« Back to Table of Content || Next »

Chapter III. The Neurological Basis of Conscious Experience >> Introduction: The Human Brain

We are ready for the most exciting part of the book in which we delve into the brain in an attempt to learn what networks of neurons make us into thinking modern humans.

Introduction: The Human Brain

The human brain contains more than 100 billion neurons (that is about 15 times the number of all people living in the world). Each neuron can be linked to as many as 10,000 other neurons. To appreciate the complexity of the brain, imagine that each neuron is a person. Then imagine that nearly each person on the planet is talking to 10,000 other people. This is the conversation that we are trying to understand. The analogy goes further. Similar to people’s sensory receptors on the skin and in the internal organs, neurons have millions of receptors both on their surface and inside the cell. These receptors are sensitive to hundreds of chemicals that can affect a neuron’s communication, spur growth, or trigger apoptosis (controlled death). We cannot expect to describe the working of the brain in the same elegant format as ideal gas thermodynamics. Nor can we expect to develop a probabilistic description of the brain similar to quantum mechanics. The molecules of ideal gas are identical for the purposes of the theory. Quantum mechanics is able to reduce a complex system to elegant equations because electrons are identical to each other. Neurons, however, are all different. They have different receptors, different shapes, different connections, and different genes are turned on and off to manufacture different proteins. Every neuron in the brain is special. However, I am convinced that we can understand the rules of the conversations going on inside the brain, albeit in a simplified model. This chapter presents one such simplified model, which attempts to incorporate existing experimental observations and makes testable predictions for future experiments.

Schematic representation of a neuron.

Image of pyramidal neurons in cerebral cortex expressing green fluorescent protein.

Neurons are small cells with a cell body about 0.02 millimeters wide (the width of a human hair is 0.02 to 0.10 millimeters) and long branch-like projections that can be as long as one meter. These long projections allow neurons to exchange information over long distances. Neurons exchange information by firing action potentials, which are short 1 millisecond (0.001 seconds) spikes in voltage from a resting membrane potential of negative 70 millivolt (0.070 Volt) to positive 40 millivolt. When a neuron in the motor cortex fires an action potential, the muscle cell connected to the neuron twitches. Action potentials travel between the neuron and the muscle via a long branch of the neuron called an axon. Action potentials travel along the axon quite slowly. The speed varies from 1 kilometer per hour to 500 kilometers per hour (0.6 to 300 miles per hour). (Compare that to the speed of electrical transmission along a copper wire, which is about one trillion kilometers per hour.) One reason for the relatively slow transmission rates is that action potentials have to be amplified every millimeter or so. The amplification stations are located at the so-called Nodes of Ranvier. You can visualize an action potential traveling along an axon as a flame racing along a fuse, each Node of Ranvier igniting the next until the action potential reaches the end of the axon, referred to as the axon terminal.

The contact between an axon terminal and the target cell is called a synapse. When an action potential reaches the axon terminal, a chemical, called a neurotransmitter, is released from the axon terminal into the space between the axon terminal and the target cell. This neurotransmitter acts upon the target cell to evoke a response. If the target cell is a muscle, the response is a contraction of the muscle. If the target cell is a neuron, the response is a change of voltage in that neuron. The voltage change in response to activity at a single synapse is relatively small. Stimuli at multiple synapses are required to induce the neuron to fire an action potential. The process is analogous to voting. The synapses can vote for the action potential or against the action potential. (Recall that there are as many as 10,000 voting synapses on each neuron.) If enough synapses vote for the action potential, so that they depolarize the neuron from negative 70 millivolt to above the critical threshold, approximately negative 55 millivolt, the neuron will respond with an action potential. Once activated, the action potential travels along the axon all the way to axon terminals where it stimulates other neurons or muscle cells connected to this axon.

Schematic Action Potential. Source.

A signal propagates from one neuron along its long branch (an axon) to the cell body and dendrites of a second neuron. The contact between the axon and the second neuron is called a synapse. One synapse is marked by a white square and expanded in the right lower corner. When an action potential reaches a synapse, a chemical, called a neurotransmitter, is released from the axon into the synapse. These neurotransmitter molecules bind to the receptors located on the surface of the second neuron and evoke a response. A response can be a change in the membrane potential from negative 70 millivolt to negative 65 millivolt. Normally stimuli at multiple synapses are required to depolarize the neuron to negative 55 millivolt and thus to induce a neuron to fire an action potential.

The brain is divided into left and right cerebral hemispheres, which are interconnected by about 250 million axons laid out in a structure called the corpus callosum. There are two groups of specialized neurons deep inside the brain that we will refer to in the text: the thalamus and the hippocampus (pleural hippocampi). The thalamus is a relay station for a number of sensory signals including visual and auditory sensory information. The hippocampus is a group of specialized neurons essential for forming long-term memories of people, places and events. These long-term memories cannot form when the hippocampi are surgically removed from both hemispheres as was demonstrated in the seminal case of the patient “HM”. Since the age of 10, HM suffered from increasing epileptic seizures. Eventually the seizures became so intense and frequent that by the age of 27 his doctors suggested removing the parts of the brain that were thought to be responsible for his disorder. The surgical removal of both hippocampi and surrounding tissue stopped the debilitating seizures but induced a serious side-effect: an inability to form new memories of people, places and events and a partial retrograde amnesia. The operation, however, had no effect on early memories, personality, general intelligence, language abilities, or short-term memory (Scoville,1957; Smith & Kosslyn, 2007).

The thalamus and the hippocampi are structures deep inside the brain. They are symmetrically located in the right and left hemispheres.

