Week 2: Mechanics of the nervous system

What is the primary focus of the cerebellum?

Movement and coordination

What factors are involved with the spinal cord in the CNS?

• Continuous with brain stem

• Long conical structure, cone-like

• Thickness of adult’s little finger, 1cm in diameter

• Mediates transmission of information
  between brain & body

What are the three major functions of the spinal cord?

1. Coordinating reflexes

2. Serving as a conduit for sensory and motor information: messages from brain to body (efferent), messages from body to brain (afferent)

3. Mediating messages between brain and body

What do the dorsal roots of spinal nerves carry? Where are they located?

Afferent neuron axons for sensory input.

• Located in the back of the spinal cord

• Afferent neuron axons enter cord in dorsal root and terminate in dorsal horn - sensory input

What do the ventral roots of spinal nerves carry? Where are they located?

Efferent neurons that send messages from the CNS to the body

• Located in the front of the spinal cord

• Efferent neurons have a cell body in ventral horn and axons leave cord in ventral root - comes out into receptive part of body

What are the functions of the PNS?

• Connects CNS to limbs & organs via cranial and spinal nerves

• Carries information from environment to CNS
  (afferent neurons), processed in CNS

• Carries messages from CNS to muscles and glands (efferent neurons)

How many pairs of nerves are there in the PNS?

• 43 pairs of nerves, nerves come in pairs

  • 12 cranial nerve pairs

  • 31 spinal nerve pairs

How many pairs of cranial nerves are there?

12 pairs

• 10 located in brainstem

• 1 and 2 located in forebrain

How many pairs of spinal nerves are there?

31 pairs

Name all of the 12 cranial nerves

Old
Owls
Often
Take
Tiny
Acorns
For
Very
Grumpy
Village
Angry
Hedgehogs

What is the function of the somatic nervous system?

• Voluntary control of body movement

• Receives sensory information and controls spinal nerves that innervate skin, joints & muscles

  • Afferent neurons carry sensory info from skin (sensory neuron)

  • Efferent neurons control skeletal muscles (motor neuron) - support contractions, muscle fibres pulling across each other

• -> Neurons are excitatory

What does the autonomic nervous system control?

• Controls involuntary functions and internal environment

• Afferent neurons carry sensory info from internal organs to CNS

• Efferent neurons control smooth muscle, cardiac muscle & glands (production of hormones)

• Neurons are excitatory or inhibitory (e.g. slow down heart rate - parasympathetic)

What are the three sub-divisions of the autonomic nervous system?

Sympathetic Nervous System

Parasympathetic Nervous System

Enteric Nervous System

What is the role of the sympathetic nervous system?

Coordinates the body's fight or flight response

Responses for activities which expand energy

What is the role of the parasympathetic nervous system?

Coordinates rest and relaxation responses

Activities involved with increase in the body’s supply of stored energy

What are the main roles of the enteric nervous system?

• The “second brain”

• Links digestive system to the brain

• Lines your gastrointestinal tract from oesophagus to rectum

• Main role is controlling digestion

  • swallowing

  • release of enzymes

  • control of blood to facilitate nutrient absorption

What is the gut-brain axis?

• The complex interplay between gut microbiota, the immune system, and the central nervous system

• Gut Microbiota (GM) regulates brain function by preserving the CNS immune homeostasis

• Prevents neuroinflammation and degredation

What are sensory neurons responsible for?

• Part of PNS

• Contain sensory receptors for detecting sensory changes

• Sends information about these changes to CNS

• Cell body in PNS, axon enters CNS (axon terminals located in CNS)

• Taste, touch, movement, pressure, temperature

What do motor neurons do?

• Part of PNS

• Synapses to skeletal muscle to command movement or onto glands (inhibition/activation) to release hormones

• Relays signal from CNS to PNS

• Dendrites & cell body in CNS, axon enters PNS

What is the role of interneurons?

• In CNS

• Receives info from sensory neurons

• Sends info to motor neurons

• Integrate/change signal

• -> Integrate - inputs from multiple afferent neurons - average signal

• -> Changer - interneurons can provide excitatory or inhibitory signals

What is the primary function of the central nervous system (CNS)?

To receive information from sensory neurons, send information to motor neurons, and integrate/change signals.

What are the two main types of signals that interneurons can provide?

Excitatory and inhibitory signals.

What is the structure of the neuronal membrane?

• Made of two layers of lipid molecules

• Lipid molecules - attracted to the intracellular and extracellular fluid

  • Hydrophilic (water attracting) heads

  • Hydrophobic (water repelling) tails

• Barrier: water soluble molecules cannot pass through

• Particularly impermeable to ions, stop ions passing through

What drives the movement of ions across the neuronal membrane?

Concentration gradients (via diffusion) and electrical forces (via electrostatic pressure).

Explain the fluid environment containing ions

• Made up of intracellular fluid and extracellular fluid

Cations (+ charged)

• Sodium (Na+)

• Potassium (K+)

• → predominantly intracellular

Anions (- charged)

• Chloride (Cl-) → predominantly extracellular

• Organic ions (A-) → only intracellular

What happens to ions under electrostatic pressure?

