Functional Subsystems of the Nervous System

We have just surveyed the development of the nervous system in terms of anatomically distinct structures. At any one time, these various structures are engaged in many simultaneous tasks—a property known as parallel processing of information. Specific tasks are carried out by subsystems that may involve several different anatomical regions or structures of the nervous system. We will now examine several of these functional subsystems.

The spinal cord receives and processes information from the body

The spinal cord conducts information in both directions between the brain and the organs of the body. It also integrates a great deal of the information coming from the peripheral nervous system, and it responds to that information by issuing motor commands.

46.3 The Spinal Cord Processes Information

Sensory (afferent) information enters the spinal cord through the dorsal horns (red pathway), and motor (efferent) output leaves it via the ventral horns (blue pathways).The extensor component of the knee-jerk response is a monosynaptic reflex circuit, but the flexor inhibition component involves a spinal interneuron (green).

Gray matter

White matter

| The motor neuron conducts an action potential to the extensor muscle, causing contraction.

3} In a monosynaptic pathway, the sensory neuron synapses with a motor neuron in the ventral horn of the spinal cord.

A stretch receptor fires an action potential.

ij A hammer tap stretches the tendon in the knee, stretching a receptor in the extensor muscle.

A cross section of the spinal cord reveals a central area of gray matter in the shape of a butterfly, surrounded by an area of white matter (Figure 46.3). In the nervous system, gray matter is tissue rich in neuronal cell bodies, and white matter contains axons. The gray matter of the spinal cord contains the cell bodies of the spinal neurons; the white matter contains the axons that conduct information up and down the spinal cord. Spinal nerves extend from the spinal cord at regular intervals on each side. Each spinal nerve has two roots, one connecting with the dorsal horn of the gray matter, and the other connecting with the ventral horn. Each spinal nerve carries both afferent and efferent information. The afferent axons enter the spinal cord through the dorsal root, and the efferent axons leave the spinal cord through the ventral root.

The conversion of afferent to efferent information in the spinal cord without participation of the brain is called a spinal reflex. The simplest type of spinal reflex involves only two neurons and one synapse and is therefore called a monosynaptic reflex. An example is the knee-jerk reflex, which your physician checks with a mallet tap just below your knee. We can diagram the wiring of a monosynaptic reflex by following the flow of information through the spinal cord.

In the case of the knee-jerk reflex, sensory information comes from stretch receptors in the leg muscle that is suddenly stretched when the mallet strikes the tendon that runs over the knee. Each stretch receptor initiates action potentials that are conducted by the axon of a sensory neuron through the dorsal horn of the spinal cord and all the way to the ventral horn. In the ventral horn, the sensory neuron synapses with motor neurons, causing them to fire action potentials that are then conducted back to the leg extensor muscle, causing it to contract. The function of this simple circuit is to sense an increased load on the limb and to increase the strength of muscle con

Gray matter

White matter

| The motor neuron conducts an action potential to the extensor muscle, causing contraction.

3} In a monosynaptic pathway, the sensory neuron synapses with a motor neuron in the ventral horn of the spinal cord.

A stretch receptor fires an action potential.

ij A hammer tap stretches the tendon in the knee, stretching a receptor in the extensor muscle.

Dorsal Ventral Horn Afferent

Dorsal root (afferent nerves)

Dorsal horn Ventral horn Ventral root (efferent nerves)

Dorsal root (afferent nerves)

Dorsal horn Ventral horn Ventral root (efferent nerves)

traction to compensate for the added load Q The leg and thereby keep muscle length constant. \ extendsJ Most spinal circuits are more complex than this monosynaptic reflex, as we can demonstrate by building on the circuit we have just traced. Limb movement is controlled by antagonistic sets of muscles—muscles that work against each other. When one member of an antagonistic set of muscles contracts, it bends, or flexes, the limb; it is therefore called a flexor. The antagonist to this muscle straightens, or extends, the limb, and is called an extensor. For a limb to move, one muscle of the pair must relax while the other contracts. Thus, sensory input that activates the motor neuron of one muscle also inhibits its antagonist. This coordination is achieved by an interneuron, which makes an inhibitory synapse onto the motor neuron of the antagonistic muscle (see Figure 46.3). Thus the reciprocal inhibition of antagonistic muscles involves an interneuron between the sensory cell and the motor neuron of the inhibited muscle, and therefore at least two synapses.

