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The Mammalian Nervous System: Structure and Higher Function 47 The Mammalian Nervous System: Structure and Higher Function •47.1 How Is the Mammalian Nervous System Organized? •47.2 How Is Information Processed by Neural Networks? •47.3 Can Higher Functions Be Understood in Cellular Terms? 47.1 How Is the Mammalian Nervous System Organized? Vertebrate nervous systems: Brain, spinal cord, and peripheral nerves that extend throughout the body. Central nervous system (CNS): Brain and spinal cord. Peripheral nervous system (PNS): Cranial and spinal nerves that connect the CNS to all tissues. 47.1 How Is the Mammalian Nervous System Organized? Neuron: An excitable cell that communicates via an axon. Nerve: A bundle of axons that carries information. The afferent part of the PNS carries sensory information to the CNS. The efferent part of the PNS carries information from the CNS to muscles and glands. 47.1 How Is the Mammalian Nervous System Organized? Efferent pathways can be divided into two divisions: •The voluntary division, which executes conscious movements •The involuntary, or autonomic, division, which controls physiological functions Figure 47.1 Organization of the Nervous System 47.1 How Is the Mammalian Nervous System Organized? The CNS receives neuronal information from the PNS and chemical information from hormones circulating in the blood. Neurohormones released by neurons can send information to other neurons or enter the circulatory system. 47.1 How Is the Mammalian Nervous System Organized? The CNS develops from the neural tube of an embryo. The anterior part of the neural tube develops into the hindbrain, midbrain, and forebrain. The rest of the neural tube becomes the spinal cord Cranial and spinal nerves also form. Figure 47.2 Development of the Central Nervous System (Part 1) Figure 47.2 Development of the Central Nervous System (Part 2) Figure 47.2 Development of the Central Nervous System (Part 3) 47.1 How Is the Mammalian Nervous System Organized? The three regions of embryonic brain develop into adult brain structures. The hindbrain becomes the medulla, the pons, and the cerebellum. Physiological functions, such as breathing and swallowing, are controlled by the medulla and pons. Muscle control is coordinated in the cerebellum. 47.1 How Is the Mammalian Nervous System Organized? The embryonic midbrain becomes structures that process visual and auditory information. Together the hindbrain and midbrain are known as the brainstem. 47.1 How Is the Mammalian Nervous System Organized? The embryonic forebrain develops the diencephalon and telencephalon. The diencephalon consists of the: •Thalamus—the final relay station for sensory information •Hypothalamus—regulates physiological functions such as hunger and thirst 47.1 How Is the Mammalian Nervous System Organized? The telencephalon, or cerebrum, consists of two cerebral hemispheres. •Telencephalization: Evolutionary trend in vertebrates; the telencephalon increases in size and complexity •Largest brain region in humans: involved in sensory perception, learning, memory, and behavior 47.1 How Is the Mammalian Nervous System Organized? The spinal cord: •Conducts information between brain and organs •Integrates information coming from PNS •Responds by issuing motor commands 47.1 How Is the Mammalian Nervous System Organized? Anatomy of the spinal cord: •Gray matter is in the center, and contains cell bodies of spinal neurons •White matter surrounds gray matter and contains axons that conduct information up and down the spinal cord •Spinal nerves extend from the spinal cord 47.1 How Is the Mammalian Nervous System Organized? Each spinal nerve has two roots. •One spinal root connects to the dorsal horn, the other to the ventral horn •Afferent (sensory) axons enter through the dorsal root •Efferent (motor) axons leave through the ventral root 47.1 How Is the Mammalian Nervous System Organized? Spinal reflex—afferent information converts to efferent activity without the brain. The knee-jerk reflex is monosynaptic: •Stretch receptors send axon potentials through dorsal horn to ventral horn, via sensory axons •At synapses with motor neurons in the ventral horn, action potentials are sent to leg muscles, causing contraction 47.1 How Is the Mammalian Nervous System Organized? Most spinal circuits are more complex—limb movement is controlled by antagonistic muscle sets. •Flexors bend or flex the limb •Extensors straighten or extend the limb Coordination of relaxation and contraction is done by interneurons. Figure 47.3 The Spinal Cord Coordinates the Knee-Jerk Reflex 47.1 How Is the Mammalian Nervous System Organized? The reticular system is a network of neurons in the brainstem. Distinct group of CNS neurons is a nucleus. Reticular formation activity high in the brainstem controls sleep and wakefulness, known as the reticular activating system. Low to mid-brainstem activity is involved with balance, coordination. 47.1 How Is the Mammalian Nervous System Organized? Structures in primitive regions of the telencephalon form the limbic system—responsible for basic physiological drives. •Amygdala—involved in fear and fear memory •Hippocampus—transfers short-term memory to long-term memory Figure 47.4 The Limbic System 47.1 How Is the Mammalian Nervous System Organized? The cerebrum is the dominant structure in mammals. Cerebral cortex—a sheet of gray matter covering each hemisphere that is convoluted to fit into the skull. •Gyri (gyrus)—ridges of the cortex •Sulci (sulcus)—valleys of the cortex Figure 47.5 The Human Cerebrum (Part 1) Figure 47.5 The Human Cerebrum (Part 2) 47.1 How Is the Mammalian Nervous System Organized? Regions of the cerebral cortex have specific functions. Association cortex is made up of areas that integrate or associate sensory information or memories. 