CHAPTER 48
NERVOUS SYSTEMS

Introduction: 

Nervous systems perform the three overlapping functions of sensory input,
 integration, and motor output All achieved by two systems

    Peripheral nervous system (PNS). 
    Central nervous system (CNS)

            Sensory receptors are responsive to external and internal stimuli.   Such sensory input
            is conveyed to integration centers, here in the input is interpreted and associated
            with a response.  Motor output is the conduction of signals from integration centers 
            to effector cells.   Effector cells carry out the body’s response to a stimulus. 

Nerve cell (Neuron)

    The neuron is the structural and functional unit of the nervous system. Nerve impulses 
    are conducted along a neuron. 

    Axons:
        Some axons are insulated by a myelin sheath.  the myelin sheath Axon endings are 
        called synaptic terminals. They contain neurotransmitters  which conduct a signal 
        across a synapse. A synapse is  the junction between a presynaptic and postsynaptic cell. 

     Supporting Cells (Glia):
 
       Astrocytes are found within the CNS (Structural and metabolic support).  By inducing 
       the formation of tight junctions between capillary cells astrocytes help form the blood-brain 
       barrier.  Like neurons, astrocytes communicate with one another via chemical signals. 

        Oligodendrocytes are found within the CNS. Form a myelin sheath by insulating axons. 

        Schwann cells are found within the PNS. Form a myelin sheath by insulating axons. 

        A ganglion is a cluster of nerve cell bodies within the PNS. 
        A nucleus is a cluster of nerve cell bodies within the CNS. 


Nerve Impulse:

    Every cell has a voltage, or membrane potential, across its plasma membrane A membrane 
    potential is a localized electrical gradient across membrane.  Anions are more concentrated
    within a cell.  Cations are more concentrated in the extracellular fluid  An unstimulated cell
    usually has a resting potential of -70mV. 

    Cations. 
    K+ is the principal intracellular cation. 
    Na+ is the principal extracellular cation. 

    Anions. 
    Proteins, amino acids, sulfate, and phosphate are the principal intracellular anions. 
    Cl– is the principal extracellular anion. 

    Ungated ion channels allow ions to diffuse across the plasma membrane. These channels
    are always open. This diffusion does not achieve an equilibrium since the sodium-potassium 
    pump transports these ions against their concentration gradients. Changes in the membrane 
    potential of a neuron give rise to nerve impulses Excitable cells have the ability to generate
    large changes in their membrane potentials. Gated ion channels open or close in response to
    stimuli.  The subsequent diffusion of ions leads to a change in the membrane potential.
 
    Types of gated ions. 
          Chemically-gated ion channels open or close in response to a chemical stimulus. 
          Voltage-gated ion channels open or close in response to a change in membrane potential. 


 Graded potentials..... changes in membrane potential
        Hyperpolarization. 
           Gated K+ channels open à K+ diffuses out of the cellà the membrane potential
           becomes more negative. 
        Depolarization. 
          Gated Na+ channels openà Na+ diffuses into the cellà the membrane potential
          becomes  less negative. 

 Action Potential
    If graded potentials sum to -55mV a threshold potential is achieved.  This triggers an 
    action potential. 
        • Step 1: Resting State 
        • Step 2: Threshold. 
        • Step 3: Depolarization phase of the action potential. 
        • Step 4: Repolarizing phase of the action potential. 
        • Step 5: Undershoot. 
·

 Vertebrate Nervous Systems

Vertebrate nervous systems have central and peripheral components
     Central nervous system (CNS). 
        Brain and spinal cord. Both contain fluid-filled spaces which contain cerebrospinal fluid (CSF).  
        The central canal of the spinal cord is continuous with the ventricles of the brain. White matter 
        is composed of bundles of myelinated axons Gray matter consists of unmyelinated axons, nuclei, 
        and dendrites. 
     Peripheral nervous system. 
        Everything outside the CNS. The divisions of the peripheral nervous system interact in maintaining
        homeostasis Structural composition of the PNS.  Paired cranial nerves that originate in the brain 
        and innervate the head and upper body.  Paired spinal nerves that originate in the spinal cord and 
        innervate the entire body. Ganglia associated with the cranial and spinal nerves. 

Brain
                                                   

The Brainstem “lower brain” , Consists of the medulla oblongata, pons, and midbrain. 
Functions in homeostasis, coordination of movement, conduction of impulses to higher 
brain centers. 
    Medulla oblongata. 
    Contains nuclei that control visceral (autonomic homeostatic) functions.  Breathing. 
    Heart and blood vessel activity. Swallowing. Vomiting. Digestion.  Relays information
     to and from higher brain centers. 
    Pons. 
    Contains nuclei involved in the regulation of visceral activities such as breathing. 
    Relays information to and from higher brain centers.
    The Midbrain. 
    Contains nuclei involved in the integration of sensory information. Relays information to 
    and from higher brain centers.  Regulates sleep and arousal.  Acts as a sensory filter. 

The Cerebellum. 
 Functions to error-check and coordinate motor activities, and perceptual and cognitive factors.
 Relays sensory information about joints, muscles, sight, and sound to the cerebrum. 
 Coordinates motor commands issued by the cerebrum. 

Thalamus. 
Relays all sensory information to the cerebrum. Contains one nucleus for each type of sensory information.
Relays motor information from the cerebrum. Receives input from the cerebrum. Receives input from brain 
centers involved in the regulation of emotion. 

Hypothalamus. 
Contains nuclei involved in thermoregulation, hunger, thirst, sexual and mating behavior, 
etc. Regulates the pituitary gland.  The biological clock is the internal timekeeper.  Experiments
in which humans have been deprived of external cues have shown that biological clockhas a 
period of about 25 hours. This, and other biological clocks, may be responsive to hormonal release, 
hunger, and various external stimuli. 

 The cerebrum
The cerebrum is divided into left and right cerebral hemispheres. The corpus callosum is the major 
connection between the two hemispheres. The left hemisphere is primarily responsible for the right 
side of the body.   Specializes in language, math, logic operations, and the processing of serial 
sequences of information, and visual and auditory details.   Specializes in detailed activities required
 for motor control. . The right hemisphere is primarily responsible for the left side of the body.  Specializes
 in pattern recognition, spatial relationships, nonverbal ideation, emotional processing, and the parallel
 processing of information. Cerebral cortex: outer covering of gray matter. Neocortex: region unique to 
mammals. The more convoluted the surface of the neocortex, the more surface area, the more neurons.