Chapter 25 Communication units of nervous systems Detect information about internal and external conditions Issue commands for responsive actions.
Download ReportTranscript Chapter 25 Communication units of nervous systems Detect information about internal and external conditions Issue commands for responsive actions.
Chapter 25 Communication units of nervous systems Detect information about internal and external conditions Issue commands for responsive actions stimulus (output) Sensory neurons Detect and relay information receptors sensory neurons Interneurons Receive and process information integrators interneurons of brain, spinal cord Motor neurons Transmit signals from interneurons to effectors motor neurons effectors muscles, glands response (output) dendrites INPUT ZONE cell body axon TRIGGER ZONE CONDUCTING ZONE OUPUT ZONE axon endings Fig. 25-1b, p.423 Cells that metabolically assist, structurally support, and protect neurons Make up more than half the volume of the vertebrate nervous system Electrical gradient across membrane About -70 mV Maintained by sodium-potassium pump Potassium (K+) higher inside Sodium (Na+) higher outside more Na+ flows into the neuron more gated channels for Na+ open neuron becomes more positive inside K+ Na+ outside plasma membrane K+ Na+ inside p.424a interstitial fluid Na+/K+ pump cytoplasm passive transporters with open channels passive transporters with voltage-sensitive gated channels active transporters lipid bilayer of neuron membrane Brief reversal in membrane potential Voltage change causes voltage-gated channels in membrane to open Inside of neuron briefly becomes more positive than outside 1 Na+ Na+ K+ K+ K+ 2 Na+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ 3 Na+ Na+ 4 interstitial fluid cytoplasm Fig. 25-4a, p.425 Na+ Na+ Na+ Fig. 25-4b, p.425 K+ K+ K+ Na+ Na+ Na+ Fig. 25-4c, p.425 Na+/K+ pump K+ K+ K+ Na+ Na+ Na+ K+ Fig. 25-4d, p.425 more Na+ ions flow into the neuron more gated channels for Na+ open neuron becomes more positive inside All action potentials are the same size If stimulation is below threshold level, no action potential occurs If stimulation is above threshold level, cell always depolarizes to same level Once action potential peak is reached, Na+ gates close and K+ gates open Movement of K+ out of cell The inside of the cell once again becomes more negative than the outside action potential Membrane potential (millivolts) +20 0 -20 threshold -40 resting membrane potential -70 0 1 2 3 4 Time (milliseconds) 5 Action potential in one part of an axon brings neighboring region to threshold Action potential moves from one patch of membrane to another Can only move one direction Action potentials cannot jump from cell to cell Signal is transmitted from axon end, across a synaptic cleft, by chemical signals called neurotransmitters Gap between the terminal ending of an axon and the input zone of another cell plasma membrane of axon ending of presynaptic cell synaptic vesicle plasma membrane of postsynaptic cell synaptic cleft membrane receptor Action potential in axon ending triggers release of neurotransmitter from presynaptic cell into synaptic cleft vesicle inside presynaptic cell synaptic cleft postsynaptic cell Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell Binding of neurotransmitter to receptors opens ion gates in membrane of postsynaptic cell neurotransmitter ions receptor for neurotransmitter gated channel protein Many signals reach a neuron at the same time Signals may suppress or reinforce one another Whether or not an action potential occurs depends on the sum of the signals the neuron receives Synapse between motor neuron and skeletal muscle fiber Neuron releases chemical neurotransmitter acetylcholine (ACh) neuromuscular junction motor neuron axons from spinal cord to skeletal muscle fibers A Neuromuscular Junction transverse slice of spinal cord part of a skeletal muscle Fig. 25-6a, p.427 A Neuromuscular Junction muscle fiber axon ending Fig. 25-6b, p.427 Acetylcholine (ACh) Norepinephrine Epinephrine Dopamine Serotonin GABA After neurotransmitter has acted, it is quickly removed from synaptic cleft Molecules diffuse away, are pumped out, or broken down sensory neuron interneuron motor neuron Neurons are bundled in nerves Nerves are organized in circuits and reflex pathways Information from sensory neurons is relayed to interneurons in spinal cord and brain Motor neurons carry signals to body axon myelin sheath A bundle of axons enclosed within a connective tissue sheath many neurons inside a connective tissue sheath • Sheath blocks ion movements • Action potential must “jump” from node to node • Greatly enhances speed of transmission A condition in which nerve fibers lose their myelin Slows conduction Symptoms include visual problems, numbness, muscle weakness, and fatigue Automatic movements in response to stimuli In simplest reflex arcs, sensory neurons synapse directly on motor neurons Most reflexes involve an interneuron STIMULUS Biceps stretches. sensory neuron motor neuron RESPONSE Biceps contracts. All animals except sponges have some sort of nervous system Nerve cells interact with one another in signalconducting and information-processing highways rudimentary brain branching nerve nerve cord ganglion (one in most body segments) Earliest fishlike vertebrates had a hollow, tubular nerve cord Modification and expansion of nerve cord produced spinal cord and brain Nerve cord persists in vertebrate embryos as