The Biology of Thought

A Neuronal Mechanism in the Generation of Thought - A New Molecular Model

Krishnagopal Dharani

• Éditeur: Academic Press
• Année: 2014
• ISBN:
  9780128011614
• Sujet: Scienze biologiche
• Sous-sujet: Neuroscienza

The question of " what is thought " has intrigued society for ages, yet it is still a puzzle how the human brain can produce a myriad of thoughts and can store seemingly endless memories. All we know is that sensations received from the outside world imprint some sort of molecular signatures in neurons – or perhaps synapses – for future retrieval. What are these molecular signatures, and how are they made? How are thoughts generated and stored in neurons? The Biology of Thought explores these issues and proposes a new molecular model that sheds light on the basis of human thought. Step-by-step it describes a new hypothesis for how thought is produced at the micro-level in the brain – right at the neuron.

Despite its many advances, the neurobiology field lacks a comprehensive explanation of the fundamental aspects of thought generation at the neuron level, and its relation to intelligence and memory. Derived from existing research in the field, this book attempts to lay biological foundations for this phenomenon through a novel mechanism termed the " Molecular-Grid Model " that may explain how biological electrochemical events occurring at the neuron interact to generate thoughts. The proposed molecular model is a testable hypothesis that hopes to change the way we understand critical brain function, and provides a starting point for major advances in this field that will be of interest to neuroscientists the world over.

• Written to provide a comprehensive coverage of the electro-chemical events that occur at the neuron and how they interact to generate thought
• Provides physiology-based chapters (functional anatomy, neuron physiology, memory) and the molecular mechanisms that may shape thought
• Contains a thorough description of the process by which neurons convert external stimuli to primary thoughts

Part I

Basic Concepts in Neurobiology

Outline

Chapter 1 Functional Anatomy of the Brain

Chapter 2 Physiology of the Neuron

Chapter 3 Memory

Chapter 1

Functional Anatomy of the Brain

Chapter 1 describes the anatomy of the human brain in relation to its functions. A general plan of the nervous system is presented, which shows how the sensory nervous system, motor system and autonomic system work, and how they are all functionally integrated. The structure of the brain is briefly described, keeping the reader abreast of the functional significance of the major anatomical structures of the brain. A brief account of the microscopic anatomy is also given. Some of the peculiar features of the brain, such as cortical neural connections and cortical asymmetry, are discussed. This information will be useful for the reader to navigate comfortably the succeeding chapters.

Keywords

Brain; sensory system; motor system; human cortex; diencephalon; basal ganglia

Overview

The human brain is the most sophisticated and complex product of biological evolution, and forms the structural basis of all human thoughts and actions. The human brain weighs about 1.5 kg – only three living species have brains larger than human beings, the elephant, the whale and the porpoise – however, in humans the weight ratio between the brain and the body far exceeds any of these.

The anatomy of the brain is better described in relation to its functions. Though there are significant advances in the understanding of human brain function, the knowledge is far from complete and the function of some significant portions of the human brain remains mysterious. The following description of the human brain may appear a little over-simplified, but it does serve as a useful guide for exploring the secrets of the brain.

General Plan of the Nervous System

Anatomical Considerations

The brain (Fig. 1.1) is the most important coordinating organ of the human body. It receives stimuli from the external environment and prepares the body to respond suitably to those stimuli. We will now study the types of nervous system and see how the complex human brain is structurally designed to receive the stimuli and to respond.

Figure 1.1 Lobes of brain.

Types of Nervous System

Nervous system is conventionally divided into central nervous system (CNS) and peripheral nervous system (PNS). The brain and the spinal cord constitute the CNS, whereas nerves and other structures outside the CNS constitute the PNS. The PNS collects information from the outside world and sends it to the CNS; the CNS processes this information, and sends it back to the PNS for appropriate action.

Functionally, the nervous system may also be divided into the following three categories:

Somatic nervous system

Autonomic nervous system

Enteric nervous system.

