- The Structure of Neurons
- The Neural Impulse
- How Neurons Communicate
- The Nervous System
- The Endocrine System
- Older Brain Structures
- The Limbic System
- The Cerebrum
In this part, I have learned the biological foundation of psychology. The details are listed below, and the summary is provided at the end.
A neuron contains the following parts.
- The bush-like dendrites.
- The cell body.
- The lengthy axon.
- The myelin sheath.
The information pathway is as follows.
(the first neuron’s) dendrite $\rightarrow$ cell body $\rightarrow$ axon $\rightarrow$ (the next neuron’s) dendrites $\rightarrow\ldots$
In short, dendrites listens, the axons speak. And the myelin sheath can speed the impulses.
When stimulated by senses or chemical signals from neighboring neurons, a neuron fires an impulse, called the action potential, the latter travels down the axon.
Ions transmit along the selectively permeable axons. Positive and negative, if strong enough, flood in or out the surface of the axon’s membrane, resulting in the depolarization, and the travel of the pulse.
Two types of impulses exist, i.e., the excitatory and inhibitory signals. They are the accelerator and brake, respectively.
A neuron’s reaction manifests in an all-or-none fashion, this is how the ReLU function of Deep Learning comes. A strong stimulus cannot increase the intensity of the impulse, but the number of neurons to fire.
The key is the neurotransmitter. Neurons are not directly connected with each other. Instead, the axon from the last neuron and the dendrite from the next neuron are separated by a synaptic gap. It is the neurotransmitter that works as the messenger in-between.
When the action potential reaches the end of the axon, it triggers the release of neurotransmitters. The latter crosses the gap, precisely injecting into the keyhole – the receptor sites on the next neuron, leading to the electrical charge, so that the next neuron is either excited or inhibited. Finally, the excess neurotransmitters either drift away and decomposed or are absorbed by the sending neuron again (called reuptake). The flow is as below.
Pulse on the terminals $\rightarrow$ neurotransmitter $\rightarrow$ bind to receiving neuron $\rightarrow$ cause the electrical charge $\rightarrow$ emerge the excitation of inhibition $\rightarrow$ excesses drift away or are reabsorbed.
Agonist increases the production or release of neurotransmitters, or blocks the reuptake in the synapse. Agonist can also be very similar to a certain neurotransmitter and play the role of it directly.
Antagonists decrease the effect of neurotransmitters by block the receptor.
The nervous system consists of the central nervous system (CNS) and the peripheral nervous system (PNS). Nerves are formed of bundles of axons, linking the CNS with the body’s sensory receptors, muscles, and glands.
Information is carried through the sensory neurons or motor neurons. These two types of neurons are afferent and efferent, respectively.
A large component of the CNS is the neurons cluster, called neural networks.
The PNS contains the somatic nervous system and the autonomic nervous system (ANS). The former enables voluntary control of our skeletal muscles. The latter controls our glands and our internal organ muscles.
Furthermore, the ANS contains the sympathetic nervous system and the parasympathetic nervous system. The former arouses and expends energy, whereas the latter conserves energy. These two work together to maintain a status of homeostasis.
The CNS and PNS are connected by the spinal cord. A simple spinal reflex pathway will not pass through the CNS.
In addition to the electrochemical information system, the endocrine system’s glands secrete another form of chemical messengers, called the hormones.
The difference between hormones and neurotransmitters are twofold, i.e., speed and duration. Hormones travel slow yet outlast the effect, yet the neurotransmitters travel fast but have much narrower durations.
The pituitary gland is the most influential endocrine gland. The information through the endocrine system pass along brain $\rightarrow$ pituitary $\rightarrow$ other glands $\rightarrow$ hormones $\rightarrow$ body and brain.
Our brain has three major components, the brainstem, the cerebellum, and the cerebra. They are all involved in a number of processes, from low-level, autopilot, to high-level, volunteered. In this section, I will introduce the low-level parts, called the older brain structures. They are “older” because they perform much as they did for our distant ancestor, features as many basic survival functions. For example, no matter we are asleep or awake, the basic functions like heartbeat are active. Thanks to this autopilot our “newer” brain regions are freed to advanced activities like think and talk.