Information entering the dorsal horn is also transmitted by axons up the spinal cord to the brain. We are aware of the mallet hitting the knee, but the reflex response actually begins before that information registers in our consciousness. A great deal of information processing takes place in the spinal cord without any input from the brain. Spinal circuits can even generate repetitive motor patterns, such as the swimming movements of the shark that had its telen-cephalon removed.

The reticular system alerts the forebrain

Sensory information ascending the spinal cord to final destinations in the forebrain passes through the brain stem. Many sensory fibers give off collateral branches that form synapses with a network of brain stem neurons called the reticular sys tem. The reticular system is a highly complex network of axons and dendrites. Within the reticular system are many discrete groups of neurons. Such an anatomically distinct group of neurons in the CNS is called a nucleus (not to be confused with the nucleus of a single cell).

The reticular system is distributed through the core of the medulla, pons, and midbrain. Afferent information passes through the reticular system, where many connections are made to neurons involved in controlling many functions of the body. Information from joints and muscles, for example, is directed to nuclei in the pons and cerebellum that are involved in balance and coordination, whereas information from pain receptors is directed to nuclei that control sensitivity to pain. This information continues upward to the forebrain, where it results in conscious sensations that can be localized to the specific sites in the body where the information originated.

The information routed through the reticular system also influences the level of arousal of the nervous system. Nuclei in the reticular system are involved in the control of sleep and waking. High levels of activity in the reticular system influence these nuclei to maintain the brain in a waking condition; low levels of activity enable sleep. Because of the alerting function of the reticular core of the brain stem, it has been called the reticular activating system.

If the brain of a person is damaged at midbrain or higher levels and the alerting action of the reticular system cannot reach the forebrain, the person loses the ability to be in a conscious, waking state and becomes comatose. Damage to the brain stem or the spinal cord below the reticular system does not interfere with the ascending alerting actions of the reticular system and leaves the person with normal patterns of sleep and waking. However, such damage can cause loss of sensation and loss of motor function.

The limbic system supports basic functions of the forebrain

The telencephalon of fishes, amphibians, and reptiles consists of only a few structures surrounding the diencephalon. In birds and mammals, these primitive forebrain structures are completely covered by the evolutionarily more recent elaborations of the telencephalon called the neocortex, but these primitive forebrain structures still have important functions. These structures are collectively referred to as the limbic system (Figure 46.4).

The limbic system is responsible for basic physiological drives, instincts, and emotions. Within the limbic system are areas that, when stimulated with small electric currents, can cause intense sensations of pleasure, pain, or rage. If a rat is given the opportunity to stimulate its own pleasure centers by pressing a switch, it will ignore food, water, and even sex, pushing the switch until it is exhausted. Pleasure and pain

Cerebral hemispheres

Cerebral hemispheres

Parts Spinal Cord

Hippocampus Spinal cord

46.4 The Limbic System The evolutionarily primitive parts of the telencephalon (blue) are referred to as the limbic system.

Olfactory bulbs Hypothalamus Pituitary Amygdala

Hippocampus Spinal cord

46.4 The Limbic System The evolutionarily primitive parts of the telencephalon (blue) are referred to as the limbic system.

centers in the limbic system are believed to play roles in learning and in physiological drives.

A component of the limbic system, the amygdala, is involved in fear and fear memory. If a certain portion of the amygdala is damaged or chemically blocked, an animal cannot learn to be afraid of a stimulus or a situation that would normally induce a strong fear reaction. Moreover, blocking protein synthesis in this part of the limbic system blocks the formation of fear memory.

Another part of the limbic system, the hippocampus, is necessary in humans for the transfer of short-term memory to long-term memory. If you are told a new telephone number, you may be able to hold it in short-term memory for a few minutes, but within half an hour it is forgotten unless you make a real effort to remember it. The phenomenon of remembering something for more than a few minutes requires its transfer to long-term memory.

