Chapter 46
The Mammalian Nervous System: Structure and Higher Functions
The Mammalian Nervous System:
Structure and Higher Functions
The Nervous System: Structure, Function, and Information Flow
Functional Subsystems of the Nervous System
Information Processing by Neuronal Networks
Understanding Higher Brain Functions in Cellular Terms
The Nervous System:
Structure, Function, and Information Flow
The brain and spinal cord together constitute the central nervous system (CNS).
Information is brought to and from the CNS by means of the enormous network of nerves that make up the peripheral nervous system.
The peripheral nervous system reaches every tissue of the body; it connects to the CNS via spinal and cranial nerves.
A nerve is a bundle of axons. Some axons in a nerve may be carrying information from the CNS while other axons in the same nerve may be carrying information to the CNS.
The Nervous System:
Structure, Function, and Information Flow
The nervous system is a complicated __________ processing system.
The afferent portion of the peripheral nervous system carries information to the CNS.
We are consciously aware of vision, hearing, pain, and limb position, which are types of information that move through afferent pathways.
We are unaware of other afferent information such as blood pressure, deep body temperature, and blood oxygen supply.
The Nervous System:
Structure, Function, and Information Flow
The efferent portion of the peripheral nervous system carries information from the CNS to the muscles and glands of the body.
Efferent pathways can be divided into a voluntary division (conscious movements) and an involuntary, or autonomic, division (physiological functions).
In addition to receiving neural information, the CNS receives chemical information from __________ circulating in the blood or from neurohormones released by neurons into the extracellular fluids of the brain.
The Nervous System:
Structure, Function, and Information Flow
Early in vertebrate development, a hollow tube of neural tissue forms.
The neural tube runs the length of the animal on its dorsal side.
At the anterior end, the neural tube forms three swellings that become the basic divisions of the brain: the hindbrain, midbrain, and forebrain.
The rest of the neural tube becomes the spinal cord.
Cranial and spinal nerves, which make up the peripheral nervous system, sprout from the neural tube and grow throughout the embryo.
The Nervous System:
Structure, Function, and Information Flow
Different brain structures develop from the three embryonic regions.
The medulla, pons, and cerebellum are derived from the hindbrain.
The medulla and pons contain distinct groups of neurons that are involved in the control of physical functions such as breathing and circulation.
All information traveling from the spinal cord must pass through the pons and medulla.
The cerebellum refines motor commands to the joints and muscles.
The Nervous System:
Structure, Function, and Information Flow
From the midbrain come structures that process aspects of visual and auditory information.
Structures that develop from the hindbrain and midbrain are collectively known as the brain stem.
The Nervous System:
Structure, Function, and Information Flow
The __________ develops a central region called the diencephalon and a surrounding structure called the telencephalon.
The diencephalon is the core of the forebrain and consists of the thalamus and the hypothalamus.
The telencephalon (cerebrum) consists of two cerebral hemispheres, left and right.
In humans, the telencephalon is the largest part of the brain and plays major roles in sensory perception, learning, memory, and conscious behavior.
Functional Subsystems of the Nervous System
The nervous system is engaged in many tasks at the same time, a property known as parallel processing of information.
Specific task are carried out by subsystems, which include the spinal cord, reticular system, limbic system, and cerebrum.
Functional Subsystems of the Nervous System
The spinal cord conducts information between the brain and organs of the body, integrates information coming from the peripheral nervous system, and issues motor commands.
In the nervous system, gray matter is tissue rich in neuronal cell bodies, and white matter contains axons.
Spinal nerves leave the spinal cord at regular intervals; each one has two roots, one connecting it to the dorsal horn of the gray matter and the other connecting with the ventral horn.
Functional Subsystems of the Nervous System
The conversion of afferent to efferent information in the spinal cord without participation of the brain is called a __________ reflex.
The simplest spinal reflex involves only two neurons and one synapse; it is called a monosynaptic reflex, such as the knee-jerk reflex.
Functional Subsystems of the Nervous System
Limb movement is controlled by antagonistic sets of muscles.
Contraction of one muscle (the flexor) bends the limb, contraction of the other (the extensor) straightens the limb.
Sensory input that activates the motor neuron of one muscle also inhibits the opposing muscle. This coordination is achieved by an interneuron, which makes an inhibitory synapse onto the motor neuron of the antagonistic muscle.
