nervous tissue

characteristics of neurons

excitability (response to stimuli), conductivity (ability to initiate and propagate signals), secretion (release neurotransmitter in response), longevity

neuron structures

soma, dendrites, axon

axon

long process emanating from cell body; originates from axon hillock and ends in several axon terminals

glial cell (neuroglia)

nnon-excitable, outnumber neurons, account for half the volume of nervous system

astrocyte

CNS glial cell; perivascular feet form association with blood vessels and constitute blood-brain barrier (control what enters nervous tissue from blood + regulate tissue fluid composition); structural framework and secrete chemicals that regulate synapse formation

ependymal cells

CNS; line internal cavities of brain and spinal cord; ciliated and allow movement of CSF; form choroid plexus with nearby blood capillaries (produce and circulate CSF)

microglia

CNS; small, rare phagocytic cells that wander and replicate in infection

oligodendrocytes

CNS; large cells with multiple slender extensions; extensions wrap aruond axons of neurons forming myelin sheath

satellite cells

PNS; arranged around neuronal cell bodies in a ganglion; electrically insulate and regulate exchange of nutrients and waste between neuronal cell and interstitial fluids

neurolemmocytes (Schwann cells)

PNS; elongated, flat cells that wrap around PNS axons to form myelin covering

myelin

several layers of lipid membrane; neurolemmocytes in PNS and oligodendrocytes in CNS

myelination in PNS

neurolemmocyte myelinates only 1 mm, so several needed for one axon; gaps between neurolemmocytes are neurofibril nodes

myelination in CNS

one oligodendrocyte myelinates 1 mm of multiple axons, each at multiple spots; neurofibril nodes are present between adjavent wrapped segments; contributes to multiple axons and extends cellular processes

unmyelinated axons

CNS- not associated with oligodendrocytes

PNS- axon sites in depressed portion of neurolemmocyte, not fully ensheathed by it

receptive segment

dendrites and soma; chemically gated Na+, K+, Cl- channels

initial segment

axon hillock (trigger zone fro action potential); voltage-gated Na+ and K+ channels

conductive segment

axon and its branches; voltage-gated Na+ and K+ channels

transmissive segment

synaptic knobs; voltage-gated Ca2+ channels and Ca2+ pump

characteristics of resting neurons

ions unevenly distributed across plasma membrane due to actions of pumps (higher [K+] ICF and higher [Na+], [Cl-}, [Ca2+] in ECF); gated channels closed in functional segments of cell; electrical charge difference across membrane (RMP is -70 mV)

reception of neurotransmitter

triggers postsynaptic potential; binds to chemically gated ion channels and open them; voltage change produces a graded potential

postsynaptic vs presynaptic neuron

post- neuron receiving neurotransmitter

pre- neuron releasing neurotransmitter

excitatory postsynaptic potentials (EPSPs)

depolarizations caused by cation (Na+) entry at chemical synapse in receptive region

inhibitory postsynaptic potentials (IPSPs)

hyperpolarizations caused by cation (K+) exit or anion (Cl-) entry at chemical synapse in receptive region

spatial summation

multiple locations on cell’s receptive regions receive neurotransmitter simultaneously and generate postsynaptic potentials

temporal summation

single presynaptic neuronrepeatedly releases neurotransmitter and produces multiple EPSPs within a very short period of time

generation of action potential along axon

1. at RMP, voltage-gated channels are closed

2. as Na+ enters from adjacent region, voltage-gated Na+ channels open

3. Na+ enters axon leading to depolarization

4. Na+ channels become inactive at +30 mV and Na+ entry into axon stops

5. voltage-gated K+ channels open; K+ diffuses out of acon leading to repolarization

6. K+ channels stay open for a long time leading to hyperpolarization

7. K+ channels close and RMP reestablished by Na+/K+ ATPase

refractory period

period of resistance to stimulation; during action potential and few msec after, it’s difficult to stimulate that region of a neuron to fire another action potential

absolute refractory period (~1 msec)

no stimulus can initiate another action potential; Na+ channels are open then inactivated

relative refractory period (just after absolute)

another action potential is possible (na+ channels at rest) but more difficult to stimulate cell; cell slightly hyper polarized and further from threshold

continuous conduction

occurs in unmyelinated axons; charge opens voltage-gated channels which allows ions to enter, spread to adjacent region, open more channels; every segment of axon must fire action potential from axon hillock to terminal

saltatory conduction

occurs in myelinated axons; action potentials occur only at nodes, where axon’s voltage-gated channels are concentrated; after Na+ enters at node, it starts rapid positive current down inside of axon’s myelinated region; current is strong enough to open voltage-gated channels at next node (impulse jumps from node to node); conduction is faster than continuous conduction in unmyelinated axons

multiple sclerosis

progressive demyelination of neurons in CNS; oligodendrocytes are destroyed by immune system; progressive decline in donduction of impulses in motor neurons

Guillain-Barré syndrome

loss of myelin from peripheral nerves due to inflammation; muscle weakness in distal limbs to proximal muscles