7 May 2009

Synaptic Circuits in Marieb & Hoehn, Human Anatomy & Physiology

by Corry Shores
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Marieb & Hoehn

Human Anatomy & Physiology

Organization of Neurons:
Neuronal Pools

The millions of neurons in the CNS are organized into neuronal pools, functional groups of neurons that integrate incoming information received from receptors or different neuronal pools and then forward the processed information to other destinations. (421c)

Types of Circuits

The patterns of synaptic connections in neuronal pools are called circuits, and they determine the pool's functional capabilities.

In diverging circuits, an incoming fiber triggers responses in ever-increasing numbers of neurons farther and farther along in the circuit. Thus, diverging circuits are often amplifying circuits. Divergence can occur along a single pathway

or along several.

These circuits are common in both sensory and motor systems. For example impulses traveling from a single neuron of the brain can activate a hundred or more motor neurons in the spinal cord and, consequently, thousands of skeletal muscle fibres. (421d)

The pattern of converging circuits is opposite that of diverging circuits, but they too are common in both sensory and motor pathways. In a converging circuit, the pool receives inputs from several presynaptic neurons, and the circuit has a funneling, or concentrating, effect. Incoming stimuli may converge from one area

or from many different areas,

which results in strong stimulation or inhibition. The former condition helps explain how different types of sensory stimuli can have the same ultimate effect. For instance, seeing the smiling face of their infant, smelling the baby's freshly powdered skin, or hearing the baby gurgle can all trigger a flood of loving feelings in parents. (421-422)

In reverberating, or oscillating, circuits, the incoming signal travels through a chain of neurons, each of which makes collateral synapses with neurons in a previous part of the pathway.

As a result of the positive feedback, the impulses reverberate (are sent through the circuit again and again), giving a continuous output signal until one neuron in the circuit fails to fire. Reverberating circuits are involved in control of rhythmic activities, such as the sleep-wake cycle, breathing, and certain motor activities (such as arm swinging when walking). Some researchers believe that such circuits underlie short-term memory. Depending on the specific circuit, reverberating circuits may continue to oscillate for seconds, hours, or (in the case of the circuit controlling the rhythm of breathing) a lifetime. (422d)

In parallel after-discharge circuits, the incoming fiber stimulates several neurons arranged in parallel arrays that eventually stimulate a common output cell.

Impulses reach the output cell at different times, creating a burst of impulses called an after-discharge that lasts 15 ms or more after the initial input has ended. This type of circuit has no positive feedback, and once all the neurons have fired, circuit activity ends. Parallel after-discharge circuits may be involved in complex exacting types of mental processing.

Patterns of Neural Processing

Input processing is both serial and parallel. In serial processing, the input travels along one pathway to a specific destination. In parallel processing, the input travels along several different pathways to be integrated in different CNS regions. Each mode has unique advantages in the overall scheme of neural functioning, but as an information processor, the brain derives its power from its ability to process in parallel.

Serial Processing

In serial processing, the whole system works in a predictable all-or-nothing manner. One neuron stimulates the next, which stimulates the next, and so on, eventually causing a specific, anticipated response. The most clear-cut examples of serial processing are spinal reflexes, but straight-through sensory pathways from receptors to the brain are also examples. Because reflexes are the functional units of the nervous system, it is important that you understand them early on. (422-423)

Reflexes are rapid, automatic responses to stimuli, in which a particular stimulus always causes the same response. Reflex activity, which produces the simplest of behaviors is stereotyped and dependable. For example, jerking away your hand after touching a hot object is the norm, and an object approaching the eye triggers a blink. Reflexes occur over neural pathways called reflex arcs that have five essential components -- receptor, sensory neuron, CNS integration center, motor neuron, and effector. (423)

Parallel Processing

In parallel processing, inputs are segregated into many pathways, and information delivered by each pathway is dealt with simultaneously by different parts of the neural circuitry. For example smelling a pickle (the input) may cause you to remember pickling cucumbers on a farm; or it may remind you that you don't like pickles or that you must buy some at the market; or perhaps it will call to mind all these thoughts. For each person, parallel processing triggers some pathways that are unique. The same stimulus -- pickle smell, in our example -- promotes many responses beyond simple awareness of the smell. Parallel processing is not repetitious because the circuits do different things with the information, and each "channel" is decoded in relation to all the others to produce a total picture. (423)

Think, for example, about what happens when you step on something sharp while walking barefoot. The serially processed withdrawal reflex causes instantaneous removal of your injured foot from the sharp object (painful stimulus). At the same time, pain and pressure impulses are speeding up to the brain along parallel pathways that allow you to decide whether to simply rub the hurt spot to soothe it or to seek first aid. (423)

Parallel processing is extremely important for higher level mental functioning -- for putting the parts together to understand the whole. For example, you can recognize a dollar bill in a split second, a task that takes a serial-based computer a fairly long time. This is because you use parallel processing, which allows a single neuron to send information along several pathways instead of just one, so a large amount of information is processes much more quickly. (423a)

Marieb, Elaine N., & Katja Hoehn. Human Anatomy & Physiology. London: Pearson, 2007.

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