The brainy brain: Oh the complexities of the human brain

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Okay, so let’s talk about the brain. First thing anyone needs to know is the 7 major regions of the brain:

1. Cerebrum

2. Cerebellum

3. Diencephalon

4. Basal Ganglia

5. Midbrain*

6. Pons*

7. Medulla Oblongata*

brain major regions

Notice the last three (5-7) are actually a part of the brainstem which ultimately becomes the spinal cord once it exits the foramen magnum, which is basically a hole in the occipital bone.

The basal ganglia isn’t as visible as the other regions when looking right at the brain model, to see it clearly you have to look at a transverse cut of the brain:

transverse cut of brain BASAL GANGLIA

Okay so before we can go into detail about why you should even care about any of this we have to go over a few more basic ideas so it can all come together in the big picture.

The brain has several coverings, called meninges which mainly serves as a protective covering for the amazing organ that basically makes you, YOU! In the brain we have:

1. Dura mater

2. Arachnoid mater

3. Pia mater

The dura mater is the most superficial layer of meninges and there are two layers of the dura mater. Dura mater literally translates as “tough mother” so we can safely assume this layer is the most durable layer in the meninges.

The arachnoid mater is the middle layer which forms a loose brain covering and is separated from the dura mater by a narrow cavity toward the subdural space. Beneath the arachnoid membrane is subarachnoid space. In this portion weblike extensions secure the arachnoid to the last layer, the pia mater.

The pia mater clings tightly to the brain and contains many tiny blood vessels to serve the brain.

So the next thing you need to know are the ventricles of the brain:

Ventricles of brain

So lets talk about what we’re looking at. At the superior part of the highlighted structure, we see two C shaped structures that are known as the Lateral ventricles (ventricles 1 and 2) they are separated by the septum pellucidum and are located in each of the cerebral hemispheres. These two ventricles connect to the interventricular foramen, which connects the lateral ventricles to each other and to the third ventricle.  As you can see, the third ventricle is located medially to the lateral ventricles and the cerebral aqueduct (which is just inferior to the third ventricle) connects the third to the fourth ventricle. The fourth ventricle, the most inferior of the ventricles is located between the pons and the cerebellum and median and lateral apertures connect the fourth ventricle to the subarachnoid space that surrounds the brain.

So now that we know all of that we can talk about the Cerebrospinal fluid. This is another protective aspect of the brain and it flows through the brain via the ventricles.

Starting at the lateral ventricles and working its way to the interventricular foramen (which we learned connects the lateral ventricles to each other and the third ventricle) so it seems natural that the next location the CSF would flow to would be the third ventricle which then makes its way to the, you guessed it, cerebral aqueduct which of course goes to the fourth ventricle. At this point it can go one of three ways 1) through the central canal, 2) through the medial aperture, or 3) the lateral aperture.The medial and lateral apertures go to the subarachnoid space then to the arachnoid villi and then the venous sinuses (blood)

Lateral ventricles –> interventricular foramen –> third ventricle –> cerebral aqueduct –> fourth ventrical –> medial or lateral apertures –> subarachnoid space –> arachnoid villi –> venous sinuses (blood)

Cerebrospinal fluid is created by the ependymal cells of the choroid plexus which filter blood plasma and use pumps to get the correct ion concentrations.

So now we move on to the major lobes, which are different from the major regions and are all located in the cerebrum:

1. Frontal

2. Parietal

3. Temporal

4. Occipital

major regions cerebrum

Birds eye view flow chart (excludes temporal bone)

Frontal lobe –> precentral gyrus –> central sulcus –> post central gyrus –> parietal lobe –> parieto-occipital sulcus –> occipital lobe –> transverse cerebral fissure –> cerebellum –> pons –> medulla oblongata

Fissures:

The longitundinal fissure separates the two cerebral hemispheres

Central sulcus separates the frontal lobe from the parietal lobe

Lateral sulcus separates the temporal lobe with the parietal and frontal lobes.

