NERVES Neurons & Supporting Cells Neurons- nerve cells o There are 100 billion in the human brain Nerves- bundles of fiber-like extensions of neurons Greater complexity of nervous systems evolved with cephalization (the clustering of neurons in a brain near the anterior (front) end in animals with elongated, bilaterally symmetrical bodies) Central nervous system- small brain and longitudinal nerve cords o Consists of brain and spinal cord o Located along the dorsal (back) side of the body Peripheral nervous system- nerves that connect the CNS with rest of body o Nerves and ganglia comprise the PNS Ganglia- ventral nerve cords containing segmentally arranged clusters of neurons Information processing: 3 stages produce reflexes o Sensory input- transmit information from sensors that detect external stimuli (light, sound, touch, heat, etc.) and internal conditions (blood pressure, blood CO2 level, etc.) o Integration- interneurons analyze and interpret the sensory input o Motor output- motor neurons leave the CNS who communicate with effector cells (muscle cells and endocrine cells) Neuron structure: depends on the ability of neurons to receive and transmit info o Most organelles are located in cell body o 2 types of extensions from cell body Dendrites: highly branched extensions that that RECEIVE signals from other neurons Axon: much longer extensions that TRANSMIT signals to other cells o Axon hillock: conical region where axon joins cells body o Myelin sheath: lipid membranes that surround the axon to provide electrical insulation o Synaptic terminal: ending of several branches of axon o Synapse: the site of communication between a synaptic terminal and another cell Presynaptic cell: transmitting neuron in synapse Postsynaptic cell: receiving cell in synapse Neurotransmitters: chemical messengers o Glia: supporting cells that are essential for structural integrity Several types of glia: Astrocytes: provide structural support for neutron and regulate the extracellular concentrations of ions and neurotransmitters; stimulate blood vessel dilation; induce formation of tight junctions (aka blood-brain barrier (restricts the passage of most substances into the CNS)) Radial glia: forms tracks along which newly formed neurons migrate from the neural tube (the structure that gives rise to the CNS) Oligodendrocytes (located in the CNS) & Schwann cells (located in the PNS): glia that form the myelin sheaths around the axons of many vertebrate neurons Vertebrate Nervous System CNS (filled with cerebrospinal fluid (which is formed in the brain by filtration of the blood) & serves as a cushion) o Brain- provides the integrative power that underlies the complex behavior of vertebrates White matter: well-defined bundles of axons whose myelin sheath give them a whitish appearance Gray matter: which consists mainly of dendrites, unmyelinated axons, and neuron cell bodies Divided into 4 ventricles o Brainstem- composed of 3 parts that function in homeostasis, coordination of movement, and conduction of information Medulla oblongata: contains centers that control homeostatic functions, including breathing, heart, and blood vessel activity, swallowing, vomiting, and digestion Pons: participates in some of these activities; it regulates the breathing centers in the medulla The medulla and the pons together help coordinate large-scale body movements; both contain centers that cause sleep when stimulated Midbrain: contains centers for the receipt and integration of several types of sensory information; it also sends coded sensory information along neurons to specific regions of the forebrain Has an area that causes arousal The brainstem and the cerebrum control sleep and arousal Reticular formation: diffuse network of neurons are present in core of brainstem; regulate sleep and arousal; selects with information reaches the cerebral cortex In the case that these brain functions do not act as needed, melatonin has been promoted as a dietary supplement o Cerebellum- important for coordination and error checking during motor, perceptual, and cognitive functions Likely involved in learning and remembering motor skills, such as those involved in riding a bicycle Receives input concerning motor commands issued by the cerebrum Is involved in hand-eye coordination o Diencephalon- composed to 3 adult brain regions Epithalamus: includes the pineal gland and coroid plexus, one of several clusters of capillaries that produce cerebrospinal fluid from blood Thalamus: main input center for sensory information going to the cerebrum and the main output center for motor information leaving the cerebrum Receives