Week 1 - nerve and muscle

Question 1

What are nerves and muscle cells known as and what does this mean?

Answer 1

• 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

Question 2

What does the nervous system consist of at the highest level?

Answer 2

• The central nervous system (CNS) and the peripheral nervous system (PNS)

Question 3

At the second, level what does the nervous system consist of?

Answer 3

• CNS:
  
  • Brain
  • Spinal chord
• PNS:
  
  • Numerous nerves in the body
  • Special sense organs required for:
    
    • Sight
    • Hearing
    • Taste
    • Smell
    • Touch

Question 4

What are the subdivisions of the PNS and what do they do?

Answer 4

• 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

Question 5

Describe the structure of nerve cells

Answer 5

Neurones, or nerve cells, are specialised cells and consist of three main parts:

1. Soma (cell body) - contains the nucleus and other cellular organelles
2. Dendrites - extend from the cell body and receive signals from other neurones
3. Axon - long extensions that transmit signals away from the cell body

Question 6

What is myelin and what does it do?

Answer 6

• 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

Question 7

How are neurons classified and what are the types of neurons?

Answer 7

• Neurons are classified according to the direction they transmit electrical impulses

1. Sensory (afferent) neurons - transmit impulses from all areas of the body to the CNS
2. Moter (effent) neurons - transmit impulses away from the CNS to muscle and glandular epithelial tissues
3. Interneurons - transmit impulses from sensory to motor neurons (sometimes called connecting neurons)

Question 8

What are glial cells?

Answer 8

• 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

Question 9

Name three types of glial cells

Answer 9

1. Astrocytes
2. Microglia
3. Oligodendrocytes

Question 10

Outline astrocytes

Answer 10

• 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)

Question 11

Outline microglia

Answer 11

• 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

Question 12

Outline oligodendrocytes

Answer 12

• 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

Question 13

Outline multiple sclerosis and how it relates to oligodendrocytes

Answer 13

• 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

Question 14

Describe the complete / greater structure of neurons

Answer 14

• 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

1. Each individual axon is surrounded by endoneurium - thin layer of fibrous connective tissue
2. Groups of these axons form fascicles
3. Each fascicle is surrounded by another thin layer of fibrous tissue called the perineurium
4. The entire nerve is covered by a tough fibrous sheath called the epineurium

Question 15

What is an action potential?

Answer 15

• 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

Question 16

How is the membrane potential of a nerve cell determined?

Answer 16

1. By the relative permeability of the membrane to specific ions such as Na⁺, K⁺ and Cl^-
2. The concentration gradient of those ions across the membrane

Question 17

Define the four terms associated with a voltage time graph of an action potential

Answer 17

• 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

Question 18

Describe what is happening at each stage of the voltage / time graph of an action potential

Answer 18

• Impulses are not continuously transmitted by neurons, they must be initiated by a stimulus e.g. temperature

1. 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
   
   1. If the magnitude of depolarisation is sufficient and reaches the threshold potential an action potential is initiated - this is an all-or-nothing event
2. 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
3. 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
   
   1. ​Depolarisation has already stimulated Na channels in the neighbouring section of membrane to open and action potential travels along the membrane
4. After depolarisation the membrane repolarised beyond the initial resting state - hyperpolarisation
   
   1. ​This is due to the v. g. K⁺ channels not closing as quickly as the v. g. Na⁺ channels & threshold potential is increased momentarily
5. This period is the refractory period in which a second action potential cannot be initiated in this time

Question 19

How does the nervous system detect changes in both the internal and external environment?

Answer 19

• It is able to detect stimuli by means of receptors that are sensitive to chemical and physical stimuli

Question 20

With regards the nervous system, what does the term “integration” mean?

Answer 20

• The process by which information from internal and external sensory receptors is coordinated to initiate appropriate responses is called integration

Question 21

Why are synapses necessary?

Answer 21

• 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

Question 22

What is a synapse?

Answer 22

• It is the site where nerve impulses are transmitted from one neuron, called the presynaptic neuron, to another, called the postsynaptic neuron

Question 23

What does a synapse consist of ?

