Rabu, 12 Juni 2013

Outline from Chapter 34: The nervous System

Why Crack the System?

  1. Crack, a cheap, potent form of cocaine, disrupts synapses by overstimulating the postsynaptic cells. 
    1. Normal impulses to eat and sleep are suppressed.
  2. The neuron, or nerve cell, is the basic unit of communication in all nervous systems. 
    1. Sensory neurons are receptors for specific sensory stimuli.
    2. Interneurons in the brain and spinal cord integrate input and output signals.
    3. Motor neurons send information from integrator to muscle or gland cells (effectors).

34.1   Neurons--The Communication Specialists

  1. Functional Zones of a Neuron 
    1. The cell body contains the nucleus and metabolic machinery for protein synthesis.
    2. Dendrites are numerous, usually short extensions that receive stimuli (input zones).
    3. An axon is usually a single, rather long extension (conducting zone) that transmits impulses to other cells at its branched endings(output zones); signals actually arise in trigger zones.
  2. What Is a "Resting" Neuron? 
    1. A neuron at rest maintains a steady voltage difference across its plasma membrane. 
      1. The inside is more negatively charged than the outside.
      2. This is called the resting membrane potential.
    2. When a neuron receives signals, an abrupt, temporary reversal--the inside becomes more positive--in the polarity is generated (an action potential).
    3. Any membrane that can produce action potentials is said to show membrane excitability.
  3. Gradients Required for Action Potentials 
    1. The resting membrane potential is the result of three factors: 
      1. The concentrations of potassium, sodium ions, and other charged molecules are not the same on the two sides of the plasma membrane.
      2. Channel proteins spanning the membrane influence the diffusion of specific types of ions.
      3. Transport proteins spanning the membrane actively pump sodium and potassium ions.
    2. There are more potassium ions inside and more sodium ions outside the resting neuron membrane. 
      1. Potassium ions have a tendency to leak out by facilitated diffusion through channel proteins.
      2. Most of the sodium channels are "gated" and remain closed most of the time, keeping the concentration outside high.
      3. However, small amounts of sodium do leak in and must be pumped out (and potassium pumped in) by the sodium-potassium pump.

34.2   How Are Action Potentials Triggered and Propagated?

  1. Approaching Threshold 
    1. "Graded" means that the signals at the input zone vary in magnitude depending on the intensity and duration of the stimulus.
    2. "Local" means the signal does not usually spread beyond the input zone; however, if the stimulation is strong enough, an adjacent trigger zone may respond.
    3. When a stimulus reaches a certain minimum--a threshold--gated channels open and sodium rushes in. 
      1. In an accelerating way, more and more gates open (example of positive feedback).
      2. At threshold, the opening of more gates no longer depends on the stimulus but is self-propagating.
  2. An All-or-Nothing Spike 
    1. Action potentials are all-or-nothing events.
    2. When depolarization in one region is ended, the sodium gates close and potassium gates open.
    3. The sodium-potassium membrane pumps also become operational to fully restore the resting potential.
  3. The Direction of Propagation 
    1. The action potential is self-propagating and moves away from the stimulation site to adjacent regions of the membrane undiminished.
    2. A brief (refractory) period follows at each depolarization site--sodium gates shut, potassium gates open--during which the membrane is insensitive to stimulation.

34.3   Chemical Synapses

  1. A chemical synapse is a junction between a neuron and an adjacent cell, separated by a synaptic cleft into which a neurotransmitter substance is released. 
    1. The neuron that releases the neurotransmitter molecules into the cleft is called the presynaptic cell. 
      1. First, gated protein channels open to allow calcium ions to enter the neuron.
      2. Calcium causes the vesicles to fuse with the membrane and release the transmitter substance into the cleft.
    2. The neurotransmitter binds to receptors on the membrane of the postsynaptic cell. 
      1. Neurotransmitters may have excitatory effects if they drive a cell's membrane to the threshold of an action potential.
      2. Neurotransmitters may have inhibitory effects if they help drive the membrane away from threshold.
    3. Acetylcholine is the transmitter at neuromuscular junctions.
  2. A Smorgasbord of Signals 
    1. Serotonin acts on brain cells to govern sleeping, sensory perception, temperature regulation, and emotional states.
    2. Norepinephrine apparently affects brain regions concerned with emotions, dreaming, and awaking.
    3. Dopamine is the specialty of neurons in brain regions dealing with emotions.
    4. GABA is the most common inhibitory signal in the brain.
    5. Neuromodulators are substances that enhance or reduce the effects of a neurotransmitter on target neurons.
  3. Synaptic Integration 
    1. Excitatory and inhibitory signals compete at the input zone. 
      1. An excitatory postsynaptic potential (EPSP) is a summation of signals that brings the membrane closer to threshold (depolarizing effect).
      2. An inhibitory postsynaptic potential (IPSP) drives the membrane away from threshold by a hyperpolarizing effect.
    2. In synaptic integration, competing signals that reach the input zone are reinforced or dampened, sent on or suppressed.
  4. How Is Neurotransmitter Removed From the Synaptic Cleft? 
    1. Neurotransmitter molecules must be removed promptly from the synaptic cleft.
    2. Some molecules diffuse out; acetylcholinesterase degrades many; others are actively pumped back into the presynaptic cells by membrane transport proteins.

