CHAPTER 48 Nervous System
Nervous System Function: coordinates and controls bodily functions with nerves and electrical impulses The human brain Contains an estimated 100 billion nerve cells, or neurons Each neuron May communicate with thousands of other neurons Communication between neurons can be long-distance electrical signals or short-distance chemical signals
Nervous System In all vertebrates, the nervous system shows a high degree of cephalization and distinct CNS and PNS components Central nervous system (CNS) The brain provides the integrative power that underlies the complex behavior of vertebrates The spinal cord integrates simple responses to certain kinds of stimuli and conveys information to and from the brain Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Ganglia outside CNS Spinal nerves Figure 48.19
Information Processing Nervous systems process information in three stages Sensory input, integration, and motor output Sensory input Sensor Integration Motor output Figure 48.3 Effector Peripheral nervous system (PNS) Central nervous system (CNS)
Anatomy The central canal of the spinal cord and the four ventricles of the brain are hollow, since they are derived from the dorsal embryonic nerve cord 4 characteristics of chordates: notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail Gray matter White matter Ventricles Figure 49.5
Anatomy Gray matter no myelin sheath Located on outside in brain and inside in spinal cord White matter has myelin sheath Located on outside in spinal cord and inside in brain
Peripheral Nervous System The PNS transmits information to and from the CNS Plays a large role in regulating a vertebrate s movement and internal environment Divisions of PNS: Sensory and Motor division sends signals to and from the CNS Motor divides into the Somatic nervous system and the Autonomic nervous system Autonomic nervous system divides into Parasympathetic, Sympathetic, and Enteric divisions
Peripheral Nervous System Somatic nervous system Carries signals to skeletal muscles Autonomic nervous system Regulates the internal environment, in an involuntary manner Carries signals to cardiac muscle, smooth muscle, and glands Sympathetic and parasympathetic divisions Antagonistic effects on target organs
Parasympathetic division Sympathetic division Action on target organs: Action on target organs: Location of preganglionic neurons: brainstem and sacral segments of spinal cord Constricts pupil of eye Stimulates salivary gland secretion Dilates pupil of eye Inhibits salivary gland secretion Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Neurotransmitter released by preganglionic neurons: acetylcholine Constricts bronchi in lungs Slows heart Cervical Sympathetic ganglia Relaxes bronchi in lungs Accelerates heart Neurotransmitter released by preganglionic neurons: acetylcholine Location of postganglionic neurons: in ganglia close to or within target organs Stimulates activity of stomach and intestines Stimulates activity of pancreas Thoracic Inhibits activity of stomach and intestines Inhibits activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Location of postganglionic neurons: some in ganglia close to target organs; others in a chain of ganglia near spinal cord Neurotransmitter released by postganglionic neurons: acetylcholine Stimulates gallbladder Promotes emptying of bladder Lumbar Stimulates adrenal medulla Inhibits emptying of bladder Neurotransmitter released by postganglionic neurons: norepinephrine Figure 49.8 Promotes erection of genitalia Synapse Sacral Promotes ejaculation and vaginal contractions
Autonomic Nervous System The sympathetic division Correlates with the fight-or-flight response The parasympathetic division Promotes a return to self-maintenance functions Resting and digesting The enteric division Controls the activity of the digestive tract, pancreas, and gallbladder Controlled by other 2 divisions
Neuron Structure Most of a neuron s organelles are located in the cell body with cytoplasmic projections off the cell body Dendrites Cell body Nucleus Axon Signal direction Axon hillock Synapse Presynaptic cell Myelin sheath Postsynaptic cell Figure 48.4 Synaptic terminals
Neuron Structure Most neurons have dendrites Highly branched extensions that receive signals from other neurons The axon is typically a much longer extension Transmits signals to other cells at synapses May be covered with a myelin sheath fatty cell that wraps around the axon
Types of Neurons Neurons have a wide variety of shapes that reflect their input and output interactions Dendrites Axon Cell body Figure 48.5 (a) Sensory neuron (b) Interneurons (c) Motor neuron
