Make your own free website on Tripod.com

THE NERVOUS SYSTEM - INFORMATION PROCESSING AND

HIGHER ORDER FUNCTIONS

INTRODUCTION

1. Many of the principles applied to our analysis of spinal reflexes also apply to brain function.

NEUROTRANSMITTERS IN THE BRAIN

1. Neurons within the brain may use 30-50 different neurotransmitters and neuromodulators.

2. Important transmitters with widespread distribution include acetylcholine,norepinephrine, dopamine, serotonin, and GABA.

3. Glutamic acid, Substance P, enkephalins, endorphins, and dynorphin are important neurotransmitters and neuromodulators.

PRINCIPLES OF INFORMATION PROCESSING

1. There are usually several levels of processing along both sensory and motor pathways.

The Sensory and Motor Pathways

1. Pathwaysinclude the peripheral nerves, tracts, and nuclei that connect higher centers with the rest of the body.

Sensory Pathways

1. The posterior column pathway carries fine touch, pressure, vibration, and proprioception information. From the peripheral receptor the information proceeds to the nucleus gracilis (or cuneatus) of the medulla via the fasciculus gracilis (or cuneatus) of the spinal cord. After synapsing the information travels to the medial lemniscus, and after decussation reaches the ventral posterolateral nucleus of the thalamus. Projection fibers then relay the information to the primary sensory cortex of the cerebrum.

2. The spinothalamic pathway carries poorly localized touch, pressure, pain, and temperature information. Interneurons send the information to the opposite side of the spinal cord, to enter the anterior and lateral spinothalamic tracts . These tracts end in synapses at the ventral posterolateral nucleus, and projection fibers transmit the information to the primary sensory cortex.

3. The precise distribution of sensory information enables you to determine that a stimulus affected a specific portion of the body.

4. Most of the afferent sensory information gets processed within the brain stem or spinal cord and does not reach the cerebral cortex.

Motor Pathways

1. A motor homunculus can be plotted on the surface of the precentral gyrus.

2. The motor neurons are called pyramidal (basket) cells and their axons form the corticospinal and corticobulbar tracts.

3. The pyramidal system proceeds the length of the brain stem and spinal cord, via the cerebral peduncles, the pyramids, and the anterior and lateral corticospinal tracts. Most of the descending fibers decussate at the pyramids; the remainder cross over within the anterior white commissure before synapsing on motor neurons in the spinal cord.

4. The extrapyramidal system includes the red nucleus, the reticular formation, the vestibular nucleus, and the mesencephalic tectum. The motor commands enter the spinal cord as the vestibulospinal, tectospinal,rubrospinal, and reticulospinal tracts.

5. The cerebral nuclei are also part of the extrapyramidal system.

6. Gross intentional movements and patterns of motion are controlled by the caudate and putamen nuclei. The positional movements and associated alterations in muscle tone are mediated by the globus pallidus.

7. The cerebral nuclei exert their control by regulating the output of other extrapyramidal nuclei or by feedback control of the primary motor cortex.

8. The cerebellum processes sensory information and provides automatic fine-tuning to the motor outputs of the extrapyramidal and pyramidal systems.

Higher-Order Functions

The Asleep, Awake, Alert, and Attentive States

1. An electroencephalogramprovides a printed record of the electrical activity underway in the brain. The brain waves provide an indication of the ongoing level of activity.

2. Alpha waves are found under normal resting conditions, with the eyes closed.Beta waves appear during mentalconcentration. Theta waves are seen in the brains of children and in stressed adults. Delta waves are seen in infants, during deep sleep, and in certain disease states.

3. Activity in the cerebral cortex determines the state of consciousness.

4. An asleep individual is unconscious but can be aroused by normal sensory stimuli. In deep sleep the cerebral cortex has its lowest level of activity; REM sleep involves active dreaming and alterations in cardiovascular and respiratory rates.

5. Arousal involves a feedback interaction with the reticular activating system (RAS) headquartered in the mesencephalon. Activation of the thalamic portion of the RAS appears to focus and heighten attention on specific tasks or sensory stimuli.

Memory and Learning

1. Primary(short-term) memoriescontain small bits of information and they can be retained for brief periods with immediate access.

2. Long-term memories last from minutes to decades. Long-term memories are considered to be secondary if they fade with time and tertiary if they are permanent aspects of consciousness.

3. Learning involves the integration of memories and their use to direct or modify behaviors.

The Integration of Information

1. The general interpretive center integrates sensory information from all of the sensory association areas. Damage to this area decreases intellectual abilities and information processing.

2. The speech center regulates the necessary patterns of breathing and vocalization for normal speech.

3. The prefrontal cortex coordinates inputs from the secondary and special association areas.

4. The dominant hemisphere, usually the left, contains the general interpretive and speech centers. In addition to these centers, the two hemispheres show other functional differences.

5. In the disconnection syndrome the two hemispheres are separated by severing the corpus callosum. Initially the cerebral hemispheres function as independent entities, with different capabilities. Over time the symptoms disappear.

GO TO VISCERAL AUTONOMIC NERVOUS SYSTEM