47.1 How Is the Mammalian Nervous System Organized? Four cortical lobes: •Temporal •Frontal •Parietal •Occipital 47.1 How Is the Mammalian Nervous System Organized? Temporal lobe: •Receives and processes auditory information •Association areas of the temporal lobe involve: •Identification •Object naming •Recognition Agnosia: Disorder of the temporal lobe Fi gur e47. 6“ FaceNeur ons”i nOneRegi onoft heTempor alLobe 47.1 How Is the Mammalian Nervous System Organized? Frontal Lobe: •Central sulcus—divides the frontal and parietal lobes •Primary motor cortex is located in front of the central sulcus and controls muscles in specific body areas •Association areas involve: •Planning •Personality Figure 47.7 The Body Is Represented in Primary Motor and Primary Somatosensory Cortex (Part 1) Figure 47.7 The Body Is Represented in Primary Motor and Primary Somatosensory Cortex (Part 2) Figure 47.8 A Mind-Altering Experience 47.1 How Is the Mammalian Nervous System Organized? Parietal lobe: •Primary somatosensory motor cortex—behind the central sulcus; receives touch and pressure information •Involves attending to complex stimuli Contralateral neglect syndrome: Unable to recognize stimuli on one side of body when the opposite parietal lobe is damaged 47.1 How Is the Mammalian Nervous System Organized? Occipital lobe: •Receives and processes visual information •Association areas involve: •Making sense of the visual world •Translating visual experience into language 47.1 How Is the Mammalian Nervous System Organized? There is a correlation between body size and brain size. In mammals the forebrain is larger than other structures. Evolutionary changes in the cortex provide for intellectual capacity. Figure 47.9 Evolution of the Human Brain 47.2 How Is Information Processed by Neural Networks? Autonomic Nervous System (ANS)— the output of the CNS that controls involuntary functions. ANS has two divisions that work in opposition—one will increase a function and the other will decrease it. Sympathetic and parasympathetic divisions are distinguished by anatomy, neurotransmitters, and their actions. Figure 47.10 The Autonomic Nervous System 47.2 How Is Information Processed by Neural Networks? Autonomic efferent pathways begin with cholinergic neurons that use ACh and have cell bodies in the brainstem or spinal cord. These preganglionic neurons synapse on a second neuron outside the CNS, in a collection of neurons called a ganglion. The second neuron is postganglionic— its axon leaves the ganglion and synapses in the target organs. 47.2 How Is Information Processed by Neural Networks? Postganglionic neurons of the parasympathetic division are mostly cholinergic. Sympathetic postganglionic neurons— noradrenergic;use norepinephrine as their neurotransmitter. Target cells respond in opposite ways to acetylcholine and norepinephrine. Example: Pacemaker cells in the heart. 47.2 How Is Information Processed by Neural Networks? Sympathetic and parasympathetic divisions have different anatomy. The sacral region contains preganglionic neurons of the parasympathetic region. The thoracic and lumbar regions contain sympathetic preganglionic neurons. 47.2 How Is Information Processed by Neural Networks? The sympathetic division is specialized to innervate the fight-or-flight response in the adrenal gland. Preganglionic sympathetic neurons send axons to the adrenal. Hormone-secreting cells in the adrenal are actually modified neurons— secrete neurotransmitters that act as hormones into the circulation. 47.2 How Is Information Processed by Neural Networks? In the retina, there is convergence of information. A receptive field of photoreceptors that receive information from a small area of the visual field activates a ganglion cell. The ganglion cell transmits information to the thalamus and then to the visual cortex, part of the occipital lobe. 47.2 How Is Information Processed by Neural Networks? Receptive fields have two concentric regions, a center and a surround. A field can be either on- or off-center. Light falling on an on-center receptive field excites the ganglion cell, while light falling on an off-center receptive field inhibits the ganglion cell. The surround area has the opposite effect so ganglion cell activity depends on which part of the field is stimulated. 47.2 How Is Information Processed by Neural Networks? Photoreceptors send their information to ganglion cells via bipolar cells. Lateral connections through horizontal cells and amacrine cells modify communication. The receptive field of a ganglion cell results from a pattern of synapses between photoreceptors, bipolar cells and lateral connections. Figure 47.11 What Does the Eye Tell the Brain? (Part 1) Figure 47.11 What Does the Eye Tell the Brain? (Part 2) 47.2 How Is Information Processed by Neural Networks? Receptive fields of the visual cortex: Neurons in the visual cortex, like retinal ganglion cells, have receptive fields. Some cortical neurons, simple cells, are stimulated by bars of light, corresponding to rows of circular receptive fields of ganglion cells. 