Chapter 12 Neurons & Nervous Tissue

Sections of the Nervous System

Central

Peripheral

Central Nervous System comprises of

brain & spinal cord

Peripheral Nervous System comprises of

nervous tissue outside CNS and ENS

Two Kinds of Cells in Nervous System

neurons

neuroglia (suppporting cells)

Special Sensory Receptors

monitor smell, taste, vision, balance, and hearing

Visceral Sensory Receptors

monitors internal organs

Somatic Sensory Receptors

monitor skeletal muscles, joints, and skin

• movement, temp, pain, pressure, vibration

Afferent Division

part of PNS that is responsible for transmitting sensory information from the body's sensory receptors to the CNS

Effectors

structures that respond to incoming neural signals

Receptors

structures that detect changes

Information Processing in the CNS

integrates, processes, and coordinates sensory input and motor commands

Efferent Division

part of the PNS that is responsible for carrying signals from the CNS to the body

Somatic Nervous System (SNS)

controls voluntary functions such as muscle movement

Autonomic Nervous System (ANS)

controls involuntary functions such as heart rate and digestion

automatically regulates smooth muscle, cardiac muscle, gladular secretions, adipose tissue

Parasympathetic Division

controls the body during rest and digestion

Sympathetic Division

controls the body during times of stress

aka “fight or flight”

Regions of the Neuron

Axon

Cell Body

• Perikaryon

• Nucleus

Dendrites

Telodendria / Axon Terminal

Nissl Bodies

region of the cell body where RER and free ribosomes are located

only found in neurons

reason for gray matter

Gray Matter

regions containing cell bodies & unmyellinated axons

White Matter

regions dominated by myelinated cells

Anterograde Transports

from soma to terminals

Retrograde Transport

from terminals to soma

Synaptic Vesicle

store neurotransmitters that is moved via a kinesin protein in the axon

Structural Classes of Neurons

Anaxonic

Bipolar

Unipolar

Multipolar

Anaxonic Neuron

have more than 2 processes and they may all be dendrites

Bipolar Neuron

have 2 processes separated by the cell body

Unipolar Neurons

have single elongated process with cell body located off to the side of its axon

most common type of sensory neuron in PNS

Multipolar Neuron

have more than 2 processes with a single axon and multiple dendrites

characteristic of all motor neurons

Functional Classes of Neurons

Sensory Neuron

Motor Neuron

Interneuron

Types of Sensory Neurons

Somatic Sensory Neurons

• external environment

Visceral Sensory Neurons

• internal environment

Sensory Receptors

Interal Systems (i.e. digestive, etc.)

Internal Senses (stretch, pain)

Somatic Senses (temp, pain, etc.)

Proprioceptors (position, movement of muscles & joints)

where do Motor (Efferent) Neurons send signals

Somatic MNs

• skeletal muscle

Visceral MNs (ANS)

• smooth, cardiac muscles, glands, adipose tissue

what chemical do MN Neurotransmitters sent to somatic MN

Achetylcholine

what chemical do MN Neurotransmitters sent to visceral MN

Acetylecholine (parasympathetic)

Norepinepherine (sympathetic)

MN Target Organ Receptors for Skeletal Muscles

nicotonic acetylcholine receptors

MN Target Organ Receptors for Smooth & Cardial Muscle, Glands, Adipose Tissue

Muscarinic AChRs

Adrenergic Receptors

Primary Location of Interneurons

brain & spinal cord

Purpose of Interneurons

distribute sensory info

cooridnate motor activity

involved in high functions (i.e., memory, planning)

Types of Neuroglia of CNS

astrocytes

ependymal cells

oligodentries

microglia

Astrocytes

• anchor nuerons to capillaries for material exchange

• maintain BBB

• provide strucutral support

• regulate ion, nutrient, dissolved gas conc

• absorb & recyle neutrotransmitters

• form scar tissue after injury

Ependymal Cells

• simple cuboidal epithelial cells that line brain and spinal cord

• produce, circulate, monitor cerebrospinal fluid (CSF)

Oligodendrocytes

• cells with sheet-like processes that wrap around axons

• help form myelin sheath

• provide structural framework

Microglia

• remove cell debris, waste, pathogens by phagocytosis

Purpose of Myelin Sheath

• provides protective insulation

• affects how fast signals travel through those nerve cells

• maintains the strength of the impulse message as it travels down the axon

Types of Neurglia of PNS

Satellite Cells

Schwann Cells (Neurolemmocytes)

Satellite Cells

• surround clusters of neuronal cell bodies or ganglia

  • regulates fluids around ganglionic neurons

Schwaan Cells

• wrap plasma membrane around axons

• outer surface called neurolemma

• each cell forms an individual myelinated internode

Rabies is a viral disease contracted from the bite of an infected animal. Rabies bypasses many immune system defenses by traveling in peripheral neurons to reach the CNS. Which methods of transport are used by the rabies virus to reach the CNS?