The outermost, convoluted layer of each hemisphere (2–4 millimeters thick) is called the cerebral cortex. It contains a lot of neuronal cell bodies. (Neuronal cell bodies have a grey color in dead, preserved brains, hence, the name 'grey matter'. Under the layer of neuronal cell bodies lies a layer of axons that connect neurons to their target cells. This layer has a white color in preserved brains, hence, the name 'white matter'). The cerebral cortex plays a key role in voluntary muscle control, sensory perception, memory, and language. Neurons in the cortex are organized territorially based on their function. Motor neurons are located in the motor cortex. Neurons sensitive to touch are located in the somatosensory cortex. Neurons responsible for language comprehension are concentrated in Wernicke's area. Neurons responsible for language production are located in Broca's area. If you think of a brain as a company, then functional cortical areas correspond to different departments. Each department minds its own business. The departments often exchange information, however the bulk of the decisions is made within each department. In the brain, sending an action potential from one department to another takes approximately five milliseconds. Within each department, the shorter distance between neurons allows for faster communication: one to three milliseconds. Thus, the neuronal organization based on function reduces the distance between neurons in one department, and, consequently, allows for a faster decision making process within the department.

Neurons in the brain are organized territorially based on their function. Motor neurons that control muscle movement are located in the motor cortex (yellow). Neurons sensitive to touch are located in the somatosensory cortex (red). Language areas responsible for comprehension are located in Wernicke's area (violet). Language areas responsible for generating speech are located in Broca's area (blue).

When a disease such as stroke destroys one functional area of the cortex, the patient loses control over that function. Damage to the motor area results in loss of control over the part of the body controlled by the damaged cortex. Damage to the somatosensory area results in loss of sensations within the body part connected to the damaged cortex. Damage to Broca's area results in loss of productive speech (motor aphasia). Damage to Wernicke's area results in loss of auditory comprehension by the patient (sensory aphasia).

The visual system also consists of multiple departments. The primary visual cortex, V1, is located in the occipital lobe. It receives information from the retina via the lateral geniculate nucleus in the thalamus. The primary visual cortex is the first cortical area that receives visual information. Bilateral damage to the primary visual cortex results in blindness; this disorder is often referred to as cortical blindness, to distinguish it from retinal blindness.

From the primary visual cortex, the visual information is passed in two directions, or streams. In the first stream, the information flows from the primary visual cortex to the inferior temporal cortex. This stream includes the departments that deal with object recognition and form representation. These departments are concerned with what the object is. They include: V2, V4, PIT (posterior inferotemporal), CIT (central inferotemporal), and AIT (anterior inferotemporal). Due to the stream’s direction from the back of the brain towards the front of the brain, it is referred to as the ventral pathway from venter, the Latin word for the abdomen.

From the primary visual cortex (V1, shown in yellow), the visual information is passed in two streams. The neurons along the ventral stream (shown in purple) are primarily concerned with what the object is. The neurons along the dorsal stream (shown in green) are primarily concerned with where the object is.

The second stream deals with an object’s position in space. The departments along this stream process information about where the object is in space. These departments are concerned with an object’s motion, location, as well as the control of eyes and arms. For example, the perception of motion is the major function of the visual area V5, also known as visual area MT. Damage to V5 leads to a specific deficit in motion perception. Due to the stream’s direction from the back of the brain towards the parietal lobe, it is referred to as the dorsal pathway from dorsum, the Latin word for the back.

The primary visual cortex (V1, located in the occipital lobe) receives information from the retina via the lateral geniculate nucleus in the thalamus. The primary visual cortex is the first cortical area that receives visual information. There is a specialization hierarchy along the visual information pathways. The departments become more specialized the farther the information moves along the visual pathway. Neurons in the lateral geniculate nuclei can be activated by visual stimulation from either one eye or the other but not both eyes. They respond to any change in activity of the retinal neuron that they are connected to. Neurons in V1 can usually be activated by either eye. Neurons in V1 are sensitive to specific attributes, such as the orientation of line segments, color, and binocular disparity. Visual information is transmitted from V1 to cortical areas with greater specificity. Neurons in V4 respond selectively to aspects of visual stimuli critical to shape identification. Neurons in the inferior temporal lobe may respond only when an entire object (such as a face) is present within the visual field. The perception of motion is the major function of the visual area V5, also known as visual area MT.

There is a specialization hierarchy along the visual information pathways. The departments become more specialized the farther the information moves along the visual pathway. As mentioned above, images from the retina at the back of each eye are first channeled to the lateral geniculate nuclei in the thalamus. Neurons in the lateral geniculate nuclei can be activated by visual stimulation from either one eye or the other but not both eyes. They respond to any change in activity of the retinal neuron that they are connected to. From the lateral geniculate nuclei, visual information moves to the primary visual cortex (V1). Neurons in V1 can usually be activated by either eye. Neurons in V1 are sensitive to specific attributes, such as the orientation of line segments, color, and binocular disparity. Visual information is transmitted from V1 to cortical areas with greater specificity. Neurons in V4 respond selectively to aspects of visual stimuli critical to shape identification. Neurons in the inferior temporal lobe may respond only when an entire object (such as a face) is present within the visual field.

Long-term visual memory is stored in the cortex in the form of connections between neurons (synapses). A new memory is formed by changing the number and properties of millions of synapses. You can visualize the process of memory formation by thinking of a neuron as a tree. When making new synapses, the tree (neuron) grows its branches to reach other trees (neurons) in the forest (brain). While you are reading this book, your neurons are growing their branches and making millions of new connections.