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Senses and Neuroanatomy

Alice Monica Koltchev

Quentin Pajot-Moric

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central nervous system

peripheral nervous system

brain

spinal cord

cranial & spinal nerves

sensory division

motor division

environment → CNS

CNS → environment

autonomic nervous system

somatic nervous system

involuntary movement

voluntary movement

sympathetic division

fight or flight

parasympathetic division

rest and digest

Overview

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a complex nervous system

a single neuron

“Nothing in biology makes sense except in light of evolution”

— Theodosius Dobzhansky, a famous evolutionary geneticist, in 1973

Understanding instead of memorising

In this lecture, we will investigate how humans’ nervous systems got to where they are today. I.e. Where did this structure (human nervous system) come from?

This lecture will be about recapitulating how we went from simple multicellular organisms to us and how that resulted in our nervous system looking the way it does. For example, how do single neurons like the one shown here organise into complex systems like the one you see on the right (which is entirely made up of these little cells.. And their supportive glia etc.)

So now that we know how cool the human NS is… let’s have a look at how we got here and how we compare to other organisms. We’ll take a trip to 650 million years ago to understand how organisms evolved and began accumulating new nervous system characteristics as they adapted to their environments to what we are today.

If the complexity of the human NS makes no sense to you.. You are not alone!

“Nothing in biology makes sense except in light of evolution” Dobzhansky 1973

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How did we get here?

In this lecture, we will investigate how humans’ nervous systems got to where they are today. I.e. Where did this structure (human nervous system) come from?

This lecture will be about recapitulating how we went from simple multicellular organisms to us and how that resulted in our nervous system looking the way it does. For example, how do single neurons like the one shown here organise into complex systems like the one you see on the right (which is entirely made up of these little cells.. And their supportive glia etc.)

So now that we know how cool the human NS is… let’s have a look at how we got here and how we compare to other organisms. We’ll take a trip to 650 million years ago to understand how organisms evolved and began accumulating new nervous system characteristics as they adapted to their environments to what we are today.

If the complexity of the human NS makes no sense to you.. You are not alone!

“Nothing in biology makes sense except in light of evolution” Dobzhansky 1973

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Neurons appear...

… together with more complex organisms

a cnidarian: hydra

How did we get here?

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an echinoderm:

sea star

More neurons → organisms start moving around...

… eating and avoiding getting eaten

How did we get here?

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an echinoderm:

sea star

a planarian:

flatworm

How did we get here?

Not just moving, but also seeing, tasting etc...

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a mollusc:

octopus

Brains! Now we can do really cool stuff..

How did we get here?

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How did we get here?

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How did we get here?

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How did we get here?

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reasoning, problem solving,

processing sensory info,

complex learning, etc

midbrain

breathing, sleeping,

movement coordination,

balance maintenance

forebrain

hunger, fear, vision, smell...

hindbrain

How did we get here?

The brain itself is separated into 3 main parts: forebrain, midbrain and hindbrain. This is the last ‘facts’ slide but it will help you see how human fit into the evolutionary history of neuroanatomy, as these structures are seen across many animals. Our huge forebrains (compared to other animals) is what make humans special in terms of reasoning and complex problem solving capacities.

Our mid- and hindbrains include the brainstem and cerebellum and can be thought of as all the parts that ‘poke out’ of the brain or are visible.

Note that the hindbrain is the oldest and most conserved structure and that subsequent structures have evolved over it (folded over it in humans)

FOREBRAIN - Process sensory info, reasoning, problem solving. Mostly made up of the cerebrum (Two 3rds of brain’s mass) and covers most other brain structures. The most important structure in the cerebrum in terms of reasoning is the cerebral cortex (grey matter).

MIDBRAIN - Helps regulate movement

Process auditory and visual info

Physically connects forebrain to hindbrain

HINDBRAIN - Coordinate movement

Maintain balance

Relay sensory info from muscles to cortex

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antennal lobe (olfaction)

optic lobe (vision)

mushroom bodies (memory)

antennal mechanosensory & motor centre

central complex (decision making)

thoracic

ganglion

brain

Neuroanatomical similarities with insects

SO now that we know what human brains are like… what if you weren’t a human on land and were a fly that needs to get through air?