 

Functional areas of the cerebral cortex:

1. Primary motor cortex- located in the precentral gyrus and controls the voluntary movement of the skeletal muscles

2. Premotor cortex- located in the frontal lobe just anterior to the precentral gyrus, its responsible for repetitive movements and plans movements this is the area of the brain responsible for typing, playing a musical instrument, etc.

3. Broca’s area- unilateral, usually on the left cerebral cortex which lies anterior to the inferior region of the premotor cortex which controls the movements necessary to speak.

4. Primary somatosensory area- located in the postcentral gyrus and is responsible for spatial discrimination which means its identifies the region of the body being stimulated.

5. Sensory association cortex- posterior to the somatosensory area and integrates sensory input (temperature, pressure, etc) relayed to it via the primary somatosensory cortex, this is what allows us to figure out what things are without your sight (feeling for your keys in your purse)

6. Primary visual cortex- located at the posterior tip of the occipital lobe which recieves and processes visual inputs from the retina

7. Association area- covers much of the occipital lobe and receives and processes visual inputs, this is what puts all the info from the visual cortex together.

8. Primary auditory cortex- located in the temporal lobe and receives and processes sound

9. Association cortex-  located in the temporal lobe and receives and processes sounds received from the auditory cortex

10. Prefrontal cortex- anterior portion of frontal lobe which is responsible for our intellect, cognition, memory and personality

So now lets talk about lateralization and cerebral dominance.

Lateralization

Left: language, math and logic

Right: visual spacial skills, intuition, emotion, artistic skills, musical skills and creativity

Cerebral is determined by the hemisphere that is dominant for language which is usually the left and which has greater control over language, math and logic.

 

Commissural Fibers connect the hemispheres to each other and run horizontally ex/ corpus callosum

Association fibers connect different parts of the same hemisphere and run horizontally

Projection fibers connect the cerebral cortex to the lower brain and spinal cord, may carry impulses to the lower brain and spinal cord and run vertically

Diencephalon and subdivisions

Location of diencephalon- surrounds the third ventricle

subdivisions of the diencephalon are the thalamus, hypothalamus, and epithalamus

The thalamus receives and integrates sensory input and sends the information to the cerebral cortex

Hypothalamus is the main visceral control center of the body

epithalamus includes the pineal gland (endocrine)

 

Major regions of brain stem

Midbrain coordinates the head and eye movements with visual tracking of an object. Its linked to the basal ganglia and contains motor tracts on their way to the spinal cord.

Pons acts as a relay station between the motor cortex and the cerebellum it contains neurons from higher centers on their way to the spinal cord

Medulla oblongata works where motor tracts cross over, the right side controlling the left side and the left controlling the right. This contains important visceral motor nuclei which control hiccuping, vomiting, swallowing, coughing and sneezing as well as cardiovascular and respiratory functions

 

Cerebellum coordinates sensory input from the motor cortex to produce smooth coordinated movement, its also responsible for predicting sequences of events to produce coordinate movement, word association and puzzle solving, it consists of 2 hemispheres the superficial being gray matter and the deep layer white, located posterior to the pons and medulla.

 

Limbic system:

its located on the medial aspect of each cerebral hemisphere and diencephalon includes the hippocampus, amygdala and other structures. This is the emotional center of the brain, although the amygdala and hippocampus play a role in memory as well.

Muscular System

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Muscle tissue

There are three  basic kinds of muscle tissue: Smooth, Skeletal and Cardiac. In the table below you can see some differences and similarities in the tissues.

type involuntary/voluntary striated vs smooth function location
Skeletal voluntary Striated Moves skeleton Skeleton
Smooth involuntary smooth propel substances hollow and tubular organs
cardiac involuntary striated pump blood heart

 

Now looking deeper into each tissue will give us a better understanding:

Skeletal muscle:

For each muscle there is-

1 nerve

1 artery

1-2 veins serving it.

These muscles have a rich blood supply due to its high demand of oxygen and glucose as well as its high waste products of carbon dioxide and lactic acid.