Charges of opposite sign attract, while charges of the same sign repel.

Outline the process of the electrical polarity of neurons

• Neuron is polarised

• At rest, neurons are negatively charged compared to extracellular fluid

• Negative charge occurs if there are less positive ions and/or more negative ions inside cell

• Whilst there is a difference in charge, an electrical force tends to move ions across the membrane

• At rest, the resting potential of the inside of the neuron is -70 mV

• 0 mV in the extracellular of the neuron

What is the resting potential of a neuron?

-70 mV, indicating that the inside of the neuron is negatively charged compared to the extracellular fluid.

Outline the role and function of border guards

• Controlled by a gate

• Ion channels (leak channels) - always open

• Passive ion specific conduits

• Selected ions rush down gradients of concentration and electric potential

• More K+ gates open compared to Na+

What role do ion channels play in neuronal function?

They allow specific ions to pass through the membrane, contributing to the resting potential and action potentials.

What is the primary ion responsible for maintaining resting potential?

Potassium ions (K+), which are highly concentrated inside the cell.

Outline the ion pump process

• Cytoplasm has less Na+ than extracellular fluid

• 3 Na+ out and 2 K+ which maintains this resting potential of -70Mz

• Energy consuming

• Active transport: against gradient

• Maintains and builds gradients

• Slower

Outline the process of diffusion of potassium ions

• K+ highly concentrated in cell

  • K+ wants to move out of cell down concentration gradient

• At rest, K+ leak channels allows K+ to leave neuron down concentration gradient

• Inside cell becomes more negative

• Ions will stop moving when opposing forces are at equilibrium: -70Mz

• → This happens in a resting cell

Outline the process of diffusion of chloride ions. Give reference to electrostatic pressure

• Generally equally distributed

• Cl- highly concentrated outside cell

• Cl- wants to move into cell down concentration gradient

• Inside of cell is + charged

• Cl- also wants to move out of cell due to repel of electric charge

Outline the process of diffusion of sodium ions. Give reference to electrostatic pressure

• Na+ is highly concentrated outside cell

• Na+ wants to move into cell down concentration gradient

• Inside of cell is - charged

• Na+ also wants to move into cell due to electric charge attraction

• → Net force for Na+ = move into cell

Outline what is meant by the resting membrane potential

• Two forces act on ions

• Membrane is a barrier to ion movement

• At rest membrane is permeable to K+ so mainly K+ ions move

• K+ ion movement stops once opposing forces reach equilibrium

• → unequal distribution of positive and negative ions on the inside and outside of membrane

Resting membrane potential = difference in charge across membrane at rest = -70mV

What occurs during depolarisation of a neuron?

The inside of the neuron becomes more positively charged as sodium ions (Na+) enter the cell.

What is the threshold potential for an action potential to occur?

Approximately -55 mV.

What is meant by action potential?

A brief electrical impulse that provides the basis for conduction of information along an axon

What are the phases of an action potential?

1. Depolarisation: inside becomes more +

2. Repolarisation: inside becomes more -

3. Hyperpolarisation: more - than at rest

What is meant by the 'all-or-nothing' phenomenon in action potentials?

An action potential occurs only if the threshold is reached; otherwise, it does not occur.

If depolarisation reaches threshold (-55mV), an AP occurs automatically

→ -55mV

What regulates the strength of a neural response?

The rate of neural firing, not the size of a single action potential.

What occurs during depolarisation of a cell?

• Stimulus causes a small amount of Na+ to move into the cell

• Na+ is + charged → neuron becomes less - (slightly depolarised)

• If depolarisation changes charge by +15mV, it activates voltage-gated channels in membrane

Outline the process of voltage-gated channels

• Activated by changes in charge of membrane

Outline the process of a voltage-gated action potential

1. Voltage-gated Na⁺ channels open. Na⁺ influx → more +ve

2. Na⁺ channels become refractory at peak (neuron is resistant at firing another AP)

3. Voltage-gated K⁺ channels open. K⁺
   efflux → less +ve

4. Open K⁺ channels allow outflow

5. Overshoot caused by slow closing K⁺ channels

What is the role of voltage-gated sodium channels during an action potential?

They open in response to depolarisation, allowing Na+ influx, which further depolarises the neuron.

What happens during repolarisation of a neuron?

Voltage-gated K+ channels open, allowing K+ to exit the cell, making the inside more negative.

What is the function of the Na+/K+ ATPase pump?

It moves 3 Na+ ions out of the cell and 2 K+ ions into the cell, maintaining the concentration gradients.

• The pump keeps Na+ concentration low in neuron

• K+ diffuses back into neuron

• → Re-establishes resting membrane potential

• Signal travels away from cell body towards axon terminals

• No decay

• AP propagation

What is the effect of K+ leak channels on resting potential?

They allow K+ to leave the neuron, contributing to the negative resting potential.

What is the net force acting on sodium ions (Na+) at rest? Why do these ions move into the cell?

Large net force

Na+ wants to move into the cell due to both concentration gradient and electrostatic pressure.

What is the role of chloride ions (Cl-) in neuronal resting potential?

Cl- is generally equally distributed but is influenced by both concentration gradient and electrostatic pressure.

