Lesson 4/5 (ch4): Neurophysiology

Nervous system

• brain, spinal cord, and nerves

central nervous system

• inside the blood brain barrier (spinal cord, brain)

spine

• carry messages to and from the brain

• spinal reflex → sometimes a message is so important it needs to be responded to before it reaches the brain

Peripheral nervous system

• carrying signals to and from different parts of the body

• 2 subsystems → autonomic and somatic

autonomic

• do not consciously control

• 2 subsyetems → sympathetic and parasympathetic

sympathetic

• controlling our fight or flight response; prepare to react to challenging situations

parasympathetic

• reverses the changes that the sympathetic system made → achieve homeostasis

somatic

• everything you do control

2 primary functions of neurons

• recieve signals

• relaease chemicals to other neurons

synapse

• space between terminal button

• most frequently located between terminal buttons and dendrites

  • can also be axodendritic (to dendrite) or axosomatic (to cell body)

cell body

• part of neuron

• does all the necessary functions to keep the neuron alive

  • nucleus, genetic info

dendrites

• part of neuron

• accept chemical signals from other cells and relay info to the cell body

axons

• part of neuron

• pass signal sent from the cell body all the way to the end of the axon terminal, which releases neurotransmitters

myelin sheath

• part of neuron

• make signal transmission faster

  • covers the axons

  • glial cells produce it (schwann cell)

white matter

• higher proportion of myelinated neurons

• help transmit signals quickly across the entire lengths and width of brain

grey matter

• lower portion of myelinated neurons

3 types of neurons

• sensory

• motor

• interneurons

conduction

• electrical signal passed from a neurons dendrites, through its cell body, to itz axon

  • works from action potential of neurons lined up in a series

transmission

• one neuron passes an electrical signal to another neuron

Action potential - what is it

• goal → pick up a chemical signal and turn it into an action potential that travels to the end of axon

• start at a cell that is not active (resting potential)

  • resting potential at -70mV → need to add positive electrical charge to get action potential started

  • when activated → spikes to 70, then drops to lower than -70, until it levels out again

Action potential - how it happens

• resting potential → The neuron is at rest, with a negative charge inside (-70mV).

  • Sodium-potassium pumps maintain this by pumping 3 positively charged sodium ions out and 2 positively charged potassium ions in.

  • The inside of the neuron is more negative compared to the outside.

• Depolarization → A stimulus causes sodium channels to open.

  • If the threshold (-55mV) is reached, sodium channels open, allowing ions to rush into the neuron.

  • The inside becomes more positive

• Repolarization → sodium channels close, and potassium channels open.

  • potassium rushes out, restoring the negative charge inside.

• Hyperpolarization → potassium channels close slowly, causing the neuron to become more negative than resting potential.

  • This prevents another action potential from happening immediately.

• Return to Resting Potential → The sodium-potassium pump restores the original balance.

  • The neuron is ready for another action potential.

Presynaptic axon

• before the synapse

postsynaptic dendrite

• dendrite on the other side of syanpse

synaptic gap/cleft

• space in the middle of the presynaptic axon and the postsynaptic dendrite

neurotransmission

• neurotransmitters are ejected into the other neuron

electrical synapse

• direct connection → ex. heart

chemical synapse

• synaptic cleft, what we will focus on

sodium potassium pump

• gate along membrane → lets in 2 positively charged potassium ions, and lets our 3 positively charged sodium

  • to remain action potential

  • lets out more positively charged eventually (more negative resting potential)

ion channels

• only works for specific kinds of ions (potassium, sodium); main force of what makes action potential happen

depolarization

• threshold for action potential is released; sodium channels open, ions rush into cell and make cell more positive

repolarization

• potassium channels open and let out potassium ions; makes the charge of the cell negative again

hyperpolarization

• overshoot during repolarization

• sodium potassium pump levels things out again

excitatory neurotransmitters

• encourage neurons to fire

  • more likely for postsynaptic cell to have an action potential

  • introduce positive voltage; depolarize

inhibitory neurotransmitters

• discourage neurotransmitters from firing

  • less likely for postsynaptic cell to have an action potential

  • discourage neurons from firing; hyperpolarize them

ionotropic receptor

• receptors in some synapses contain binding sites that are connected directly to an ion channel