Organization of Skeletal muscle:

image

As you can see at the microscopic level we have the muscle fiber which is wrapped by the endomysium, then groups of muscle fibers create fasicles which are then wrapped by perimysium and groups of fasicles are brought together to create muscle which is then wrapped by epimysium.

 

Now lets take a closer look at the muscle fiber. Muscle fiber is just a fancy way of saying a singular muscle cell. For some reason, muscles have all sorts of substitute names for things we find in most cells. Lets break those down now:

Muscle fiber = muscle cell

Sarco = Muscle

Sarcolemma = plasma membrane

Sarcoplasm = Cytoplasm

Sarcoplasmic reticulum = endoplasmic reticulum

Why change the names?

Recall the meaning of the prefix cyto to mean ‘cell’? Well, the prefix sarco means muscle.

So now lets discuss the skeletal muscle fiber.

Unlike a stereotypical cell, the skeletal muscle fiber has about 100 or more nuclei and instead of being centrally located, they’re located peripheral to the sarcolemma.

Now in the sarcoplasm which is the cytoplasm of the muscle fiber we have several components:

Glycosomes store glycogen for the fiber

Myoglobin stores the oxygen

Myofibrils contain the contractile elements of the fiber and there are several different types of myofibrils:

Thick myofilaments contain myosin which acts as a cross bridge to thin myofilaments

Thin myofilaments which contain actin which is the contractile protein that forms a helix

regulatory proteins are troponin and tropomyosin

elastic filaments are composed of titan

Sarcomere is the smallest functional unit of muscle it is what causes the striations we see in skeletal muscle.

image

Sarcoplasmic reticulum aka endoplasmic reticulum stores the calcium of the fiber and releases calcium when the fiber receives the signal to contract. It also contains the terminal cisternae which acts as a reservoir.

Sarcolemma aka plasma membrane transmits the signal to contract and T-tubules which are the in folding’s of the sarcolemma that reach every myofilament.

Now lets discuss the bigger question: How on earth do all these components make our muscles move everyday?

The answer is simple but also complex. Lets break it down into a couple of charts:

image

here we have the Neuromuscular junction. Sounds pretty complex but its actually pretty simple because all of the six steps are intertwined. If the action potential never arrived at the axon terminal, voltage gated calcium channels would have never opened and the calcium wouldn’t be able to flood into the axon terminal which would make it impossible for acetylcholine to be released. And that just brings us to the whole mess of activating the receptors on the sarcolemma, which would allow for sodium to get into the cell and potassium to get out which furthers the process of an action potential for the muscle fiber! Basically, what I’m trying to say is, the neuromuscular junction is important because it allows the fiber to initiate an action potential which is what causes contraction which brings me to…………….

The excitation contraction coupling which is how the action potential in the sarcolemma leads to the sliding of the filaments which occurs in the t tubules in the sarcolemma.

image

Okay so lets talk about what’s happening in these pictures:

Step 1: the action potential propagates along the sarcolemma and down T-tubules

Step 2: calcium ions are released from the t tubules

Step 3:  Calcium travels down the t tubule and binds to troponin ( this is the regulatory protein which blocks actin) this action removes the blocking action of tropomyosin on actin.

Step 4: Contraction is then able to begin as myosin binds to actin and forms a cross bridge.

Which then brings me to…. The cross bridge cycle:

image

Step 1 is known as Cross Bridge Formation and what happens here is the myosin head attaches to actin myofilament thus forming the cross bridge.

image

Step 2 aka Power/ working stroke

3 things happen:

1. ADP and inorganic phosphate are released

2. myosin head pivots and bends

3.myosin head pulls the actin filaments toward the M-line (middle of the sarcomere)

image

Step 3 aka cross bridge detachment:

Myosin and actin link weakens and the myosin head detaches when ATP binds to myosin.

image

Step 4 aka Cocking of myosin head:

ATP is hydrolyzed to ADP and inorganic phosphate and the myosin head returns to high energy position.