What is the role of K+ ions in a neuron?

K+ ions diffuse back into the neuron, re-establishing the resting membrane potential.

What is meant by AP propagation?

AP propagation refers to the signal traveling away from the cell body towards the axon terminals without decay.

• Na+ ions spread away from site of AP which changes the charge in nearby area of cell be be more + charged (depolarised)

• This triggers another action potential

• Next AP occurs as previous AP starts to die out

• APs are triggered one after another all the way to axon terminals

• → ‘mexican wave effect’

How do Na+ ions contribute to action potential propagation?

Na+ ions spread away from the site of the action potential, depolarising nearby areas of the cell and triggering subsequent action potentials.

What prevents action potentials from traveling backwards?

The refractory period prevents action potentials from traveling backwards and determines the upper limit on action potential frequency.

What is the function of neurotransmitters?

Neurotransmitters are released from vesicles in the terminal ends of axons to excite, inhibit, or modulate postsynaptic cells.

• 2 (or more) neurotransmitters are released from each neuron

Name 7 neurotransmitters.

• Acetylcholine

• Serotonin

• Dopamine

• Nor/epinenphrine

• Endorphins

• GABA

• Glutamate

What role does acetylcholine play in the nervous system?

Acetylcholine is an excitatory neurotransmitter that regulates heart rate, blood pressure, gut motility, muscle contractions, memory, and learning.

• Imbalances linked with Alzheimer’s disease, seizures and muscle spasms

Name 4 monoamines

Serotonin

Dopamine

Epinephrine (adrenaline)

Norepinephrine

Name some factors involved within serotonin

Serotonin is an inhibitory neurotransmitter that regulates mood, sleep patterns, libido, anxiety, appetite, and pain.

• Imbalances include SAD, anxiety, fibromyalgia and chronic pain

• Medications which regulate serotonin include selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs)

What is dopamine's role in the brain?

Dopamine is involved in the reward system, facilitating pleasure, heightened arousal, focus, concentration, and learning.

• Dysfunctions of the dopamine system include Parkinson’s disease, Sz, bipolar disease, restless legs syndrome and ADHD

What is the function of epinephrine and norepinephrine?

They are responsible for the 'fight-or-flight response' to fear and stress, increasing heart rate, breathing, blood pressure, and focus.

• Excess epinephrine can lead to high blood pressure, diabetes, heart disease and other health problems.

• As a drug, epinephrine is used to treat anaphylaxis, asthma attacks, cardiac arrest and severe infections

What are endorphins and their role in the body?

Endorphins are pain relievers that contribute to the perception of pain and create 'feel good' feelings.

• Low levels may play a role un fibromyalgia and some types of headaches

Name 2 amino acids

Glutamate

Gamma-aminobutryic acid (GABA)

What is glutamate's role in the brain?

Glutamate is the most common excitatory neurotransmitter and plays a key role in cognitive functions like thinking, learning, and memory.

• Imbalances associated with Alzheimer’s disease, dementia, Parkinson’s disease and seizures

What is GABA and its function?

GABA is the most common inhibitory neurotransmitter in the brain, regulating brain activity to prevent anxiety, irritability, and seizures.

What are the two types of glial cells that perform myelination?

Schwann Cells (peripheral nervous system) and Oligodendrocytes (central nervous system)

Outline structural features of glial cells (support cells)

• Contain fatty tissue (myelin) which wraps around neuron axons

• Forms insulating coating (myelin sheath)

• Schwann cells - wrap individual axons

• Oligodendrocyte - wrap several axons

• Axons then become myelinated

• → Cells jump down the axon - nodes of Ranvier

How do Schwann cells and oligodendrocytes differ in their function?

Schwann cells wrap individual axons, while oligodendrocytes wrap several axons.

Outline the anatomy of a myelinated axon

• Yellow areas: Schwann cells, production of myelin sheaths

• Purple areas: ‘naked’ axon - nodes of Ranvier

• Ions can only cross the membrane at nodes of Ranvier

• No ion leakage outside of nodes of Ranvier, so influence of AP spreads quicker in myelinated axons

Outline differences between unmyelinated and myelinated axons

What are the advantages of myelinated axons compared to unmyelinated axons?

1. Myelinated axons conduct action potentials faster due to saltatory conduction.

2. Much more economical: less Na+ enters during APs, more efficient AP regeneration

What is saltatory conduction?

The process where action potentials jump from node to node in myelinated axons between gaps (nodes of Ranvier)

How much faster are myelinated axons compared to unmyelinated axons?

Myelinated axons can be up to 10 times faster.

What is one economic advantage of myelination?

Less Na+ enters during action potentials, reducing the need for ATP to remove it.

What are some factors that can lead to demyelination?

• Hereditary: Tay-sachs disease, Niemann-pick disease, Gaucher disease, and Hurler syndrome

• Stroke

• Infections: viruses, bacteria

• Immune disorders

• Metabolic disorders

• Nutritional deficiencies (e.g. lack of vitamin B12)

• Poisons, e.g. carbon monoxide

• Drugs or medications, e.g. antibiotic ethambutol

• Excessive use of alcohol