Nervous and Sensory Systems 38 Nervous and Sensory Systems

Aim: How can we describe the peripheral and central nervous system Aim: How can we describe the peripheral and central nervous system? Do Now: Describe the main differences between the peripheral and central nervous system. Homework: Read chapter 38 and complete concept questions.

Concept 38.1: Nervous systems consist of circuits of neurons and supporting cells The ability to sense and react originated billions of years ago in prokaryotes Hydras, jellies, and cnidarians are the simplest animals with nervous systems In most cnidarians, interconnected nerve cells form a nerve net, which controls contraction and expansion of the gastro-vascular cavity

(b) Planarian (flatworm) Figure 38.2 Eyespot Brain Nerve cords Transverse nerve Nerve net (a) Hydra (cnidarian) (b) Planarian (flatworm) Brain Brain Ventral nerve cord Spinal cord (dorsal nerve cord) Figure 38.2 Nervous system organization Sensory ganglia Segmental ganglia (c) Insect (arthropod) (d) Salamander (vertebrate) 4

In more complex animals, the axons of multiple nerve cells are often bundled together to form nerves These fibrous structures channel and organize information flow through the nervous system Animals with elongated, bilaterally symmetrical bodies have even more specialized systems

Cephalization is an evolutionary trend toward a clustering of sensory neurons and interneurons at the anterior Nonsegmented worms have the simplest clearly defined central nervous system (CNS), consisting of a small brain and longitudinal nerve cords

Annelids and arthropods have segmentally arranged clusters of neurons called ganglia In vertebrates: The CNS is composed of the brain and spinal cord The peripheral nervous system (PNS) is composed of nerves and ganglia

Glia Glia have numerous functions to nourish, support, and regulate neurons: Embryonic radial glia form tracks along which newly formed neurons migrate Astrocytes (star-shaped glial cells) induce cells lining capillaries in the CNS to form tight junctions, resulting in a blood-brain barrier

CNS PNS VENTRICLE Neuron Cilia Oligodendrocyte Schwann cell Figure 38.3 CNS PNS VENTRICLE Neuron Cilia Oligodendrocyte Schwann cell Microglial cell Capillary Ependymal cell Astrocytes Figure 38.3 Glia in the vertebrate nervous system Intermingling of astrocytes with neurons (blue) 50 m LM 9

Astrocytes Intermingling of astrocytes with neurons (blue) 50 m LM Figure 38.3a Astrocytes Intermingling of astrocytes with neurons (blue) Figure 38.3a Glia in the vertebrate nervous system (LM) 50 m LM 10

Organization of the Vertebrate Nervous System The spinal cord runs lengthwise inside the vertebral column (the spine) The spinal cord conveys information to and from the brain It can also act independently of the brain as part of simple nerve circuits that produce reflexes, the body’s automatic responses to certain stimuli

Central nervous Peripheral nervous system (CNS) system (PNS) Brain Figure 38.4 Central nervous system (CNS) Peripheral nervous system (PNS) Brain Cranial nerves Spinal cord Ganglia outside CNS Spinal nerves Figure 38.4 The vertebrate nervous system 12

The brain and spinal cord contain: Gray matter, which consists mainly of neuron cell bodies and glia White matter, which consists of bundles of myelinated axons

The CNS contains fluid-filled spaces called ventricles in the brain and the central canal in the spinal cord Cerebrospinal fluid is formed in the brain and circulates through the ventricles and central canal and drains into the veins It supplies the CNS with nutrients and hormones and carries away wastes

The Peripheral Nervous System The PNS transmits information to and from the CNS and regulates movement and the internal environment In the PNS, afferent neurons transmit information to the CNS and efferent neurons transmit information away from the CNS

The PNS has two efferent components: The motor system carries signals to skeletal muscles and can be voluntary or involuntary The autonomic nervous system regulates smooth and cardiac muscles and is generally involuntary

The autonomic nervous system has sympathetic, parasympathetic, and enteric divisions The enteric division controls activity of the digestive tract, pancreas, and gallbladder The sympathetic division regulates the “fight-or-flight” response The parasympathetic division generates opposite responses in target organs and promotes calming and a return to “rest and digest” functions

Aim: How can we describe the various parts of the brain and how they function? Do Now: Describe the different functions of the left and right hemispheres of the brain?

