This is from a science magazine I read recently…
Our ears usually pick up the first news of a threat (since our eyes see in limited directions at a time, and the ears hear 360 degrees). Sound is the movement of molecules through the air or another medium, like a wave in water. The curved outer portion of the ear, called the pinna or auricle, collects these sound waves and channels them down the ear canal.
The initial stage of the acute fear response – the “freeze” – allows us to assess the situation by stopping, watching and listening. At this point, the pupil dilates to let in additional light, and thus more data.
Light enters the eye and passes through the lens: a solid, crystalline mass just behind the pupil. The lens can change shape to better focus incoming light onto the retina, a thin sheet of neural tissue that lines the rear wall of the eye. There, special vision cells (cones and rods) general electric signals that travel into the brain.
The tympanic membrane, about the size of a pinky nail and commonly known as the eardrum, stretches across the end of the ear canal. Waves of molecules crash against this membrane. The eardrum reverberates under this pressure, shaking the three tiny bones of the middle ear; the ossicles. These bones, the smallest in our bodies, transmit the danger of the outside world to the inner ear and on to the soft inner tissues of the brain
The rattling ossicles of the middle ear jiggle fluid in the cochlea, a snail-shaped structure that houses the organ of Conti. The 15,000 special hair cells in this organ detect the vibrations in the fluid, and their resulting displacement yields electrical impulses that the cochlear nerve transmits to the brain.
The optic nerve, about 45mm long, conducts neural impulses from the eye to the processing centers in the brain. A bit of slack in the nerve allows the eyeball to swivel in its socket. The two optic nerves cross en route to the visual cortices at the rear of the brain. Routing visual information to opposite sides of the brain aids our sense of depth perception to help us locate an assailant.
Billions of neural cells connect all parts of the brain, funneling sensory information from the auditory and visual cortices to core regions for further analysis. Like computers, neurons link to become a network. Axons, the long lines hooking up one cell to the next, act like internet links. Short, bristly dendrites receive impulses from other neurons. Bundled together, neurons are the nerves that run throughout the body.
In milliseconds, molecules called neurotransmitters ferry electric signals from one neuron to the next. This mechanism sends news through the brain’s neural circuitry to quick-response regions such as the amygdala. Neurotransmitters are stored in little sacs called vesicles. Activated vesicles extrude through a neuron’s membrane, cross a tiny gap, and bind to receptors on the neighboring cells, thus relaying the neural impulse.
The neural signals from the ears and eyes are routed through the amygdala, often called the brain’s “fear center”. In this almond-shaped bundle of nerve cells, the ultimate decision is made as to whether the events outside the body are cause for temporary alertness or a full-fledged fear response. If the latter, the amygdala cues the needle-thin locus ceruleus in the brain stem to release gobs of the neurotransmitter norepinephrine, also known as noradrenaline. This chemical acts like a siren sounding throughout the brain.
And then we decide: flight or fight.
Isn’t that amazing? Fearfully and wonderfully made, indeed.