Nervous Tissue

Two body systems that provide regulation and control of the other body systems

* nervous system
* endocrine system

nervous system function

* maintains homeostasis
* respond rapidly to stimuli
* rapid adjustments to loady processes

endocrine system

* repsons slowly to stimuli
* affect target tissue

Three basic functions of nervous system

* sensory function
* integrative function
* motor function

sensory function

* detect changes in environment (stimuli)
* sensory receptors that respond to stimuli
* internal/external environment stimuli

integrative function

* recieve sensory input and analyze it
* will store input and learn what the brain analyze
* previous experience of same stimuli (recognition)

motor function

* reponse to the stimulis
* motor function activates state (effector organs)

effector organs

activate

* skeletal muscles
* glands

Two major divisions of the nervous system

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

Anatomic components of two major divisions of the nervous system

* Central nervous system
* brain
* spinal cord
* Peripheral nervous system
* nerves
* spinal
* cranial
* ganglia (collection of neuron cell bodies)

Cranial nerves

carry nerve fibers (axon) to and from the brain and the brain stem

Spinal nerves

carry nerve fibers or axon to and from spinal cord

Afferent function of PNS

* sensory (afferent) divisions
* somatic sensory fibers
* carry input from skin, skeletal muscle, and joints
* visceral sensory fibers
* carry input from visceral organs
* Motor (efferent) divisions
* somatic nervous system (SNS)
* voluntary nervous system = consciously control skeletal muscle
* conducts impulses from CNS to skeletal muscles
* autonomic nervous system (ANS)
* sympathetic nervous system
* parasympathetic nervous system
* involuntary nervous system
* viscera nerve fibers that carry input to smooth, cardiac muscle and glands
* subdivided into sympathetic and parasympathetic nervous system

Divisions of the ANS

* symphatetic nervous system
* parasymphatetic nervous system

involuntary division of the PNS

autonomic nervous system (ANS)

Two basic cell types in nervous tissue

* neuroglial cells
* neurons

neuroglial cells

* supporting cells of the nervous system
* 4 types of glia in CNS: astrocytes, microglial cells, ependymal cells, oligodendrocytes
* 2 types of glia in the PNS: satellite cells, Schwann cells
* most numerous cells in the body

neurons

* excitable cells
* tissue are excitable because of neurons
* conduct impulses and generate them
* large, highly specialized
* long-lived
* maintains a pathway and can perform for a long time
* high metabolic rate
* amitotic = don’t undergo mitosis

Basic anatomy of a neuron

* cell body
* dendrites
* axon
* axon collaterals

Cell body of neuron

* also called the soma or perikaryon
* most nueron cell bodies are in the CNS
* cluster of nueron cell bodies in the CNS is called a nucleus
* nucleus is round
* prominent nucleolus in nucleus with RNA
* make lots of RNA to make neurotransmitters

Dendrites

* short, tampering, highly branched processes
* extend from cell body
* input is recieved by the neuron
* Function
* provide a lot of surface area
* ligand-gated ion channels which allow dendrites to generate grated potentials
* travels towards the cell body of the neuron
* not active potentials

axon

the neuron’s afferent process

* axon arises from cell body at axon hillock (cone shaped region)
* single, cylindrical acon
* goes into initial segment of axon
* carries output away from cell body

nerve fiber name in CNS

tract

nerve fiber name in PNS

nerve

axon collaterals

* branches are considered collaterals
* axon and axon collaterals are branched at terminal end, forming the terminal branches
* end of the axon, many terminal branches can exist

terminal branches

* end in a bulb-shaped synaptic knobs (axon terminal), which contains many synaptic vesicles

synaptic vesicles

contain thousands of molecules of neurotransmitter

myelin

a white, fatty substance that protects and electrically insulates nerve fibers

* surrounds axon/nerve fibers
* insulate axons electrically
* protects axons

types of myelin nerve fibers

* myelinated fibers
* wrapped in many layers of myeline
* filled with phospholipids


