neurons, synapses, and signaling chapter 48. figure 48.1 overview of a vertebrate nervous system
TRANSCRIPT
Neurons, Synapses, and Signaling
CHAPTER 48
Figure 48.1 Overview of a vertebrate nervous system
NERVOUS SYSTEM
• Central nervous system (CNS) – brain and spinal cord
• Peripheral nervous system (PNS) – nerves that communicate motor and sensory signals between CNS and rest of body
NEURON• Functional unit of nervous system• Relatively large cell body• Processes:
– Dendrites – convey signals from tips to cell body; often branched
– Axons – conduct signals away from body and toward tip; often single
• Myelin sheath – protective, insulating layer that covers many axons
• Axon ends at synaptic terminals– Synapse – site of contact between
synaptic terminal and target cell (neuron or effector cell – for example a muscle cell)
– Neurotransmitter – chemical messengers between neurons and other cells
Figure 48.2 Structure of a vertebrate neuron
Figure 48.0 A neuron on a microprocessor
Figure 48.0x1 Aplysia neuron
Figure 48.5 Schwann cells
ORGANIZATION OF NEURONS
• Sensory neurons – communicate sensory information from eyes and other senses and internal conditions– Senses, blood pressure, muscle tension,
CO2 levels)
• Interneurons – integrate sensory input and motor output; communicate only between neurons; make up vast majority of brain neurons
• Motor neurons – convey impulses from CNS to effector cells (muscles and glands)
• Ganglion – cluster of nerve cell bodies in PNS
• Nuclei – clusters of nerve cell bodies in brain– Both allow activities without entire
nervous system involved– Knee-jerk reflex
•Reflex – automatic reaction to stimuli mediated by spinal cord and lower brain
Figure 48.3 The knee-jerk reflex
MEMBRANE POTENTIAL
• Voltage measured across the membrane (like a battery)
• Inside of cell more negative• Typically –50 to –80 mV (resting
potential)• Sodium-potassium pump keeps ionic
gradient (3Na+ out, 2K+ in)
Figure 8.15 The sodium-potassium pump: a specific case of active transport
Figure 48.6 Measuring membrane potentials
Figure 48.7 The basis of the membrane potential
Charges Across Membranes
• Neurons have ability to generate changes in their membrane potential
• Resting potential – membrane potential of cell at rest (-60mV to -80mV)
• Gated ion channels control membrane potential – open to different stimuli– Hyperpolarization – increase in
electrical gradient•Open K+ channel (K+ moves out)
•Cell becomes more negative•No action potential because it makes it harder to depolarize
– Depolarization – decrease in electrical gradient•Open Na+ channel (Na+ moves in)•Cell becomes more positive•Action potential generated if threshold is reached (-50mV to -55mV)
–Massive change in voltage
•Threshold causes all-or-none event
Figure 48.8 Graded potentials and the action potential in a neuron
Figure 48.9 The role of voltage-gated ion channels in the action potential
ROLE OF GATED CHANNELS• Depolarizing – Na+ gates open rapidly so Na+
moves into cell• Repolarizing – K+ gates finally open and K+
moves out; Na+ gates close• Undershoot (Refractory Period) - K+ still open
(they are slower to close) and Na+ still closed so cell becomes even more negative than resting and cannot be depolarized
• Stronger stimuli result in greater frequency of action potentials and NOT from stronger action potentials
• Propagation– Action potentials move in one direction due to
refractory period
Propagation of the action potential
Na+ moves into cell starting action potential.
Depolarization spreads and K+ repolarizes initial area. Prevents action potential on that side.
