Ch 4 + 5 : Communication Within the Nervous System

Neurons

= cells that convey sensory info to the brain, do operations within the brain & transmit commands to the body

• polarized

• movement, thoughts, memory, sensations

• diff. shapes based on fxn

• have cell membrane?

structure:

• cell body

• nucleus

• dendrites - RECIEVE INFO

• axon

• axon terminals - SEND INFO TO NEXT NEURON

Neurons : Types (3)

1. motor neuron = carry commands to muscles & organs

• bi/multipolar

• PNS

2. sensory neuron = carry info from body/outside world (senses) to brain & spinal cord

• uni/bipolar

• PNS

3. interneurons = connect neuron in same are of brain or spinal cord

• the majority of our neurons

unipolar neuron

= only 1 thing is attached to cell body

• sensory

• most

bipolar neuron

= 2 things attached to the cell body

• sensory

multipolar neuron

= multiple things attached to cell body

• motor

interneuron

• multipolar neuron

Cell membrane

Energy & the Cell membrane

Polarization = a difference in electrical charge b/w the inside & outside of the neuron

Voltage = difference in electrical charge b/w 2 points

Resting Potential = difference in charge b/w inside & outside of a neurons membrane at rest

• neurons : -70mv (inside is MORE (-), than outside the cell)

• a result of ions (atoms that lose/gain electrons)

NOTE: size of anions is responsible for the -70mv

Resting Potential : What moves the Ions?

1. force of diffusion = tendency of ions to move thru membrane to less [ ] side

2. electrostatic pressure = force where ions are repelled from similar charges & attracted to oppositely charged

• allowed with channels

3. Sodium potassium pump = protein molecule that moves 3 sodium ions outside the cell & 2 potassium ions inside the cell

• against [ ] gradient

• maintains -70mv

Ion Channels

= gated pores in the membrane, formed by proteins, that limit the flow of ions in/out of cell

• found along ENTIRE neuron (chemically gated more common on dendrites)

• chemically OR electrically gated

Chemically = by neurotransmitters or hormones

Electrically = by a change in electrical potential of the membrane

ex. closed at -70mv → must create a local potential to open

Depolarization : Local Potential

Local potential = partial depolarization, where an area’s polarity shifts twd 0

• a graded potential → thus varies in magnitude w the strength of stimulus that produces it

THUS: more neurotransmitters → stronger signal → MORE depolarization

→ MUST meet threshold (ex. -60mv) to open ion channel

Depolarization : Action Potential

= abrupt depolarization of membrane, allowing neurons to communicate

• occurs when the local potential (partial depolarization) exceeds the threshold for an electrically-gated channel

ex. Na+ floods cell, K+ leaves cell

• an UNGRADED potential = occurs at full strength, or NOT at all (all-or-none law)

• nondecremental → thus potential travels down axon w/o any decrease in size + is propagated at points along the way

Refractory Periods

Absolute refractory period = Na+ channels are Unresponsive to further stimulation → a new action potential CANT occur

Relative refractory period = Na+ channels could support another action potential, BUT the K+ channels are still open

• a new action potential CAN occur, if the stim. can overcome the charge

Rate law

= axons encode the stimulus intensity by its FIRING RATE, NOT by the size of its action potential

• HIGHER # of signals = higher intensity/ importance of signal

• HIGHER rate = higher importance

Neurons : Myelination & conduction speed

Myelin = fatty tissue around axon to insulate it (faster)

• keeps cells separate from extracellular fluid & other neurons

• created by glial cells

Nodes of Ranvier = gaps in myelin sheath

• Saltatory conduction = transmission of action potentials appearing to jump b/w nodes

Neurons : Benefits of Myelin Sheath

1. reduces capacitance (resistance) → makes it faster

• capacitance slows down movement of ions across axon

2. signal regeneration at nodes

3. use less energy

Glial Cells

= NONneuronal cells that support neurons functioning

• makes myelin

Glial Cells : Myelin-producing Glial Cells

2 types:

1. Oligodendrocytes = produce myelin in the brain & spinal cord (CNS)

• can make multiple myelin sheaths

2. Schwann cells = produce myelin in the rest of neurons (PNS)

• only makes one myelin sheath

Glial Cells : other functions - KNOW

Radial Glia = form scaffold that guide new neurons to their destinations - DURING fetal development

Microglia = provide energy to neurons + respond to injury and diseases by removing cellular debris

• “brain immune system”

Astrocytes = trigger the formation of 7x as Manny connections in neurons

How neurons communicate

Synapse = connection b/w 2 neurons

Synaptic cleft = gap separating neurons, so they’re not in direct contact

Presynaptic = transmitting neurons (w axon terminals)

Postsynaptic = receiving neuron (w dendrites)

via : chemical transmission at the synapse → changes behaviour of another cell

• 1st shown by Lower in 1900s using a frogs vagus nerve

→ neurons release at least 2 diff. chemicals that have opposite effects

Neurons: Chemical transmission at the synapse

Vesicles = membrane-enclosed bubbles at axon terminals that store neurotransmitters

Ionotropic receptors = form the ion channel + open quickly to produce IMMEDIATE reactions

• chemically-gated ion channel

• opened by neurotransmitters - thus ions can enter/exit

Metabotropic receptors = open channels INDIRECTLY thru a second messenger

• NOT ion channels

Neurons : Excitation & Inhibition

Excitatory Postsynaptic Potential (EPSP) = when receptors open Na+ channels to produce a partial depolarization of the dendrites & cell body

• more (+) than resting

• Partial depolarization = excitatory → facilitates occurrence of an action potential

Inhibitory Postsynaptic Potential (IPSP) = when receptors open K+ channels, Cl- channels or both to produce a hyperpolarization of the dendrites & cell body

• more (-) than resting

• Hyperpolarization = inhibitory → makes an action potential LESS likely to occur

Neurons : Postsynaptic Integration

Spatial Summation = combines potentials occuring simultaneously @ different locations on the dendrites & cell body

Temporal Summation = combines potentials arriving a short time apart from the same/separate inputs

→ for both excitatory & inhibitory

Removing Neurotransmitters : Reuptake

Reuptake = transporter proteins take transmitters back into the axon terminals → repackaged into vesicles for reuse

NOTE: sometimes done by astrocytes (glial cell) instead

Regulating Synaptic Activity

1. Presynaptic excitation = increases the presynaptic neuron’s release of neurotransmitters to the postsynaptic

2. Presynaptic inhibition = decreases the presynaptic neuron’s release of neurotransmitters to the postsynaptic

Autoreceptors = receptors on presynaptic terminals, that sense the amount of transmitter in cleft

Neurotransmitters : false beliefs

• Dale’s principle = FALSE belief that a neuron could ONLY release one transmitter

Neurotransmitters: Release

Corelease = SAME vesicle of a terminal + MULTIPLE transmitters

Cotransmission = multiple transmitters in SEPARATE vesicles of a terminal

Spatial Segregation = multiple transmitters in SEPARATE vesicles are released from DIFF terminals

→ multiple neurotransmitters can be released from a neuron

Neurotransmitters: Differential Ca2+ sensitivity

Ca2+ enters terminal → neurotransmitters are released

• diff amounts of Ca2+ = diff neurotransmitters released

Neural Codes + Neural networks

Neural code = varied intervals b/w spikes in nerve signals

• timing of signals + differ b/w pathways

Neural network = groups of neurons that function together

Human Connectome project = effort to map the brain’s circuits

In perspective

• impossible to understand the brain, or the behaviour it produces w/o understanding a neurons limitations & capabilities

• modern tools + cooperative efforts are the key to moving fwd in biological-psych research