Somatic nervous system

Question 1

Somatic nervous system

Answer 1

Coordinates the voluntary movements of the body,
Takes messages from the CNS through the motor (efferent - away from) to the muscles

Question 2

Sensory neurons

Answer 2

Pick up information about sense touch, stretch, pain etc…
-> That message enters the spinal cord
That information can be relayed to the brain via the ascending tract (nerves that go up the spinal cord to the brain)
Will go to a region called the somatic sensory cortex
-> Alternatively if that message applies to a reflex it will go through a reflex arc (don’t have a message that is integrated in the brain) -> goes via a reflex arc to the muscle, quick response that bypasses the involvement of the brain

Question 3

Reflex arc

Answer 3

Neuron detects a sensation in the periphery (external boundary or surface of a body) from a sensory receptor
-> Message is taken to the spinal cord where it either goes up to the brain or a reflex arc and the message stimulates a muscle to contract
MOTOR neurons -> Carry messages from the CNS to the muscle to cause movement
Motor neurons and sensory neurons travel together in nerves -> nerves contain both sensory and motor information
Only branch just before they enter or leave the spinal cord then they rejoin again
(pic pg 5)

Question 4

Ascending tracts

Answer 4

Alternative to a reflex arc is that the sensory message is taken to the brain
Ascending tracts relay information from the spinal cord to the sensory cortex (somatosensory cortex)
One side of the spinal cord is for ascending and one side of the spinal cord is for ascending
(pic pg 6)

Question 5

Descending tracts

Answer 5

Descending tracts relay information from the motor cortex to the spinal cord
ascending and descending tracts occur in different parts of the spinal cord

Question 6

Somatosensory cortex and motor cortex

Answer 6

sensory receptor -> sensory neuron (ascending) -> somatosensory cortex -> motor cortex (where messaged are generated) -> motor (descending) neurons
In the somatosensory cortex there are regions of this that are dedicated to receiving messages from different parts of the body
In motor cortex there are regions dedicated to controlling muscles in different areas of the body
somatosensory cortex can talk to the motor cortex - can generate a signal which will travel in the descending tracts to the muscles
Side note -> (anterior, front of brain - posterior, back of brain)
(pic pg 7)

Question 7

Motor neurons

Answer 7

2 types that occur in series;
->Upper motor neurons take message to relevant area of
spinal cord
Synapses onto another (lower) motor neuron
->Lower motor neuron relays nerve impulses from the spine to trigger contraction of skeletal muscle -> exit spine
Cell body is in the spinal cord, it send out an axon, the end branches of the axon connect to a muscle
ONE Alpha motor neuron (exits the spine and travels to a muscle) ->myelinated

Question 8

Schwann cells

Answer 8

Myelinate neurons in the periphery

Question 9

Neuromuscular junction

Answer 9

Where the neuron meets the muscle its a specialised synapse called a neuromuscular junction
->Synapse somatic motor neurone and a muscle fibre: Neuromuscular Junction (NMJ)
Allows action potentials (nerve impulses), which travel down an axon to communicate with a muscle
Particular type of neurotransmitter -> released at the neuromuscular junction - Acetylcholine
Action potential travels down the axon and reaches a terminal -> presynaptic terminal, synaptic esicles will come to the presynaptic membrane, merge and release acetylcholine, acetylcholine travels across the synaptic cleft (the gap) and binds to and activates nicotinic actylcholine receptors (ionotrophic, ligand gated ion channels) on the postsynaptic membrane
In a neuromuscular junction, the postsynaptic membrane is called a motor end plate (MEP)
Generally these allow sodium into the motor end plate to allow that to depolarise

Question 10

Types of muscle

Answer 10

->Skeletal (striated)
->Cardiac (striated)
->Smooth
(pic pg 10)

Question 11

Skeletal muscle

Answer 11

-Enables movement of limbs and other parts of the skeleton
-Connected to bone via tendons

Question 12

Cardiac muscle

Answer 12

-The pump in the circulation (heart)

Question 13

Smooth muscle

Answer 13

-Around many hollow internal organs

Question 14

Skeletal muscle structure

Answer 14

An individual skeletal muscle cell consists of a single muscle fibre , these are packaged together in parallel into other muscle structures
These structures lie in parallel again in the muscle itself
Skeletal muscles have lots of nuclei
Plasma membrane -> sarcolemma
Cytoplasm -> sarcoplasm
Stretching the length of the muscle fibre is a structure called a myofibril
Structure of myofibrils is important for muscle contraction

Question 15

Structure of a muscle fibre

Answer 15

Sarcoplasmic reticulum -> stores a lot of calcium
T - tubules (transverse tubules) -> inversions of bits of the plasma membrane which delve deep into the myofibril, within the sarcoplasmic reticulum
Darker and lighter regions that make sure the striated (banding) pattern seen in muscle
(pic pg 12)

Question 16

Key proteins involved in contraction (in myofibril)

Answer 16

-Myosin
-Actin
-Troponin (binds calcium)
-Tropomyosin (supports actin)
All four of these proteins join together in different ways to form two types of filament -> thick filament and thin filament

Question 17

Thick filament

Answer 17

Consists of predominantly myosin molecules
Structure - myosin tails, myosin heads, 2 at opposing angles
Shaped like a golf club with two heads
(Pic pg 13)

Question 18

Thin filament

Answer 18

Consisting of mostly actin molecules
Tropomyosin is important as it gives a backbone structure which actin can hold onto -> backbone stabilises the structure, also important for regulating contraction
Troponin is a molecule which binds to calcium

Question 19

Ultrastructure of muscles

Answer 19

In muscles thin and thick filaments line up in characteristic ways -> this is called a sarcomere