Thus the reciprocal inhibition of antagonistic muscles involves an interneuron between the sensory cell and the motor cell and, therefore, at least two synapses.
Functional Subsystems of the Nervous System
Many sensory fibers give off collateral branches that form synapses with a network of brain stem neurons called the reticular system.
Within the reticular system are many discrete groups of neurons. In the CNS, an anatomically distinct group of neurons is called a nucleus.
The reticular system is distributed through the core of the medulla, pons, and midbrain.
Afferent information coming up the neural axis passes through the reticular system.
Functional Subsystems of the Nervous System
Information routed through the reticular system also influences the level of arousal of the nervous system.
The reticular system controls sleep and waking, for example.
Because of this alerting function of the system, it has been called the reticular activating system.
If the brain stem is damaged at midbrain or higher levels, reticular activity is not conveyed to the forebrain and the person is rendered comatose.
Damage to the brain stem below the reticular system leaves a person with normal sleep and wake patterns but may cause paralysis.
Functional Subsystems of the Nervous System
The evolutionarily primitive parts of the forebrain are collectively known as the limbic system.
The __________ system is responsible for basic physiological drives, instincts, and emotions.
__________ stimulation of certain areas of the limbic system can cause intense sensations of __________, pain, or rage.
The amygdala is involved in __________ and fear memory. If part of the amygdala is damaged, an animal cannot learn to be afraid of a stimulus.
The hippocampus is necessary, in humans, for transfer of short-term memory to long-term memory.
Functional Subsystems of the Nervous System
The cerebral hemispheres are the dominant structures in the mammalian brain.
A sheet of gray matter called the cerebral cortex covers each cerebral hemisphere.
The cerebral cortex is folded into ridges, called gyri, and valleys, called sulci.
Under the cerebral cortex is white matter.
The left and right cerebral hemispheres receive sensory information from the opposite sides of the body.
Functional Subsystems of the Nervous System
Different regions of the cerebral cortex have specific functions.
Although many of these functions are easily defined, most of the cortex is involved in higher-order information processing. These regions are called the association cortex.
Each hemisphere is divided into four lobes: temporal, frontal, parietal, and occipital.
Functional Subsystems of the Nervous System
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.
Facial recognition is one function of the temporal lobe.
Functional Subsystems of the Nervous System
A strip of the frontal lobe just in front of the central sulcus is called the primary __________ cortex.
The neurons in this region have axons that project to muscles in specific parts of the body, and areas with fine motor control (e.g., face, hands) have the greatest representation.
The association functions of the frontal lobe have to do with planning and contribute significantly to personality.
A famous case of frontal lobe damage is that of Phineas Gage.
Functional Subsystems of the Nervous System
The strip of parietal lobe cortex just behind the central sulcus is the primary somatosensory cortex.
This area receives touch and pressure information through the thalamus. The whole body surface can be mapped onto the primary somatosensory cortex.
Areas of the body that are capable of making fine discrimination in touch (e.g., lips, fingers) have large representation.
A major association function of the parietal lobe is the ability to attend to complex stimuli.
Functional Subsystems of the Nervous System
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.
Damage to the left parietal field does not cause the same degree of impairment.
Functional Subsystems of the Nervous System
The occipital lobes receive and process visual information.
The association areas of the occipital lobe are essential to make sense of the visual world and to translate experience into language.
Functional Subsystems of the Nervous System
The size of the telencephalon relative to the rest of the brain increases substantially from amphibians, to reptiles, to birds and mammals.
Among mammals, it is much larger in size and complexity in animals with complex behaviors (e.g., primates) than it is in animals with relatively simple ones (e.g., rodents).
Humans and dolphins have the largest __________ of brain size to body size, and they have the most highly developed cerebral cortex.
Information Processing by Neuronal Networks
The autonomic nervous system is categorized into two parts: the sympathetic and parasympathetic divisions.
The two divisions work in opposition to each other, one causing an increase in activity, the other a decrease.
The __________ division produces the flight or fight response (increasing heart rate, blood pressure, and cardiac output); the __________ division slows the heart and blood pressure.
However, the sympathetic division slows down the digestive system, whereas the parasympathetic speeds it up.
Information Processing by Neuronal Networks
Both divisions of the autonomic nervous system are efferent pathways.
Each pathway begins with a neuron that has its cell body in the brain stem or spinal cord and uses acetylcholine as its neurotransmitter. These are preganglionic neurons.