Regions of the cerebrum interact to produce consciousness and control behavior

The cerebral hemispheres are the dominant structures in the mammalian brain. In humans, they are so large that they cover all other parts of the brain except the cerebellum (Figure 46.5a). A sheet of gray matter called the cerebral cortex covers each cerebral hemisphere. It is about 4 mm thick and covers a total surface area over both hemispheres of 1 square meter. The cerebral cortex is convoluted, or folded, into ridges called gyri (singular, gyrus) and valleys called sulci (singular, sulcus). These convolutions allow it to fit into the skull.

Gyri Function
46.5 The Human Cerebrum (a) Each cerebral hemisphere is divided into four lobes. (b) Different functions are localized in particular areas of the cerebral lobes.

Under the cerebral cortex is white matter, made up of the axons that connect the cell bodies in the cortex with one another and with other areas of the brain.

A curious feature of our nervous systems is that the left side of the body is served (in both sensory and motor aspects) mostly by the right side of the brain, and the right side of the body is served mostly by the left side of the brain. Thus, sensory input from the right hand goes to the left cerebral hemisphere, and sensory input from the left hand goes to the right cerebral hemisphere. The two hemispheres, however, are not exactly symmetrical. Language abilities, for example, reside predominantly in the left hemisphere, as we will see below.

Different regions of the cerebral cortex have specific functions (Figure 46.5b). Some of those functions are easily defined, such as receiving and processing sensory information, but most of the cortex is involved in higher-order information processing that is less easy to define. These latter areas are given the general name of association cortex.

To understand the cerebral cortex, it helps to have an anatomical road map. As viewed from the left side, a left cerebral hemisphere looks like a boxing glove for the right hand with the fingers pointing forward, the thumb pointing out, and the wrist at the rear (see Figure 46.5a). The "thumb" area is the temporal lobe, the fingers the frontal lobe, the back of the hand the parietal lobe, and the wrist the occipital lobe. A mirror image of this arrangement characterizes the right cerebral hemisphere. Let's look at each lobe of the cerebrum separately.

Functional Areas The Cerebrum

the temporal lobe. The upper region of the temporal lobe receives and processes auditory information. The association areas of the temporal lobe are involved in the recognition, identification, and naming of objects. Damage to the temporal lobe results in disorders called agnosias in which the individual is aware of a stimulus, but cannot identify it.

Damage to one area of the temporal lobe results in the inability to recognize faces. Even old acquaintances cannot be identified by facial features, although they may be identified by other attributes such as voice, body features, and characteristic style of walking. Using monkeys, it has been possible to record the activity of neurons in this region that respond selectively to faces in general (Figure 46.6). These neurons do not respond to other stimuli in the visual field, and their responsiveness decreases if some of the features of the face are missing or appear in inappropriate locations. Damage to other association areas of the temporal lobe causes deficits in understanding spoken language, even though speaking, reading, and writing abilities may be intact.

This neuron responds maximally to a

This neuron responds maximally to a

Subsystems The Nervous System
46.6 Neurons in One Region of the Temporal Lobe Respond to Faces The traces represent the firing rate of a neuron in the temporal lobe of a monkey in response to the pictures shown below them.

Somatosensory

Primary Somatosensory Cortex Damage

Somatosensory

Teeth Gums

The left side of the cerebral cortex communicates with the right side of the body, and vice versa.

Motor cortex

The left side of the cerebral cortex communicates with the right side of the body, and vice versa.

The Cerebral Membrane

Somatosensory cortex

Teeth Gums

46.7 The Body Is Represented in the Primary Motor Cortex and the Primary Somatosensory Cortex Cross sections through the primary motor and primary somatosensory cortexes can be represented as maps of the human body. Body parts are shown in proportion to the brain area devoted to them.

Gage's head below his left eye, passed through his frontal lobe, and exited the top of his head (Figure 46.8).