Question 20

Sarcomere

Answer 20

-> Smallest contraction unit of a muscle
Light and dark banding is due to the way the myosin and the actin folaments line up in parallel
Actin filaments -> thin filaments
Myosin -> thick filaments
Line up with gaps in between
Gives the striated bands
(Pic pg 14)

Question 21

Z-disk

Answer 21

Holds actin filaments together

Question 22

M-line

Answer 22

Holds the myosin filaments together

Question 23

H-Zone

Answer 23

Non-overlapping region of the myosin filaments, myosin but not actin

Question 24

A-band

Answer 24

Length of myosin filaments

Question 25

I-band

Answer 25

Length of non-overlapping actin filaments, at end by z disk, actin filaments but no myosin filaments

Question 26

Muscle contraction

Answer 26

The myosin heads walk along the actin filament and pull the actin filament towards the centre -> size of the bands changes

Question 27

Sliding filament theory

Answer 27

During contraction -> Z - disks move closer together until the actin filaments from each side begin to overlap –» sarcomere shortens, happens all the way along the length of the muscle so that it contracts and gets shorter -> H zone disappears and I band also disappears
The maximum amount a muscle can contract is the width of the A-band

Question 28

Cross-bridge cycling

Answer 28

Myosin heads moving along an actin filament
Myosin has two binding sites, can bond to actin and can bond to ATP, which gives this system energy - its a cyclical process, -> this is the start of a contraction - myosin bound to actin but not very tightly
ATP then binds to myosin, causes the head of myosin and actin to become separated a little bit
Myosin is an ATPase, this means it can hydrolyse ATP (can break down ATP into ADP and a phosphate -> gives the system energy
ATP causes the myosin head to change angle (bent to straight) - At this point it will bind to actin (still a weak binding) - remains weakly bound until calcium enters the system
When calcium enters the system you get the generation of a power stroke

Question 29

Power stroke

Answer 29

Means that myosin an bind to actin more strongly
-> Calcium binds to troponin, changes the structural conformation of these molecules, it nudges tropomyosin out of the way as tropomyosin covers the binding site where actin and myosin normaly bind
Power stroke is where the myosin head pushes actin along (influx of calcium which pushes that along)
After this -> myosin releases ADP and inorganic phosphate -> causes it to become less tightly bound and the cycle is ready to start again

Question 30

Excitation - contraction coupling

Answer 30

Links the action potential in the neuron to the contraction
Action potential arrives at the NMJ -> causes depolarisaion of the motor end plate, when this happens a number of changes cause calcium to be released
Calcium release in muscle fibre is the key link!!

Question 31

Events at the neuromuscular junction

Answer 31

1) RESTING STATE
-> All neurotransmitters are packaged within synaptic vesicles in the presynaptic membrane
2) AP ARRIVAL
-> AP arrives at the presynaptic terminal of the NMJ
Depolarisation (becomes more positive) of the presynaptic membrane and acetylcholine is released and travels across the synaptic cleft towards the postsynaptic terminal (the motor end plate)
3)MEP DEPOLARISATION
->Motor end plate becomes more positive as it is depolarised (wave of depolarisation across fibre) -> presynaptic membrane repolarises
Acetylcholine binds to a receptor on the motor end plate -> causes a ligand gated ion channel to open, sodium ions flow into the MEP and it becomes depolarised -> wave of depolarisation passes down muscle fibre cell
4)CONTRACTION
->When that wave of depolarisation passes down that muscle cell you get contraction -> while you are contracting the motor end plate repolarises and another contraction can build up (back to resting state)

Question 32

Calcium intracellular signal for contraction

Answer 32

When the action potential, neurotransmitter is released, depolarisation of MEP, action potential spreading along the muscle fibre
Spreads along the length of the sarcolemma and the bit of the plasma membrane that forms T-tubules -> electrical impulse carried to the cells interior via T-tubules, into the sarcoplasmic reticulum -> electrical impulse triggers the release of calcium ions from the sarcoplasmic reticulum
-> That calcium will go on to bind with troponin

Question 33

After contraction

Answer 33

During the cross bridge, to stop muscle from continuing contraction, calcium is pumped back into the sarcoplasmic reticulum -> less calcium, not binding to troponin -> tropomyosin moves back into its original position and actin and myosin can’t bind to eachother strongly and you get relaxation of the muscle fibre
Skeletal muscle -> not spontaneously contractile
Nerve supply cut -> flaccid paralysis in skeletal muscles

Question 34

Latent period

Answer 34

Involves;
-Time taken for motor end plate to depolarise
-AP (depolarisation) to be transmitted down the T-tubules, calcium channel open in the sarcoplasmic reticulum -> increase of calcium in the sarcoplasm
-Calcium binds to troponin and reveals myosin binding site on actin
Influx of calcium leads to contraction as;
- Myosin is bound strongly to actin and you get a power stroke -> this occurs many times - a cyclical period of events so that the sarcomere can shorten to the maximum degree
Relaxation;
-Calcium is transported back into the sarcoplasmic reticulum -> reformation of troponin and tropomyosin complex -> prevents myosin binding strongly to actin
-> Muscle fibre lengthens (passively) -> relaxes

Question 35

Tetanus

Answer 35

When muscles contract and they do not have enough time to relax before the next action potential arrives -> another AP arrives and causes another sarcomere to contract
->This continues as a frequency of stimulation until all of the sarcomeres of the mujscle fibre have contracted
State of tetanus -> happens when you get stimulation at high frequency

Question 36

Motor unit

Answer 36

Nerve arriving at a muscle fibre;
-> One motor neuron branches and contacts several muscle fibres connecting to a MEP
-> The number of muscle fibres depends on the particular type of muscle
->Fine muscle control -> requires a smaller ratio of muscle fibres to nerve fibres e.g., extraocular muscles (eye) - 1:10
Bigger ratio - stronger less precise response