The brainstem is the oldest and innermost region of our brain. It is a crossover point where most nerves pass through from each side of the brain and body. Four components are important and listed as below (though the reticular formation and thalamus do not belong to the brainstem, I tend to put them together for better understandings).
- The medulla, controlling the heartbeat and breathing.
- The pons, coordinating movements and control sleep.
- The reticular formation, net-like tissue extending from the spinal cord right up through the thalamus. It enables arousal.
- The Thalamus, the main hub for sensory information to be received.
Note that everything above the brainstem usually appears in paired (from the thalamus above to the rest of things in this note).
The cerebellum is involved in the judgment of time, emotion modulation, and voluntary movement coordination. Without it, one cannot maintain balance.
The limbic system lies between the older and newer brain areas, a fun fact is that limbus means border. It contains three components as listed below.
- The amygdala, linked to aggression and fear. A person with amygdala lesions often becomes fear-free. A cat with its amygdala being stimulated will cower in terror.
- The hypothalamus, linked to bodily maintenance, influences hunger, thirst, body temperature, and sexual behavior. It maintains the homeostatic we mentioned before. It is remarkable that it also plays the role of reward centers.
- The hippocampus, which processes conscious, explicit memories. Lesions on it may make the forming new memories disabled.
The Cerebrum belongs to the “newer” brain structures. It contributes $95%$ of the brain’s weight. It is involved in specialized work including our perceiving thinking and speaking.
The cerebral cortex covers the hemispheres. It is the body’s ultimate control and information-processing center. Humans have much larger cerebral cortexes than other animals. It has four lobes.
- The frontal lobes, involved in speaking and muscle movements, plan-making, and judgments. The motor cortex locates here.
- The parietal lobes, receiving sensory input for touch and body position. The somatosensory cortex locates between frontal and parietal lobes, receiving the information of touch.
- The occipital lobes, receiving visual information. The visual cortex locates here.
- The temporal lobes, receiving auditory information. The auditory cortex locates here.
Apart from the four lobes locates the association areas, which are found in all four lobes. These areas have no observable response if being triggered electrically, however, they serve crucial purposes to our mind. They interpret, integrate, and act on sensory information and link it with memories. Humans have much larger association areas than other animals.
The association area in frontal lobes is called prefrontal cortex, it enables judgment, planning, and processing of new memories. Once damaged, one’s inhibitions may be removed, and the personality changed. The association area in the temporal lobe enables us to recognize faces.
Biologically, two systems are involved in our mind and behavior, the neural and hormonal systems. The neural system, from low- to high-level, from small- to large-scale, has the following components.
- Neurons with dendrites, cell bodies, axons, and neurotransmitters.
- Nervous system.
- Brainstem for automatic survial function
- Thalamus as sensory information hub
- Medulla for heartbeat and breathing
- Pons for movement and sleep
- Limbic System
- Amygdala for fear
- Hypothalamus for pleasure
- Hippocampus for memory
- Frontal lobes for motor
- Parietal lobes for touching
- Occipital lobes for visual
- Temporal lobes for auditory
- Association areas for high-level functions
- Spinal cord
- Sympathetic for arousing
- Parasympathetic for calming
- Sensory input
- Motor output
However, it is extremely crucial that most functions of our brain are not related to particular regions. The list above is just for the ease of memorization, for a humble computer science student like me. Keep in mind that complex mental functions do not reside in any one place. Our memory, language, emotion, etc., result from the synchronized activity among distinct brain areas and neural networks. Ditto for our love.
Our perception of moving flows not from the movement itself, but rather from our intention and the results we expected.
The brain has great plasticity. Brains are sculpted not only by our genes but also by our experiences. There are two facts.
- Severed brain structures usually do not regenerate, unlike skin.
- The function of the severed parts can be re-assigned to other brain areas.
Our brain is divided into two hemispheres. These two intrinsically have different functions. This property is called lateralization. The connection is realized by the corpus callosum, the wide band of axon fibers. If split, “one skull, two minds” can happen.