Ch. 11 - Functional Organization of Nervous Tissue

functions of the nervous system

1. sensory input

2. integration

3. control of muscles and glands

4. homeostasis

5. mental activities

central nervous system

processes, integrates, stores, responds to information from PNS

• brain & spinal cord

• encased in bone

peripheral nervous system

detects stimuli, transmits info to and receives info from CNS

• sensory receptors & nerves

• nervous tissue outside of CNS

• 2 divisions: sensory & motor

sensory division (PNS)

transmits action potentials from sensory receptors to the CNS

motor division (PNS)

carries action potentials away from the CNS in cranial or spinal nerves

• 2 subdivisions: somatic & autonomic

somatic nervous system (m)

innervates skeletal muscle

autonomic nervous system (m)

innervates cardiac muscle, smooth muscle, and glands

• 3 subdivisions: sympathetic, parasympathetic, enteric

sympathetic division (ANS)

“fight-or-flight”; most active during physical activity

parasympathetic division (AMS)

“rest-and-digest”; regulates resting functions

enteric nervous system (ANS)

controls the digestive system

neurons

excitable cells that transmit electrical signals

• 3 components: soma, dendrites, axon

nuclei

clusters of cell bodies in the CNS

ganglia

clusters of cell bodies in the PNS

soma (cell body)

• nissl substance: primary site of protein synthesis

  • rough ER and free ribosomes

• contains nucleus, nucleolus, Golgi apparatus, mitochondria

axons (nerve fibers)

• slender processes: uniform diameter, length varies

• trigger zone: where axon originates and action potential is generated

presynaptic terminal

branched terminus of an axon

synapse

junction between a nerve cell and another cell

multipolar neurons

have several dendrites and a single axon

• interneurons & motor neurons

bipolar neurons

have a single axon and dendrite

• components of sensory organs

unipolar neurons

have a single axon

• most sensory neurons

glial cells

cells that surround neurons, accounting for over half of the brain’s weight

• supportive scaffolding: “neuroglia”

• segregate and insulate neurons

• guide young neurons to proper connections

• promote health and growth

glial cells of the CNS

• astrocytes

• microglial cells

• ependymal cells

• oligodendrocytes

glial cells of the PNS

• satellite cells

• Schwann cells

astrocytes

most abundant, branched, and versatile; cling to neurons/synapses and cover capillaries

• support neurons & blood vessels

• anchor neurons to nutrient supplies

• influence function of blood-brain barrier

• produce secondary energy to neurons

• process substances: clean up K ions & recycle neurotransmitters

• isolate damaged tissue & limit spread of infection

ependymal cells

range in shape and are often ciliated; line ventricles of brain and central canal of spinal cord

• produce cerebrospinal fluid (CSF)

• help to circulate CSF using cilia

microglia

small and ovoid with tiny processes; migrate to areas of inflammation/damage/death

• macrophages (immune cells)

• phagocytes, monitor the health of neurons

oligodendrocytes

form myelin sheaths around the axons of several CNS neurons

• ‘octopus’

• MS: attacks, breaks down myelin sheaths

Schwann cells (PNS)

form myelin sheath around part of one axon of a PNS neuron

satellite cells (PNS)

• support/nourish cell bodies within ganglia

• protect cell from heavy metals

myelin

whitish, fatty segmented sheath around most long axons

• protects axon

• electrically insulates fibers from one another

• increases speed of nerve impulse transmission

nodes of Ranvier

gaps in myelin sheath

unmyelinated axons

• rest in invaginations of Schwann cells (PNS) or oligodendrocytes (CNS)

• conduct action potentials slowly

• still technically myelinated, but much less so

white matter

consists of myelinated axons

• propagates action potentials

• CNS: forms nerve tracts

• PNS: forms nerves

gray matter

collections of neuron cell bodies or unmyelinated axons

• CNS: forms cortex & nuclei

• PNS: forms ganglia

resting membrane potential

charge difference across the plasma membrane when the cell is not being stimulated

• inside of the cell is more negatively charged

  • K+ diffuses out of cell

action potentials

electrical signals produced by cells

• occur when a graded potential causes depolarization of the the PM to a level called threshold

• all-or-none fashion, of the same magnitude no matter the stimulus strength

results of action potentials

• sensations

• complex mental activities

• contraction of muscles

• secretion of certain glands

electrical properties of cells result from:

permeability characteristics and ionic concentration differences across the plasma membrane

channels within cell body

50-70 channels per micrometer

channels within trigger zone

~350 channels per micrometer

concentration differences across PM

• Sodium, Calcium, and Chloride ions are in much greater concentration outside the cell

• Potassium ions and negatively charged molecules (e.g. proteins) are in much greater concentration inside the cell

  • negatively charged proteins are synthesized inside the cell, cannot diffuse out of it

sodium-potassium pump

• moves ions by active transport

• K+ ions moved into the cell

• Na+ ions moved out of the cell

leak channels

• non-gated (always open)

• K+ channels more numerous

  • plasma membrane is more permeable to K+ when at rest

gated ion channels

include ligand-gated channels, voltage-gated channels, etc.

ligand-gated ion channels

open/close with the binding of a specific ligand (neurotransmitter)

• common in nervous & muscle tissue, glands

voltage-gated ion channels

open/close in response to small voltage changes across the plasma membrane

• common in nervous & muscle tissues

depolarization

inside of the cell becomes more positive

• Na+ diffuses into the cell through voltage-gated ion channels

repolarization phase

return of the membrane potential to the resting membrane potential

• voltage-gated Na+ channels close

• voltage-gated K+ channels open, K+ diffuses out of the cell

afterpotential

brief period of hyperpolarization following repolarization

graded potentials

magnitude varies from small to large depending on stimulus strength or frequency

• can by hyperpolarizing or depolarizing

• can summate (add on to each other)

• decrease in magnitude as they spread across membrane

• can cause generation of APs

propagation of action potentials

• unmyelinated: immediately adjacent to previous APs

• myelinated: at successive Nodes of Ranvier

synapse

junction between two cells where communication takes place

• presynaptic cell: transmits signal

• postsynaptic cell: receives the signal

electrical synapses

gap junctions in which tubular proteins called connexons allow ionic currents to move between cells

• APs are conducted rapidly between cells, synchronized activity

• common in cardiac muscle and many types of smooth muscle where coordinated contractions are essential

chemical synapses

have three anatomical components:

• presynaptic terminals

• postsynaptic membranes

• synaptic cleft

presynaptic terminals

enlarged ends of the axon containing synaptic membranes

postsynaptic membranes

contain receptors for the neurotransmitter

chemical synapse activity

1. APs arriving at presynaptic terminal cause voltage-gated Ca channels to open

2. Ca ions diffuse into the cell, synaptic vesicles release neurotransmitters

3. neurotransmitters diffuse from presynaptic terminal across synaptic cleft

4. neurotransmitters combine with their receptor sites and cause ligand-gated channels to open. ions diffuse in/out of cell, membrane potential changes

excitatory post-synaptic potential

a depolarizing graded potential of the postsynaptic membrane

• increases neurotransmitter release

inhibitory postsynaptic potential

a hyperpolarizing graded potential of the postsynaptic membrane

• decreases neurotransmitter release

• decreases likelihood of action potential by moving membrane potential farther from threshold