Topic 3.1: Neuromuscular system - neuromuscular junctions and chemical synapses

propagation of action potential - steps (4)

1. Influx of sodium ions at Point A due to open voltage-gated sodium channels -> depolarisation of membrane at point A

2. High concentration of sodium ions in the cytosol of the axon -> movement of sodium ions left and right of Point A

3. Downstream of Point A, increased positive charge intracellularly results in depolarisation of membrane at Point B etc

4. Relative refractory period at point A -> voltage-gated sodium channels are closed but the potassium channels are open and membrane is hyperpolarised

orthodromic transmission of action potential - meaning

Although sodium ions move upstream during an action potential, unlikely for another action potential to occur as voltage gated potassium ion channels are open and membrane is hyperpolarized

allows orthodromic transmission of action potential along axon only

myelination - meaning

wrapping of oligodendrocyte around axon

myelination - purpose

acts as insulator around axons

prevents leak of ions across axonal membrane and allows for jumping transmission of action potential

node fo Ranvier - meaning

small region of unmyelinated axon between myelinated regions

myelinated and unmyelinated axons - action potential transmission type

myelination: saltatory action potential transmission

unmyelinated: continuous transmission

saltatory action potential propagation

jumping transmission of action potential between myelinated segments of axon

voltage gated ion channels = restricted to nodes of Ranvier → current dlows form one node to the next

continuous vs saltatory action potential transmission - speed

continuous is slower because more sodium channels have to open → longer for action potential to travel down axon

myelinating cells types - list (2)

Central nervous system = oligodendrocytes

Peripheral nervous system = Shwann cells

myelinating cell types - cell:axons wrapped

oligodendrocytes: one cell = many axons wrapped

Swann cells: one cell = one axon wrapped

multiple sclerosis - summary

autoimmune condition resulting from scaring of the myelin sheath surrounding axons -> prevents action potential transmission along axon

Diagnosis is difficult -> symptoms determined by nerve damage

multiple sclerosis - symptoms list (3)

Different symptoms can appear at different times

Vision problems

Tingling and numbness in specific regions of the body

Bladder and bowel dysfunction

neuromuscular junctions - structure list (3)

1. nerve terminal

2. synaptic cleft

3. motor end plate

neuromuscular junctions - nerve terminal summary (3)

Presynaptic nerve terminal filled with vesicles containing neurotransmitters

Opening of voltage-gated calcium channels will result in high concentration of calcium ions -> triggers endocytosis of neurotransmitter

Axon is myelinated -> nerve terminal is unmyelinated

neuromuscular junctions - motor end plate summary (2)

Postsynaptic portion of sarcolemma

Junctional folds increase surface area and thus nicotinic acetylcholine-receptor density

nicotinic acetylcholine receptors - def

ligand-gated ion channels

cholinergic receptors → binding of two acetylcholine moelcules opens channel

channel increases permeability to sodium ions → depolarisation of sarcolemma

sarcolemma - location

extracellular membrane of skeletal muscle cell

neuronal synapses vs neuromuscular junctions (2)

neuronal synapses dont have junctional folds on post-synaptic membrane

NMJ is larger than neuronal synapses

motor unit - def

motor neuron and all the muscle fibres it innovates

end plate potentials - membrane potential

always super threshold under normal conditions → one action potneital leads to a msucle twitch

action potential across NMJ to RyR activation - steps (8)

1. Action potential travels down axon towards nerve terminal

2. Depolarisation of membrane opens voltage-gated calcium channels at nerve terminal -> influx of calcium ions

3. Increase [calcium ions] triggers exocytosis of neurotransmitters stored in vesicles

4. Neurotransmitters diffuse across the synaptic cleft and bind to nicotinic acetylcholine receptors on the post-synaptic cell

5. Binding of acetylcholine opens receptors, allowing sodium ions into the muscle cell -> depolarisation of sarcolemma

6. Depolarisation and influx of sodium ions travels along the membrane, opening DHP receptors on the muscle cell

7. Ryanodine receptors (RyR) on the sarcoplasmic reticulum open, allowing calcium ions to leave the reticulum and into the sarcoplasma

