David Sadava David M Hillis H Craig Heller

David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia. blu Anatomia e fisiologia dei viventi S

Neurons, Glia, and Nervous Systems

What Cells Are Unique to the Nervous System? Nervous systems have two types of cells: Neurons, or nerve cells, are excitable—they generate and transmit electrical signals, called action potentials. Neurons have four regions: • Cell body—contains the nucleus and organelles • Dendrites—bring information to the cell body • Axon—carries information away from the cell body • Synapse

What Cells Are Unique to the Nervous System? Glia, or glial cells, modulate neuron activity and provide support. In brain and spinal cord, glia called oligodendrocytes wrap around neuron axons. Glia called Schwann cells wrap the axons of other nerves. Oligodendrocytes and Schwann cells produce myelin that covers axons.

What Cells Are Unique to the Nervous System? Glia called astrocytes contribute to the blood–brain barrier, which protects the brain from toxic substances in the blood.

How Do Neurons Generate and Transmit Electrical Signals? Membrane potential is the electrical charge difference across the membrane. Resting potential is the steady state membrane potential of a neuron. Voltage (electric potential difference): force that causes charged particles to move between two points. The resting potential of an axon is – 60 to – 70 millivolts (m. V).

How Do Neurons Generate and Transmit Electrical Signals? The inside of the cell is negative at rest. A stimulus that changes the permeability of the membrane allows ions to move quickly across.

How Do Neurons Generate and Transmit Electrical Signals? In solutions and across cell membranes, electric current is carried by ions. Major ions in neurons: sodium (Na+), potassium (K+), calcium (Ca 2+), chloride (Cl–). Ion transporters and channels generate membrane potentials. The sodium–potassium pump moves Na+ to the outside and K+ to the inside; requires energy; establishes concentration gradients.

Ion Transporters and Channels Sodium-potassium pumps in all animal cells create gradients of Na+ and K+ across the cell membrane. Ion channels in the membrane allow ions to pass through.

How Do Neurons Generate and Transmit Electrical Signals? Some ion channels are “gated”: • Voltage-gated channels respond to change in voltage across membrane • Chemically-gated channels depend on specific molecules that bind or alter the channel protein • Mechanically-gated channels respond to force applied to membrane

How Do Neurons Generate and Transmit Electrical Signals? If Na channels open suddenly, Na+ diffuses in and the inside of the cell becomes less negative— plasma membrane is depolarized. If gated K+ channels open and K+ efflux, the membrane potential becomes even more negative, and the plasma membrane is hyper-polarized.

How Do Neurons Generate and Transmit Electrical Signals? When the membrane is depolarized about 5 to 10 m. V above resting potential, a threshold is reached. A large number of sodium channels open and an action potential is generated.

The Course of an Action Potential

Action Potentials Travel along Axons

How Do Neurons Generate and Transmit Electrical Signals? The axon returns to resting potential as voltage-gated Na+ channels close and voltage-gated K+ channels open. Voltage-gated Na+ channels cannot open during the refractory period.

How Do Neurons Generate and Transmit Electrical Signals? Action potentials travel faster in myelinated than in non -myelinated axons. The nodes of Ranvier are regularly spaced gaps in the myelin along an axon. Action potentials are generated at the nodes and appear to jump from node to node, a form of propagation called saltatory conduction.

How Do Neurons Generate and Transmit Electrical Signals? Action potentials can travel over long distances with no loss of signal. An action potential is an all-or-none event—positive feedback to voltage-gated Na+ channels ensures the maximum action potential. An action potential is self-regenerating because it spreads to adjacent membrane regions.

How Do Neurons Communicate with Other Cells? Neurons communicate with other neurons or target cells at synapses. Axons carry information as action potentials away from the originating cell body (presynaptic cell) to the receiving target cell (postsynaptic cell). Chemical synapse: chemicals from a presynaptic cell induce changes in a postsynaptic cell. Electrical synapse: the action potential spreads directly to the postsynaptic cell.

How Do Neurons Communicate with Other Cells? Neuromuscular junctions are chemical synapses between motor neurons and skeletal muscle cells. The neurotransmitter is acetylcholine (ACh). ACh diffuses across the synaptic cleft to the motor end plate on the muscle cell.

How Do Neurons Communicate with Other Cells? An action potential causes release of the neurotransmitter ACh when voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Vesicles release ACh into the synaptic cleft by exocytosis, ACh diffuses across the cleft and binds to receptors on the motor end plate.

How Do Neurons Communicate with Other Cells? Synapses between motor neurons and muscle cells are excitatory. ACh always causes depolarization. Other synapses can be inhibitory if the postsynaptic response is hyper-polarization. Neurons have many dendrites that can form synapses with axons of other neurons. The mix of excitatory and inhibitory activity determines whether the graded membrane potential is more positive or more negative than resting.