Types of Neurons Sensory neurons transmit information from sensory receptors to the CNS Detects external stimuli and internal conditions Interneurons integrate the information in the CNS Motor output leaves the CNS via motor neurons and travels to the PNS Neuron communicates with effector cells (muscles and glands)
Three stages of information processing Reflex body s automatic responses to certain stimuli 2 Sensors detect 3 Sensory neurons a sudden stretch in the quadriceps. convey the information to the spinal cord. Quadriceps muscle Cell body of sensory neuron in dorsal root ganglion 4 The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. White matter Gray matter 5 Sensory neurons from the quadriceps also communicate with interneurons in the spinal cord. 1 The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle. Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron 6 The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. Figure 49.3
Supporting Cells (Glia) Essential for the structural integrity of the nervous system and for the normal functioning of neurons CNS Astrocytes nutrients to neurons in CNS Oligodendrocytes protection Ependymal cells lines ventricles and has cilia to move cerebrospinal fluid Microglial cells protection against microorganisms PNS Schwann cells protection
50 µm Astrocytes Provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters Helps create the blood-brain barrier that regulates substances entering the brain tissue Figure 49.6
Oligodendrocytes and Schwann Cells Glia that form the myelin sheaths around the axons of many vertebrate neurons Helps to transmit signals faster down the axon Node of Ranvier Layers of myelin Axon Axon Myelin sheath Schwann cell Nodes of Ranvier Schwann cell Nucleus of Schwann cell Figure 48.12 0.1 µm
Nerve Physiology Across the plasma membrane, every cell has a voltage called a membrane potential The inside of a cell is negative relative to the outside and is measured using a voltmeter The resting potential is the membrane potential of a neuron that is not transmitting signals Resting membrane potential = - 70mV
Resting Membrane Potential In all neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane Ion pumps and ion channels maintain the resting potential of a neuron CYTOSOL [Na + ] 15 mm [K + ] 150 mm [Cl ] 10 mm [A ] 100 mm EXTRACELLULAR FLUID + [Na + ] 150 mm + + + + [K + ] 5 mm [Cl ] 120 mm Figure 48.6 Plasma membrane
Resting Membrane Potential The concentration of Na + is higher in the extracellular fluid than in the cytosol while the opposite is true for K + A neuron that is not transmitting signals contains many open K + channels and fewer open Na + channels in its plasma membrane The diffusion of K + and Na + through these channels leads to a separation of charges across the membrane, producing the resting potential
Why is charge -70 mv? Inner chamber Outer 90 mv chamber Inner +62 mv chamber Outer chamber 150 mm KCL + 5 mm KCL 15 mm NaCl + 150 mm NaCl + Cl + Potassium channel + K + Artificial membrane Cl Na + Sodium channel + Figure 48.7 (a) Membrane selectively permeable to K + (b) Membrane selectively permeable to Na +
Action Potential Gated ion channels open or close in response to the binding of a specific ligand or a voltage change The response is a change in the membrane potential When ion channels open, two different responses can occur: hyperpolarization or depolarization Both are called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus
Membrane potential (mv) Cell Responses Stimuli +50 Some stimuli trigger a hyperpolarization An increase in the magnitude of the membrane potential 0 50 Threshold Resting potential Hyperpolarizations Figure 48.9 100 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K +. The larger stimulus produces a larger hyperpolarization.
Membrane potential (mv) Cell Responses +50 Stimuli Other stimuli trigger a depolarization A reduction in the magnitude of the membrane potential Figure 48.9 0 50 Threshold Resting potential Depolarizations 100 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+. The larger stimulus produces a larger depolarization.
Membrane potential (mv) Cell Responses A stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response, called an action potential Threshold = membrane voltage amount that causes an action potential - 55 mv Figure 48.9 +50 0 50 Stronger depolarizing stimulus Action potential Threshold Resting potential 100 0 1 2 3 4 5 6 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold.
