Week 1: Thorough Explanation

Nervous system fundamentals, synapses, communication, action potentials, basic brain anatomy

What are the main differences between the CNS and the PNS?

CNS: UPS store (receives all the mail and packages that drivers bring)

• brain and spinal cord

• function: interpret sensory signals and determine motor response

• gray matter (collection of cell bodies): nuclei

• white matter: tracts

PNS: UPS drivers, roads that lead to UPS store

• cranial nerves and spinal nerves

• function

  • links the body to the CNS (drivers link UPS mail to UPS store)

  • carries signals from tissues to the brain (sensory) and brain to the tissues (motor) (roads that UPS drivers use)

• gray matter: ganglia (only located close to the spinal cord)

  • ex: dorsal root ganglion

• white matter: tracts

Explain afferent vs. efferent neurons

• afferent

  • sensory signals

  • go from PNS —> CNS

  • somatic

    • touch, pressure, temperature, pain

    • hearing, balance, vision, proprioception

  • visceral

    • stretch, pain, nausea, hunger, thirst

    • smell and taste

• efferent

  • motor signals

  • go from CNS —> PNS

  • somatic: acting on skeletal muscle

  • visceral: acting on smooth or cardiac muscle

    • sympathetic and parasympathetic response

What are the two different nerve cell types?

neuron: actor

glia: surrounding crew

• way more crew for a movie set compared to number of actors

• way more glia cells supporting the neurons than the number of neurons

What are the parts of a neuron?

• dendrites: receives signals

• cell body: nucleus, organelles

• axon: send signals

• nodes of ranvier: gaps between myelin

Explain the different variations neurons and examples of each

• multipolar

  • 1 axon + many dendrites

  • ex: motor neurons, interneurons

• unipolar

  • 1 axon + one dendrite

  • ex: sensory neurons (not all sensory neurons are unipolar, but if you find a unipolar neuron, it will be sensory)

• bipolar

  • 1 axon + 2 dendrites

  • ex: eye

What are the four general PNS neuron types (give examples of each)

1. GSA: general somatic afferent

   1. touch, pain, temp, pressure, proprioception, itch

2. GVA: general visceral afferent

   1. homeostatic senses

3. GSE: general somatic efferent

   1. skeletal muscle, coughing, sneezing, breathing

4. GVE: general visceral efferent

   1. smooth, cardiac muscle

   2. digestion, heart beat, etc.

Explain the pathway of a signal from touching a hot pan with your hand to removing it

1. sensory signal carrying: GSE (touch, pain, temperature) body tissues —> dorsal root ganglion of spinal cord

2. dorsal root ganglion —> dorsal horn and synapses with interneuron

3. interneuron synapses with motor neuron

4. motor neuron sends signal out through ventral horn —> ventral root

5. spinal nerve and then out ventral ramus

6. muscles that control hand movement to remove hand from stove

How can glia cells be problematic?

• have the ability to divide

• form scar tissue and impede healing

What are the 4 types of Glia cells in the CNS?

1. astrocytes

   1. nourish

   2. form part of blood-brain barrier

   3. maintain ionic concentration

   4. assist with cell to cell signaling

2. microglia

   1. pac-man, garbage and police of CNS

   2. destroy invading pathogens

   3. clean up dead, injured neurons

3. ependymal

   1. form epithelium that lines brain and spinal cord

   2. help make cerebral spinal fluid (CFS)

   3. surround central canal

4. oligodendrocytes

   1. produce myelin that covers axons

How does multiple sclerosis affect oligodendrocytes?

• MS destroys them which means neurons don’t have the myelin sheath

• affects nerve signaling and conduction speed

What are the 2 types of glia cells in the PNS?

1. satellite cells

   1. similar function as astrocytes in CNS

2. schwann cells

   1. produce myelin for axons in PNS

   2. very ordered with injuries

Name the parts of a nerve from innermost to outermost

1. axon

   1. covered in myelin

   2. endoneurium

2. fascicle

   1. bundles of axons

   2. perineurium

3. nerve

   1. bundles of fascicles

   2. epineurium

What are the components of the cell membrane (neuron)

• hydrophilic head

  • repels other polar moclecules

  • ex: Na, Cl, water

• hydrophobic tail

  • allows nonpolar molecules to pass through via diffusion

  • O2, CO2

What are the different methods of passive transport?

• diffusion

  • natural motion of cells moving with concentration gradient, they want to spread out (people bunched up wanting to separate)

    • especially when in a liquid

  • affected by: temperature, mechanical forces

• facilitated diffusion

  • allows things that would normally be repelled by the cell membrane to pass through cell membrane

  • non-gated channels: tunnels

    • always open

    • ex: aquaporins for water

  • gated channels: doors

    • take energy to open/close

    • ex: Na, K

Explain active transport and give some examples

• moving molecules against their concentration gradient (going UP hill, requires energy)

• membrane pumps

  • Na/K pump: 3 Na out, 2 K in

• endocytosis

  • cell membrane wraps around something to bring it in

• exocytosis

  • cell membrane breaks off to push something out

  • hormones

What is the resting membrane potential of a cell?

• slightly + charge outside cell

  • Na/K pump

• slightly - charge inside cell

  • Cl ions near cell surface

  • negative charge proteins

What does depolarization mean?

• charges between the inside of the cell and outside of the cell become more equal

• inside: becomes more positive

What does hyper-polarization mean?

• charge difference between inside/outside cell increases

• inside: becomes more negative; outside: becomes more positive

What are the events of a local potential?

