Neural Communication and Brain Anatomy Fundamentals

Sensory deprivation, such as losing sight, necessitates a transition into a "whole different world" that requires significant practice.
In the absence of vision, individuals naturally begin to pay more attention to their other senses, which may functionally become more keen through use and focus.
Neuroplasticity allows the brain to adapt to unused sensory areas. For instance, in blind individuals, the visual area located at the back of the brain (the occipital lobe) is often innervated by other parts of the brain. This means other brain regions start wiring into the unused tissue to repurpose it.

Neural communication begins with specialized cells called neurons. A common example is the motor neuron, which typically originates in the motor cortex within the frontal lobe.
Motor neurons carry signals through the spinal cord and connect to muscles to facilitate movement, such as moving an arm, running, or throwing a ball.
The basic anatomy of a neuron includes:
- Cell Body (Soma): Derived from the Latin word for body, the soma contains the nucleus, which regulates the cell's activities.
- Dendrites: These are the reaching structures that receive signals from other neurons.
- Axon: Often described as the "tail" of the neuron, this is the pathway along which the electrical signal travels. Communication always proceeds from the axon of one neuron to the dendrite of another.
- Synapse: This is a microscopic gap between neurons measuring approximately billions of a meter ( $nm$ ). Despite this gap, for communicative purposes, the cells function as if they are nearly touching.
- Myelin Sheath: A layer of fatty tissue that coats the axons of motor neurons, resembling "hot dog buns." This sheath enables faster communication.
- Nodes: Open spots or gaps on the axon between the segments of the myelin sheath.

Saltatory Conduction: In myelinated neurons, the chemical message does not simply slide down the axon; it "hops" from one node to the next. This hopping motion makes neural transmission significantly faster.
Multiple Sclerosis (MS): This condition occurs when the immune system mistakenly attacks the body's own myelin. While there is no current cure, various treatments and immunoboosters are available today to manage the loss of motor activity associated with the disease.

The human brain contains approximately $86 \times 10^{9}$ neurons.
Humans do not possess the largest brains on the planet (elephants and sperm whales have larger brains by volume and weight), but humans have the largest brain relative to body size.
Neuron counts vary wildly across species, from animals with $1 \times 10^{6}$ or $200 \times 10^{6}$ neurons to others with $3 \times 10^{9}$ .
Glia Cells: These are non-neuronal brain cells that provide support. "Glia" comes from the Latin word for "glue." They provide nutrients, assist with myelin insulation, and hold the brain structure together.
Distribution of Neurons:
- The cerebellum alone contains about $70 \times 10^{9}$ neurons.
- The remaining parts of the brain account for approximately $16 \times 10^{9}$ neurons.

Dopamine:
- Often associated with the reward circuit and addictive behavior.
- It does not necessarily feel like "pleasure," but rather acts as a motivational signal that makes a person want to revisit an activity, such as playing a video game or eating.
- In the basal ganglia, dopamine is crucial for motor activity. A lack of dopamine in this area is involved in Parkinson’s disease and the presence of tremors.
Acetylcholine: Vital for learning, memory, and muscle contraction.
Serotonin: Regulates mood, hunger, and sleep.
- Antidepressants like Prozac and Paxil work by preventing the reuptake of serotonin to help manage depression and anxiety.
- Serotonin has an inverse relationship with cortisol (the stress hormone). When stress/cortisol increases, serotonin levels typically drop.
- Managing cortisol can lead to physical changes, such as the loss of belly fat.
- People often self-medicate with food; eating carbohydrates can help metabolize serotonin, providing a temporary mood boost.
Norepinephrine: Acts like adrenaline in the brain to control alertness and arousal.

Resting State: A neuron at rest maintains an electrical gradient of approximately $-70\,mA$ .
Firing: When activity reaches a "threshold of excitement," the neuron fires. This is an all-or-nothing process, similar to flushing a toilet or firing a gun.
Refractory Periods:
- Absolute Refractory Period: A period of a few milliseconds during which the neuron cannot fire again under any circumstances.
- Relative Refractory Period: A following period where the neuron can fire again, but only with sufficient stimulation.
Sensory Conversion: Every sensation—sight, sound, taste, touch, smell, and pain—is transmitted via action potentials. For example, lidocaine at a dentist's office works by canceling action potentials so the brain never receives the pain signal, even though the physical injury remains.
Without senses, the brain is effectively "locked in the skull" with no knowledge of the external world.

Peripheral Nervous System (PNS): Consists of all nerves emanating from the spinal cord, extending south of the neck (though some connections exist in the neck).
- Somatic Nervous System: Controls voluntary muscle movements.
- Autonomic Nervous System (ANS): Operates involuntary functions like heart rate, respiration, and digestion. It has two main divisions:
- Sympathetic Nervous System: The arousal system that turns on in response to stress or challenges.
- Parasympathetic Nervous System: Works to return the body to a state of equilibrium or homeostasis.
Constant activation of the sympathetic nervous system (hyper-vigilance) can lead to burnout or "nervous breakdowns."

The endocrine system is a network of glands that communicate using hormones released into the bloodstream.
Pituitary Gland: Known as the "master gland," it is located behind the bridge of the nose.
Comparison with the Nervous System: Neural communication is extremely fast. Glandular communication via hormones is slower to start but the effects are much longer-lasting.

Hindbrain (Brainstem): Includes the pons and the medulla. This area is essential for life support, controlling heart rate and respiration. It must be "on" at all times. Damage or tumors in this area are often inoperable because of its extreme delicacy.
Cerebral Cortex: The wrinkled, visible outer layer of the brain.
The Four Lobes:
- Occipital Lobe: Located at the back of the head; responsible for vision.
- Parietal Lobe: Responsible for the sense of touch.
- Frontal Lobe: Manages motor activities, language, and executive decision-making.
- Temporal Lobe: Responsible for hearing and memory.
Hemispheres: The brain is divided into left and right hemispheres. Generally, the left hemisphere controls the right side of the body, and the right hemisphere controls the left side of the body.
Subcortical Areas: Structures located underneath the cerebral cortex.