Answer 23

• 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

Question 24

What is the purpose of the synaptic vesicles in the presynaptic knob?

Answer 24

• The vesicles contain neurotransmitters which, when a action potential reaches the presynaptic terminal, are stimulated to release the neurotransmitter into the synaptic cleft

Question 25

What is the function of receptors in the postsynaptic plasma membrane?

Answer 25

• When the released neurotransmitters bind to these specific receptors a nerve impulse in the postsynaptic neuron is initiated

Question 26

In the synapse, how is neurotransmitter activity terminated?

Why is this important?

Answer 26

• 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:

1. Released neurotransmitters are transported back into the presynaptic knob (neurotransmitter reuptake)
2. Specific enzyme metabolise the neurotransmitter into inactive compounds

Question 27

What are the different ways in which neurotransmitters work?

Answer 27

• 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

Question 28

Describe the two ways in which stimuli can be detected by sensory neurons

Answer 28

• 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

Question 29

Name four sense organs

Answer 29

1. Eyes
2. Tongue
3. Ear
4. Nose

Question 30

Describe the eye as a sense organ

Answer 30

• The eye is the organ that senses light

1. Light passes through the cornea - clear protective outer layer of the eye
2. Light enters the pupil and passes through the lens
3. Ciliary muscles can alter the shape of the lens so that light is focused on the retina
4. The retina contains light receptors that trigger nerve impulses in the optic nerve which relays the signal to the brain

Question 31

Describe the ear a sense organ

Answer 31

1. The ear senses sound waves that enter the ear canal and trtavel to the ear drum causing it to vibrate
2. These vibrations are transmitted to the cochlea which is a spiral shaped, fluid filled cavity - the fluid is called perilymph
3. These vibrations cause the fluid to move
4. The movement of the fluid is detected by the bending of cilia on thousands of hair cells
5. This mechanical stimulus triggers a nerve impulse in the auditory nerve which relays the signal to the brain

Question 32

Describe the tongue as a sense organ

Answer 32

• 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

Question 33

Describe how the tongue detects different substances

Answer 33

• 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

Question 34

What are the different nerves that are stimulated on the tongue and what locations do they correspond to?

Answer 34

• 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

Question 35

Describe the nose as a sense organ

Answer 35

• Smell is also a chemical sense
• The system used to sense smell is the olfactory system

1. Supporting cells of the olfactory epithelium secrete mucus which covers all the surfaces of the nasal cavity
2. Odour molecules travel to the nasal cavity where they dissolve in the mucus
3. The odour molecules bind to the olfactory receptors present on the cilia of olfactory neurons
4. 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
5. This impulse is transmitted along the neuron which synapses with a glomerulus in the olfactory bulb of the brain
   
   1. Olfactory glomeruli are thought to bevital for odourant signal detection

Question 36

How are we able to smell different odours?

Answer 36

• 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

Question 37

What is important about the spatial organisation of olfactory glomeruli?

Answer 37

• 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

Question 38

What happens once an action potential is triggered?

Answer 38

• 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

Question 39

What is the route called that an impulse travels to complete the “detection-integration-response” process?

Answer 39

• The route that an nerve impulse travels to complete the “detection-integration-response” process is called the neuron pathway

Question 40

What is a basic type of neuron pathway?

Answer 40

• A basic type of neuron pathway is a reflex arc

Question 41

Outline the simplest type of reflex arc and the neurons that are involved

Answer 41

• The knee-jerk reflex uses just two of the three types of neuron: sensory and motor neurons

1. Sensory receptors located inside the quadricep muscle are stimulated when the muslce is stretched in response to a tap on the patella ligament
2. 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
3. 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
4. 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
5. The motor neuron synapses with the effector, the quadricep muscle, resulting in the contraction of the muscle

Question 42

What is the dorsal root ganglion?

Answer 42

• The dorsal root ganglion is a cluster of nerve cell bodies near the spinal chord

Question 43

What is the function of the knee-jerk reflex?

