Cells of the Nervous System

Division of Nervous system

Central Nervous System containing the brain and spinal cord, Peripheral nervous system containing the cranial nerves, spinal nerves, and ganglia

Division of the Peripheral nervous system

the somatic nervous system and the autonomic nervous system

Somatic nervous system

controls voluntary movement

Autonomic nervous system

controls involuntary functions and is divided into sympathetic and parasympathetic

Nervous tissue

Consists of neurons and all their supporting cells

Neurons

Electrically excitable cells that transmit electric signals, major structural and functional unit of nervous system that live long with a high metabolic rate

Neuroglia

helper cells

Astrocytes

Cover capillaries, support, brace, anchor neurons to nutrient supply, guide migration of new neurons, control chemical environment

Microglia

small, ovoid cells with spiny processes, phagocytes, monitor neuron health

Ependymal Cells

line central brain and spinal cavities, produce cerebrospinal fluid

Olidenrocytes

branched cells, wrap around neuron axons

Schwann cells

maintain myelin sheaths around PNS nerves cell

Satellite Cells

surround neuron cell bodies with ganglia

Sensory (afferent) neurons

detect changes in body and environment, information transmitted to brain and spinal cord

Interneurons

between sensory and motor pathways in CNS, 90% of neurons, process, store, and retrieve information

Motor (efferent) neurons

send signals out to muscles and gland cells, organs that carry out responses = effectors

Dendrites

branch-like structures of a neuron that receive information from other neurons and transmit it to the cell body

soma

cell body, biosynthetic, receptive region

nissl bodies

similar to rough ER, protein synthesis

Axon hillock

action potential generations, summing center of impulse

Axon

conducts electrical impulses away from the cell body

axon terminals

secretion of neurotransmitters

Nerves impulses

are electrical signals

Voltage

potential energy generated by separated charge

Potential difference

voltage between 2 points

current

flow of electrical charge

resistance

hindrance to flow of charge

insulator

substance with high electrical resistance

conductor

substance with low electrical resistance

resting membrane potential (Vr)

potential difference across the plasma membrane generated by different concentrations of Na,K,Cl and protein anions

due to: differential permeability to Na+ and K+, operation of the sodium potassium pump

at rest: Sodium, chloride high OUTSIDE, protein anions, potassium high INSIDE

Electrochemical Gradient

ions flow down their chemical gradient (high→low) ions will also flow down their electrical gradient (toward area of opposite charge)

Electrochemical gradient = Electrical + chemical gradient

Passive ion channels

always open

chemically gated ion channels

open with binding of specific neurotransmitter

mechanically gated ion channels

open and close due to physical deformation

Voltage Gated ion channels

open and close in response to membrane potential

graded potentials

short-lived, localized changes in membrane potential, decrease in intensity with distance from the initial site, magnitude varies directly with the strength of stimulus, if graded potential is strong enough, can initiate action potential

Excitatory postsynaptic potential (EPSP)

causes local depolarization, increases membrane potential, in favor of action potential

inhibitory postsynaptic potential (IPSP)

causes local membrane hyperpolarization, more negative, inhibits action potential

Action potientials

short reversal of membrane potential, total amplitude of 100 mV, only generated by muscle cells and neurons, does not decrease in strength over distance, all or nothing response

AP: resting state

Na and K channels are closed, each Na channel has 2 voltage regulated gates (activation gate closed in resting state) (inactivation gate open in resting state)

AP: Depolarization phase

Na permeability increases; membrane potential reverses, Na gate open; K gates closed

Repolarization phase

Na inactivation gate close, membrane permeability to Na to resting levels, as Na gates close, voltage-sensitive K gates open, K exits and internal negativity of resting neuron restored

hyperpolarization

K gates remain open, excessive efflux of K, neuron insensitive to stimulus and depolarization

Absolute refractory period

time from the opening of Na activation gates until inactivation gates prevent the generation of AP, ensure that AP is separate, enforces transmission of nerve impulses

relative refractory period

the interval following the absolute refractory period when Na gates close, K gates open, and repolarization occurring

axon condition velocities

vary widely among neurons, rate of impulse propagation determined by axon diameter and myelin sheath presence

axon diameter

larger diameter = faster impulse

myelin sheath presence

myelination dramatically increases impulse speed

Myelination

fatty, white, segmented sheath around many long axons

function: protection, electrical insulation, increase speed of electric impulse

nodes of Ranvier

gaps in the myelin sheath between adjacent Schwann cells

unmyelinated axons

schwann cells still associate with unmyelinated nerve fibers, surround but not coiling, one Schwann cell can partially enclose 15 or more unmyelinated axons

saltatory conduction

myelinated axon, current only passes at nodes on ranvier, voltage gate Na channels concentrated at these nodes, AP triggered only at nodes and jump from node to node, faster than conduction along unmyelinated axons

presynaptic neurons

conducts impulses toward synapse

postsynaptic neurons

transmits impulses away from synapse

synaptic delay

0.3 - 0.5 ms, rate-limiting step of neural transmission

Neurotransmitter fate

diffusion, reuptake, enzyme degradation