Neurotransmition, Neuroanatomy and Functional Localization

 

I.  EARLY HISTORY

1.     Phrenology: Assumed that the brain is made of functionally discrete modules

2.     Early psychosurgery: leucotomy, frontal lobotomy.  Moniz.  Render aggravated, aggressive, anxious patients calm, quiet.  Also deadened, flat, lifeless. 1940s – 20,000+ lobotomies

3.     First effective psychotropic drugs in the 1960s – psychosurgery largely abandoned. Suspicion over brain surgery as a treatment for mental illness

4.     Increasing knowledge of both mental illness and normal experience

a.      For example, no single pain center.

b.     Pain comes from activation of a constellation of areas associated with attention and emotion.  More stress, more pain.  Religious experience of spiritual transcendence, mystical presence, reproduced by stimulating certain parts of the temporal lobe.

 

II.  NEUROTRANSMITION

 

Brain Cells: [see image of neuron]

There are two main types of brain cells: neurons and neuroglia.

1.     Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system. Each neuron is composed of a cell body called a soma, a major fiber called an axon, and a system of branches called dendrites. Axons, also called nerve fibers, convey electrical signals away from the soma and can be up to 1 m (3.3 ft) in length.

2.     Most axons are covered with a protective sheath of myelin, a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.

3.     Neuroglial cells are twice as numerous as neurons and account for half of the brain's weight. Neuroglia (from glia, Greek for “glue”) provide structural support to the neurons. Neuroglial cells also form myelin, they guide developing neurons, take up chemicals involved in cell-to-cell communication, and contribute to the maintenance of the environment around neurons.

 

Synapse: [see image of synapse]

1.     Synaptic Sequence: Five Step Process

a.      A message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse.

b.     When it reaches the terminal end of a nerve cell, the impulse triggers tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters.

c.     The neurotransmitters float across the synapse, or gap between adjacent nerve cells.

d.     When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to “fire,” or generate an electric message-carrying impulse.

e.      As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic vessicles in a process called neurotransmitter reuptake.

 

III.  NEUROANATOMY

“The human brain is as big as a coconut, the shape of a walnut, the color of uncooked liver and the consistency of chilled butter.”  3 lbs., 100 billion neurons, glial cells

 

Cerebrum: [see image of ]

Most high-level brain functions take place in the cerebrum. Its two large hemispheres make up approximately 85 percent of the brain's weight.

1.     The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter.

2.     The convolutions are made up of ridge-like bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortex—about 16 ft2 in an adult—to fit within the cranium. The pattern of these convolutions is similar, although not identical, in all humans. The two cerebral hemispheres are partially separated from each other by a deep fold known as the longitudinal fissure.

3.     Each half of the cerebrum is divided into four major lobes: the frontal, parietal, temporal, and occipital lobes.  The insula, a fifth lobe, lies within the deep folds of the lateral sulcus.

4.     Each lobe processes its own group of things:

a.      Frontal lobe: the largest of the five and consists of all the cortex in front of the central sulcus. Broca's area, a part of the cortex related to speech, is located in the frontal lobe.  Concerned with thinking, conceptualization, planning (integrated functions).  Conscious appreciation of emotion.

b.     Parietal lobe: consists of the cortex behind the central sulcus to a sulcus near the back of the cerebrum known as the parieto-occipital sulcus. Parietal functions are connected with movement, calculation and some recognition.

c.     Occipital lobe: at the back.  Almost entirely visual processing areas.

d.     Temporal lobe: The temporal lobe is to the side of and below the lateral sulcus. Wernicke's area, a part of the cortex related to the understanding of language, is located in the temporal lobe.  Temporal lobe deals with sound, speech comprehension (usually on left only), some aspects of memory.

5.     Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering.

a.      Cortical regions known as associative cortex are responsible for integrating multiple inputs, processing the information, and carrying out complex responses.

b.     Communication between the two hemispheres is through several concentrated bundles of axons, called commissures, the largest of which is the corpus callosum. [see image of right hemisphere]

 

Limbic System: Mammalian Brain [see image of limbic system]

Beneath the cortex.  Contraption of tubes, chambers, lumps and blobs.  Each little unit has its own function, all are interconnected by criss-crossing axons. The limbic system is a group of brain structures that play a role in emotion, memory, and motivation.

 

1.     For example, electrical stimulation of the amygdala in laboratory animals can provoke fear, anger, and aggression.

2.     The thalamus and the hypothalamus lie underneath the cerebrum and connect it to the brain stem.

a.      The thalamus consists of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres.

b.     The thalamus is the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory input to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.

c.     The hypothalamus lies beneath the thalamus on the midline at the base of the brain. It regulates or is involved directly in the control of many of the body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity. It also controls the function of internal body organs by means of the autonomic nervous system, interacts closely with the pituitary gland, and helps coordinate activities of the brain stem.

1.     Brain injury can cause deregulation of limbic functions.  Four Fs

 

Brain Stem and Cerebellum: Reptilian Brain [see image of brain stem]

3.     The brain stem, shown here in cross section, is the lowest part of the brain. It serves as the path for messages traveling between the upper brain and spinal cord but is also the seat of basic and vital functions such as breathing, blood pressure, and heart rate, as well as reflexes like eye movement and vomiting.

4.     The brain stem has three main parts: the medulla, pons, and midbrain. A canal runs longitudinally through these structures carrying cerebrospinal fluid. Also distributed along its length is a network of cells, referred to as the reticular formation, that governs the state of alertness.

 

The Cerebellum is at the back of the main body of the brain, tucked under its tail, partly fused to it.  The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity depends on the cerebellum.

 

Neuroscience shining light on human qualities associated with the soul:

1.     Functional localization and will:  Research by Chris Frith in London identifies particularly areas of the brain involved in making choices; Phineas Gage (rail worker 19th C.) loses self-control after brain injury.