8. Calcium ions initiate excitation contraction coupling

dihydropyridine receptor and ryanodine receptor

DHP receptor = modified voltage-gated receptor

DHP receptors are physically connected to RyR (ryanodine receptors) → activation of DHP receptor involves conformational change that opens RyR

division of nervous system - sections listed (3)

1. somatic

2. autonomic

3. sensory

division of nervous system - somatic summary (3)

efferent pathways

skeletal muscle

voluntary movement

division of nervous system - autonomic summary (3)

efferent pathways

target visceral organs → smooth muscle, cardiac muscle, glandular tissue

divided into sympathetic and parasympathetic

division of nervous system - sensory summary (2)

afferent pathways

divided into somatic (conscious awareness) and visceral (homeostasis)

axon hillock - trigger potential

acts as classic trigger zone of neuron for primary sensory neurons

integrates incoming graded electrical signals and initiates action potential if combined depolarisation reaches threshold voltage

location of primary sensory afferent neuron cell bodies - general

ganglia of PNS

location of primary sensory afferent neuron cell bodies - spinal vs cranial nerves

spinal = dorsal root ganglia

cranial = sensory ganglia

somatic efferent pathway - summary

motor neuron leaves CNS and synapses directly with skeletal muscle

autonomic efferent pathway - summary (2)

pre-ganglionic motor neuron leaves CNS and synapses on post-ganglionic neuron at peripheral ganglia

peripheral ganglia sends axon projections to smooth muscle, cardiac muscle, or glands

general somatic receptors for sensations - list (4)

1. Nociceptors -> pain

2. Thermoreceptors -> temperature (related to pain receptors)

3. Proprioceptive → position and movement

4. Tactile → touch and pressure

Change stimuli into biological signal -> action potential

general visceral receptors for sensations - list (4)

1. Chemoreceptor -> water-soluble and lipid-soluble substances dissolved in body fluids

2. Mechanoreceptors -> sensitive to stimuli that distort their plasma membranes

3. Pain → referred pain

4. Temperature → core temp

somatic and visceral sensation - receptor types list (3)

1. free nerve receptor

2. encapsulated sensory receptor

3. specialised receptor

somatic and visceral sensation - free nerve receptor (3)

• Receptor where its dendrites acts as receptor for stimuli

• Typically associated with pain perception and temperature perception

• Primary sensory neuron is unmyelinated axon

somatic and visceral sensation - encapsulated sensory receptor (3)

• Typically associated with touch receptors

• Primary sensory neuron is myelinated axon

• Primary sensory neuron acts as sensory receptor

somatic and visceral sensation - specialised receptor (2)

• Specialised cell acts as sensory receptor -> releases neurotransmitter onto dendritic terminals of primary sensory neuron -> primary

• Primary sensory neuron is myelinated axon

skeletal muscle reflex - summary steps (4)

1. Stimulus detected by receptor (sensor)

2. Sensory neuron carries signal to integration centre

Eg. spinal cord

1. Efferent response which signals to effector

Eg. skeletal muscle

1. Response from effector

Eg. skeletal muscle contraction

monosynaptic vs polysynaptic reflex - def

Monosynaptic reflex = single synapse between the afferent and efferent neurons

Polysynaptic reflex = two or more synapses between afferent and efferent neurons

• interneurons can be inhibitory or excitatory

monosynaptic reflex - steps (20

1. Primary sensory neuron synapses with neuron whose cell body is in the spinal cord

2. Efferent neuron acts on target cell effector for response

monosynaptic reflex - steps (2)

1. Primary sensory neuron synapses onto neuron whose cell body is in the dorsal horn (interneuron) of the spinal cord

2. Neuron whose cell body is in the dorsal horn will synapse with an efferent neuron leading to a response

joint receptors - def

group of receptors which monitor angle of joints and send signals to CNS

Range of tactile receptors

joint receptors - firing rate

Receptor firing rate can change depending on angle of joint -> relationship could be exponential or linear

joint receptors - free nerve receptors

Free nerve ending along ligament -> pain, restrict range of motion to prevent injury

muscle proprioceptors - types (2)

muscle spindles

golgi tendon organs

muscle proprioceptors - muscle spindles summary (3)

wrapped around intrafusal muscles inside of muscle bodies -> monitor muscle length for muscle stretch reflexes

