PSYC 3800: Final Review - Ch 24: Memory Systems

Nerves

The elementary signaling units

Charles Sherrington

Coined the term synapse

• ~1000 connections

• Receives input from ~10,000 neurons

How many connections does the average neuron form? How many neurons does it receive input from?

10^11 (1 trillion)

How many neurons are in the brain/body?

10^14

How many synapses are in the brain/body?

• Electrical synapse

• Chemical synapse

What are the 2 types of synapses in the brain?

Synaptic transmission

The transfer of information at the synapse

Synapse

Specialized sites of contact between two neurons that facilitate the transfer of information

Both; electrical synapses are common in the brains of invertebrates, and vertebrates, including mammals

Are electrical synapses found in invertebrates or vertebrates?

Chemical synapse

Which type of synapse is more common in the brains?

Electrical synapses

Type of synapse where two neurites are connected by a gap junction with channels comprised of connexons that allow direct flow of ions from one neuron to another. Involves the conformational change of the synapse.

• Allow ionic current to pass equally well in each direction

Connexons

Hexameric protein structures composed of six connexin subunits that span the cell membrane

Chemical Synapses

A type of synapses where an axon terminal has machinery to release NTs stored into synaptic vesicles.

Gap junctions

Gaps that occur between cells in nearly every part of of the body an inter-connect many non-neural cells

Electrically coupled

Cells connected by gap junctions are said to be

• Fire rapidly

• If synapse = large, nearly failsafe

What are the advantages of electrical synapses?

• small/weak PSP

What are the cons of electrical synapses/signaling?

Furshpan & Potter

Discovered electrical synapses

• Heart

• Smoothie Muscle

• Liver

• Crustaceans

Where are electrical synapses found?

• Astrocytes & glial processes

• Neuron-neuron communication

Where do gap junctions occur?

• Small molecular transfer between cells

• Calcium wave (‘calcium tsunami’)

• Repetitive firing

• Synchronous activity

What is the process of electrical signaling in Astrocytes?

• Brainstem Networks (synchronous firing to regulate hormones release and control breathing !)

What are some examples of synchronous firing occurring in Electrical synapses

Characteristics of Electrical synapses

• Channels = low resistance & high conductance

• Speed (very fast)

• Bi-directional signaling

Loewi

Discovered that the electrical stimulation of a frog’s heart caused muscles to release a specific chemical (Vagusstoff/acytlcholine ) which could mimic the effects of electrical stimulation

Benefits of chemical synapses:

• Mediate excitatory or inhibitory signally

• More complex - plasticity

• Amplify signals

• Axodendritic

• Axosomatic

• Axoaxonic

• Dendrodendritic

What are the 4 different synaptic arrangements in the CNS?

Gray’s type I

A type of asymmetrical, excitatory synapse. High postsynaptic density

Gray’s type II

A type of symmetrical, inhibitory synapse. Low postsynaptic density

Neuromuscular Junction (NMJ)

Specialized synapse between motor neuron and skeletal muscle. Convert nerve signals into muscle contractions.

• Studies of this system established principles of synaptic transmission

• High clinical relevance

• Easy accessibility

• Synthesis

• Load into vesicles

• Fusing of vesicles to presynaptic terminal

• Neurotransmitter spills into synaptic cleft → release

• Binds to postsynaptic receptors

• Biochemical/electrical response

• Removal of NT from cleft

What are the 7 basic steps of neural communication?

Amino acids

Small organic molecules

• contain at least one N atom

• Fast synaptic transmissioN

• GABA (inhibitory)

• Glutamate (inhibitory)

• Glycine (excitatory)

Examples of amino acids

Biogenic amines

Small organic molecules that also contain at least one small ‘N’ atom

• Dopamine

• Norepinephrine

• Histamine

Examples of biogenic amines

Peptides

Short amino acid chains stored in and released from secretory granules

• Dynorphin

• Enkephalins

Examples of peptides

Dales Law

Law that states that “A single neuron always produces the same transmitter at every one of its synapses

• Now known to be not always the case (co-release)

Concept cells

Neurons that are selective for a specific person, place, or concept, but the response does not depend on the features of the stimulus.