Axoplasmic Transport

• virus particles can travel w other materials along molecular motors in cytoplasm

• specifically Retrograde Flow (type of axoplamic)

  • substance flow from axon to cellbody and destroys neuron

Osmosis

• viruses can dlow in & among cells by traveling along water routes

Response to Injury in PNS

1) Fragmentation of axon & myelin occurs in region distal to injury

2) Cord of Schwaan cell grows into injrt & unites ends

• macrophages engulf degrading axon & myelin

3) Axon sends buds into network of Schwann cells & then starts growing along cord of Schwaan cells

4) Axon continues to grow into distal stump & is enclose by Schwann cells

Neuronal Membrane Potential

movement of ions across plasma membrane of neurons

measured in voltage (mV)

Resting Membrane Potential

net positive charge outside neuron

net negative charge inside neuron

charges separated by plasma membrane

stabilized by action of Na+/K+ Exchange Pump

Features of Resting Membrane Potential

• different ionic conc outside vs inside neuron

  • outside: high [Na+] & {Cl-]

  • inside: high [K+] & [negatively charged proteins]

• neruonal membrane are high selective about what crosses in & out of neurons

  • @ rest, ions move through leak channels

• neuronal membranes have different permeabilities for different ions

Graded Potential

transient (temporary), typically small, local change in membrane potential (mV)

caused by stimmulus intensity (i.e., neurotransmitter binding to receptor)

reversible

can be polarizing or depolarizing

Action Potential in Neurons

large change in membrane potential (+100mV) produced by summation of graded potentials at axon hillock

sends cascade of electrical changes that travels length of axon, ending at synaptic terminal

Current (I)

movement of ions

Resistance (R)

how much a barrier restricts ion movement

Voltage (V)

product of current & resistance is the membrane potential

V = IR

Passive Chemical Gradient

more K+ ions leak out of neuron than Na+ ions leak into neuron

Active Na+/K+ Pump

aka Na+/K+ ATPase

pumps 3 Na+ out & 2 K+ in

results in net negative inside neuron (favoring Na+ to enter cell)

• however, pu expels any Na ions entering cell

uses ATP to power pump

balances out passive forces (espcially after action potential)

maintains conc of Na & K ions across plasma membrane

Electrochemical Gradient

sum of chemical & electrical forces acting on an ion across the neural membrane

Equilibrium Potentials

membrane potential at which there is no net movement of a particular ion across cell membrane

Potassium Ion Gradients

at neuron’s resting potential, the chemical & electrical gradients are opposites for K+ ions

net electrochemical gradient tends to force K+ out of cell

K+ Equilibrium Potential = -90 mV

• only possible if there is no resistance to flow of K+ (free permeable membrane)

Sodium Ion Gradients

chemical & electrical gradients for Na+ are combined at resting membrane potential of neuron

net electrochemical gradient forces Na+ into cell

Na+ Equilibrium Potential = +66 mV

• only possible if there is no ressitance to flow of Na+ ions (free permeable membrane)

Passive Channels

Na+ & K+ Leak Channels

always open

more K+ leak channels than Na+

Active Channels

gated ion channels

• open & close by stimulus

• most active channels are closed when neuron is at rest

Types of Active Channels

Chemically Gated

Voltage-gated

Mechanically gated

Chemically (Ligand-) Gated Ion Channels

open or close when they bind to specific chemicals or ligands (i.e., neurotransmitters)

found on soma (cell body) and dendrites of neurons

Voltage-gated Ion Channels

respond to changes in memebrane potential

found in axon, especially conc @ axon hillock, skeletal & cardial muscle cell membrane

Mechanically Gate Ion Channels

respond to distortion of neural membrane

found in sensory neurons

• skin

found in sensory cells

• hair cells in ear canal & vestibular system

Graded Potential

transient, local change in membrane potential

• change is stimulated by opening ion channel such as ligand or mechanically

• change in voltage is proportional to stimulus

• degree of depolarization decreases with increased distance from stimulation site

  • cyotosol offers resistance to ion movement

Example of Graded Poteintial Triggering Specific Cellular Function

exocytosis of glandular secretions are stimulated by graded potential

ACh stimulated nAChRs & cause graded potentials at neuromusclar junction (NMJ)