34.4   Paths of Information Flow

  1. Blocks and Cables of Neurons 
    1. Neuron circuits or pathways will determine the direction a signal will travel. 
      1. In the brain, neurons are organized into regional blocks that receive, integrate, and then send out signals.
      2. The circuits may be divergent, convergent, or reverberating.
    2. Signals between brain or spinal cord and body regions travel by nerves. 
      1. Axons of sensory neurons, motor neurons, or both, are bundled together in a nerve.
      2. Within the brain and spinal cord, such bundles are called nerve pathways, or "tracts."
    3. Many axons are covered by a myelin sheath derived in part from Schwann cells. 
      1. Each section of the sheath is separated from adjacent ones by a node where the axon membrane (plentiful in gated sodium channels) is exposed.
      2. The action potentials jump from node to node, which is fast and efficient.
  2. Reflex Arcs 
    1. Reflexes are simple, stereotyped movements made in response to sensory stimuli.
    2. In the simplest reflex, the reflex arc, sensory neurons synapse directly on motor neurons. 
      1. In the stretch reflex, receptors of sensory neurons (muscle spindles) transmit impulses to the spinal cord where direct synapses with motor neurons occur.
      2. In the withdrawal reflex, interneurons in the spinal cord can activate or suppress motor neurons as necessary for a coordinated response.

34.5   Invertebrate Nervous Systems

  1. Regarding the Nerve Net 
    1. Nearly all animals have a nervous system, forming communication lines for detecting stimuli and responding in suitable ways.
    2. The more complex the life-style of an animal, the more elaborate are its modes of receiving, integrating, and responding to information in the external and internal worlds.
    3. The nerve net in the cnidarians reflects their radially symmetrical bodies.
    4. Reflex pathways result in simple, stereotyped movements that provide the basic operating machinery of nervous systems such as the nerve net.
  2. On the Importance of Having a Head 
    1. Flatworms are the simplest animals with bilateral symmetry, which is reflected in their arrangement of muscles and nerves. 
      1. The ladderlike nervous system includes two longitudinal nerve cords, associated ganglia, and nerves.
      2. Some flatworms have a small brainlike clump of nervous tissue at the head end of the nerve cords. (an example of cephalization).
    2. Perhaps this arrangement evolved from the nerve net of cnidarian planula larvae.
    3. Cephalization (formation of a head) is the evolutionary result of the layering of more and more nervous tissue over reflex pathways of ancient origin.

34.6   Vertebrate Nervous Systems--An Overview

  1. Evolution of the Spinal Cord and Brain 
    1. Early evolutionary forms of vertebrates relied more on the interaction of notochord and muscles to accomplish movement. 
      1. Above the notochord, a hollow, tubular nerve cord was evolving also.
      2. In early vertebrates, simple reflex pathways predominated.
      3. In existing vertebrates, the oldest parts of the brain deal with reflex coordination, other parts deal with storage of information, and most recent layerings are the basis of memory, learning, and reasoning.
    2. The neural tube in vertebrate embryos undergoes expansion to form the brain and spinal cord along with their associated nerves; the spinal cord is enclosed within the vertebral column.
  2. The System's Functional Divisions 
    1. The central nervous system includes the brain and spinal cord. 
      1. The communication lines within the brain and spinal cord are called tracts; those in the white matter contain axons with glistening myelin sheaths and specialize in rapid transmission of impulses.
      2. Gray matter consists of unmyelinated axons, dendrites, nerve cell bodies, and neuroglia cells, that protect and support neurons.
    2. The peripheral nervous system includes all of the nerves carrying signals to and from the brain and spinal cord.

34.7   What Are the Major Expressways?

  1. Peripheral Nervous System 
    1. Somatic and Autonomic Subdivisions 
      1. The human peripheral system has two types of nerves based on location: 
        1. Spinal nerves (31 pairs) connect with the spinal cord and innervate most areas of the body.
        2. Cranial nerves (12 pairs) connect vital organs directly to the brain.
      2. Spinal and cranial nerves can also be classified on the basis of function: 
        1. The somatic nerves relay sensory information from receptors in the skin and muscles and motor commands to skeletal muscles (voluntary control).
        2. The autonomic nerves sends signals to and from smooth muscles, cardiac muscle, and glands (involuntary control).
    2. Sympathetic and Parasympathetic Nerves 
      1. Parasympathetic nerves tend to slow down body activity when the body is not under stress.
      2. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they also call on the hormone epinephrine to increase the "fight-flight" response.
  2. Spinal Cord 
    1. The spinal cord is a pathway for signal travel between the peripheral nervous system and the brain. 
      1. The cord is also the center for controlling some reflex actions.
      2. The spinal cord (and also the brain) is covered with tough membranes--the meninges--and resides within the protection of the stacked vertebrae.
    2. Signals move up and down the spinal cord in bundles of sheathed axons.

