Information Processing by Neuronal Networks

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The functions of the nervous system can be understood in terms of neuronal networks. In this section we will use two subsystems of the nervous system to demonstrate the functioning of such networks. The first example, the autonomic nervous system, consists of efferent pathways; the second, the visual system, consists of afferent and integrative pathways. Techniques that have allowed neurobiologists to trace neuronal connections, identify neurotransmitters at synapses, and record action potentials in single cells and groups of cells have advanced our understanding of how certain subsystems of the nervous system work.

The autonomic nervous system controls the physiological functions of organs and organ systems

The autonomic nervous system is divided into two parts: the sympathetic and parasympathetic divisions. These two divisions work in opposition to each other in their effects on most organs, one causing an increase in activity and the other causing a decrease. The two divisions of the autonomic nervous system are easily distinguished from each other by their anatomy, their neurotransmitters, and their actions (Figure 46.10).

The best-known functions of the autonomic nervous system are those of the sympathetic division that produce the "fight-or-flight" response, increasing heart rate, blood pres-

Parasympathetic division

Sympathetic division

Constricts pupils

Stimulates salivation

Constricts airways

Slows heartbeat

Stimulates digestion

Stimulates release of glucose, stimulates gallbladder

Stimulates activity of intestines

Stimulates urinary bladder to contract

Stimulates penile or clitoral arousal

Noradrenergic neurons (postganglionic) Cholinergic neurons (preganglionic) Cholinergic neurons (postganglionic)

46.10 Organization of the Autonomic Nervous System The autonomic nervous system is divided into the sympathetic and parasympathetic divisions, which work in opposition to each other in their effects on most organs (one causing an increase and the other a decrease in activity).

Parasympathetic division

Sympathetic division

Stimulates release of glucose, stimulates gallbladder

Stimulates activity of intestines

Stimulates urinary bladder to contract

Ganglion Sympathetic System

Dilates pupil

Inhibits salivation

Relaxes airways

Accelerates heartbeat

Inhibits digestion

Inhibits release of glucose, inhibits gallbladder

Inhibits activity of intestines

Relaxes urinary bladder

Stimulates orgasm, vaginal contraction

Noradrenergic neurons (postganglionic) Cholinergic neurons (preganglionic) Cholinergic neurons (postganglionic)

Dilates pupil

Inhibits salivation

Relaxes airways

Accelerates heartbeat

Inhibits digestion

Inhibits release of glucose, inhibits gallbladder

Inhibits activity of intestines

Relaxes urinary bladder

Stimulates orgasm, vaginal contraction sure, and cardiac output and preparing the body for emergencies (see Chapter 42). In contrast, the parasympathetic division slows the heart and lowers blood pressure. It is tempt ing to think of the sympathetic division as the one that speeds things up and the parasympathetic division as the one that slows things down, but that is not always a correct distinction. The sympathetic division slows the digestive system, and the parasympathetic division accelerates it.

Both divisions of the autonomic nervous system are efferent pathways. Each autonomic efferent pathway begins with a cholinergic neuron (one that uses acetylcholine as its neuro-transmitter) that has its cell body in the brain stem or spinal cord. These cells are called preganglionic neurons because the second neuron in the pathway with which they synapse resides in a ganglion (a collection of neuronal cell bodies that is outside of the CNS). The second neuron is called a post-ganglionic neuron because its axon extends out from the ganglion. The axon of the postganglionic neuron synapses with cells in the target organs.

The postganglionic neurons of the sympathetic division are noradrenergic (use norepinephrine as their neurotrans-mitter), while the postganglionic neurons of the parasympa-thetic division are cholinergic. In organs that receive both sympathetic and parasympathetic input, the target cells respond in opposite ways to norepinephrine and to acetyl-choline. A region of the heart called the pacemaker, which generates the heartbeat, is an example. Stimulating the sympathetic nerve to the heart or dripping norepinephrine onto the pacemaker region depolarizes the pacemaker cells, increases their firing rate, and causes the heart to beat faster. Stimulating the parasympathetic nerve to the heart or dripping acetylcholine onto the pacemaker region hyperpolarizes the pacemaker cells, decreases their firing rate, and causes the heart to beat more slowly. In contrast, in the digestive tract, norepinephrine hyperpolarizes muscle cells, which slows digestion, and acetylcholine depolarizes muscle cells, which accelerates digestion.

