CH 11. FUNCTIONAL ORGANIZATION OF NERVOUS TISSUE

**functions of nervous system:**

* sensory input: 
* sensory receptors monitor external & internal stimuli
* integration:
* the brain & spinal cord
* control of muscles & glands:
* … when stimulated by nervous system
* smooth muscle, cardiac muscle, & glands are also under nervous control
* homeostasis:
* regulates other systems to maintain constant internal environment
* center for mental activities:
* brain is center of metal activities including memory, emotions, thinking, & consciousness

**divisions of nervous system:**

* central nervous system (CNS)
* peripheral nervous system (PNS)

**divisions of nervous system**

central nervous system (CNS):

composed of brain & spinal cord

**divisions of nervous system**

peripheral nervous system (PNS):

* nervous tissue external to the CNS
* composed of sensory receptors & nerves

**peripheral nervous system (PNS)**

subdivisions:

* sensory receptors
* nerve
* sensory division
* motor division

**peripheral nervous system (PNS) subdivisions**

sensory receptors:

ends of neurons or separate specialized cells that detect temperature, pain, touch, pressure, light, sound, & odors

**peripheral nervous system (PNS) subdivisions**

nerve:

bundle of axons & sheaths that connect CNS

* cranial nerves (12)
* spinal nerves (31)

**peripheral nervous system (PNS) subdivisions**

sensory division:

afferent, transmits action potentials from sensory receptors to CNS

**peripheral nervous system (PNS) subdivisions**

motor division:

efferent, transmits action potentials away from CNS to effector organs

* composed of two subdivisions:
* somatic nervous system
* autonomic nervous system (ANS)

**PNS -- motor division -- subdivisions**

somatic nervous system:

innervates skeletal muscle

**PNS -- motor division -- subdivisions**

autonomic nervous system (ANS):

innervates cardiac muscle, smooth muscle, & glands

* consists of its own subdivisions:
* sympathetic division: 
* always active at basal levels, but most active during states of heightened stress
* parasympathetic division: 
* regulates resting functions, slows down the body
* enteric division: 
* controls digestive activities; houses within digestive tract

**general functions of CNS & PNS**

central nervous system (CNS):

processes, integrates, stores, & responds to information from the PNS

**general functions of CNS & PNS**

peripheral nervous system (PNS):

detects stimuli & transmits info to CNS & receives info from CNS

**list components of neuron structure:**

* cell body (soma)
* dendrites
* axons

**neuron structure**

cell body (soma):

contains:

* nucleus
* nissl substance

**neuron structure**

dendrites:

short, branched cytoplasmic extensions of cell body

**neuron structure**

axons:

* cytoplasmic extension of cell body
* trigger zone:
* part of neuron where axon originates
* one un-branched axon
* presynaptic terminal

**functional classification of neurons**

* sensory (afferent) neurons
* motor (efferent) neurons
* interneurons (association)
* multipolar
* bipolar
* unipolar

**functional classification of neurons**

sensory (afferent) neurons:

conduct action potentials towards CNS

**functional classification of neurons**

motor (efferent) neurons:

conduct action potentials away from CNS towards muscles or glands

**functional classification of neurons**

interneurons (association):

conduct action potentials from one neuron to another neuron within CNS

**functional classification of neurons**

multipolar:

have several dendrites, single axon, interneurons & motor neurons

**functional classification of neurons**

bipolar:

have a single axon & dendrite

**functional classification of neurons**

unipolar:

have single axon

**neuroglia of CNS:**

* astrocytes
* microglia
* ependymal cells
* oligodendrocytes

**neuroglia of CNS**

astrocytes function:

* support & brace neurons & blood vessels
* regulate substances from blood

**neuroglia of CNS**

astrocytes location:

* cling to neurons & cover capillaries
* blood brain barrier:
* protects neurons from toxic substances

**neuroglia of CNS**

ependymal cells function:

* line ventricles of brain & central canal of spinal cord
* secrete cerebrospinal fluid
* cilia help move fluid through cavities of brain

**neuroglia of CNS**

ependymal cells structure:

specialized to form choroid plexuses

**neuroglia of CNS**

ependymal cells location:

found within certain regions of ventricles

**neuroglia of CNS**

microglia function:

* become mobile & phagocytic in response to inflammation
* phagocytes monitor health of neurons

**neuroglia of CNS**

microglia structure:

specialized macrophages

**neuroglia of CNS**

microglia location:

found in brain & spinal cord

**neuroglia of CNS**

oligodendrocytes function:

* form myelin sheaths around axons of CNS neurons
* single oligodendrocyte can form myelin sheaths around portions of several axons

**neuroglia of CNS**

oligodendrocytes location:

found in gray & white matter of CNS

**neuroglia of PNS:**

* satellite cells
* schwann cells

**neuroglia of PNS**

schwann cells (neurolemmocytes):

* form myelin sheaths around part of axon of PNS neuron
* form around portion of only one axon

**neuroglia of PNS**

satellite cells:

* support & nourish neuron cell bodies within ganglia
* protect neurons from heavy metal poisons by absorbing & reducing access to neuron cell bodies