1. dendrite or cell body receives a stimulus

   1. mechanical (touch, pressure)

   2. chemical (neurotransmitters)

      1. motor/interneurons ONLY have chemical, sensory can have both

2. Na+ channels to open (stimulus-gated)

3. Na+ ions flood into cell (Na+ is moving down its concentration gradient)

4. membrane becomes depolarized (less difference btw charges of inside/out)

What causes a local potential to be stronger/more likely to result in an action potential?

• location

  • closer to axon, less distance for Na+ ions to travel, more likely to generate action potential

• speed/frequency of Na+ flooding in

  • increases, more likely to result in action potential

How is an action potential different from a local potential?

• only occurs on the axon

  • local: dendrite, cell body

• sends messages to generate a response in other cells

  • local: receives messages

• voltage-gated channels open in response to electrical stimulation

  • local: stimulus-gated

• all or nothing

  • local: can be stronger or weaker depending on location/tempo

• conduction over long distances

  • local: short distances

What must happen in order for an action potential to occur?

• Enough Na+ ions enter cell —> depolarization strong enough to reach threshold potential inside membrane —> voltage-gated Na+ to open —→ all Na+ channels open

What are the events of an action potential?

1. dendrite receives signal

2. local potential/depolarization

3. depolarization reaches threshold

4. voltage-gated Na+ channels open

5. this continues until signal reaches end of axon

6. repolarization: absolute refractory period

   1. Na+ close

   2. K+ open, let out K+ ions inside cell

7. hyper polarization: relative refractory period

8. resting membrane potential

What is the difference between absolute and relative refractory period?

• absolute

  • impossible for channels to re-open

  • Doctor on vacation, can’t reach them at all

• relative

  • possible to re-open but just takes bigger threshold stimulus

  • doctor on call, just need to bother them a ton and then they’ll come in

What is different when lifting 50lbs vs. 25lbs (in terms of signals)

• 50lbs

  • increase in frequency of action potentials

  • stronger muscle contraction

What affects conduction speed of axons?

1. myelination

   1. MUCH MORE IMPORTANT

2. diameter

Why are nodes of ranvier necessary for action potentials?

• no Na/K channels on myelinated parts of axons

• nodes allow the signal to jump (stepping stones across a river = faster than swimming)

What are the names of the different synapses on a neuron? (order from strongest to weakest)

1. axo-axonic: directly on axon

2. axosomatic: on cell body

3. axodendritic: on dendrite

What is the mechanism of a neurotransmitter stimulating an adjacent neuron? (hint: lock and key)

• neurotransmitter acts like a key and binds to a receptor with a specific shape

  • neurotransmitter: key

  • receptor: lock

• this stimulates a reaction in the cell

What is the process of releasing a neurotransmitter at a synapse?

1. axon potential reaches axon terminal

2. Ca voltage channels open

3. Ca stimulates synaptic vesicles to open

4. Neurotransmitters are released via exocytosis

5. NT binds to a receptor on the next cell (neuron, or tissue being affected)

What is the effect of excitatory post-synaptic potentials?

depolarization

What is the effect of inhibitory post-synaptic potentials?

hyperpolarization

What is the physiological effect of excitatory potentials?

• depolarization, making inside of cell more positive (so inside/outside cell more neutral)

• causes Na or Ca to enter post-synaptic cell

What is the physiologic response of inhibitory potentials

• hyper-polarization

• K+ ions to exit OR Cl- ions to enter

Explain the reason for excitatory/inhibitory potentials regarding the patellar reflex

• tendons have muscle spindles which detect stretch causing a reflexive muscle contraction

• if muscle spindles in patellar tendon send sensory signal to the spinal cord, this causes an excitatory potential in the motor neuron which then results in contraction of the quads and extension of the knee

• if quads are contracting and we want knee ext, then hamstrings (which cause knee flexion) need to be relaxed

• so inhibitory signal is then released and affects motor neurons which turn off motor neuron that is responsible for knee flexion

What is the inhibitory process?

1. sensory neuron sends excitatory signal to an inhibitory interneuron

2. inhibitory interneuron is stimulated to release inhibitory signal to a motor neuron

3. motor neuron stops sending signal to muscle, causing muscle to relax

Explain spatial vs. temporal summation

spatial: number

• adding up all the signals that are nearby which then determines which signal gets through

• ex: if there are 2 + signals and 1 - signal, result will be an excitatory potential which would then activate/stimulate the neuron/effector cell

temporal: pace/speed

• pace at which a signal is being delivered

• ex: excitatory signal can ramp up the number of action potentials to override a conflicting inhibitory signal

What is the difference between a divergent, convergent, and feedback neuron network (give example of each)

divergent: pyramid scheme

• one neuron —> many neurons

• ex: sensory neuron then stimulates interneuron then motor neuron

convergent: everyone complaining to the president

• many neurons influence one cell

• ex: sensory input all gathers into the cerebral cortex of brain

feedback: one neuron influences the neurons that provide it input

• teacher giving critique to student who turned in homework

• ex:

What two things determine the function of a neurontransmitter?

1. type of neurotransmitter

2. type of receptor molecule it binds to on postsynaptic cell

How does norepinephrine (NE) affect blood vessels?

• smooth muscle

  • released by GVE neurons

  • binds to alpha receptors

  • creates excitatory potential

  • causes blood vessels to constrict

  • decrease in binding —> dilation of vessels

• skeletal muscle

  • binds to beta receptors

  • causes blood vessels to dilate

  • this brings more O2 and glucose to muscle cells

What are the major categories of neurotransmitters?

• small molecules

  • AcH

  • Amines (serotonin, dopamine, histamine)

  • amino acids (GABA, glutamate)

• peptides

  • endorphins

  • substance P (for pain)

• other

  • nitrous oxide

  • carbon monoxide

What is the process of neurulation and where does it occur?

• location: ectoderm

• process of forming spinal cord