* unmyelinated fibers
* small nerve fibers in diameter
* surrounded by myelin
* doesn't form segmented sheets
* insulates it from surrounding fibers
* dendrites are always unmyelinated

produces myelin in PNS

Schwann cells

produces myelin in CNS

oligodendrocytes

serves as a phagocyte in the CNS

microglia

largest glial cell in the CNS

astrocytes

most numerous glial cell in the CNS

astrocyte

multipolar neurons

have 3 or more processes

* one axon
* others are dendrites

bipolar neurons

* have 2 processes (an axon and dendrite) that extend from opposite sides of the cell body
* dendrite is on both sides (opposite sides0
* sensory function
* provide input
* sight, smell, hearing
* dendrite (input)
* axon (output)

unipolar neurons

* have one short processs that emerges from the cell body, the divides (like a T) into peripheral process and a central process
* peripheral process
* acts like a dendrite
* psuedopolar neurons
* single, short unipolar neurons
* very shot and spilys
* sensory reception
* located in ganglia in the PNS
* output to somewhere in the body
* peripheral process of the body
* central process enters as CNS
* have synaptic knobs (axon terminals) that contain nuerotransmitters
* has receptive endings

nuerofibrils

* bundles of intermediate filaments that maintain cell shape and integrity.


* form a netwrok through the body and its processes

nissil bodies (chromatophilic substance)

* the rough ER stains darkly with basic dyes
* prominent in soma of neurons because they synthesize proteins

axonal transport

bidirectional movements

* anterograde - movement away from the cell body
* includes mitochondria, cytoskeletal elements, membrane components used to renew the axon plasma membranes, and enzymes needed to synthesize of nuerotransmitters
* Retrograde - movement toward the cell body
* mostly organelles returning to the cell body to be degraded or recycled. Important means of intracellular communication. Allows cell body to be advised of the condition at the axon terminals. Delivers vesicles to cell body containing signal molecules.
* overall transports proteins

myelination in the PNS

* myelin sheaths in the PNS are formed by Schwann cells, which indent to reciev an axon and then wrap themselves arround in a jelly roll fashion
* wrapping is loose initially, but Schwann cell cytoplasm is still gradually squeezed from between the membrane layers
* many concentric layers of Schwann cell plasma membrane enclose the axon, much like gauze wrapped around an injured finger, when wrapping process is complete
* nucleus and most of cytoplasm of Schwann cell end up as a budlge just external to the myelin sheath (outer collar of perinuclear cytoplasm)

myelination in the CNS

* forms one myelin sheath with an oligodenrocyte, which has multiple flat processes that can coil around many as 60 axons at the same time
* myekline sheatth gap seperate adjacent section of an axon’s myelin sheath
* CNS sheaths lack an outer collar of perinuclear cytoplasm because cell extensions do the coiling and the squeezed-out cytoplasm is forced back toward the centrally located nucleus instead of peripherally

nodes of Ravier

* gaps in the sheath
* occur at regular interverals along a myelinated axon
* axon collaterals emerge at these gaps

neurilemma (outer collar of perinculear cytoplasm)

* CNS myelin sheaths do not have a neurilemma
* Schwann cells in the PNS have one
* nucleus and most of the cytoplasm end up as a bulge just external to myelin sheath

Association between schwann cells and unmyelinated fibers

* unmyelinated fibers are surrounded by schwann cells, but do not wrap around to form myelin sheath

Gray matter

* consists of short, nonmyelinated neurons and nueron cell bodies

White matter

consists mostly of myelinated axons with some nonmyelinated axons, primarily fiber tracts. The dense coating of fatty myeline is what gives white matter its color

nerve

a bundle of axons in PNS

tract

a collection of axons in the CNS having the same origin, termination, and function

nucleus

clusters of nueron bodies in the CNS

ganglion

collection of nerve cell bodies in CNS

most common type of neuron in the nervous system

multipolar neruons

afferent neurons (sensory)

conveys impulses from the CNS from sensory perceptors in the skin or internal organs

