Cerebral Cortices

Cerebral Cortex

The word cortex means “bark.”  Not bark like a dog but like the bark on that tree.  The cortex is the outer chocolate candy shell of the brain or the outer covering of gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses..  The hills are called gyri, or gyrus for the singular version.  The valleys are called sulci or sulcus for the singular.  These gyri all do something specific.  We can name them according to their location or we can name them according to what they do.  We can even do both!

Primary Motor Cortex

There is a sulcus that divides the brain into front and back or rostral and caudal sections.  This is aptly named the central sulcusA groove separating the frontal and parietal lobes..  The gyrus that is rostral or in front of the central sulcus is called the precentral gyrusThe area of the frontal lobe responsible for voluntary movements..  Guess what the gyrus behind or caudal to the central sulcus is called….the post central gyrus.   These are locational names.  But what do these gyri do?  What is their functional name? 

The precentral gyrus houses the primary motor cortex.  This is the last stop for outgoing information.  After all sensory information has been gathered, patterns of movementA fundamental property of life involving motion of the body or its parts. are determined. Everything is ready for descending the spinal cordThe central nervous system structure that relays signals between the brain and body.. It all stops at the primary motor cortex.

The primary motor cortex doesn’t actually determine anything.  It makes no decisions other than where to send the info. This includes the sciatic nerveThe largest nerve in the body, arising from the sacral plexus., the brachial nerve, etc. The primary motor cortex is attached to the corticospinal pathway.  This is the motor or descending pathway that transmits voluntary info to your skeletal muscles.  This cortex can also be referred to as the primary somatomotor cortex since it controlsVariables that remain constant to ensure a fair test. skeletal muscle.

Look at this lateralAway from the midline of the body. view of the brain up here with the primary motor cortex colored in green.  It’s like a headband.  Now, look at the picture at the bottom with the funny looking person on it.  This is how your primary motor cortex sees your body.  There are many muscles to control in the face and hands. They make very fine, very precise movements.  Arms and legs – not so precise.  The limbs have very large-scale or coarse movements compared to the face and hands.  This little man is called the motor homoculus.

Premotor Cortex

The information spends time in this cortex called the premotor cortex before it even goes to the primary motor cortex. It’s in the pre-precentral gyrus.  No, I’m kidding, it’s a general area in the frontal lobeThe brain region responsible for decision-making, personality, and voluntary movements.. 

The premotor cortex consolidates info from different parts of the brain.  It will take the info from the cerebellumThe brain region responsible for balance, coordination, and fine motor control. regarding balance. It will use the info from your occipital lobeThe brain region responsible for visual processing. about how far away that curb is. It will also gather other info to determine the output.  It’s this part of your brain that prevents you from tying your shoes as a child. 

You can understand the sequence of movements needed to tie your shoes once it develops a little more.  Playing an instrument, knitting, hitting a baseball, and a tons of other patterned movements are consolidated here. 

Primary Sensory Cortex

In the previous two slides, we saw how information exits the brain. It stops at the premotor cortex, then the primary motor cortex.  Caudal to the central sulcus is the postcentral gyrusThe area of the parietal lobe responsible for processing sensory input. I mentioned earlier.  This post central gyrus houses the primary somatosensory cortexA region in the postcentral gyrus that processes touch and sensation.. 

This is the cortex that would receive any incoming information from somatic tissues. It includes skeletal muscles, skinThe body’s largest organ, providing protection and regulation., and bones, but no squishy organs.  This cortex receives information about external temperature, pain, touch, and other general sensesSensory modalities distributed throughout the body: touch, pressure, temperature, pain, etc.. Most of this information is taken in by free nerve endingsUnencapsulated receptors that detect pain, temperature, and crude touch..  It’s a lot of different info. Just like the little man for how your brain sees your muscles, there is a homunculusA map of sensory or motor control in the brain, representing different body parts. for sensory input as well.  Again, your face and hands are prioritized so you can be very discrete as to where the sensation is.

If a dust particle lands on your eyelid, you can determine its position. It could be on the inside, middle, or outside of the eyelid.  But with the back and legs, the receptorA structure that detects stimuli. areas are really large and overlapping. This makes it hard to scratch someone’s back.

Info incoming to the primary sensory cortex is coming from peripheral tissues and up the ascending sensory spinothalamic tract.  The thalamusThe brain’s relay center, sending sensory information to the cerebral cortex. then determines exactly where in the sensory cortex the info should go and sends it there.  Remember that there is usually a unipolar sensory neuron taking in the sensory info. Then, another neuron brings it up the spinal tract. Finally, another neuron, the neuron of the third order, brings it from the thalamus to the primary sensory cortex.

Somatosensory Association Cortex

OK, so the primary sensory cortex says, “Your skin is hot.”  The somatosensory association areaA brain region that processes touch-related information. says, “It’s hot, I’m wearing a seater and standing in the sun.”  This area puts meaning to the sensation.  This is called perception.  Sensation and perception are two different things. 

This somatosensory cortex is located in the gyrus. It is posterior to the post central gyrus, just like that premotor cortex.  Let’s also highlight that premotor cortex right here.  So, I want to draw an abridged circuit for you.  Info comes in to the primary sensory cortex from the spinothalamic tract. Then, it goes to the somatosensory cortex. After that, it probably pings around the brain. Before it leaves, any output is consolidated at the premotor cortex. Finally, it goes to the primary motor cortex and descends the corticospinal pathway.

Special Senses’ Primary and Association Cortices

The somatosensory association cortex is for general senses, but the special senses all have their own cortices.  Most have a primary cortex and an association cortex, just as we have for the general senses.  Olfaction, or info from cranial nerve I and info concerning taste share an association area.  Visual information has its own primary and association cortices as does hearing.