**myelin sheath:**

plasma membrane of schwann cells or oligodendrocytes repeatedly wraps around a segment of axon to form myelin sheath

**myelin:**

whitish, fatty, segmented sheath around most long axons

* protects, electrically insulates, increases speed of nerve

**cells that myelinate in CNS & PNS**

axons: myelinated:

* node of ranvier: gap in myelin sheath
* conduct action potentials rapidly

**cells that myelinate in CNS & PNS**

axons: unmyelinated

* rest in invaginations of schwann cells or oligodendrocytes
* surround each axon, do not wrap around it many times
* leads to conduction of action potentials slowly

**gray matter vs white matter**

gray matter:

* collections of neuron cell bodies, unmyelinated axons, dendrites, & neuroglia
* forms cortex & nuclei in CNS & ganglia in PNS
* integrative functions

**gray matter vs white matter**

white matter:

* myelinated axons
* propagates action potentials
* forms nerve trans in CNS & nerves in PNS

**leak channels vs gated channels**

leak channels:

* always open
* K+ leak channels are more numerous than Na+ leak channels
* plasma membrane more permeable to K+ than to Na+ when at rest

**leak channels vs gated channels**

gated channels:

* consist of ligand-gated channels, voltage-gated ion channels, & other gated ion channels
* open & close in response to stimuli

**leak channels vs gated channels**

gated channels -- ligand-gated ion channels:

open or close w/the binding of specific ligand (neurotransmitter)

**leak channels vs gated channels**

gated channels -- voltage-gated ion channels:

* open & close in response to small voltage changes across plasma membrane
* abundant in nervous & muscle tissue

**concentration differences across plasma membrane (K+ vs Na+)**

* Na+, Ca2+, & Cl- in greater concentration outside cell
* K+ & negatively charged molecules, such as proteins in greater concentration inside cells
* more K+ leak channels, so potassium can travel easier

**resting membrane potential:**

* Na+, Cl-< & Ca2+ do not have great impact on RMP, very few leakage channels for these ions
* maintained by Na+/K+ pump

**how resting membrane potential is established:**

when movement of K+ out of cell are = to their movement into cell

**role of Na+/K+ pumps, leak channels, & intracellular proteins:**

* at RMP, inside cell is negatively charged; compared with outside of cell due to tendency of positively charged K+ to diffuse out of cell
* opposed by negative charge that develops inside of plasma membrane

**depolarization vs hyperpolarization**

depolarization:

decrease in resting membrane potential (less polar)

**depolarization vs hyperpolarization**

depolarization caused by:

* decrease in K+ concentration gradient
* decrease in membrane permeability to K+
* increase in membrane permeability to Na+ or Ca2+
* decrease in extracellular Ca2+ concentrations

**depolarization vs hyperpolarization**

hyperpolarization:

increase in resting membrane potential (more polar)

**depolarization vs hyperpolarization**

hyperpolarization caused by:

* increase in K+ concentration gradient
* increase in membrane permeability to K+, Cl-
* decrease in membrane permeability to Na+
* increase in extracellular Ca2+ concentrations

**graded potential:**

smaller changes in resting membrane potential; confined to a small area of plasma membrane

**graded potential**

what can cause graded depolarization?

increase in membrane permeability to Na+

**graded potential**

what can result in graded hyperpolarization:

increase in membrane permeability to K+ or Cl-

**phases of action potential & ion movement:**

* depolarization
* repolarization
* afterpotential

**phases of action potential & ion movement**

depolarization:

inside of membrane becomes more positive

* Na+ diffuses into cell through voltage-gated ion channels

**phases of action potential & ion movement**

repolarization:

return of membrane potential towards negative resting membrane potential 

* voltage-gated Na+ channels close
* voltage-gated K+ channels open & K+ diffuses out of cell

**phases of action potential & ion movement**

afterpotential:

brief period of hyperpolarization following repolarization

**all-or-none principle as it applies to action potentials**

* action potential starts w/graded depolarization to level called threshold
* action potentials work in all-or-none-fashion, meaning that if threshold is not met/exceeded, no stimulus will occur
* all stimuluses that meet threshold are same magnitude, no matter how strong

**absolute vs relative refractory periods**

absolute refractory period:

time during an action potential when second stimulus cannot initiate another action potential

**absolute vs relative refractory periods**

relative refractory period:

time during which a stronger-than-threshold stimulus can evoke another action potential

**factors that determine action potential frequency:**

* subthreshold stimulus
* threshold stimulus
* submaximal stimulus
* maximal/supramaximal stimulus

**factors that determine action potential frequency**

subthreshold stimulus:

produces a graded depolarization

**factors that determine action potential frequency**

threshold stimulus:

produces a single action potential

**factors that determine action potential frequency**

submaximal stimulus:

produces an increasing frequency of action potentials as strength of stimulus increases

**factors that determine action potential frequency**

maximal/supramaximal stimulus:

produces a maximum frequency of action potentials

**myelination on speed of action potential propagation**

* in myelinated axon, action potentials are generated at successive nodes of ranvier
* occurs most rapidly in myelinated, large-diameter axons