input from the cerebrum and other parts of the brain that regulate emotion and arousal Hypothalamus: source of two sets of hormones, posterior pituitary hormones and releasing hormones that act on the anterior pituitary Also contains the body’s thermostat, as well as centers for regulating hunger, thirst, sexual and mating behaviors Controls circadian (daily) rhythms o Biological clock: pair of hypothalamic structures called the suprachiasmatic nuclei; involved in maintaining circadian rhythms; require external cues to remain synchronized with environment o Cerebrum- divided into right and left cerebral hemispheres Left hemisphere: control language, math, logical operations, and serial processing of sequences Right hemisphere: stronger at pattern recognition, face recognition, spatial relations, nonverbal thinking, emotional processes, and simultaneous processing of info Each hemisphere consists of an outer covering of gray matter, the cerebral cortex, internal white matter, and basal nuclei (important centers for planning and learning movement sequences) Cerebral cortex: largest, most complex part of the brain; sensory information is analyzed, motor commands are issued and language is generated; composed of 4 lobes Frontal: associated with reward, attention, shortterm memory, planning, and motivation Temporal: auditory info processed here Occipital: visual information processed here Parietal: somatosensory information about touch, pain, pressure, tempterature, and the position of muscles and limbs **Cortical surface area devoted to each body part is related to the number of sensory neurons that innervate that part or to the amount of skill needed to control the muscles of that** Neocortex: forms the outermost part of the mammalian cerebrum, consisting of 6 parallel layers of neurons arranged tangential to the brain surface o Highly convoluted and accounts for 80% of total brain mass Corpus Callosum: thick band of axons that enables communication between the right and left cerebral cortices o Spinal cord- integrates simple responses to certain kinds of stimuli and conveys information to and from the brain Central canal- narrow nerve cord PNS Transmits information to and from the CNS & plays a large role in regulating the vertebrate’s movement and internal environment Consists of left-right pairs of cranial and spinal nerves and their associated ganglia Cranial nerves- originate in the brain and terminate mostly in organs of the head and upper body Spinal nerves- originate in the spinal cord and extend to parts of the body below the head o Divided into 2 functional components: Somatic nervous system: carried and signals to and from skeletal muscles, mainly in response to EXTERNAL stimuli Is considered voluntary because it is subject to conscious control Autonomic nervous system: regulates the internal environment by controlling smooth and cardiac muscles and the organs of the digestive, cardiovascular, excretory and endocrine systems Is considered involuntary because it is internal 3 subdivisions of ANS: o Sympathetic: corresponds to arousal and energy generation (e.g.: heart beats faster, liver converts glycogen to glucose, bronchi of lungs dilate, etc.) o Parasympathetic: corresponds to calming and a return to self-maintenance functions (e.g.: decrease in heart rate, increase glycogen production, and enhances digestion, etc.) o Enteric: consists of networks of neurons in the digestive tract, pancreas, and gallbladder; control secretions Resting and Action Potential Membrane potential: all cells have an electrical potential difference (voltage) across their plasma membrane; typically between -60 & -80 mV when the cell is not transmitting signals (negative sign indicates that inside of the cell is negative relative to the outside) Resting Potential: membrane potential of a neuron that is not transmitting signals o Depends on the ionic gradients that exist across the plasma membrane o Na+ & K+ gradients are maintained by the sodium-potassium pump If the pump is disabled by the addition of a specific poison, the gradients gradually disappear & so does the resting potential Membrane contains many ion channels that allow only K+ to diffuse across membrane K+ tends to diffuse down its concentration gradient, from an area of higher concentration to an area of lower concentration At equilibrium, there is no net diffusions of K+ across the membrane Equilibrium potential (Eion): magnitude of membrane voltage at equilibrium Given by Nerst equation (which applied to any membrane that is permeable to a single type of ion) Equilibrium potential for K+ (Ek): -92 mV (@ 37 °C) Equilibrium potential of Na+ (ENa): +62 mV (@ 37 °C) NEITHER K+ & Na+ IS AT EQUILIBRIUM SO THERE IS ALWAYS A NET FLOW OF EACH ION ACROSS THE MEMBRANE AT REST THIS IS THE BASIS OF ELECTRICAL SIGNALS IN THE NERVOUS SYSTEM: Gated Ion Channels: channels that open or close in response to one of three kinds of stimuli; responsible for generating signals to the nervous system o Stretch-gated ion channels: found in cells that sense stretch and open when the membrane is mechanically deformed o Ligand-gated ion channels: found at synapses and open or close when a specific chemical, such as a neurotransmitter, binds to the channel o Voltage-gated ion channels: found in axons and open or close when the membrane potential changes Action Potential: a stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response; once triggered, it has a magnitude that is independent of the strength of the triggering stimulus; the signals that carry information along axons; very brief (1-2 msec) o Composed of hyperpolarization and depolarization Hyperpolarization: an increase in the magnitude of the membrane potential (the inside of the membrane becomes more negative) Depolarization: a reduction in the magnitude of the membrane potential (the inside of the membrane becomes less negative) Graded only up to a certain membrane voltage (threshold) *****These changes in membrane potential are called gradient potentials because the magnitude of the hyperpolarization or depolarization varies with the strength of the stimulus***** o Production 1. Resting state: the activation gates on the Na+ and k+ channels are closed, and the membrane’s resting potential is maintained 2. Depolarization: a stimulus opens the activation gates on some Na+ channels; Na+ influx through those channels depolarizes the membrane; if the depolarization reaches the threshold, it triggers an action potential 3. Rising phase of the action potential: depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed; Na+ influx makes the inside of the membrane positive with respect to the outside 4. Falling phase of the action potential: the inactivation gates on most Na+ channels close, blocking Na+ influx; the activation gates on most K+ channels open, permitting K+ efflux makes the side of the cell negative 5. Undershoot: both gates of the Na+ channels are closed, but the activation gates on some K+ channels are still open; as these gates close on most K+ channels, and the inactivation gates open on Na+ channels, the membrane returns to its resting state 6. Refractory period: the downtime following an action potential when a second action potential cannot be initiated; sets a limit on the maximum frequency at which action potentials can be generated o Conduction o Action potential must function over a long distance w/o diminishing from the cell body to the synaptic terminals 1. An action potential is generated as Na+ flows inward across the membrane at one location 2. The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there; the membrane behind the reinitiated action potential is repolarizing as K+ flows outward 3. The depolarization-repolarization process is repeated in the next region of the membrane; in this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon Synapses o Contain gap junctions which allow electrical current to flow directly from cell to cell o Chemical synthesis: involve the release of a chemical neurotransmitter by the presynaptic neuron o Presynaptic neuron: synthesizes the neurotransmitter and packages it in synaptic vesicles, which are stored in the neuron’s synaptic terminals 1. Action potential depolarizes the plasma membrane of the synaptic terminal 2. It opens voltage-gated Ca2+ in the membrane, triggering an influx of Ca2+ 3. Elevated Ca2+ concentration in the terminal causes synaptic vesicles to fuse with the presynaptic membrane 4. Vesicles release neurotransmitter into the synaptic cleft 5. The neurotransmitter binds to the receptor portion of ligand-gated ion channels to the postsynaptic membrane, opening the channels 6. The neurotransmitter releases from the receptions and the channels close o Direct Synaptic Transmission Ligand-gated ion channels capable of binding to the neurotransmitter are clustered in the membrane of the postsynaptic cell, directly opposite the synaptic terminal The receptor opens the channel and allows specific ions to diffuse across the postsynaptic membrane Results in a postsynaptic potential (a change in the membrane potential of the postsynaptic cell) 2 types of postsynaptic membrane o Excitatory postsynaptic potentials: depolarizations that bring the membrane potential