Answer 43

• It serves to protect the body from harmful stimuli

Question 44

After the knee-jerk reflex, what does the next simplest reflex arc involve?

Outline three neuron reflexes

Answer 44

• 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
  *

Question 45

What is a characteristic of reflex arcs that contain interneurons?

Answer 45

• Reflex arcs that contain interneurons are typically under greater control as the interneuron can control and modify the response to a stimuli

Question 46

What does the CNS contain and where are they located?

How are the components of the CNS protected?

Answer 46

• 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

Question 47

What are the subdivisions of the brain?

Answer 47

1. The brainstem
   
   • Medulla oblongata
   • Pons
   • Midbrain
2. The cerebellum
3. The diencephalon
   
   • Hypothalamus
   • Thalamus
4. The cerebrum

Question 48

Describe the brainstem region of the brain

Answer 48

• 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

Question 49

Describe the cerebellum region of the brain

Answer 49

• 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

Question 50

Describe the diencephalon region of the brain

(hypothalamus)

Answer 50

• 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

Question 51

Describe the diencephalon region of the brain

(thalamus)

Answer 51

• 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

Question 52

Describe the division of the cerebrum region of the brain

Answer 52

• 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

Question 53

Describe the surface & interior of the cerebrum region of the brain

Answer 53

• 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

Question 54

What is the function of the cerebrum?

Answer 54

• 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

Question 55

What functions are associated with the cerebrum?

Answer 55

• 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

Question 56

What can be the effect of damaging the cerebrum?

Answer 56

• 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

Question 57

Outline the spinal chord

Answer 57

• 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

Question 58

Describe the meninges

Answer 58

• 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

Question 59

What is inflammation of the meninges called?

Answer 59

• Meningitis

Question 60

Describe the vertebral body

Answer 60

• 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

Question 61

Outline the different groups of vertebrae

Answer 61

• 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

Question 62

What can be the result of injury to the spinal chord?

Answer 62

• Can lead to the loss of sensation and movement
• Generally speaking the higher the injury on the spinal chord the more dysfunction can occur

Question 63

Describe the intervertebral disks

Answer 63

• 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

Question 64

What is the peripheral nervous system?

Answer 64

• 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

Question 65

Describe the how the PNS interacts with the spinal chord

Answer 65

• 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)

Question 66

Give two examples of rami

Answer 66

• Dorsal rami innervate the skin and muscles of the back
• Ventral rami form plexus and innervate the anterior trunk and limbs

Question 67

What is a nerve plexus?

Answer 67

• 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

Question 68

What are the four nerve plexuses?

Answer 68

1. Cervical - innervates muscles of the neck and areas of skin on the head, neck and chest.
2. Brachial - innervates all areas of the shoulder and upper limb
3. 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
4. Sacral - innervates the buttocks, pelvis, perineum and lower limb (except for the thigh)

Question 69

Describe the innervation of the skin the neurons of the PNS

Answer 69

• 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.

Question 70

Summarise the PNS

Answer 70

• 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.

Question 71

What % body mass is muscle tissue?

What are the different types of muscle?

Answer 71

• Muscle accounts for 40-50% body mass of an adult
• There are three different types of muscle
  
  1. Skeletal - attached to bone via tendons & allow for movement about joints
  2. Smooth - major component of internal muscular organs such as arteries, veins, the urinary bladder and the gastrointestinal tract
  3. Cardiac - composes the bulk of the heart and is a highly specialised muscle

Question 73

Briefly outline muscle cells and how they are formed

Answer 73

• 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

Question 74

Describe the structure of muscles

Answer 74

• 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

Question 75

What are sarcomeres?

Answer 75

• 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.

Question 76

Describe the appearance of sarcomeres and how the muscle contracts

Answer 76

• 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.