Remarkably, Gage survived this terrible accident, but he was a completely different person. He was quarrelsome, bad-tempered, lazy, and irresponsible. He was impatient and obstinate, and he used profane language, which he had never done before. He spent the rest of his days as a drifter, earning money by telling his story, exhibiting his scars and the tamping iron. If you are in Cambridge, Massachusetts, you can pay him a visit. His skull, death mask, and the tamping iron are on display in the Museum of the Medical College of Harvard University.*

Somatosensory cortex

*The careful reader may have noted that we have mentioned two Gages in this chapter: Phineas and Fred. Fred Gage, the neuroscientist, is the great-grand-nephew of Phineas Gage.

the frontal lobe. The frontal and parietal lobes are separated by a deep valley called the central sulcus. A strip of the frontal lobe cortex just in front of the central sulcus is called the primary motor cortex (see Figure 46.5b). The neurons in this region control muscles in specific parts of the body. The parts of the body can be mapped onto the primary motor cortex, from the head region on the lower side to the lower part of the body at the top. Areas with fine motor control, such as the face and hands, have the greatest representation (Figure 46.7). If a neuron in the primary motor cortex is electrically stimulated, the response is the twitch of a muscle, but not a coordinated, complex behavior.

The association functions of the frontal lobe are diverse. They are best described as having to do with planning, and they contribute very significantly to personality. People with frontal lobe damage have drastic alterations of personality because they cannot create an accurate view of themselves in the context of the world around them and cannot plan for future events. A dramatic case of frontal lobe damage is the story of Phineas Gage, who was an industrious, responsible, considerate young railroad construction foreman in 1848. Then a blasting accident shot a meter-long, 3-cm-wide iron tamping rod through his brain. The tamping iron entered

Tamping Iron
46.8 A Mind-Altering Experience In a nineteenth-century railroad construction accident,an explosion blew a tamping iron through the brain of Phineas Gage. Unbelievably, Gage survived, but his personality was radically changed.This drawing of Gage's skull was made at the time of his death.

the parietal lobe. The strip of parietal lobe cortex just behind the central sulcus is the primary somatosensory cortex (see Figure 46.5fr). This area receives touch and pressure information through the thalamus.

The whole body surface can be mapped onto the primary somatosensory cortex (see Figure 46.7). Areas of the body that have a high density of tactile mechanoreceptors and are capable of making fine discriminations in touch (such as the lips and the fingers) have disproportionately large representation. If a very small area of the primary somatosensory cortex is stimulated electrically, the subject reports feeling specific sensations, such as touch, in a very localized part of the body.

A major association function of the parietal lobe is attending to complex stimuli. Damage to the right parietal lobe causes a condition called contralateral neglect syndrome, in which the individual tends to ignore stimuli from the left side of the body or the left visual field. Such individuals have difficulty performing complex tasks, such as dressing the left side of the body; an afflicted man may not be able to shave the left side of his face. When asked to copy simple drawings, a person who exhibits this syndrome can do well with the right side of the drawing, but not the left (Figure 46.9). The parietal cortex is not symmetrical with respect to its role in attention, however. Damage to the left parietal cortex does not cause the same degree of neglect of the right side of the body. We will see similar asymmetries in cortical function when we discuss language.

the occipital lobe. The occipital lobe receives and processes visual information; we'll learn more about the details of that process later in this chapter. The association areas of the occipital cortex are essential for making sense of the visual world and translating visual experience into language. Some deficits resulting from damage to these areas are specific. In one case, a woman with limited damage was unable to see motion. Her vision was intact, but she could see a waterfall only as a still image, and a car approaching only as a series of scenes of a stationary object at different distances.

Model

Model

Patient's copy

/

ffl

ffl

\

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.

Get My Free Ebook


Responses

  • katja
    Which "parts of the body" have the "greatest representation" on the "primary motor cortex"?
    5 years ago
  • jaden anderson
    How temporal lobe controls auditory system?
    5 years ago
  • antje
    Why we are aware of patellar reflex?
    5 years ago
  • Patrick
    What are gyria and sulci of the cerebellum?
    5 years ago
  • Kerstin
    What is the function of the sulci?
    5 years ago
  • Lalia
    What other kinds of efferent neurons leave the spinal cord via the ventral horn?
    5 years ago
  • frodo
    What information enters dorsal horn?
    5 years ago

Post a comment