47.2 How Is Information Processed by Neural Networks? Other cortical neurons, complex cells, respond to light with a particular orientation, across the retina. Action potentials from one retinal ganglion cell are received by hundreds of cortical neurons. Figure 47.12 Cells in the Visual Cortex Respond to Specific Patterns of Light (Part 1) Figure 47.12 Cells in the Visual Cortex Respond to Specific Patterns of Light (Part 2) 47.2 How Is Information Processed by Neural Networks? Binocular vision results from overlapping fields of view. Optic nerves from each eye join at the optic chiasm. Half of the axons from each retina go to the opposite side of the brain. Figure 47.13 Anatomy of Binocular Vision (Part 1) Figure 47.13 Anatomy of Binocular Vision (Part 2) 47.2 How Is Information Processed by Neural Networks? Visual cortex is organized in stripes and columns. Stripes refer to areas across the cortex. Columns refer to areas across the depth of the cortex. 47.2 How Is Information Processed by Neural Networks? Cells in the border of a stripe or column are binocular cells: •Receive input from both eyes •Measure disparity of the stimulus and where it falls on each retina 47.3 Can Higher Functions Be Understood in Cellular Terms? Electroencephalogram (EEG): •Measures neuronal activity and records changes in electrical potential between electrodes, over time Electromyogram (EMG) records skeletal muscle activity. Electrooculogram (EOG) measures eye movement. Figure 47.14 Stages of Sleep (A) 47.3 Can Higher Functions Be Understood in Cellular Terms? In humans, there are two main sleep states: •Rapid-eye-movement (REM) sleep is when dreams occur. The brain inhibits skeletal muscle activity •Non-REM sleep has four progressive stages; stages 3 and 4 are slow-wave sleep Figure 47.14 Stages of Sleep (B) 47.3 Can Higher Functions Be Understood in Cellular Terms? In non-REM sleep neurons in thalamus and cerebral cortex are less responsive. When awake, the reticular formation is active and cells depolarize often. At sleep onset, activity slows in the reticular formation—less neurotransmitter is released, cells hyperpolarize and are less excitable. Cells in non-REM sleep fire in bursts. 47.3 Can Higher Functions Be Understood in Cellular Terms? During non-REM and REM transition: •Brainstem nuclei become active again •Firing bursts cease •Cortex can process information as cells at threshold can depolarize •Sensory and motor pathways are still inhibited; without this feedback the cortex may produce bizarre dreams 47.3 Can Higher Functions Be Understood in Cellular Terms? The lateralization of language functions shows that 97 percent occurs in the left brain hemisphere. Brain hemispheres are connected by the corpus callosum, a bundle of axons. An aphasia is a deficit in the ability to use or understand words; occurs after damage to the left hemisphere. 47.3 Can Higher Functions Be Understood in Cellular Terms? Language areas: •Br oca’ sar ea—in frontal lobe; damage results in slow or lost speech; still can read and understand language •Wer ni cke’ sar ea—in temporal lobe. Damage results in inability to speak sensibly; written or spoken language not understood. Still can produce speech 47.3 Can Higher Functions Be Understood in Cellular Terms? •Angular gyrus—adjacent area essential for integrating spoken and written language Spoken language input flows from audi t or ycor t ext oWer ni cke’ sar ea. Written language input flows from the visual cortex to the angular gyrus to Wer ni cke’ sar ea. 47.3 Can Higher Functions Be Understood in Cellular Terms? Speech commands are formulated in Wer ni cke’ sar ea,t r avel t oBr oca’ s area, and then to the primary motor cortex for production. Brain imaging shows metabolic differences in brain regions using language. Figure 47.15 Language Areas of the Cortex (Part 1) Figure 47.15 Language Areas of the Cortex (Part 2) Figure 47.16 Imaging Techniques Reveal Active Parts of the Brain 47.3 Can Higher Functions Be Understood in Cellular Terms? Learning: Modification of behavior by experience. Memory: What the nervous system retains. Long-term potentiation (LTP) and long-term depression (LTD) describe how synapses become more or less responsive to repeated stimuli. Both may be fundamental to learning and memory. 47.3 Can Higher Functions Be Understood in Cellular Terms? Associative learning occurs when two unrelated stimuli become linked to a response. A conditioned reflex is a type of associative learning. Exampl e:Sal i var yr ef l exi nPavl ov’ s dog. 47.3 Can Higher Functions Be Understood in Cellular Terms? Complex, or observational learning in humans has a pattern of three elements: •Wepayat t ent i ont oanot her ’ s behavior •We retain a memory of what we observe •We try to copy or use that information 47.3 Can Higher Functions Be Understood in Cellular Terms? Memories can be associated with specific brain regions and neuronal properties. Types of memory: •Immediate—events happening now •Short-term—lasts 10 to 15 minutes •Long-term—lasts from days to a lifetime 47.3 Can Higher Functions Be Understood in Cellular Terms? Memories are transferred from shortto long-term. Hippocampal or limbic system damage may prevent this transfer. 47.3 Can Higher Functions Be Understood in Cellular Terms? Declarative memory is of people, places, and things that can be recalled and described. Procedural memory is how to perform a motor task and cannot be described. Formation of fear memories involves amygdala. 47.3 Can Higher Functions Be Understood in Cellular Terms? Consciousness refers to being aware of yourself, your environment, and events occurring around you. Conscious experience requires a perception of self, using integration of information from the physical and social environment, with information from past experience.