 

In contrast lets discuss the components of a smooth muscle fiber:

As we saw in the chart, smooth muscle is found in the respiratory, digestive, urinary, reproductive tracts, arteries and veins.

It’s functions are to mix and propel contents of lumen of an organ which is commonly known as peristalsis. It also increases and decreases the diameter of airways and blood vessels which allows for blood and air flow.

Unlike Skeletal muscle, smooth muscle does not have any sarcomeres present which means its has no striations, hence the name smooth muscle.

Delving deeper into the smooth muscle down to the cellular level we find that smooth muscle fibers:

contain one central nucleus

have thick and thin filaments but unlike skeletal muscle, its disorganized and not arranged in sarcomeres.

On an organizational level the smooth muscle fiber is organized into layers:

Longitudinal layer– along the longest axis of the organ

circular layer– encircle the organ

oblique layer– not always present

 

Now how does it contract?

It contracts by way of sliding filaments, just like skeletal muscle.

 

Comparing and contrasting skeletal and smooth muscle:

Feature Skeletal Smooth
Location Skeleton – attached to bones Hollow and tubular organs
Voluntary vs. involuntary voluntary involuntary
Regulators Motor neurons (nervous system) Nervous system, hormones, intrinsic nervous system, stretching
neurotransmitters acetylcholine acetylcholine
norepinephrine
microscopic features > 100 nuclei (peripheral)
striated
cylindrical
1 nucleus (central)
smooth
spindle shaped
Sarcomeres? Yes No (scattered thick and thin filaments
Neuromuscular junction/ diffuse function Neuromuscular junction (highly structured) Diffuse junction (poorly structured)
Function Movement mix and propel contents of a body cavity
regulate airflow
changes diameter to regulate blood flow
connective tissues epimysium (muscle)
perimysium (fasicle)
endomysium (fiber)
endomysium
regulatory proteins troponin calmoduim
     

Articulations

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What’s an articulation?

it’s a joint, which is a site where two or more bones meet.

Now, joints are classified in two ways:

1. By structure

2. By function

Structure Fibrous Cartilaginous Synovial
Function Synarthrosis Amphiarthrosis Diarthrosis

These categories interrelate in the sense that:

Fibrous joints are synarthrotic, meaning they are generally immovable joints

Cartilaginous joints are generally amphiarthrotic which is defined as a semi movable joint

Synovial joints are characterized as diarthrotic which means they are freely moveable.

Now lets get into detail:

Fibrous joints

Fibrous joints are made of dense fibrous connective tissue and there is no joint cavity present. There are three different types of fibrous joints:

1. Suture

2. Syndesmosis

3. Gomphosis

 

Moving onto Cartilaginous joints:

Much like the synovial joints, cartilaginous joints don’t have a joint cavity. Instead of dense fibrous connective tissue, cartilaginous joints are comprised of cartilage which connects articulating bones. These types of joints are not highly moveable and are usually classified as amphiarthrotic. There are two different types of cartilaginous joints:

1. Synchondrosis:

2. Symphysis:

Lastly, Synovial joints are defined as articulating bones separated by a fluid containing joint cavity. This is what sets synovial joints apart from cartilaginous joints and fibrous joints. Additionally, synovial joints are diarthrotic which means they are freely movable.

The components of a typical synovial joint include:

articular cartilage- lines the ends of bones in a joint

articular capsule-  encloses the joint cavity of a synovial joint.

synovial fluid- lubricates the joint surfaces and nourishes articular cartilages.

synovial cavity

reinforcing ligaments

nerve endings

Stabilizing factors:

1. Deep sockets

2. Close fit of articular surfaces

3. More ligaments

4. Muscle tone

Structures of the synovial joints:

Function Structure
Bursae act as cushion or buffer flattened fibrous sacs filled with synovial fluid
Tendon Sheaths reduce friction elongated tubelike bursae which wraps around tendons
Articular fat pads act as a cushion between fibrous capsule and synovial membrane or bone
Menisci Separate articular surfaces fibrocartilage pads

And there you have it. Joints and articulations in a nutshell, hope you enjoy 🙂