Action Potential An action potential is a brief all-or-none depolarization of a neuron s plasma membrane that carries information along axons Both voltage-gated Na + channels and voltage-gated K + channels are involved in the production of an action potential
Action Potential When a stimulus depolarizes the membrane Na + channels open, allowing Na + to diffuse into the cell As the action potential subsides K + channels open, and K + flows out of the cell A refractory period follows the action potential during which a second action potential cannot be initiated
Conduction of Action Potentials An action potential can travel long distances by regenerating itself along the axon At the site where the action potential is generated, usually the axon hillock an electrical current depolarizes the neighboring region of the axon membrane The speed of an action potential increases with the diameter of an axon In vertebrates, axons are myelinated causes the speed of an action potential to increase
Conduction of Action Potentials Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Schwann cell Depolarized region (node of Ranvier) Myelin sheath Figure 48.13 Cell body + + + + + + + + + Axon
Synapse Synapse connects one cell to another Space between the two cells is called the synaptic cleft In an electrical synapse electrical current flows directly from one cell to another via a gap junction The vast majority of synapses are chemical synapses
5 µm Synapse In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal Postsynaptic neuron Synaptic terminal of presynaptic neurons Figure 48.14
Synapse When an action potential reaches a terminal the final result is the release of neurotransmitters into the synaptic cleft Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Presynaptic membrane 5 Na + K + Neurotransmitter Postsynaptic membrane Voltage-gated Ca 2+ channel Ligandgated ion channel 1 Ca 2+ 2 Synaptic cleft 3 4 Postsynaptic membrane 6 Figure 48.15 Ligand-gated ion channels
Direct Synaptic Transmission The process of direct synaptic transmission involves the binding of neurotransmitters to ligand-gated ion channels Neurotransmitter binding causes the ion channels to open, generating a postsynaptic potential Postsynaptic potentials fall into two categories: Excitatory or Inhibitory
Direct Synaptic Transmission After its release, the neurotransmitter diffuses out of the synaptic cleft May be taken up by surrounding cells and degraded by enzymes
Neurotransmitters Chemical messengers that act on cells to create a response The same neurotransmitter can produce different effects in different types of cells Types: Acetylcholine, biogenic amines, various amino acids and peptides, and certain gases
Neurotransmitters Acetylcholine is one of the most common neurotransmitters in both vertebrates and invertebrates Can be inhibitory or excitatory Used in muscle contraction Biogenic amines: include epinephrine, norepinephrine, dopamine, and serotonin Are active in the CNS and PNS
Neurotransmitters Various amino acids and peptides are active in the brain Gases such as nitric oxide and carbon monoxide are local regulators in the PNS
Animal Nervous Diversity All animals except sponges have some type of nervous system What distinguishes the nervous systems of different animal groups is how the neurons are organized into circuits
Animal Nervous Diversity The simplest animals with nervous systems, the cnidarians have neurons arranged in nerve nets Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring Radial nerve Nerve net Nerve ring (a) Hydra (cnidarian) Figure 49.2 (b) Sea star (echinoderm)
Animal Nervous Diversity In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident Annelids and arthropods have segmentally arranged clusters of neurons called ganglia These ganglia connect to the CNS and make up a peripheral nervous system (PNS) Eyespot Brain Nerve cord Transverse nerve Brain Ventral nerve cord Segmental ganglion Brain Ventral nerve cord Segmental ganglia Figure 49.2 (c) Planarian (flatworm) (d) Leech (annelid) (e) Insect (arthropod)
Animal Nervous Diversity Nervous systems in molluscs correlate with the animals lifestyles Sessile molluscs have simple systems while more complex molluscs have more sophisticated systems Anterior nerve ring Ganglia Brain Longitudinal nerve cords Ganglia Figure 49.2 (f) Chiton (mollusc) (g) Squid (mollusc)
Animal Nervous Diversity In vertebrates, the central nervous system consists of a brain and dorsal spinal cord The PNS connects to the CNS Brain Spinal cord (dorsal nerve cord) Sensory ganglion Figure 49.2 (h) Salamander (chordate)
Embryonic Development of the Brain In all vertebrates, the brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain By the fifth week of human embryonic development, five brain regions have formed from the three embryonic regions As a human brain develops further the most profound change occurs in the forebrain, which gives rise to the cerebrum
Functional magnetic resonance imaging Technology that can reconstruct a three-dimensional map of brain activity The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks Figure 48.1
Brainstem The brainstem consists of three parts: medulla oblongata, pons, and midbrain The medulla oblongata contains centers that control heart rate, blood pressure, breathing, swallowing, and vomiting The pons controls breathing The midbrain contains centers for the passing ascending and descending signals