They synapse with neurons that reside in a ganglion (a collection of neuronal cell bodies that is outside the CNS).
The second neuron is called a postganglionic neuron. Axons of these neurons end on the cells of the target organs.
Information Processing by Neuronal Networks
The postganglionic neurons of the sympathetic division use norepinephrine as their neurotransmitter and are called noradrenergic.
Those of the parasympathetic division use acetylcholine and are called cholinergic.
Information Processing by Neuronal Networks
In organs that receive both sympathetic and parasympathetic input, the target cells respond in opposite ways to norepinephrine and acetylcholine.
In the pacemaker of the heart, for example, a norepinephrine drip causes the heart to beat faster, whereas acetylcholine slows it down.
The effects in the digestive system are the opposite.
Information Processing by Neuronal Networks
The preganglionic neurons of the parasympathetic division come from the brain stem and the sacral region of the spinal cord
The parasympathetic ganglia are close to the target organs.
The preganglionic neurons of the sympathetic division come from the thoracic and lumbar regions of the spinal cord
Most of the sympathetic ganglia are lined up on either side of the spinal cord.
Information Processing by Neuronal Networks
Neurons and circuits in the occipital cortex integrate visual information.
Information from the retina is transmitted through the optic nerve to a relay station in the thalamus, and then to the brains visual processing area, the occipital (or visual) cortex.
Information Processing by Neuronal Networks
In their famous studies, Torsten and Wiesel discovered that cells in the visual cortex have receptive fields.
Cells called simple cells are maximally stimulated by bars of light that have specific orientations.
Complex cells are also maximally stimulated by a bar of light with a particular orientation.
Complex cells seem to receive input from several simple cells that share a certain stimulus orientation but have receptive fields in different places on the retina.
Information Processing by Neuronal Networks
The brain assembles a __________ image of the visual world by analyzing edges of patterns of light falling on the retina.
The analysis is conducted in a massively parallel fashion (millions of axons from each retina and hundreds of millions of neurons in the visual cortex).
Information Processing by Neuronal Networks
To create three-dimensional vision, the eyes see overlapping, yet slightly different, visual fields (binocular vision).
The brain __________ information from two eyes in a crossed pattern.
The optic nerves from the two eyes appear to join together at the optic chiasm just under the hypothalamus.
Axons from half of each retina cross in the optic chiasm and go to the other side of the brain.
Information Processing by Neuronal Networks
The result of this crossing is that all visual information from your left visual field goes to the right side of the brain, and information from the right visual field goes to the left side of the brain.
Cells in the visual cortex are organized in columns; the columns alternate left eye, right eye, and so on.
Cells closest to the border between two columns receive input from both eyes and are called binocular cells.
Binocular cells interpret distance by measuring the disparity between where the same stimulus falls on the two retinas.
Understanding Higher Brain Functions
in Cellular Terms
The processes responsible for the higher brain functions are extremely complex.
Very few functions of the nervous system have been worked out to the point of identifying the underlying neuronal networks.
However, some progress is being made in understanding some of the cellular and molecular mechanisms.
Understanding Higher Brain Functions
in Cellular Terms
All birds and mammals, and probably all other vertebrates, sleep.
Loss of sleep impairs alertness and performance. Most people in our society are chronically sleep deprived.
The electroencephalogram (EEG) is a common tool of sleep researchers. It records the electrical activity in the cerebral cortex.
Understanding Higher Brain Functions
in Cellular Terms
In mammals other than humans, two major sleep states are recorded: slow-wave sleep and rapid-eye-movement (REM) sleep.
In humans, the sleep states are characterized as non-REM sleep and REM sleep.
Human non-REM sleep is divided into 4 stages.
Only stages 3 and 4 (which are deep, restorative sleep) are considered true slow-wave sleep.
The first episode of non-REM sleep is followed by an episode of REM sleep, during which dreams and nightmares occur.
Throughout the night we experience alternating states of non-REM sleep and REM sleep.
Understanding Higher Brain Functions
in Cellular Terms
Non-REM sleep is characterized by a decrease in the responsiveness of neurons in the thalamus and cerebral cortex.
During waking, nuclei in the brain stem are continuously active and influence the thalamus and cortex, keeping their resting potentials near thresholdmaintaining wakefulness.
