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