Concept 38.2: The vertebrate brain is regionally specialized The human brain contains 100 billion neurons These cells are organized into circuits that can perform highly sophisticated information processing, storage, and retrieval

Embryonic brain regions Brain structures in child and adult Figure 38.6b Embryonic brain regions Brain structures in child and adult Cerebrum (includes cerebral cortex, white matter, basal nuclei) Telencephalon Forebrain Diencephalon (thalamus, hypothalamus, epithalamus) Diencephalon Midbrain Mesencephalon Midbrain (part of brainstem) Metencephalon Pons (part of brainstem), cerebellum Hindbrain Myelencephalon Medulla oblongata (part of brainstem) Mesencephalon Cerebrum Diencephalon Metencephalon Midbrain Diencephalon Myelencephalon Hindbrain Figure 38.6b Exploring the organization of the human brain (part 2: brain development) Midbrain Pons Medulla oblongata Spinal cord Forebrain Telencephalon Cerebellum Spinal cord Embryo at 1 month Embryo at 5 weeks Child 20

Adult brain viewed from the rear Figure 38.6c Left cerebral hemisphere Right cerebral hemisphere Cerebral cortex Corpus callosum Cerebrum Basal nuclei Figure 38.6c Exploring the organization of the human brain (part 3: cerebrum and cerebellum) Cerebellum Adult brain viewed from the rear 21

Diencephalon Thalamus Pineal gland Brainstem Hypothalamus Midbrain Figure 38.6d Diencephalon Thalamus Pineal gland Brainstem Hypothalamus Midbrain Pituitary gland Pons Figure 38.6d Exploring the organization of the human brain (part 4: diencephalon and brainstem) Medulla oblongata Spinal cord 22

Arousal and Sleep Arousal is a state of awareness of the external world Sleep is a state in which external stimuli are received but not consciously perceived Arousal and sleep are controlled in part by clusters of neurons in the midbrain and pons

Sleep is an active state for the brain and is regulated by the biological clock and regions of the forebrain, which regulate the intensity and duration of sleep Some animals have evolutionary adaptations that allow for substantial activity during sleep For example, in dolphins, only one side of the brain is asleep at a time

Biological Clock Regulation Cycles of sleep and wakefulness are examples of circadian rhythms, daily cycles of biological activity Mammalian circadian rhythms rely on a biological clock, a molecular mechanism that directs periodic gene expression Biological clocks are typically synchronized to light and dark cycles and maintain a roughly 24-hour cycle

The SCN acts as a pacemaker, synchronizing the biological clock In mammals, circadian rhythms are coordinated by a group of neurons in the hypothalamus called the suprachiasmatic nucleus (SCN) The SCN acts as a pacemaker, synchronizing the biological clock 26

Emotions Generation and experience of emotions involve many brain structures including the amygdala, hippocampus, and parts of the thalamus These structures are grouped as the limbic system Generation and experience of emotion also require interaction between the limbic system and sensory areas of the cerebrum The brain structure that is most important for emotional memory is the amygdala 27

The Brain’s Reward System and Drug Addiction The brain’s reward system provides motivation for activities that enhance survival and reproduction The brain’s reward system is dramatically affected by drug addiction Drug addiction is characterized by compulsive consumption and an inability to control intake

Addictive drugs such as cocaine, amphetamine, heroin, alcohol, and tobacco enhance the activity of the dopamine pathway Drug addiction leads to long-lasting changes in the reward circuitry that cause craving for the drug

Functional Imaging of the Brain Functional imaging methods are transforming our understanding of normal and diseased brains In positron-emission tomography (PET) an injection of radioactive glucose enables a display of metabolic activity

In functional magnetic resonance imaging, fMRI, the subject lies with his or her head in the center of a large, doughnut-shaped magnet Brain activity is detected by changes in local oxygen concentration Applications of fMRI include monitoring recovery from stroke, mapping abnormalities in migraine headaches, and increasing the effectiveness of brain surgery

Concept 38.3: The cerebral cortex controls voluntary movement and cognitive functions The cerebrum is essential for language, cognition, memory, consciousness, and awareness of our surroundings The cognitive functions reside mainly in the cortex, the outer layer Four regions, or lobes (frontal, temporal, occipital, and parietal), are landmarks for particular functions 32

(comprehending language) Cerebellum Figure 38.11 Somatosensory cortex (sense of touch) Motor cortex (control of skeletal muscles) Parietal lobe Frontal lobe Sensory association cortex (integration of sensory information) Prefrontal cortex (decision making, planning) Visual association cortex (combining images and object recognition) Broca’s area (forming speech) Occipital lobe Figure 38.11 The human cerebral cortex Temporal lobe Visual cortex (processing visual stimuli and pattern recognition) Auditory cortex (hearing) Wernicke’s area (comprehending language) Cerebellum 33