* transmit impulse

efferent neurons (motor)

transmits impulses from CNS to effector organs like muscles and glands

* carry output from CNS to efferent

association neurons (interneurons)

lies between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs

* always CNS
* lie between sensory and motor neuron
* integration function

transmembrane potential

separation of electrical charge across a plasma membrane

* charges separate
* all cells in the body have a membrane potential
* excitable cells can use it to go to
* nerve
* muscles
* carries information to another part of the body

resting membrane potential (RMP)

transmembrane potential in a resting neuron

* not excited
* in resting state, small build of of negatively charge ions under of inside the membrane and positive charge over and outside the plasma membrane
* outside of the cell will be zero and the inside would be slighly negative making it considered to be negative

cause of voltage difference across the plasma membrane

change of voltage difference across the plasma membrane

current

region where K+ highest in concentration

inside the cell membrane (more negative) → cytosol

region where Na+ in the highest concentration

outside the cell membrane (more positive) → extracellular fluid

equilibrium potential

the membrane potnetial at which the electrical force of repulsation/attraction of an ion equals the chemical force of the concentration gradient for ion

* closer to potassium (Ek+) → -90mV
* RMP is -70mV
* affecting by change of along membrane
* diffuse to area with positive charge
* electrochemical gradient
* exactly opposes concentration
* no net movment of ion
* neurons would never depolarize to -65mV (sodium (ENa+)) because of it has few leakage channels

In resting state, the membrane is more permeable to

K+

Channels that are more numerous

K+ ion channels

Types of ion leakage channels that help create RMP in nerve and muscle cells

Na+ and K+

Different types of gated channels

* chemically gated
* volted-gated ion channels
* mechanically-gated channels

chemically-gated (ligand-gated) channels

open/close in response to a chemical stimulus

* binds to chemical ligand or receptor (neurotransmitter bind to receptor)
* will open and it flows through

Voltage-gated ion channels

* open/close in direct response to a voltage change in membrane potential

mechanically-gated channels

open/close in response to physical stimulation or mechanical deformation

* sensory receptors for touch or pressure
* when tissue is deformed or stressed, it allows for ions to travel
* ions are allowed to move more rapidly

prevents the resting cell from reaching equilibrium with respect to the diffusion of Na+ and K+ across the membrane

ATP-driven sodium-potassium pumps

Sodium-Potassium pump

* maintains the RMP
* ATP-driven
* eject 3 Na+ out of the cell
* carry 2K+ back into the cell with each turn
* concentration gradient for Na+/K+

3 factors that are at work to establish and maintain the RMP

* unequal distribution of ions across the plasma membrane
* differential permeability of the plasma membrane to K+ and Na+
* Na+/K+ pump
* concentration gradient against Na+/K+

Term that describes the decrease in the membrane potential

depolarization (increased flow of Na+)

* reduction in potential energy
* moves forward or above 0mV
* bring neuron closer to firing action potential

Term that describes a membrane potential which is greater than the resting membrane potential

hyperpolarization

* an increase in membrane potential
* cell probability of producing action potnetial goes down
* moves away from threshold
* increased in flow of K+ ions

In flow of Na+ promotes this

depolarization

In flow of K+ promotes this

hyperpolarization

Graded potential

* short-lived, local changes in the membrane potential that may be either depolarizations of hyperpolarizations
* triggered by stimuli
* open gated ion channel
* stimulus
* chemical
* electrical
* mechanical
* local region of plasma membrane
* channel open
* move just inside and outside of the plasma membrane
* set current at site of original stimulus
* magnitude of a graded potential varies with the strength of stimulus which produces it
* travel away from plasma membrane
* decay in distance
* called a “local current”

different types of graded potentials

* generator potentials
* receptor potentials
* postsynaptic potentials
* all can occur in dendrites and cell body NOT an axon

