactions
- During times of stress or danger, sypathetic nerve impulses are fired more frequently and produce widespread changes such as; increased heartbeat, blood vessel constriction (except in muscles where vessels dilate to increase blood flow), increased secretion of sweat and adrenaline.
94
Q
Outline the parasympathetic nervous system
A
- Autonomic ganglia for parasympathetic neurons are located near or in the visceral effector organ
- Parasympathetic preganglionic neurons have cell bodies located in the brainstem and sacral regions of the spinal cord and therefore is also known as the craniosacral system
- Usually much longer than sympathetic preganglionic neurons as they extent over a greater distance before terminating in the parasympathetic ganglion
- Synapse with just one other postganglionic parasympathetic neuron and only effect one organ
95
Q
What is the function of the parasympathetic nervous system?
A
- Control organs under normal, everyday conditions, activating the rest and digest response
- Parasympathetic stimulation generally counterbalances sympathetic responses as it results in the slowing of the heartbeat, increased peristalsis and secretion of insulin
96
Q
What is the term for when effectors receive inputs from both the parasympathetic and the sympathetic nervous system?
What is the significance of this?
What is an exception?
A
- This is referred to as dual innervation
- Organs that receive dual innervation are generally excited by one branch and inhibited by the other
- Some organs however only receive single innervation from one branch of the autonomic nervous system, such as most blood vessels, which only receive sympathetic innervation
97
Q
What are the types of neurotransmitter released in the ANS and which sub-system do they apply in?
A
- ACh is the neurotransmitter used between pre- and postganglionic neurons of both sympathetic and parasympathetic branches
- he transmitter used between postganglionic and visceral effectors differs between sympathetic and parasympathetic branches
- As a general rule, sympathetic neurons release norepinephrine and parasympathetic neurons release ACh
98
Q
What is the somatic nervous system and what is it responsible for?
A
- Known as the voluntary nervous system as it controls our voluntary muscle movements
- Voluntary movements therefore require conscious awareness (perception)
99
Q
Where does conscious awareness (perception) take place?
A
- Takes place in the somatosensory cortex, located in the postcentral gyrus (an area of the parietal lobe of the brain)
*
100
Q
What does the somatic nervous system control?
A
- The somatic nervous system consists of sensory and motor divisions
101
Q
What is the sensory division of the somatic nervous system?
A
- Aka afferent division
- Consists of neurons that transmit information via nerve impulses from the sensory organs (skin, eyes, ears, nose, and tongue), muscles, tendons, joints, and many other organs, to the somatosensory cortex in the brain
102
Q
What does the pathway from periphery to somatosensory cortex require?
A
- Requires three neurons and these are often described as first, second and third order neurons in the sensory pathway.
103
Q
What are first, second and third order neurons?
A
- First order neurons: sensory neurons that relay information from the internal and/or the external environment to the CNS. The cell body of all first order neurons is located in the dorsal root ganglion.
- Second order neurons: interneurons (stimulated by the first order neurons), that are located entirely in the CNS. These interneurons relay sensory information to the thalamus
- Third order neurons: interneurons that extend from the thalamus and terminate in the primary somatosensory cortex of the postcentral gyrus.
104
Q
What is the signficance of projections from the thalamus in the somatic nervous system?
What can be constructed using this information?
A
- Projections from the thalamus to the somatosensory cortex can produce maps of the body surface
- This is called a somatosensory homunculus
- Exaggerates areas of the body to varying degrees depending on the density of receptors i.e. very sensitive areas such as the lips and the fingertips have a huge representation
105
Q
What us the precentral gyrus?
A
- It is almost parallel to the postcentral gyrus
- It houses the motor division (also known as the efferent division or primary motor cortex) of the somatic nervous system
- The precentral gyrus works with the postcentral gyrus to plan and execute voluntary movements.
106
Q
How can the portion of the brain directly responsible for movement can be shown?
A
- Can be shown pictorially in a motor homunculus
107
Q
What does the motor division of the precentral gyrus contain?
What is the difference between them?
A
- Consists of upper and lower motor neurons
- Upper motor neurons from different areas of the precentral gyrus group together and form a bundle of neurons (tract) that descends through the cerebrum to the medulla (brainstem)
- In the medulla most of the neurons cross over; therefore the left hemisphere controls the right-hand side of the body whilst the right hemisphere controls the left-hand side of the body
- The upper motor neurons descend into and synapse with lower motor neurns in the anterior horn of the spinal cord
- The lower motor neurons then extend out of the spinal cord via the ventral roots to skeletal muscles.