Language and Speech The mapping of cognitive functions within the cortex began in the 1800s Broca’s area, in the left frontal lobe, is active when speech is generated Wernicke’s area, in the posterior of the left frontal lobe, is active when speech is heard

Lateralization of Cortical Function The left side of the cerebrum is dominant regarding language, math, and logical operations The right hemisphere is dominant in recognition of faces and patterns, spatial relations, and nonverbal thinking The establishment of differences in hemisphere function is called lateralization The two hemispheres exchange information through the fibers of the corpus callosum Severing this connection results in a “split brain” effect, in which the two hemispheres operate independently

Information Processing The cerebral cortex receives input from sensory organs and somatosensory receptors Somatosensory receptors provide information about touch, pain, pressure, temperature, and the position of muscles and limbs The thalamus directs different types of input to distinct locations

Frontal Lobe Function Frontal lobe damage may impair decision making and emotional responses but leave intellect and memory intact The frontal lobes have a substantial effect on “executive functions”

Evolution of Cognition in Vertebrates In nearly all vertebrates, the brain has the same number of divisions The hypothesis that higher order reasoning requires a highly convoluted cerebral cortex has been experimentally refuted The anatomical basis for sophisticated information processing in birds (without a highly convoluted neocortex) appears to be a cluster of nuclei in the top or outer portion of the brain (pallium)

Aim: How can we explain how organisms maintain equilibrium Aim: How can we explain how organisms maintain equilibrium? Do Now: What human structure/organ contributes to maintaining equilibrium the most?

Neural Plasticity Neural plasticity is the capacity of the nervous system to be modified after birth Changes can strengthen or weaken signaling at a synapse Autism, a developmental disorder, involves a disruption of activity-dependent remodeling at synapses Children with autism display impaired communication and social interaction, as well as stereotyped and repetitive behaviors

(a) Synapses are strengthened or weakened in response to activity. Figure 38.14 N1 N1 N2 N2 (a) Synapses are strengthened or weakened in response to activity. Figure 38.14 Neural plasticity (b) If two synapses are often active at the same time, the strength of the postsynaptic response may increase at both synapses. 41

Memory and Learning Neural plasticity is essential to formation of memories Short-term memory is accessed via the hippocampus The hippocampus also plays a role in forming long-term memory, which is stored in the cerebral cortex Some consolidation of memory is thought to occur during sleep

Concept 38.4: Sensory receptors transduce stimulus energy and transmit signals to the central nervous system Much brain activity begins with sensory input A sensory receptor detects a stimulus, which alters the transmission of action potentials to the CNS The information is decoded in the CNS, resulting in a sensation

Sensory Reception and Transduction A sensory pathway begins with sensory reception, detection of stimuli by sensory receptors Sensory receptors, which detect stimuli, interact directly with stimuli, both inside and outside the body Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor This change in membrane potential is called a receptor potential Receptor potentials are graded; their magnitude varies with the strength of the stimulus 44

(a) Receptor is afferent neuron. Figure 38.15 (a) Receptor is afferent neuron. (b) Receptor regulates afferent neuron. To CNS To CNS Afferent neuron Afferent neuron Receptor protein Neurotransmitter Sensory receptor Figure 38.15 Neuronal and non-neuronal sensory receptors Stimulus leads to neuro- transmitter release. Stimulus Sensory receptor cell Stimulus 45

Transmission Sensory information is transmitted as nerve impulses or action potentials Neurons that act directly as sensory receptors produce action potentials and have an axon that extends into the CNS Non-neuronal sensory receptors form chemical synapses with sensory neurons They typically respond to stimuli by increasing the rate at which the sensory neurons produce action potentials 46

The response of a sensory receptor varies with intensity of stimuli If the receptor is a neuron, a larger receptor potential results in more frequent action potentials If the receptor is not a neuron, a larger receptor potential causes more neurotransmitter to be released 47

Perception Perception is the brain’s construction of stimuli Action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus The brain thus distinguishes stimuli, such as light or sound, solely by the path along which the action potentials have arrived Amplification is the strengthening of stimulus energy by cells in sensory pathways Sensory adaptation is a decrease in responsiveness to continued stimulation 48