Exist somewhere on the continuum between sensory coding & memory; are not encoding sensory input in any simple way, as they can be activated by visual stimuli that have noting in common (e.g., a photograph vs. a written name) and they can also be driven by both visual and auditory inputs.

• May serve a role in the formation of new memories of people and things we already recognize (e.g., such as a patient’s memory of Halle Berry)

• May be a part of the machinery that forms associations between experiences (a fundamental role of the hippocampus)

What is the proposed role of concept cells/selective cells in the hippocampus?

• Common objects and famous faces continue to be recognized even after lesions to the hippocampus suggesting other regions outside of the hippocampus are involved in this process

What evidence supports that concept cells or selective cells in the hippocampus are likely not essential for perception and recognition?

• It made a strong case that the brain was not equipotential for memory, as Lashley had concluded, and it demonstrated that different types of memory involve different brain structures

• Medial temporal lobes were removed from both hemispheres of H. M.’s brain to alleviate epileptic seizures. 2/3rds of his anterior hippocampi and parts of his cortex were also removed.

• Surgery had little effect on perception, intelligence, or personality but resulted in severe anterograde amnesia (could forget events almost as quickly as they occurred)

• Some degree of retrograde amnesia (retained memories of his childhood but little to no memory for events just before his surgery → extending back several years, possible decades)

• Working memory largely normal; could also learn new tasks (procedural memory) but no recollection

Why was the study of H. M.’s amnesia so important to neuroscience, especially understanding memory?

True

T/F: lesions to areas outside of the hippocampus and elsewhere in the brain that communicate with the hippocampus can produce amnesia

• Studies of Korsakoff Syndrome

• Typically, there are lesions in the dorsomedial thalamus & mammillary bodes, which receive direct input from the hippocampus via the fornix

• Can involve severe retrograde amnesia

What evidence supports the role of the diencephalon in memory?

Damage to:

• Thalamus

• Mammillary bodies

What area(s) of the brain do researches suspect are involved in anterograde amnesia associated with diencephalic lesions?

• Memory consolidation

  • Serves as a temporary storage system that organizes and abilities new episodic and declarative memories before they are transferred to the neocortex for long-term storage

What aspect of memory are the temporal lobes (medial) and the connecting systems (hippocampus, amygdala, adjacent cortex) important for?

Place field

The specific, localized area in an environment that causes a hippocampal neuron (aka place cell) to fire potentials at a high rate.

• In ways, similar to the receptive field of neurons in sensory systems

Place cells

Specialized neurons, primarily pyramidal neurons in the hippocampus, that fire to create a spatial maps of the environment

• typically neurons in CA1 & CA3

True; place cells have been shown to remap its place fields when placed in a new environment

T/F: Place cells & fields are dynamic

Grid cells

Specialized neurons in the brain’s medial entorhinal cortex that act as a navigation system, creating a hexagonal, coordinate-like map of the environment

Edvard & May-Britt Moser

Discovered grid cells

• Place cells

• Grid cells

• Head-direction cells

• Border cells

• Speed cells

What are the notable cell types of and around the hippocampus that store spatial information?

• Length of experience as a taxi driver

What does the size of the posterior hippocampus seem to correlate with?

Cognitive map theory

Theory that states that the hippocampus is part of a memory system that creates a map representing an animal’s environment along with objects encountered at different places

O’Keefe & Nadel

Who proposed the cognitive map theory?

Eichenbaum & Cohen

Who proposed the alternative to the cognitive map theory?

• The hippocampus is a machine that builds memories for experiences based on the relationships between sensory inputs.

• Memory for the spatial locations of objects is one type of relationship that is useful for navigation, but spatial mapping is not the central function of the hippocampus

• Evidence = odor discrimination test

What is the alternate theory to the cognitive map theory?