significance of graded potentials

help initiate action potentials

* travels throgh dendritic membranes and cell body membranes
* \

action potential

sequence of rapidly occuring events that produce a brief, large depolarization and subsequent repolarization of the plasma membrane

threshold stimulus

\
minimum stimulus needed to achieve an action potential

* depolarization becomes self-generation
* -55mV--50Mv

Membrane potential (voltage) where action potential becomes all or none

\-55mV - 50 mV

Depolarization’s influence membrane permeability of Na+

* at threshold the opening of voltage-gated Na+ channels dramatically increase sodium permeability

Depolarization is a phase for positive feedback (why)

feedback cycle is initiated and the depolarization becomes self-generated

sodium channels open → more sodium enters the plasma membrane → more depolarize → more permeable to sodium ions → repeats until all sodium channels are open

When Na+ rushes into the cell

the membrane potential ultimately revereses and overshoots to 30mV

stops the flow of Na+ into the cell

closing of inactivation gates in voltage-gated channels

absolute refractory period

the time during which a second AP cannot be generated, even with a very strong stimulus

* occurs when a neuron reaches threshold
* each action potential is a separate all or none event
* can only move in one direction along an axon

cause voltage-gated K+ channels to open

closing of inactivation gates of voltage-gated channels

membrane potential when K+ rushes out of the cell

it increases (more negative)

causes after-hyperpolarization (undershoot of AP)

opening of potassium ions

restores the RMP following an action potential

ARP-driven sodium-potassium pumps

relative refractory period

* the time during which a second AP can be generated, but only by a larger-than-threshold stimulus
* some reset back to resting position
* need stronger than normal stimulus to open sodium channels even faster than normal
* threshold is elevated
* bring cell’s threshold
* stimuli no longer needed and the AP is not produced

Explanantion of why generation of action potential is all or none

active potential (AP) is either generated and reaches threshold or not generated and doesn’t reach threshold

Impulse propagation in unmyelated fibers

* Na+, which entered the cell during AP moves laterally through the axoplasm, away from the depolarized region
* this sets up a local current which depolarizes adjacent patches of membrane causing the impulse to self-propagate along the membrane
* the impulse travels in only one direct: from axon hillock to axon terminals
* called continuous conduction

impulse propagation in myelinated fibers

* **myeline serves as an insulator to prevent the leakage of almost all ions across the membrane**
* only occurs in nodes of Ranvier in myelinated fiber
* called salutory conduction (much faster)
* thicker myelin = faster the conduction

speed of ipulse propagation

* diameter of the fiber
* larger diamter = smaller resistance to current flow and faster impulse conduction
* presence/absence of myelin sheath
* presence = faster
* absence = slower
* propagation speed is not related to stimulus strength (all or none principle)

Three types of nerve fibers (diamertes, degree of myelination, speed of impulse conduction)

ent fiber A fibers:

* largest diameter of fibers
* heavily myelinated
* conduction speeds of 15-150 m/sec
* mostly these are somatic sensory and motor fibers serving the skin, skeletal muscles, and joints

B fibers

* diameter is intermediate in size (diameter of axon)
* lightly myelinated
* impulses conducted at a rate of 3-15 m/sec
* afferent & efferent ANS fibers serving visceral organs

C fibers

* smallest diameter fibers
* unmyelinated
* impulses conducted relatively slowly at 0.5-2 m/sec
* sensations of poorly localized pain, touch, pressure, and visceral (ANS) efferent fibers

Synaptic vesicles

junctions where information is transferred form one cell to another (one neuron to an effector)

function of neurotransmitters

* act directly are those that bind to and open ion channels.


* provoke rapid responses in postsynaptic cells by altering membrane potential
* ACh and the amino acid neurotransmitters are typically direct-acting neurotransmitters.

Ions trigger exocytosis of neurotransmitters from the synaptic vesicles

Calcium ions