How Do Neurons Communicate with Other Cells? Excitatory and inhibitory postsynaptic potentials are summed over space and over time. Temporal summation adds up potentials generated at the same site in a rapid sequence. Spatial summation adds up messages at different synaptic sites.

How Do Neurons Communicate with Other Cells? The three main neurotransmitters in the brain are amino acids: • Glutamate—excitatory • Glycine—inhibitory • γ-aminobutyric acid (GABA)—inhibitory • monoamines—includes dopamine, norepinephrine, and serotonin.

How Is the Mammalian Nervous System Organized? Vertebrate nervous systems: • Central nervous system (CNS): brain and spinal cord—the sites of most information processing, storage, and retrieval • Peripheral nervous system (PNS): nerves that connect the CNS to all tissues and sensors of the body.

How Is the Mammalian Nervous System Organized? The CNS develops from the neural tube of an embryo. • The anterior part develops into the brain • The rest becomes the spinal cord

How Is the Mammalian Nervous System Organized? Gray matter is rich in neural cell bodies; white matter contains myelinated axons. Afferent (sensory) axons in a spinal nerve enter the spinal cord through the dorsal root; efferent (motor) axons leave through the ventral root. An anatomically distinct group of CNS neurons is a nucleus. Brainstem nuclei are involved in keeping higher brain areas awake or allowing them to sleep.

How Is the Mammalian Nervous System Organized? The diencephalon consists of the thalamus and hypothalamus. The thalamus communicates sensory information to the cerebral cortex, the hypothalamus regulates many homeostatic functions. The telencephalon (cerebrum) consists of left and right cerebral hemispheres. The cerebral cortex (outer layer) has a large surface area; it is folded and these foldings, or convolutions, allow the large surface of the cortex to fit in the skull.

How Is the Mammalian Nervous System Organized? The limbic system is responsible for instincts, longterm memory formation, physiological drives such as hunger and thirst, and emotions such as fear. • Amygdala—involved in fear and fear memory • Hippocampus—transfers short-term memory to longterm memory

How Is the Mammalian Nervous System Organized? The cerebral cortex is divided into lobes: • Temporal • Frontal • Parietal • Occipital

How Is the Mammalian Nervous System Organized? Different regions of the cerebral cortex have specific functions. Association cortex: many areas that integrate or associate sensory information or memories; higherorder information processing. The left hemisphere of the brain controls the right side of the body; the right hemisphere controls the left side, except in the head. The two hemispheres are not symmetrical with respect to all functions, e. g. , language abilities reside in the left hemisphere.

How Is the Mammalian Nervous System Organized? Temporal lobe: • Receives and processes auditory information • Association areas involve identifying and naming objects Agnosias—inability to identify objects Damage in certain areas cause inability to recognize faces.

How Is the Mammalian Nervous System Organized? Frontal Lobe: • Central sulcus—divides frontal and parietal lobes • Primary motor cortex is located in front of the central sulcus; controls muscles in specific body areas Parietal lobe: primary somatosensory motor cortex —just behind the central sulcus. Receives touch and pressure information from the thalamus.

How Is the Mammalian Nervous System Organized? The midbrain, medulla, and pons are known as the brainstem. All information traveling between the spinal cord and higher brain areas must pass through the brainstem. Medulla and pons control physiological functions, such as breathing. Cerebellum coordinates muscle activity and maintaining balance.

How Is the Mammalian Nervous System Organized? The spinal cord: • Conducts information to and from the brain • Integrates information coming from the PNS and issues motor commands (e. g. , knee-jerk reflex) • Complex motor programs also exist in the spinal cords of many vertebrates

How Is the Mammalian Nervous System Organized? Nerve: a bundle of axons that carries information. Some axons in a nerve may be carrying information to the CNS (Afferent), while others in the same nerve are carrying information from the CNS to the body’s organs and glands (Efferent). The brainstem regulates many autonomic functions (involuntary physiological functions). It has 12 paired cranial nerves, including the olfactory, optic, and auditory nerves.

How Is Information Processed by Neural Networks? Spinal reflex: conversion of afferent to efferent information in the spinal cord without participation of the brain. It is a monosynaptic reflex—only one synapse between the afferent and efferent neurons.

How Is Information Processed by Neural Networks? Autonomic Nervous System (ANS)— output pathways of the CNS that control involuntary functions. Two divisions work in opposition: sympathetic and parasympathetic. One causes increase in an activity, the other decreases it.

What Are the Major Diseases? Some diseases affect myelin and impair conduction of action potentials. Multiple sclerosis is an autoimmune disease; antibodies to proteins in myelin in the brain and spinal cord are produced.

How Are Neurons and Glia Organized into Information-Processing Systems? Nervous systems vary in complexity. Cnidarians have simple networks of neurons called nerve nets. There is little or no integration or processing of signals. More complex animals must process and integrate larger amounts of information. Neurons are organized into clusters called ganglia. In bilaterally symmetrical animals, the ganglia may be enlarged at the anterior end to form a brain.

Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved