Neural Pathways

All Invertebrates have a simple nervous system. Most of them have few neurons that are arranged in nets. In some invertebrates, there are aggregations of neurons. The nervous system has evolved in vertebrates especially mammals.

In mammals, the nervous system is specialized because of the complex network of neurons and their communication through neural pathways.

What are the Neural pathways?

As the name indicates, Neural: related to nervous system Pathway: passage. So the neural pathway is a passage between the nervous system. An area of the nervous system is connected to another area by neural pathways. So, neural pathways communicate information from one area to another area of the nervous system. A neuron is made up of:

1. Cell
   body
2. Axon
   (away from the cell body)
3. Dendrite
   (towards the cell body)

As axon carries impulses away from the cell body, the major bulk of neural pathways is composed of axons. These connections via axons either axons are single or in the bundle are called nerve tracts.

Short neural pathways are present in grey
matter. Long neural pathways are present in white matter. (1)

How Neural Pathways are Discovered?

Neural pathways were discovered on the brain of cadaver. These pathways were discovered during the examination of the brain as these pathways are large and long. They are easily identifiable on macroscopic examination of the brain.

For example, corpus callosum can be seen while dissecting the brain of a cadaver. It communicates information between two cerebral hemispheres. Axons which forms neural pathways are of two types,

1. Myelinated
   axons
2. Unmyelinated
   axons

Myelinated
axons make the pathways appear bright because myelin contains fat content.
Unmyelinated axons make the pathway appear grey because they don’t contain any
fat content.

How Neural Pathways are Named?

During the early days when anatomy was not much advanced, neural pathways are named based on their macroscopic appearance. For example cerebral peduncle, in which peduncle refers to the stalk at the end of which fruit is attached.

This name tells us about the shape of this pathway. As anatomy advances, there is precision in naming pathways. Pathway name should determine either function, location or specific characteristics.

Dorsal column medial lemniscus pathway is an example of such a naming system. (2)

Types of Neural Pathways

There are many types of neural pathways from the simple one to way too complex. Some important neural pathways are monosynaptic reflexes, corpus callosum, reward pathway, pain pathway, visual pathway, dorsal column medial lemniscus pathway, pyramidal tracts, cerebral peduncles, etc.

Monosynaptic reflex

One of the simplest neural pathways is the mono-synaptic
reflex pathway.

Only two neurons are involved in this pathway. The sensory component(sensory neuron) of the pathway has its receptors on muscles and motor component(motor neuron) is present in the spinal cord.

Knee jerk reflex is a classic example of this pathway. This reflex is produced when we stimulate third, fourth and fifth lumbar nerves.

Knee jerk reflex can
be observed by tapping patellar tendon which causes contraction of the
quadriceps muscle. This process does not involve higher centers of the brain.
Another similar reflex is Achilles reflex.

These reflexes are clinically significant as
just by testing them, we can identify lesions affecting areas of the spinal
cord. (3)

Corpus Callosum

Corpus callosum is a collection of nerve fibers through which the right and left sides of the brain are connected. It is the largest collection of a bundle of nerve fibers in the human body.

It is made up of commissural fibers. Besides communication between hemispheres, it has a significant role in eye movements and eyesight by adding the visual inputs from each cerebral hemisphere. It helps us to focus by improving our attentive behavior and maintains balance or equilibrium of the body.

It helps a person to locate sensations of pain, touch, and pressure on a particular area of the body.

Any damage or defect to corpus callosum can result in impairment of behavior as coordination is lost between the right and left halves of the brain. It can lead to disorder "Split Brain Syndrome"

Visual pathway

In the visual pathway, many cells and synapses are involved. In this pathway, visual images are carried from different parts of the eye to the brain. The optic nerve is the main component of the visual pathway.

Photoreceptors change light energy to nerve impulse which goes to a bipolar cell. From bipolar cells impulse is transferred to amacrine cell. These impulses terminate on ganglia. This whole process takes place on the retina. From here, the role of optic nerve starts which carries information from retina to brain via optic chiasma.

Hence, information is finally transferred to the visual cortex which is the control center for vision. The visual cortex is located in the occipital lobe of the brain. Different types of defects in the visual field occur when brain lesions involve any part of the visual pathway. Examples of defects are temporal hemianopia, homonymous hemianopia, heteronymous hemianopia, etc.(4)

Subcortical lesions affect visual pathways. Any defect in the visual pathway can be an indication of the subcortical lesion. The visual pathway is not affected by cortical lesions.