Organisms that need to perform similar functions have evolved (even if through different processes) to have similar brain sections/functions. For example, even flies and humans have similarities:

Humans have 86 billion neurons compared to fruit flies’ 135 thousand

Insect brains are organised in functional modules, just like the human (mammalian) brain:

- sensory modules (olfaction, bilateral vision, audition, mechanosensation)

- central complex ~ basal ganglia (action selection, action maintenance, visuomotor integration) ← when this affected, fruit flies express symptoms akin to those seen in some human conditions (sleep disturbances, attention deficits, memory impairment)

- mushroom body (associative learning, memory) ← a three-layer expand-and-converge architecture, similar to cerebellum & perceptron, good for pattern separation; protein expression resembles hippocampus

Insect brains also have loops

Their spinal cord equivalent is called thoracic ganglion

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The octopus nervous system

Octopus brains are a lot more de-centralised than human brains. Different to the ‘homunculus’, which is very organised so that useful part of your bodies get more representations, in the octopus, most of their neurons are in the arms. That’s how they’ve evolved and adapted to their environment.

However, humans also have LOTS of neurons in the cerebellum, which is what we need for fine motor control. So you can see that brains have evolved to move organisms.

Octopuses are very intelligent animals: they can solve puzzles, learn through observation, and use tools.

Part of the class Cephalopoda which comes from the greek (means head-feet)

Brain to body ratio similar to that of other intelligent animals and the highest within invertebrates.

Complex nervous system with about 500 million neurons (similar to a rabbit)

Big difference is that the neurons are not as centralized as in most species:

Central brain - less than 10% of neurons
2 optic lobes have about 30% of neurons - This allows them to take advantage of their highly developed eye structure.
Arm nerve chords - actually more than half (about 60% - 70%) of neurons are in the arm nerve chords.

Without a skeleton, their limbs can move practically in any direction - and interestingly, scientists have not found a map of the octopus body in their brain. Instead, their brain has a behavior map: nearby groups of neurons in the brain can create responses like crawling or jet propelled swimming that make use of their entire body.

Each octopus arm is able to process sensory information, then organizes and executes a rudimental motor action by “itself”, although the brain seems to keep control of the overall activity.

One example paper on the movement control by the octopus brain

https://www.cell.com/current-biology/fulltext/S0960-9822(09)01546-2?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982209015462%3Fshowall%3Dtrue

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an appealing idea but many exceptions

The human advantage : brain size?

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rodent and primate brains scale up in different ways

The human advantage : packing density

Herculano-Houzel came up with a clever idea: she essentially made mashed brains and counted nuclei.

Rodent and primate brains are fundamentally different, so the EQ comparisons on the previous slide was like comparing apples and oranges.

As rodent brains get bigger and heavier, the number of neurons increases at a relatively modest rate

- rodents don’t compensate efficiently for the communication bottlenecks that emerge as brains increase in size: axon fibres get longer and thicker, so white matter ends up occupying relatively more space, leaving less space for more neurons

The human brain is a scaled-up primate brain, not a scaled-up mammalian brain

- some axons (the ones that travel long distances) get thicker as well, but primate brains show increased compartmentalisation, enhanced local computations that do not require an increase in white matter and leaves place for more neurons

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In summary

The brains of different species have evolved for their particular environments

The human brain has evolved to adapt to many environments

But how do we sense which environment we’re in???

Depending on environmental pressures you develop different capabilities

Some animals have evolved to survive/thrive in very specific environments e.g. some mammals only live in Australia or only in specific temperatures in the sea.

However, humans have adapted to live on all parts of the globe. Potentially, this may be because we have evolved to pack billions of neurons into brains we can actually carry around. Making us multifunctional.

As we have also seen in this lecture, our brains are different to other species but we also conserve many similarities with other species. As the human brain is so complicated, it is very difficult to study, but we can make use of all of these other organisms to understand fundamental brain functions. You will learn lot about this in your animal models lecture!

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What are senses for?

First take home message:

We need senses to experience the world. That’s how our brain gets information about what’s happening inside and outside of our body.

We need this information to make good decisions about what to do next.

Senses provide an evolutionary advantage.

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How many senses can you think of?

Humans have 5 main senses, but there’s a lot more in nature...

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How many senses can you think of?

Bats use echolocation to detect objects

Platypus uses electric fields to detect objects

Birds use the earth’s

magnetic field to orient themselves

Ants use ultraviolet light to orient themselves

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How many senses can you think of?