Depolarization

a change in membrane potential fron negative value toward 0 mV

Local Current

the movement of positive charges parallel to iner and outer surfaces of membrane to spread depolarization

Repolarization

return of membrane back to resting state

Hyperpolarization

movement of membrane potential away from normal resting potential and farther fron 0mV

Action Potential (Nerve Impluses)

wave of membrane depolarization that affects the entire neuronal membrane

• begins at axon initial segment to axon terminals

• propagated by opening voltage-gated ion channel

• results in large depolarization that does not diminish as wave moves away from site of stimulus

• occurs only if graded potentials change membrane potensial to threshold

Threshold

membrane potential at which an action potential begins

for an axon, between -60 and -55 mV

All-or-None Principle

an action potential will always be propagated if stimulus reaches threshold

all action potnetials depolarize by the same amount

if theshold is not reached, action potential will not be triggered

Refractory Period

time between initiation of action potential and restoration of normal resting membrane potential

membrane will not respond normally to stimulation

Absolute Refractory Period

first part of refractory period that lasts 0.4-1.0 msec

all voltage-gated Na+ channels are either open or inactive

mambrane cannot respond to any further stimullation

Relative Refractory Period

begins when Na+ channels regain resting condition

continues until resting memebrane potential stabilizes

only strong stimulus can initate another action potential

Flushing a Toilet Analogy

Nothing happens while pressing handle, until water stars flowing (threshold is reached)

after, the amount of water released is independent of how hard or quickly the handle is pressed (all-or-nothing principle)

finally, toilet cannot be flushed again until tank refills (refractory period)

Propagation

steps for moving an action potential along an axon

Continuous Propagation

occurs in unmyelanated axons

affects one segment of an axon at a time

Saltatory Progagation

occurs in myelinated axons

faster than continous propagation & requires less energy

myelin sheath prevents continous propagation

local current “jump” from node to node

depolarization occurs only at nodes

how does refractory period affect propagation of action potentials

neuronal membrane upstream of depolarizing signal is either difficult or impossible to fire action potential

keeps propagation of action potential toward acon terminals

how does axon diameter affect propagation of action potentials

large diameter is faster, has less resistance

small diameter is slower, has more resistance

how does myelination affect propagation of action potentials

saltatory propagation or cunduction is faster

continuous conduction is slower

Type A Nerve Fibers

myelinated, large diameter neurons

fast transmission

sensory fibers transmitting info about body position & balance

motor fibers send signals to skeletal muscles

Type B Nerve Fibers

myelinated, large diameter neurons

intermediation transmission speeds

sensory & motor fibers from internal organs transmit signals via ANS

Type C Nerve Fibers

unmyelinated, small diameter neurons

slow neurotransmission

most sensory info is transmitted to brain via C fibers, including temp & pain

Which neuron fiber types takes priority

Type A because sensory & motor messages are transmitted according to priority

i.e., life-threatening sensory info or motor commands that prevent injury

Synapses

specialized sites where neurons communicate w another cell

Presynaptic Neuron

sends message

Postsynaptic Neuron

receives message

Electrical Synapses

direct physical contact between cells

• i.e., gap junctions

pre- & postsynaptic membranes work together to allow ions to pass through pores

fast propagation of action potentials

• found in some areas of brain & cardiac smooth muscle

Chemical Synapses

NTs cross gap (synaptic cleft) to target cell

most common synaptic connection between neurons

only way for neurons to communication w non-neural cells

Function of Chemical Synapses

NTs from presynaptic cell are released into synaptic cleft

NTs bind to receptors in postsynaptic cell

binding events may trigger opening of ion channel, leading to graded potentials

if change in potential is large enough to reach threshold, action potential will be generated

What happens during cholinergic synapse

1) action potential arrives at axon terminal & depolarizes membrane

2) voltage-gated Ca2+ channels open in terminal & Ca2+ flow into terminal

3) increase in extracellular Ca2+ trigger fusion of synaptic vesicles w presynaptic membrane

4) synapse releases NT, ACh

• e.g., all synpases involving skeletal muscles & may CNS synapses

5) ACh binds to AChRs on postsynaptic membrane & depolarizes membrane

6) ACh is metabolized by acetylecholiinesterase (AChE), which is found in high conc in synaptic cleft

7) AChE metabolizes ACh into acetate & choline

8) transporter proteins bring choline back into terminal to allow for synthesis of ACh

Neurotransmitters

chemical messengers

packaged into synaptic vesicles

released into synaptic cleft response to action potential

often transported back into nerve terminal unchanged (or as metabolite)