Arousal and Sleep A diffuse network of neurons called the reticular formation is present in the core of the brainstem A part of the reticular formation, the reticular activating system (RAS) regulates sleep and arousal Eye Reticular formation Input from touch, pain, and temperature receptors Input from ears Figure 49.10
Cerebellum The cerebellum is important for coordination and balance Also involved in learning and remembering motor skills
Diencephalon The embryonic diencephalon develops into three adult brain regions: epithalamus, thalamus, and hypothalamus The epithalamus includes the pineal gland (releases melatonin) and the choroid plexus (capillaries that produce cerebrospinal fluid) The thalamus sends sensory information to the cerebrum and sends motor information from the cerebrum
Diencephalon The hypothalamus regulates homeostasis Basic survival behaviors such as feeding, fighting, fleeing, and reproducing Also known as the limbic center The hypothalamus regulates circadian rhythms Such as the sleep/wake cycle (biological clock) Biological clocks usually require external cues to remain synchronized with environmental cycles
Cerebrum The cerebrum contains right and left cerebral hemispheres Each consist of cerebral cortex overlying white matter and basal nuclei (regions of gray matter inside brain) centers for planning and learning movement sequences Left cerebral hemisphere Corpus callosum Right cerebral hemisphere Basal nuclei Figure 49.13
Cerebrum A thick band of axons, the corpus callosum provides communication between the right and left cerebral cortices In humans, the largest and most complex part of the brain is the cerebral cortex, where sensory information is analyzed, motor commands are issued, and language is generated
Cerebrum Each side of the cerebral cortex has four lobes Frontal, parietal, temporal, and occipital Frontal lobe Parietal lobe Frontal association area Speech Smell Hearing Auditory association area Speech Taste Somatosensory association area Reading Visual association area Vision Figure 48.27 Temporal lobe Occipital lobe
Cerebrum In the somatosensory cortex and motor cortex neurons are distributed according to the part of the body that generates sensory input or receives motor input Frontal lobe Parietal lobe Toes Genitalia Lips Jaw Tongue Figure 48.28 Primary motor cortex Tongue Pharynx Abdominal organs Primary somatosensory cortex
Lateralization The left hemisphere becomes more adept at language, math, logical operations, and the processing of serial sequences The right hemisphere is stronger at pattern recognition, nonverbal thinking, and emotional processing
Language and Speech Studies of brain activity have mapped specific areas of the brain responsible for language and speech Max Hearing words Seeing words Figure 48.29 Speaking words Generating words Min
Emotions The limbic system is a ring of structures around the brainstem Thalamus Hypothalamus Prefrontal cortex Figure 48.30 Olfactory bulb Amygdala Hippocampus
Emotions This limbic system includes three parts of the cerebral cortex: amygdala, hippocampus, and olfactory bulb These structures attach emotional feelings to survival-related functions Structures of the limbic system form in early development and provide a foundation for emotional memory, associating emotions with particular events or experiences
Memory and Learning The frontal lobes are a site of short-term memory Interact with the hippocampus and amygdala to consolidate long-term memory Many sensory and motor association areas of the cerebral cortex are involved in storing and retrieving words and images
Neural Stem Cells The adult human brain contains stem cells that can differentiate into mature neurons The induction of stem cell differentiation and the transplantation of cultured stem cells are potential methods for replacing neurons lost to trauma or disease Figure 49.24
Nervous Disorders Unlike the PNS, the mammalian CNS cannot repair itself when damaged or assaulted by disease Current research on nerve cell development and stem cells may one day make it possible for physicians to repair or replace damaged neurons Mental illnesses and neurological disorders take an enormous toll on society, in both the patient s loss of a productive life and the high cost of long-term health care
Schizophrenia About 1% of the world s population suffers from schizophrenia Schizophrenia is characterized by hallucinations, delusions, blunted emotions, and many other symptoms Available treatments have focused on brain pathways that use dopamine as a neurotransmitter
Depression Two broad forms of depressive illness are known Bipolar disorder and major depression Bipolar disorder is characterized by manic (high-mood) and depressive (low-mood) phases In major depression patients have a persistent low mood Treatments for these types of depression include a variety of drugs such as Prozac and lithium
Alzheimer s Disease Alzheimer s disease (AD) is a mental deterioration characterized by confusion, memory loss, and other symptoms AD is caused by the formation of neurofibrillary tangles and senile plaques in the brain A successful treatment for AD in humans may hinge on early detection of senile plaques
Parkinson s Disease Parkinson s disease is a motor disorder caused by the death of dopamine-secreting neurons in the substantia nigra Characterized by difficulty in initiating movements, slowness of movement, and rigidity There is no cure for Parkinson s disease although various approaches are used to manage the symptoms