Psychology 2821 - Chapter 2

Behavioural Neuroscience

Two Divisions of the Nervous System

1. Central Nervous System (CNS)

2. Peripheral Nervous System (PNS)

Central Nervous System (CNS)

Brain and spinal cord - communicates to the rest of the body via nerves

Peripheral Nervous System (PNS)

Outside the brain and spinal cord

Nerves

Bundles of axons (part of neurons)

Cranial Nerves

12 pairs from the brain

Spinal Nerves

31 pairs from the spinal cord

3 Functional Types of Neurons

1. Sensory Neurons

2. Motor Neurons

3. Interneurons

Sensory Neurons

Carry sensory information from the PNS to CNS

Motor Neurons

Sends outputs from CNS to PNS - mostly to muscles

Interneurons

Relay information within the CNS - local networks or tracts

Neurons

Communicate with each other and body, Process and transmit information, Generate action potentials (electrical), and Release chemicals (neurotransmitters)

Glial Cells

Support and protect neurons, Supply energy and nutrients to neurons, Form blood brain barrier around blood vessels, Form myelin sheath around axons, Keep brain healthy (clean debris, attack viruses), Synthesize neurotransmitters and buffer ions, and Do not generate action potentials!

Types of Glial Cells

1. Astrocytes

2. Oligodendrocytes

3. Microglia

Astrocytes

Star shaped, Supply nutrients to neurons, Form blood brain barrier, Alter chemical composition of fluid surrounding neurons (ion concentrations; neurotransmitters levels), Engulf and digest debris, and Form scar tissue following injury

Phagocytosis

Engulf and digest debris

Gliosis

Form scar tissue following injury

Blood Brain Barrier

Formed by tightly packed cells lining capillaries in brain - Regulates what chemicals can enter brain

Oligodendrocytes

Form myelin sheath around axons (creating nodes of Ranvier) - Found in CNS only (Schwann cells in the PNS)

Microglia

Engages in phagocytosis, Protects brain from invasive agents (e.g. viruses), and Secrete pro-inflammatory chemicals (cytokines)

4 Parts of a Neuron

1. Soma

2. Dendrites

3. Axons

4. Terminal Buttons

Soma (Cell Body)

Integrates incoming signals (decides whether cells “fires”) and Directs cell function (DNA, neurotransmitter synthesis, etc.)

Dendrites

Transmit incoming signals to soma and have graded potentials

Axons

Generate action potentials

Terminal Buttons

Release neurotransmitters

Myelin Sheath

Surrounds and insulates axons and improves conduction of action potentials

3 Parts Within Neurons

1. Nucleus

2. Cytoplasm

3. Cytoskeleton

Nucleus

Contains chromosomes consisting of deoxyribonucleic acid (DNA) -Transcription of genes for protein synthesis

Cytoplasm

Contains organelles such as mitochondria (generates ATP for energy) and ribosomes (translation)

Cytoskeleton

Includes microtubules

Microtubules

Protein strands that transport vesicles

Transcription

Occurs within the nucleus

Translation

Occurs outside the nucleus on ribosomes

Action Potentials

Wave of changed polarity from negative to positive inside the axon

Axon Hillock

Where action potentials begin

Resting Membrane Potential

Before the action potential is generated, the neuron has a… (inside of axon is negative relative to the outside: -70mV)

Depolarized

Less negative inside

Firing Threshold

-55mV

Refractory Period

Until the resting membrane potential is re-established, neuron cannot generate another action potential

Sodium Potassium Pump

Na+ that entered cell must be pumped out and K+ that left cell must be taken back into cell - Requires energy (push ions against force of diffusion)

All or None Law

Neurons never “partially” fire, an action potential either occurs (if firing threshold is reached) or it doesn’t occur - The size of the action potential (change in polarity ) is always the same (always from -70mV to +40mV) all the way along
the axon

The Rate Law

Since the magnitude (size) of the action potential is always the same, the strength of a stimulus is represented in the nervous system by the rate of firing of an axon

Conduction in Myelinated Axons

Ion channels only exist at the Nodes of Ranvier (between myelinated segments) - Action potential “skips” along the axon (saltatory conduction - Increases overall speed of conduction (compared to non-myelinated axons)

Neurotransmission

Communication between neurons - chemical in nature.

Synapse

When action potential arrives at the nerve terminal, neurotransmitters are released in to…

Post-Synaptic Receptors

Neurotransmitters bind to … on adjacent neurons (lock and key analogy)

EPSPs

Depolarizing effects

IPSPs

Hyperpolarizing effects

Locations of Synapses

1. Axodendritic

2. Axosomatic

3. Axoaxonal

Axodendritic

On dendrites of post-synaptic neurons

Axosomatic

On somas of post-synaptic neurons

Axoaxonal

On axons of post-synaptic neurons

Release of Neurotransmitters

Begins when action potential arrives at nerve terminal, Ca++ enters pre-synaptic cell, Vesicles storing neurotransmitters fuse with pre-synaptic membrane, Pores on pre-synaptic membrane open and molecules of
neurotransmitter are expelled into the synaptic cleft, Neurotransmitter molecules diffuse across synapse (move from areas of high concentration to low) and bind to post-synaptic receptors

Two Mechanisms of Termination of Postsynaptic Potentials

1. Reuptake

2. Enzymatic Deactivation

Reuptake

Removal of a neurotransmitter from the synaptic cleft by the terminal button (back into presynaptic neuron) - Transporters on presynaptic membrane

Enzymatic Deactivation

Accomplished by enzyme that degrades molecules of the neurotransmitter - Ex. Acetylcholinesterase (AChE) deactivates acetylcholine (ACh)

Two General Categories of Receptors

1. Ionotropic

2. Metabotropic

Ionotropic

Coupled to an ion channel (ligand-gated) - Immediate (direct) effects (ion channel opens and ion enters cell)

Metabotropic

Coupled to a G protein - Activates a second messenger (within cells) - Changes take longer to begin and last longer

Ionotropic Receptors

Ligand-gated ion channel opens when a molecule of a neurotransmitter attaches to the binding site - Ions (not the neurotransmitter!!) enter the post-synaptic neuron causing EPSPs or IPSPs (depending on the ion)

Metabotropic Receptors

When a molecule of neurotransmitter binds with a receptor, a G protein is activated within the post-synaptic cell - Triggers a chain of events (including a second messenger) which: indirectly opens an ion channel or produces another intracellular change in the cell (e.g. activation of a gene)

Autoreceptors

Receptors located on pre-synaptic neurons, Respond to the neurotransmitter that they themselves release, Mostly regulate internal processes, such as synthesis and release of neurotransmitters, Mainly inhibitory effects when too much neurotransmitter is located in the synapse (feedback mechanism)