34.8   The Vertebrate Brain

  1. The body's master control panel, the brain, is a continuation of the anterior end of the spinal cord, and is also protected by meninges and bones. 
    1. The forebrain, midbrain, and hindbrain form from three successive portions of the neural tube.
    2. The most primitive of the tissue is the brain stem, which contains simple, basic reflex centers.
  2. Hindbrain 
    1. The medulla oblongata has influence over respiration, blood circulation, motor response coordination, and sleep/wake responses.
    2. The cerebellum acts as reflex center for maintaining posture and coordinating limbs.
    3. The pons ("bridge") possesses bands of axons that pass between brain centers.
  3. Midbrain 
    1. The midbrain originally coordinated reflex responses to visual input; the tectum still integrates visual and auditory signals in vertebrates such as amphibians and reptiles.
    2. In mammals it is now mostly a pathway switching center.
  4. Evolution of the Forebrain 
    1. The large olfactory lobes dominated early vertebrate forebrains.
    2. The cerebrum integrates sensory input and selected motor responses.
    3. The thalamus (below cerebrum) relays and coordinates sensory signals.
    4. The hypothalamus monitors internal organs and influences responses to thirst, hunger, and sex.
  5. Reticular Formation 
    1. The reticular formation is an ancient mesh of interneurons that extends from the uppermost part of the spinal cord, through the brain stem, and into the cerebral cortex.
    2. It serves as a pathway and activates centers in the cerebral cortex.
  6. Protection at the Blood-Brain Barrier 
    1. The brain and spinal cord are bathed with cerebrospinal fluid that exists within a system of cavities and canals.
    2. The fluid cushions vital nervous tissue from sudden, jarring movements.
    3. The blood-brain barrier operates at the plasma membranes of cells forming the capillaries that service the brain. 
      1. Tight junctions fuse the capillary cells together forcing substances to move through the cells to reach the brain.
      2. Membrane transport proteins allow essential nutrients (glucose) to move through but bar wastes (urea) and certain toxins.

34.9   The Human Cerebrum

  1. Functional Divisions of the Cerebral Cortex 
    1. The human cerebrum is divided into left and right cerebral hemispheres. 
      1. The left hemisphere deals with speech, math, and analytical skills; the right half controls nonverbal skills, such as music.
      2. The two halves communicate with each other by means of nerve tracts called the corpus callosum.
    2. Motor areas control voluntary motor activity. 
      1. The frontal lobe includes the motor cortex, which coordinates instructions for motor responses.
      2. The frontal lobe also includes the premotor cortex ((learned patterns of motor skills), Broca's area (sppech); and the frontal eye field (voluntary eye movements).
    3. Sensory areas deal with the meaning of sensations. 
      1. The parietal lobe contains the somatosensory cortex--the main receiving area for signals from the skin and joints.
      2. The occipital lobe, which is located in the rear, has centers for vision.
      3. The temporal lobe, near each temple, is a processing center for hearing and houses centers for influencing emotional behavior.
    4. Association areas--occupying all parts of the cortex except primary and sensory areas--integrate, analyze, and respond to many inputs.
  2. Connections With the Limbic System 
    1. The limbic system, located at the middle of the cerebral hemispheres, governs out emotions and has roles in memory.
    2. It is distantly related to olfactory lobes an still deals with the sense of smell, especially as it relates to memory.
    3. Connections from the cerebral cortex pass through the limbic system allowing us to correlate organ activities with self-gratifying behavior, such as eating and sex.

34.10   Focus on Science: Sperry's Split-Brain Experiments

34.11   How Are Memories Tucked Away?

  1. "Memory" is the storage and retrieval of information about previous experiences. 
    1. Association is the linkage of information into larger packages that can be sent to other brain regions for storage.
    2. Information becomes stored in "memory traces"--chemical and structural changes in brain regions. 
      1. Short-term memory lasts from seconds to hours and is limited to seven to eight bits of information.
      2. Long-term memory is more permanent and seems to be limitless.
    3. Persons suffering from retrograde amnesia lose short-term memory, but long-term memory remains intact.
  2. Information is moved into long-term storage with the cooperation of epinephrine, which increases a person's state of arousal.

34.12   The Not-Quite-Complete Teen Brain
34.13   Focus on Health: Drugging the Brain

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