  • either increase or decrease resting potential

  • depolarize or hyperpolarize

metabotropic receptor

• indirect; start a cascade of events that occurs more slowly, contains a number of things, and are longer lasting in the cell

  • not directly connected to ion channel → situated on outside portion of long sifnal protein which allows neurotransmitters to stick on outside and g cells inside

  • activated on outside of cell by neurotransmitter, and releases something in the cell it was already holding on to

g protein

• composed of subunits that when the receptor is not activated, are bound together (stuck to receptors); when the neurotransmitter binds to receptor, it breaks off and reacts with other things inside the cell

secondary messengers

• can interact with gated ion channels

• alter other ion channels that changes resting potential

• change how other proteins are made inside of the cell

agonists

• increase action of neurotransmitter

  • bind to receptor and facilitate its excitatory/inhibitory function

antagonists

• inhibit actions of the neurotransmitters

  • bind to a receptor and block its excitatory/inhibitory function

full agonist

• facilitate the actions of the receptor

partial agonist

• kind of facilitates, but is not getting it all the way there

inverse agonist

• blocks up the receptor so completely that the receptor shuts down

To be considered a neurotransmitter, you have to be:

• synthesized within the neuron by coexisting enzymes

• released in response to cell depolarization

• bind to receptors to alter the postsynaptic cell

• are removed or deactivated by some mechanism within the synaptic cleft

small molecule neurotransmitters

• synthesized from precursor molecules with enzymes

• transported to axon terminals in vesicles

• released into synaptic cleft to stimulate ionotropic or metabotropic receptors

• reuptake process absorb some amount back into neuron

• examples —> acetylcholine (tobacco/nicotine), monoamines (dopamine, noepinephrine, etc), amino acids (glutamate, GABA, etc), unconventional neurotransmitters (endocannabinoids, etc)

Neuroactive peptides

• hormones → may act as a neurotransmitter

• may act over larger area than the small molecule neurotransmitters

• made up of short chains of amino acids

  • packaged into larger amino acid chains in cell body and transported to the axon terminal

  • during transport, enzymes cleave chains to expose neuropeptides

  • after release, deactivated by enzymes in the synaptic cleft

• do not undergo reuptake

• examples —> opioid peptides, hypothalamic peptides, pituitary peptides, brain-gut peptides

Glutamate

• the main excitatory neurotransmitter

• involved in basic survival function (learning, sensitivity to pain, etc)

• receptors found throughout almost the entire brain → widespread connections across cerebral cortex and the cortex and midbrain

• 3 kinds → Ampa, Nmda, Kainat

  • open to allow an influx of sodium ions into the cell, and positively charged potassium ions to go out

GABA

• most widespread inhibitory neurotransmitter

• 20-30% of the neurons in the CNS

• 2 different types → A and B

• receptors contain numerous binding cells, both internal and external

  • binding sire is found between 2 of its 5 subunits (alpha and beta)

GABA A

• ionotropic

• channel that can open

• control ion channels to allow negatively charged ions into the cell

  • reduces resting potential, harder for action potential to happen

GABA B

• metabotropic

• g protein controlled

• synthesize the second messenger cAMP

orthosteric site

• the natural molecule neurotransmitter GABA binds to its designated receptor

• when activated, the ion channel that forms the core of GABA A ones and allows in negatively charged ions