toward the threshold o Inhibitory postsynaptic potentials: postsynaptic membrane hyperpolarizes o Indirect synaptic transmission A neurotransmitter binds to a receptor that is not part of an ion channel Effects have a slower onset but last longer o Neurotransmitters Each neurotransmitter binds to its own group of receptors Acetylcholine: one of the most common neurotransmitters In cardiac muscle, it activates a signal transduction pathway that reduce the strength and rate of contraction of cardiac muscle cells Biogenic amines: neurotransmitters derived from amino acids Includes epinephrine, norepinenphrine, dopamine, and serotonin Most often involved in indirect synaptic transmission Amino acids and peptides 4 amino acids know to function as neurotransmitters in CNS o Gamma aminobutyric acid (GABA), glycine, glutamate, and aspartate Neuropeptides: short chains of amino acids that serve as neurotransmitters o Include substance P and endorphins CNS Functions and Disease Functions o Language and Speech Broca’s area: located in front part of primary cortex that control muscles of face; controls mouth movement for speech Wernicke’s area: posterior portion of temporal lobe; if damaged, it can abolish the ability to comprehend speech but the person him/herself can still speak Frontal and temporal areas become active when meaning must be attached to words, such as when a person generate verbs to go with nouns or groups related words o Emotions Limbic system: ring of structures around the brainstem; includes 3 parts (amygdala, hippocampus, and olfactory bulb) Interact with neocortex to mediate primary emotions that manifest themselves in behaviors such as laughing, crying, anger, fear, distress Phineas Gage: was working on a railroad construction site when a dynamite explosions drove a meter-long iron rod through his head; the rod entered his skull just below his left eye and exited through the top of his head; he recovered, but his personality had changed drastically; after his death, neuroscientists his skill and determined that the rod had destroyed portions of his frontal lob known to mediate emotions o Memory and Learning Short-term memory: located in frontal lob Long-term memory: activated in process that requires the hippocampus Motor skills like walking is learned by repetition Once a memory is learned, it is difficult to unlearn Cellular mechanisms of learning Long-term potentiation: involves an increase in the strength of synaptic transmission that occurs when presynaptic neurons produce a brief, highfrequency series of action potentials o Because LTP can last for days or weeks, it may be a fundamental process by which memories are stored or learning takes place o Consciousness Consciousness is really studied in psychology, but brainimaging technique reveal and increasingly detailed picture of how neural activity correlates with conscious experiences Nothing much is really known… Diseases o Schizophrenia: a severe mental disturbance characterized by psychotic episodes in which patients lose the ability to distinguish reality Have hallucinations, delusions, blunted emotions, etc. Cause is unknown, but it does have a strong genetic component Treatments for schizophrenia have focused mostly on brain pathways that use dopamine as a neurotransmitter o Depression: 2 broad categories Bipolar disorder: swings of mood from high to low; composed of manic and depressive phases Affect 1% of people in world Manic phase: Characterized by high self esteem, increased energy, etc. Depressive phase: lowered ability to feel pleasure, loss of interest, sleep disturbances, etc. Major depression: have a low mood most of the time Affect 5% of people in world Have genetic component, but indications of stress may be an important factor Several treatments, but they have many other side effects o Alzheimer’s disease: mental deterioration, or dementia, characterized by confusion, memory loss, and variety of other symptoms Incidence in age related (as medicine is allowing humans to live longer, it is increasing the proportion of AD patients in the population) Has a characteristic pathology to it: neurons die in huge areas Enormous effort devoted to developing treatment for AD o Parkinson’s disease: motor disorder characterize by difficulty in initiating movements, slowness of movement, and rigidity Often have masked facial expression, muscle tremors, poor balance, flexed posture, and shuffling gait Affects 1 million people in the US Progressive brain illness whose risk increases with advancing age Symptoms: result from the death of neurons in midbrain nucleus No known cause and there is no cure