Somatic Nervous System

When a pin pricks the heel, for instance, the pain sensation is transmitted by the peripheral nerves to the spinal cord, upon which the spinal cord sends back signals through the nerves to the leg, instructing it to withdraw. This is an involuntary action. As you can see, there must be two types of nerves – one to transmit signals to the CNS, and the other to pass messages from the CNS to the PNS (see Fig. 2.1). The former is called the sensory system, which carries the information to the CNS; and the latter is called the motor system, which transmits the CNS commands to the periphery. Take another example: when you look at a beautiful rose (this is a sensory input reaching the brain) you may try to pick the flower (this is a motor action initiated by the brain and transmitted to peripheral nerves). This is a voluntary action. The system which controls both the voluntary and involuntary mechanisms is called the somatic nervous system, where there is a stimulating sensory input and a responding motor output. This looks simple enough.

Autonomic Nervous System

However, when you look at a horrifying scene, for example, your heart beats faster, you may sweat profusely, your hair stands on end and perhaps you may shudder – all this certainly without your conscious commands – the body’s system has actually worked automatically. This is executed by another type of integrated nervous system called the autonomic nervous system (ANS), which regulates the body functions without your knowledge. Several life-saving mechanisms in the body are controlled by this system – when a sprinter runs fast his heart rate increases, as does his respiratory rate to keep up with his muscle’s increased oxygen demands, and at the same time he sweats to regulate his body temperature – but for these adaptations, our sprinter would have been long since dead in his tracks.

Enteric Nervous System

There is another sort of nervous system called the enteric nervous system (ENS), which spreads along the gut and is not under the direct control of the CNS. When you eat, the food passes automatically down the esophagus and stomach to the bowels for digestion and absorption – this is coordinated by the ENS, and it is somewhat functionally integrated to the ANS. However, this system keeps functioning of its own accord, even without the supervision of the CNS and ANS; for example, even in deep coma (when CNS functions are interrupted) the gut still accepts food, digests it and propels it. Another example is that of the vagus nerves (nerves of the ANS to the gut) – even when they are cut surgically, as needed for duodenal ulcer therapy, the intestines function almost normally with the help of the ENS.

Types of Neurons

Sensory Neurons, Motor Neurons and Interneurons

In order to carry out the above functions, the nervous system employs three types of neurons (nerve cells; see Fig. 2.1). They are:

Sensory neurons

Motor neurons

Interneurons.

In order to recognize the sensations reaching the brain (such as light, sound, touch, etc.), a set of special neurons are required, called sensory neurons. So, to execute the desired actions (such as moving an arm or leg), another set of neurons called motor neurons are required. And to mediate between these two, a set of really versatile neurons called interneurons are needed. The interneurons need a special mention here. For instance, when a medical lab technician jabs a needle into your arm to collect blood, you do not withdraw your arm, right? At least not violently. In this case the sensations pass to the sensory neurons of the brain, and before they pass to the motor neurons they pass through the interneurons present in between the sensory and motor neurons, from where the message passes to different areas of the brain to carry out intelligent decisions before responding – they tell you that it is only a lab technician who has pricked you and there is no need to react. A needle prick in this case has caused the mind to make an elaborate analysis of the situation before allowing the body to act. However, the sensory neurons do send a little information to some motor neurons as well, and this may let you jerk your arm slightly.

Whereas if a needle pricks you unawares, it sends a shock wave across the body and you suddenly withdraw your arm. In this case the message passes to the sensory neurons of the spinal cord, which sends an instruction immediately to the adjacent motor neuron to withdraw the limb reflexly (spinal reflex). And there are interneurons in the spinal cord between sensory and motor neurons too, which send signals to many of the brain’s other neurons, letting your brain know that a needle has pricked you and perhaps producing a myriad other somatic and autonomic effects such as wincing and producing a few tears. However, most of the interneurons keep the signals in check before reaching the motor neurons, otherwise each and every sensation would invariably result in an action akin to a puppet show – which is all the more unpleasant and disturbing (see Chs 2 & 5 for more detail).

Functions of Nervous System

The functions of the human CNS can be summarized into the following five categories:

Sensory functions

Motor functions

Autonomic functions

Involuntary functions

Intelligent functions.

Sensory Functions

The sensory neurons can detect a wide variety of stimuli. The signals pass upwards to the brai...