Sensory Mechanisms | slpneurology

WHAT IS SENSORY INFORMATION?

When we hear the word “senses” what do we think? See, smell, taste, touch, hear, etc. Little do we know that each of those senses have pathways to the brain to let us know what we are seeing, tasting, touching, etc.

There are these things called sensory receptors that our bodies have to receive sensory stimuli in our everyday lives. We have extroreceptors and proprioreceptors.

extroreceptors: Responsible for sight, sound, smell and touch

proprioreceptors: Respnsible for sensations on our skin, pain, movement, etc. Proprioreceptors can be broken down into interoreceptors (sensations from our main organs) and nociceptors (sensations like pain from an injury to the skin)

These receptors send information through one of these three pathways:

1. First order neuron: carries information from the grey matter of the spinal column

2. Second order neuron: these synapse with our first order neurons and carry impulses to the thalamus, and then crosses over to the contralateral (opposite) side of the nervous system

3. Third order neurons: these synapse with our second order neuron and carries the impulses from the thalamus to the somatosensory cortex, which can be found in the parietal lobe of the brain

TASTE

Another type of receptor is known a mechanoreceptors. These are found on our tongues! They send information to the rest of the oral structure that the tongue comes in contact with. These types of receptors can sense pressure.

HOW DO WE SEE??

Did you know that in the beginning of our sight process, everything is upside down!? Thank goodness for our optic nerve and sensory pathways, otherwise everything would be upside down!

Here is a step by step process of how we see:

Light rays from our environment travel to our rentinas
From the retina, the light rays travel to our optic nerve, which is cranial nerve II
From CN (II), the impulse travels through to the optic chiasm: this is located between the optic nerves and optic tracts
crossing over happens at the optic chiasm: this means that impulses can become ipsilateral (stay on the same side), or contralateral (move to opposite side).
after becoming either contralateral or ipsilateral, the impulses travel to the lateral geniculate body, which can be found in the thalamus
then the impulses travel to the superior colliculus, which can be found in the midbrain

After passing through each of those “stations,” the impulse ends up in the primary and associated visual cortices, which are located in our occipital lobe

Associated Visual Cortex: there are 5 parts (V1-V5)

V1 and V2 are the most simplistic: responsible for color, orientation and spatial frequency
V3 is responsible for integrating the information that is seen
V4 is responsible for object recognition and color perception
V5, which is most complex, is responsible for motion perception

HOW DO WE HEAR??

So here is our ear, but the only part we can see is the outside. Little do we know that there is a lot more to the ear that we cannot see!

Here is a step by step process on how we hear noises:

When we hear a noise, that acoustic signal gets “captured” by our pinna, which is the part of the ear we can see.
The pinna funnels the noise through the auditory canal, which is like a tube for the acoustic signal.
At the end of the canal is your eardrum, which is also known as the tympanic membrane. The signal makes the tympanic membrane vibrate (when that happens the energy changes from acoustic to mechanical).

Once the energy becomes mechanical it has officially reached the middle ear
The first stop of the mechanical energy in the middle ear is the ossicular chain (the only bones in the ear). There are three of them: malleus, incus and stapes. The mechanical signal forces these three bones to vibrate.
The last bone in the chain, which is the stapes, looks like a horseshoe and has a footplate that moves in and out of the oval window of the cochlea when it vibrates. This way the signal can get into the cochlea, which is part of the inner ear.
The cochlea almost looks like a snails shell. It has the organ of corti in it which consists of the basilar and tectorial membrane.
As the footplate of the stapes moves, it forces the tectorial membrane to move the fluids in the cochlea (these fluids are called endolymph and perilymph). Since the signal is moving through water, it changes from mechanical energy to hydraulic energy.

The tectorial membrane is also hovering over stereo cilia (hair cells) of the cochlea. The movement of the membrane forces the stereo cilia to bend back and forth, which opens calcium gates.
These “gates” release a neurotransmitter knows as glutamate, and action potential starts to occur. As these neurotransmitters are released the hydraulic energy becomes electrochemical energy.
The stereo cilia of the cochlea and the fibers of the VIII cranial nerve (Auditory) start to synapse.
This synapse then travels to the superior olivary complex. Here is when the synapse decides to travel ipsilaterally (same side) or contra laterally (opposite side). The SOC is important for time information and localization of sound.
Then the signal reaches the lateral lemniscus, travels to the inferior colliculus of the midbrain, and then through the medial geniculate body, which is found in the thalamus.
The thalamus then relays the signal’s information to the auditory cortex, which contains Heschel’s gyrus.
Heschel’s gyrus is important for localizing, perceiving and discriminating sounds that we hear.
The signal also travels to the auditory association cortex, which has a fancy name called Wernicke’s Area (BA 22). This part of our brain comprehends speech.

Here's a cool video on hearing and balance!

HOW DO WE KNOW WE CAN HEAR?

When we are first born, how do our moms and dads know that we can hear them speaking to us? There is a test known as an Auditory Brainstem Response, which is a test administered bt an Audiologist, that can be done when a baby is sleeping! This test can see how the auditory nerve responds to sounds. Electrodes are put on the head and behind the ear. It checks hearing at different pitches.

SLP528 Neurology

Guide to neurology for clinicians and students in speech-language pathology