allosteric site

• something other than GABA binds to the GABA receptor

Acytylcholine

• first neurotransmitter discovered

• synthesized when combining acetate and choline

• molecules released into synaptic cleft, broken down by acetylcholinesterase

• where its found

  • ventral tegmental area → reward and addiction

  • mesopontine tegmentum → sleep functions

  • basal forebrain → learning and memory functions

3 types of monoamines

• dopamine

• serotonin

• norepinephrine

nigrostriatal pathway

• dopamine → smooth movements

  • substantia nigra, project towards the striata of the brain

  • important for coordinating smooth movement

  • becomes severely damaged in parkinsons

mesolimbic pathway

• dopamine → addiction

  • connect the ventral tegmental area to the nucleus accumbens

  • every drug of abuse increases the amount of dopamine being sent down this pathway

mesocortical pathway

• dopamine → complicated

  • ventral tegmental to the cortex/frontal lobe

  • in combo with mesolimbic, importance in understanding schizophrenia and other psychotic disorders

serotonin + Raphe nuclei

• serotonin → involve its effect with mood

• group of related regions in the brain stem and serotonergic projections from this go to important parts of the brain

  • thalamus, hippocampus, amygdala, basal ganglia

• regulates mood, sleep wake cycle, aggression, appetite

• all but one receptor is metabotropic

norepinephrine

• distributed from another midbrain region that projects axons all over the cortex

• involved in attention, sleep and wakefulness, feeding behaviors, and emotions

• dysfunction leads to depression, attention defecit disorders

• all receptors are metabotropic

oxytocin

• involved in

  • childbirth (contractions and milk release)

  • bonding and monogamy in coles

Endocannabinoids

• lipids that act as neuromodulators → metabotropic receptors

• made by our bodies and bind to the same receptors as THC

• 2 main receptors

  • CB2 → peripheral nervous system; used to contribute to immune system and anti inflammatory

  • CB1 → in brain; psychoactive functions; indirect effect on brain

endogenous opioids

• mimic endogenous neurotransmitters, attaching themselves to opioid receptors, and through this they are impacting other neurotransmitter systems that can increase abuse liability

• (g) Endorphins - μ  and δ receptor affinity 

• Enkephalins - δ receptor affinity 

• Dynorphins - к receptor affinity

CNS - the spine

• central part → grey matter

• hard bony part and squishy neuron part

• 24 vertebrates

• carry sensory information into the brain and motor info out

Parts of the brain

• CNS - the spine

• the hindbrain

• the midbrain

• the forebrain

• cortex

the hindbrain

• first encounter; behind the other parts of the brain

• large part of the brain stem; spinal cord connects

• contains important functions you can’t live without

• 4 main regions

  • medulla → heart rate, blood circulation, breathing

  • reticular formation → sleep, wakefulness, alertness

  • cerebellum → fine motor skills

  • pons → bridge between cerebellum and other parts of the brain

the midbrain

• contains 2 things we will talk about

  • tectum → enables orienting

  • tegmentum → movement and physiological arousal

the forebrain

• contains everything else in the brain

• 2 main parts

  • cerebral cortex → left and right hemispheres

  • inside of that → subcortical structures

limbic system

• subcortical structure

• hippocampus → memory

• anygdala → emotion

basal ganglia

• subcortical structure

• learning, muscle memory, developing habits

thalamus

• subcortical structure

• every sense other than smell involves it

  • acts as a relay center → take input data and decide what gets passed to the cerebral cortex for processing

  • conscious experience

hypothalamus

• subcortical structure

• work with the rest of the body to release hormones that regulate temp, hunger and thirst, stress, sexual functioning

• hormone systems

endocrine system

• subcortical structure

• distributed all over body

• glands that release hormones into blood stream to remain in homeostasis

  • pancreas, adrenal glands, thyroid, ovaries/testes

pineal gland

• releases melatonin

pituitary gland

• essential for releasing hormones for sexual functioning

4 parts of the cortex

• the occipital lobe

• temporal lobe

• parietal lobe

• frontal lobe

the occipital lobe

• process raw visual information

• damage typically

temporal lobe

• process and understanding auditory information

• helps with some visual processing

parietal lobe

• processing aspects of visual information (where things are located), attention

• damage → neglect of one region of space

frontal lobe

• primary motor cortex

• main job → handles executive functions