Reward pathway

When a stimulus is presented to the body that is considered rewarding in the human dictionary, the brain raises the concentration of dopamine via dopamine neural pathways in the brain.

There are many dopamine pathways but the reward center is mainly mediated by the mesolimbic pathway.

Pain
pathway

The most common complaint of patients is pain.
To relieve pain, we need to understand its pathway. Sensations of pain are
perceived by A-delta and c fibers. These sensations are carried to the brain
through the pain pathway.

Cerebral
peduncle

Cerebral
peduncles originate from the cerebellum, cerebral cortex and spinal cord
of the nervous system. There are three types of the cerebral peduncle.

1. Superior cerebral peduncle
2. Middle cerebral peduncle
3. Inferior cerebral peduncle

They are very important as they serve as a passage for motor and sensory neural tracts. For example, corticospinal tracts and corticobulbar tracts. These tracts arise from the cerebrum and end at pons.

The majority of the midbrain comes under the heading of the cerebral peduncle. The cerebral peduncle plays an important role in achieving accuracy in fine motor skills, learning of new motor skills, and maintaining equilibrium.

Damage to the cerebral peduncles results in
various diseases like multiple sclerosis, Herpes encephalitis,
leukoencephalopathy, neurofibromatosis, and infarction, etc.

Pyramidal
tracts

Pyramidal tracts are upper motor neurons. They arise from the cerebral cortex and end either at the level of the brainstem thus forming corticospinal tract or pathway. They can also end at brainstem thus forming corticobulbar tract or pathway.

The corticospinal tract is involved in motor functions of the body. This tract innervates muscles of the opposite side of the body. Corticobulbar tract controls movements of muscles of the head. For example, swallowing and smiling, etc.

 If there is an injury to the
corticospinal tract by hemorrhage, stroke, disc slip or any lesion, upper motor
neuron syndrome occurs. This syndrome is characterized by hyperactive

Reflexes, loss of fine movements and positive Babinski
sign.

Injury to the corticobulbar tract can affect movements that involve lower face like speaking. (5)

Dorsal Column Medial Lemniscus pathway

The dorsal column–medial lemniscus pathway is an important sensory neural pathway of the nervous system. Another name for this pathway is the PCML pathway. It helps a person to locate the difference between two points, we can preceive touch and vibration sensation.

It is because of the Dorsal column medial lemniscus pathway that we can perceive the position of the body. This system has derived its name from two structures

1. A posterior column of spinal cord
2.   Medial lemniscus of the brainstem

This pathway works on three types of neurons.

1. First-order
   neurons
2. Second-order
   neurons 
3. Third-order
   neurons

Dysfunction
of neural pathways

Millions of people suffer from dysfunction of neural pathways. These impairments of neural pathways occur from stroke, brain lesions, or any injury to the spinal cord. These pathways are also disrupted in some diseases like multiple sclerosis, Parkinson's disease, and cerebral palsy.

As a result, the patient faces difficulty in performing routine tasks. There is a loss of sensation of pain, touch, temperature depending on the pathway involved. Sometimes cranial nerves are affected and as result muscles are affected.

For example in facial palsy, muscles of facial expression are involved. Medicine, surgery, and physiotherapy improve symptoms. Full recovery is a bit difficult as neurons can’t regenerate.

The
Bottom Line

The nervous system controls our body via communication through neural pathways.
Based on our goals, desires, and habits, the brain tries to modify these pathways.

For example, pathways involved in memory can be improved and extended via multiple repetitions. On a daily basis, we train our brain to create a new pathway.We repeat an action over and over again until it is defined in the brain as a new pathway.

This pathway then becomes part of our brain. According to research, it takes twenty-one days to make or break a habit. This habit formation is dependent on training our brain to create new neural pathways. Complex behaviors like driving a car is an example of new neural pathways formed.

As doctors, we can guide our patients to help
them make new pathways in the brain so they can achieve their health-related
goals. (6)

References

1. https://en.wikipedia.org/wiki/Neural_pathway
2. https://www.ncbi.nlm.nih.gov/pubmed/20358106
3. https://courses.lumenlearning.com/boundless-ap/chapter/reflexes/4.                              https://www.sciencedirect.com/topics/neuroscience/visual-pathway
4. https://www.sciencedirect.com/topics/medicine-and-dentistry/pyramidal-tract
5. https://www.forbes.com/sites/vanessaloder/2015/03/18/how-to-rewire-your-brain-for-happiness/#1897bfb759ef