Figure 48.11 Saltatory conduction• Voltage leaps from node to node
SYNAPSES• Presynaptic cell – transmitting cell• Postsynaptic cell – receiving cell• Two types of synapses
– Electrical •Need gap junctions (channels between
neurons)•No delays
– Chemical•Narrow gap, synaptic cleft, between cells•More common than electrical in vertebrates
and most invertebrates•Require neurotransmitters (chemical
intercellular messengers)
•Depolarization of presynaptic membrane causes influx of Ca2+
•Increased Ca2+ in cell causes synaptic vesicles to fuse to cell membrane and release neurotransmitters via exocytosis
•Neurotransmitters diffuse to postsynaptic cell
•Postsynaptic membrane has gated channels that open when neurotransmitters bond to specific receptors
Figure 48.12 A chemical synapse
• A single neuron may receive many inputs simultaneously
• Neurotransmitters cause 2 different responses depending on the gates that are opened– Inhibitory
•(hyperpolarization)– Excitatory
•(depolarization)• Neurotransmitters are quickly degraded• Excitatory postsynaptic potential
(EPSP) – Na+ in and K+ out = depolarization• Inhibitory postsynaptic potential (IPSP)
- K+ out or CL- in = hyperpolarization
Figure 48.13 Integration of multiple synaptic inputs
Figure 48.14 Summation of postsynaptic potentials
NEUROTRANSMITTERS
• Acetylcholine – one of the most common – can excite skeletal muscle and
inhibit cardiac muscle• Epinephrine and norepinephrine
– also function as hormones
• Dopamine – Usually excitatory– Excess dopamine can cause schizophrenia– Lack of dopamine can cause Parkinson’s
• Sertonin – Usually inhibitory
• Endorphins – Natural painkillers (morphine and opium
mimic endorphins shape)• Nitric Oxide (NO)
– Released during sexual arousal (increasing blood flow)
– Nitroglycerin used to treat chest pain
Nervous System
Chapter 49
NERVOUS SYSTEM
• Verebrate Nervous Systems– All have brain and spinal cord– Brain is integrative– White matter – axons with
white meylin in CNS– Gray matter – dendrites and
cell bodies in CNS
CNS
• The brain and spinal cord contain fluid filled spaces (called ventricles in the brain)
• The spinal cord is hollow and has a central canal
Figure 48.16 The nervous system of a vertebrate
Figure 48.16x Spinal cord
• Cerebrospinal fluid – Fills canal and ventricles– Brain filtered blood– Contains nutrients and WBC– Circulated and eventually
empties into veins– Major function is as a shock
absorber
PNS
– Cranial nerves – innervate organs of head and upper body
– Spinal nerves – innervate entire body•Mammals have 12 pairs of cranial and 31 pairs of spinal nerves
– Hierarchy of PNS•Afferent Division – convey info to CNS
•Efferent Division – convey info from CNS
Efferent Division•Motor –controls skeletal muscles
(voluntary)•Autonomic – controls smooth and
cardiac muscle (involuntary)•Sympathetic – arousal and energy generation (flight or fight)
•Parasympathetic - calming and self-maintenance (rest and digest)
•Enteric – digestive tract, pancreas, and gall bladder
Functional hierarchy of the peripheral nervous system
Figure 48.18 The main roles of the parasympathetic and sympathetic nerves in regulating internal body functions
BRAIN
• Brainstem– Medulla oblongata – breathing,
heart rate, swallowing, vomiting, digestion
– Pons – breathing– Midbrain – receives sensory
information
Figure 48.19 Embryonic development of the brain
• Cerebellum– Coordination of movement, hand-eye
coordination, learning and remembering• Diencephalon
– Hypothalamus, thalamus, and epithalamus•Hypothalamus - regulates hunger, thirst, sexual response, mating behaviors, fight or flight, biological clock–Contains the Suprachiasmatic nuclei – make proteins in response to light/dark (biological clock)
• Cerebrum– Most complex integration– Controls learning, emotion,
memory, and perception– Divided into right and left
hemispheres– Cerebral cortex
•Most complex, most evolved, and surface area is 0.5 m2 which is ~80% of total brain mass
– Corpus callosum – connects hemispheres
Figure 48.20 The main parts of the human brain
Figure 48.20x1 Cerebral cortex, gray and white matter
Neural Plasticity
• The overall organization of the brain is developed in the embryo; however neural plasticity can change
• Neural plasticity – ability of brain to be remodeled– Synaptic connections can increase or
decrease– Formation of memory