Question 77

Outline the sliding filament theory of muscle contraction

Answer 77

• Individual myofilaments do not change length but overlap to shorten the sarcomere

1. At rest, myosin has both ADP + P_i attached to the individual myosin heads
2. Ca²⁺ released from the sarcoplasmic reticulum
3. Ca²⁺ bind to troponin molecules causing tropomyosin molecules to mvoe and expose the myosin attachment site on the actin myofilaments
4. The P_i on the myosin head is released as the myosin head moves to connect with the attachment site on actin
   
   1. This forms a cross-bridge
5. ADP is expended in the “powerstroke” when the myosin head pull the actin inwards
6. Once ADP released, ATP binds to the myosin heads triggering the release of myosin heads from the actin attachment sites
   
   1. ATP is immediately broken down to ADP + P_i as the head smove back to the resting position
7. Cycle repeats until all Ca²⁺ transported back into the sarcoplamsic reticulum

Question 78

What are the key points for the sliding filament theory of muscle contraction?

Answer 78

• 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

Question 79

What is necessary for a muscle to contract?

Answer 79

• 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)
  *

Question 80

What does the term excitable cell mean regarding muscle cells?

Answer 80

• They contain voltage-gated channels which enable them to respond to and conduct electrical impulses

Question 81

Describe the NMJ

Answer 81

• 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

Question 82

Describe the process of impulse transmission across the NMJ

Answer 82

• 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

Question 83

What is a T tubule?

Answer 83

• Invaginations of the sarcolemma that are rich in L-type calcium channels

Question 84

What is the minimum level of stimulation required before musckle contraction called?

Answer 84

• 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

Question 85

What is the link between action potential in the sarcolemma and a muscle contraction called?

Answer 85

• Excitation-contraction coupling

Question 86

Describe the process of releasing Ca²⁺ required for muscle contraction

Answer 86

• 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

Question 87

What is the autonomous nervous system for?

Answer 87

• ANS controls involuntary functions such as heartbeat, contractions of the stomach and intestines and secretion from glands

Question 88

What does the ANS consist of?

Answer 88

• Cardiac muscle tissue
• Smooth muscle tissue
• Glandular epithelial tissue

Question 89

What are the systems under autonomic control known as?

Answer 89

• Visceral effectors

Question 90

What neurons are autonomic neurons? What is the significance of this?

Answer 90

• 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

Question 91

What are the divisions of the ANS?

Answer 91

• The sympathetic and parasympathetic nervous systems

Question 92

Outline the sympathetic nervous system

Answer 92

• 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

Question 93

What is the function of the sympathetic nervous system?

Answer 93

• 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.

Question 94

Outline the parasympathetic nervous system

Answer 94

• 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

Question 95

What is the function of the parasympathetic nervous system?

Answer 95

• 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

Question 96

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?

Answer 96

• 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

Question 97

What are the types of neurotransmitter released in the ANS and which sub-system do they apply in?

Answer 97

• 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

Question 98

What is the somatic nervous system and what is it responsible for?

Answer 98

• Known as the voluntary nervous system as it controls our voluntary muscle movements
• Voluntary movements therefore require conscious awareness (perception)

Question 99

Where does conscious awareness (perception) take place?

Answer 99

• Takes place in the somatosensory cortex, located in the postcentral gyrus (an area of the parietal lobe of the brain)
  *

Question 100

What does the somatic nervous system control?

Answer 100

• The somatic nervous system consists of sensory and motor divisions

Question 101

What is the sensory division of the somatic nervous system?

Answer 101

• 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

Question 102

What does the pathway from periphery to somatosensory cortex require?

Answer 102

• Requires three neurons and these are often described as first, second and third order neurons in the sensory pathway.

Question 103

What are first, second and third order neurons?

Answer 103

• 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.

Question 104

What is the signficance of projections from the thalamus in the somatic nervous system?

What can be constructed using this information?

Answer 104

• 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

Question 105

What us the precentral gyrus?

Answer 105

• 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.

Question 106

How can the portion of the brain directly responsible for movement can be shown?

Answer 106

• Can be shown pictorially in a motor homunculus

Question 107

What does the motor division of the precentral gyrus contain?

What is the difference between them?

Answer 107

• 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.