a neural tube Central nervous system (CNS) Brain Spinal cord Peripheral nervous system Nerves that thread through the body Major Nerves Brain cervical nerves (eight pairs) cranial nerves (twelve pairs) Spinal Cord thoracic nerves (twelve pairs) ulnar nerve (one in each arm) lumbar nerves (five pairs) sacral nerves (five pairs) coccygeal nerves (one pair) sciatic nerve (one in each leg) Fig. 25-12, p.431 Somatic nerves Motor functions (Shown in green) Autonomic nerves Visceral functions (Shown in red) Sympathetic Parasympathetic Most organs receive input from both Usually have opposite effects on organ optic nerve eggs medulla oblongata salivary glands heart larynx bronchi lungs midbrain vagus nerve cervical nerves (8pairs) stomach liver spleen pancreas thoracic nerves (12 pairs) kidneys adrenal glands small intestine upper colon (all ganglia lower colon in walls of rectum organs) (most ganglia near spinal cord) Autonomic Nervous System bladder uterus genitals pelvic nerve lumbar nerves (five pairs) sacral nerves (five pairs) Fig. 25-13, p.432 Originate in thoracic and lumbar regions of spinal cord Ganglia are near the spinal cord Respond to stress or physical activity (fight-or-flight response) Originate in brain and sacral region of spinal cord Ganglia are in walls of organs Promote housekeeping responses such as digestion Most organs receive both sympathetic and parasympathetic signals Example: Sympathetic nerves signal heart to speed up; parasympathetic stimulate it to slow down Synaptic integration determines response White matter Tracts with myelin sheaths Sensory and motor neurons Gray matter Unmyelinated Cell bodies, dendrites, neuroglia Meninges Protective coverings Table 25-1, p.434 Expressway for signals between brain and peripheral nerves Sensory and motor neurons make direct reflex connections in spinal cord Spinal reflexes do not involve brain ventral spinal cord dorsal meninges (protective coverings) spinal nerve vertebra location of intervertebral disk Spinal Cord Fig. 25-14, p.433 The Brain corpus callosum hypothalamus thalamus pineal gland location part of optic nerve midbrain cerebellum pons medulla oblongata Fig. 25-15, p.434 Brain develops from a hollow neural tube Forebrain, midbrain, and hindbrain form from three successive regions of tube Most evolutionarily ancient nervous tissue persists as the brain stem Division Main Parts Forebrain Cerebrum Olfactory lobes Thalamus Hypothalamus Limbic system Pituitary gland Pineal gland Tectum Midbrain Hindbrain Pons Cerebellum Medulla oblongata Surrounds the spinal cord Fills ventricles within the brain Blood-brain barrier controls which solutes enter the cerebrospinal fluid Largest and most complex part of human brain Outer layer (cerebral cortex) is highly folded A longitudinal fissure divides cerebrum into left and right hemispheres primary somatosensory cortex primary motor cortex frontal parietal occipital temporal Controls emotions and has role in memory (olfactory tract) cingulate gyrus amygdala thalamus hypothalamus hippocampus Convert stimulus into action potentials Mechanoreceptors Chemoreceptors Thermoreceptors Osmoreceptors Pain receptors Photoreceptors Action potentials don’t vary in size Brain integrate information by Which pathway carries the signal Frequency of action potentials along each axon Number of axons recruited Touch Pressure Temperature Pain Motion Position Free nerve ending Ruffini ending Pacinian corpuscle Bulb of Krause Meissner’s corpuscle A special sense Olfactory receptors Receptor axons lead to olfactory lobe olfactory bulb receptor cell A special sense Chemoreceptors Five primary sensations: sweet, sour, salty, bitter, and umami Sensitivity to light is not vision Vision requires Eyes Capacity for image formation in the brain Perceives visual field Lens collects light Image formed on retina Contains visual pigments Stimulate photoreceptors sclera retina choroid iris lens pupil cornea aqueous humor ciliary muscle vitreous body fovea optic disk part of optic nerve Image on retina is upside down and reversed right to left compared with the stimulus Brain corrects during processing Photoreceptors at back of retina, in front of pigmented epithelium For light to reach photoreceptors, it must pass layers of neurons involved in visual processing Signals from photoreceptors are passed to bipolar sensory neurons, then to ganglion cells Axons of ganglion cells form the two optic nerves Cone Rod Ganglion cell Bipolar sensory neuron Rods Contain the pigment rhodopsin Detect very dim light, changes in light intensity Cones Three kinds; detect red, blue, or green Provide color sense and daytime vision Rods and Cones cone cell stacked, pigmented membrane rod cell Fig. 25-28, p.443 Macular degeneration Cataract Glaucoma fovea start of an optic nerve in back of the eyeball Outer ear Middle ear Inner ear Ear detects pressure waves Amplitude of waves corresponds to perceived loudness Frequency of waves (number per second) corresponds to perceived pitch stirrup anvil auditory nerve hammer auditory canal eardrum cochlea Sound waves make the eardrum vibrate Vibrations are transmitted to the bones of the middle ear The stirrup transmits force to the oval window of the fluid-filled cochlea hair cells in organ of Corti tectorial membrane basilar membrane lumen of cochlear duct to auditory nerve lumen of scala tympani Hair cells Mechanoreceptors located in the inner ear Maintains body position semicircular canals vestibular apparatus