Allows us to reflexively adjust muscle tension to carry extra weight -> addition of load stretches muscle and the spindles, creating a reflex contraction

activates sensory afferent pathways and send info to CNS for relex muscle constriction

muscle proprioceptors - muscle spindles when stretched, unstretched, and flaccid

Unstretched: muscle spindle has constant action potential discharge rate -> tells CNS that muscle is at rest

Stretched: muscle spindle action potential discharge rate increases -> tells CNS that muscle is stretched  -> leads to reflex response (constriction)

Flaccid: muscle spindle has no action potential discharge rate -> tells CNS that muscle is flaccid -> reflex response (constriction)

muscle proprioceptors - muscle spindles response to stretching steps (5)

1. Stretching of muscle sensory receptors changes discharge rate of first order sensory neurons

2. Increased discharge rate is detected by the spinal cord and integrated in the spinal cord

3. Efferent output to increase efferent output through alpha and gamma motor neurons

4. Muscle contraction

5. Muscle spindle no longer stretched and discharge rate return to basal levels

muscle proprioceptors - Golgi tendon organs summary (2)

found on muscle tendons -> stimulated by tension from contraction or force

Prevents injury to muscle from carrying excessive weight by causing muscle to relax and drop the load

muscle proprioceptors - Golgi tendon organs response to stretching steps (5)

1. Muscle contraction stretches Golgi tendon organ -> neuron from golgi tendon organ fires

2. Afferent neuron synapses with inhibitory interneuron

3. Inhibitory interneuron synapses with motor neuron inhibited

4. Muscle relaxes

5. Load is dropped

extrafusal muscle fibres - summary (2)

Innervated by alpha motor neurons

Responsible for generating contractile force

intrafusal muscle fibres - summary (2)

Innervated by gamma motor neurons

Provide proprioceptive input to the CNS

patellar tendon knee jerk reflex - summary (2)

Stretch on tendon connecting the quadriceps to the lower limb leads to reflex extension of the knee

Used to detect health of the CNS -> injury to CNS will result in abnormal knee jerk reflex

patellar tendon knee jerk reflex - steps (4)

1. Tapping tendon stretches muscle -> muscle stretching activates the muscle spindles

2. Muscle spindles send action potentials back to the CNS

3. At integrating centre:

• Primary sensory neuron synapses with alpha motor neuron leading to contraction of extensor agonist muscle (quadriceps)

• Primary sensory neuron synapses with an inhibitory interneuron in the dorsal horn -> interneuron synapses with alpha motor neuron and inhibits it -> flexor antagonist muscle (hamstrings) relaxes

4. Extension of knee

crossed extensor reflex - summary

involves reflex coordination of both sides of body → flexion of ipsilateral limb and extension of contralateral limb

withdraw form pain without falling over

crossed extensor reflex - steps (4)

1. Painful stimulus activates nociceptor

2. Primary sensory neuron enters spinal cord and diverges

3. At integration centre:

• Action of ascending pathways for sensation and postural adjustment

• Withdrawal reflex pulls foot away from painful stimulus

• Cross extensor reflex supports body

4. Withdraw from pain without falling over

crossed extensor reflex - withdrawal reflex components that pulls foot away from painful stimulus (2)

Contraction of posterior muscles (flexors)-> activate alpha motor neuron via excitatory interneuron

Relaxation of extensors -> inhibit alpha motor neuron via inhibitory interneuron

crossed extensor reflex - crossed extensor reflex components that supports body (2)

Contraction of extensors -> activate alpha motor neuron via excitatory interneuron

Relaxation of posterior muscles -> inhibit alpha motor neuron via inhibitory interneuron