Second take home message:

Different species have different sets of senses
How each species experiences the world depends on which senses it has.

We have no idea what if feels like to sense the earth’s magnetic field and a dog has no idea what it’s like to see something red.

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Why do species have different sets of senses?

We saw that we need senses to gather information about the environment

Different species need to capture different types of information to survive

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But WHAT are senses exactly?

PHYSICAL PHENOMENA

light

magnetic field

sound

taste

BRAIN

HOW ?

Sensory organs take care of translating physical phenomena into patterns of neural activity: we call this process transduction.

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How does transduction work?

Taking sight as an example:

the physical phenomenon

a photon

the sensory organ

the sensory neurons

neural activity

Can you think of any other example of sensory organ?

The sensory neurons have specific proteins that transform photons into neural activity

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Taste

Sensory organ: tongue’s taste buds
Sensory cells: taste receptor cells

The sensory neurons have specific proteins that detect the presence of food molecules and turn it into neural activity.

A second example aimed at showing how the brain doesn’t perceive “reality” but only what the senses tell it, which is sometimes ‘wrong’.

Food molecules get in touch with taste receptor cells. Different taste receptor cells are sensitive to different types of molecules or stimuli, which ones are activated at a given time tells the brain about what’s going on in the mouth. What a cell is activate by depends by which types of taste receptor proteins it has (no need to explain in detail what they are).

An example of such protein is TRPV1 which is sensitive, among other things, to heat. When cells with this receptor are activated you experience hot and even pain. That is true even though what’s activating these receptors/cells is not hot but a specific molecule called capsaicin. WHen this molecule, found in spicy foods, is present you experience heat and pain even though nothing is actually “hot” in your mouth

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Taste

TRPV1 makes taste receptor cells fire when they are exposed to hot food.

However, capsaicin can activate TRPV1 receptor

When that happens you experience ‘hot’ and ‘pain’ even though nothing actually hot is in your mouth.

A second example aimed at showing how the brain doesn’t perceive “reality” but only what the senses tell it, which is sometimes ‘wrong’.

Food molecules get in touch with taste receptor cells. Different taste receptor cells are sensitive to different types of molecules or stimuli, which ones are activated at a given time tells the brain about what’s going on in the mouth. What a cell is activate by depends by which types of taste receptor proteins it has (no need to explain in detail what they are).

An example of such protein is TRPV1 which is sensitive, among other things, to heat. When cells with this receptor are activated you experience hot and even pain. That is true even though what’s activating these receptors/cells is not hot but a specific molecule called capsaicin. WHen this molecule, found in spicy foods, is present you experience heat and pain even though nothing is actually “hot” in your mouth

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The brain believes what it’s told

If TRPV1 gets activated you experience ‘hot’ when nothing hot is there.

If you rub your eyes you see something even though your eyes are closed.

What’s going on?

Our experience of the world happens in the brain, but the brain has no way to experience reality: it can only make sense of what the sensory organs are telling it.

Can you think of another example where something similar happens?

A second example aimed at showing how the brain doesn’t perceive “reality” but only what the senses tell it, which is sometimes ‘wrong’.

Food molecules get in touch with taste receptor cells. Different taste receptor cells are sensitive to different types of molecules or stimuli, which ones are activated at a given time tells the brain about what’s going on in the mouth. What a cell is activate by depends by which types of taste receptor proteins it has (no need to explain in detail what they are).

An example of such protein is TRPV1 which is sensitive, among other things, to heat. When cells with this receptor are activated you experience hot and even pain. That is true even though what’s activating these receptors/cells is not hot but a specific molecule called capsaicin. WHen this molecule, found in spicy foods, is present you experience heat and pain even though nothing is actually “hot” in your mouth

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People with synesthesia experience things differently:

when seeing a number they might hear a sound too

or

they might see a color when reading a letter

To them those colors look as real as this slide does to you: what we experience is not what’s out there, it’s what our brain makes of the neural inputs it receives

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Perception, beyond senses

What animal is this?

What about these shoes?

Perception: making sense of sensory stimuli.

Perception is hard: the brain makes use of cues to make sense of the world

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SUMMARY

Sensory organs translate information from the world around us into neural activity

Different species have evolved to sense different things depending on their environment