The sympathetic and parasympathetic divisions of the au-tonomic nervous system can also be distinguished by anatomy (see Figure 46.10). The preganglionic neurons of the parasympathetic division come from the brain stem and the last segment of the spinal cord (the sacral region). The pre-ganglionic neurons of the sympathetic division come from the upper regions of the spinal cord below the neck (the thoracic and lumbar regions). Most of the ganglia of the sympathetic division are lined up in two chains, one on either side of the spinal cord. The parasympathetic ganglia are close to— sometimes sitting on—the target organs.

The autonomic nervous system is an important link between the CNS and many physiological functions of the body. Its control of diverse organs and tissues is crucial to homeostasis. In spite of its complexity, work by neurobiolo-gists and physiologists over many decades has made it possible to understand its functions in terms of neuronal properties and circuits. In Chapter 49, for example, we will see how information from pressure receptors in the blood vessels is transmitted to the CNS, where it produces autonomic signals that control the rate of the heartbeat.

Neurons and circuits in the occipital cortex integrate visual information

In Chapter 45, we learned that the information conveyed to the brain in the optic nerve consists of action potentials that are stimulated by light falling on small circular areas of the retina called receptive fields. A receptive field contains many photoreceptor cells connected together in a circuit in such a way that the signals they produce are integrated and transmitted to the brain by a single retinal ganglion cell. The axon of each ganglion cell travels to the brain in the optic nerve. How does the brain construct visual images from this information about circular patches of light falling on the retina?

Information from the retina is transmitted through the optic nerve to a relay station in the thalamus, and then to the brain's visual processing area, in the occipital cortex at the back of the cerebral hemispheres (see Figure 46.5b). David Hubel and Torsten Wiesel of Harvard University studied the activity of neurons in this visual cortex. They recorded the activities of single cells in the brains of living animals while they stimulated the animals' retinas with spots and bars of light. They found that cells in the visual cortex, like retinal ganglion cells, have receptive fields—specific areas of the retina that, when stimulated by light, influence the rate at which the cells fire action potentials.

Cells in the visual cortex, however, have receptive fields that differ from the simple circular receptive fields of retinal ganglion cells. Cortical cells called simple cells are maximally stimulated by bars of light that have specific orientations. Simple cells probably receive input from several ganglion cells whose circular receptive fields are lined up in a row.

Complex cells in the visual cortex are also maximally stimulated by a bar of light with a particular orientation, but the bar may fall anywhere on a large area of retina described as that cell's receptive field. The receptive field of a complex cell appears to be built from the receptive fields of several simple cells that share a certain stimulus orientation, but have receptive fields in different places on the retina (Figure 46.11). Some complex cells respond most strongly when the bar of light moves in a particular direction.

The concept that emerges from these experiments is that the brain assembles a mental image of the visual world by analyzing edges in patterns of light falling on the retina. This

46.11 Receptive Fields of Cells in the Visual Cortex Cells in the visual cortex respond to specific patterns of light falling on the retina.Ganglion cells that transmit information about circular receptive fields converge on simple cells in the cortex in such a way that the simple cells have linear receptive fields. Simple cells transmit information to complex cells in such a way that the complex cells can respond to linear stimuli falling on different areas of the retina.

analysis is conducted in a massively parallel fashion. Each retina sends a million axons to the brain, but there are hundreds of millions of neurons in the visual cortex. Each bit of information from a retinal ganglion cell is received by hundreds of cortical cells, each responsive to a different combi-

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • edward
    Why is glucose released by gallbladder during the sympathetic division?
    8 years ago
  • russell
    What are sympathetic efferents?
    8 years ago
  • amalia
    How sympathetic and parasympathetic systems work together?
    8 years ago
  • James
    Where is ganglia located vagina?
    8 years ago
  • cameron
    Where are pre and postganglionic neurons located?
    8 years ago

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