With the onset of sleep, these nuclei cease activity, cells in the thalamus and cortex become hyperpolarized and insensitive to synaptic input, and consciousness is lost.
Understanding Higher Brain Functions
in Cellular Terms
Interestingly, the hyperpolarization results in the synchronized firing of action potentials. This results in the EEG slow-wave pattern of non-REM sleep.
At the transition from non-REM to REM sleep, the brain stem nuclei become active again, causing depolarization of cortical neurons. The synchronized firing ceases, and the EEG resembles that of a waking brain.
During REM sleep, the cortex can process information and vivid dreams occur, but there is no motor output or sensory input.
Understanding Higher Brain Functions
in Cellular Terms
Learning is the modification of behavior by experience.
Memory is the ability of the nervous system to retain what is learned and what is experienced.
Learning that leads to long-term memory and modification of behavior must involve long-lasting __________ changes.
Understanding Higher Brain Functions
in Cellular Terms
High-frequency electrical stimulation of the mammalian hippocampus makes it more sensitive to subsequent stimulation. This phenomenon is called long-term potentiation (LTP).
Continuous, repetitive, low-level stimulation of the same hippocampal circuit reduces its responsiveness, a phenomenon called long-term depression (LTD).
LTP and LTD have been demonstrated in circuits other than hippocampal circuits, and they may be fundamental cellular or molecular mechanisms involved in learning and memory.
Understanding Higher Brain Functions
in Cellular Terms
Associative learning occurs when two unrelated stimuli become linked to the same response.
The simplest example of associative learning is the conditioned reflex, discovered by Ivan Pavlov.
In studies led by Richard Thompson of conditioned learning in rabbits, a small, specific area of the cerebellum was discovered to be necessary for a conditioned eye-blink response.
Understanding Higher Brain Functions
in Cellular Terms
The first evidence that memories have anatomical locations in the brain came from studies of people with epilepsy, who reported vivid memories when some regions of association cortex were stimulated.
Immediate memory lasts only a few seconds and is almost perfectly photographic.
Short-term memory contains less information than immediate memory but lasts longer (1015 minutes).
Long-term memory can last for years.
Understanding Higher Brain Functions
in Cellular Terms
Memory of people, places, events, and things is declarative memory; these things can be consciously recalled and described.
The memory of how to perform a motor task is procedural memory; it cannot be consciously recalled and described.
Knowledge about neural mechanisms involved in different kinds of memory has come from observations of people who have lost parts of the limbic system, most notably the hippocampus.
Our understanding of learning and memory in cellular terms is still rudimentary.
Understanding Higher Brain Functions
in Cellular Terms
Language ability in humans is located in one cerebral hemisphere onlythe left hemisphere in 97 percent of the population. This phenomenon is called lateralization.
The two cerebral hemispheres are connected by a tract of white matter called the corpus callosum.
In split-brain individuals (people who have had their corpus collosum cut as a treatment for epilepsy), the knowledge or experience of the right hemisphere can no longer be expressed in language, nor can language be used to communicate with the right hemisphere.
Understanding Higher Brain Functions
in Cellular Terms
Several language areas in the left hemisphere have been identified from persons who suffer brain damage and aphasia.
Brocas area, in the frontal lobe in front of the primary motor cortex, is essential for speech.
People with damage to Brocas area have speech deficits, though they still can read and understand language.
Wernickes area in the temporal lobe is more involved with the sensory aspects of language.
Damage to Wernicke s area leads to an inability to speak sensibly or understand spoken or written language.
Understanding Higher Brain Functions
in Cellular Terms
Normal language ability depends on the flow of information among various areas of the left cerebral cortex.
Input from spoken language travels from the auditory cortex to Wernickes area.
Input from written language travels from the visual cortex to the angular gyrus to Wernickes area.
Using modern methods of functioning brain imaging, it is possible to see the metabolic activity in different brain areas when the brain is using language.
Understanding Higher Brain Functions
in Cellular Terms
Neurobiologists face an incredible challenge in understanding the organization and functions of the human brain.
The question of __________ resolves into two types of problems: easy and hard.
The easy problems deal with all the cells and circuits that process the information involved in conscious experience.
The hard problems involve explaining how properties of cells and networks result in consciousness. We lack the proper tools or concepts even to begin to solve these kinds of questions.
Animation 46.1 Information Processing in the Spinal Cord
Video 46.1 Brain waves of slow-wave and REM sleep cycles