Chapter 12: Nervous Tissue

Organization of the Nervous System

Nervous System divided in the PNS and CNS

PNS divides into Somatic Nervous System and Autonomic nervous system

CNS divides into the brain and spinal cord

Autonomic nervous system splits into parasympathetic and sympathetic

PNS (peripheral)

• everything except CNS

• includes 12 pairs of cranial nerves and 31 pairs of spinal nerves

Somatic NS

• voluntary

• skeletal muscles

Autonomic NS

• involuntary

• visceral motor (smooth m, cardiac m, glands)

Functions of the NS

3 basic functions - sensory (input), integrative (process), motor (output)

2 types of cells

Neurons and Neuroglia

neurons

transmit action potential (nerve impulses)

nueroglia

structural and function support to neurons (CNS vs PNS)

Neurons

• possess electrical excitability - the ability to respond to a stimulus and convert it into an action potential

• extreme longevity, amitotic, high metabolic rate

Parts of a neuron

cell body, dendrites, axon

cell body of neuron contains what

nucleus and other organelles

2 types of neurons

dendrites and axons

dendrites

• highly branched, usually short

• receptive portion of neuron (Have R’s for NTs)

• respond by initiating electrical signals (local potential)

• conduct electrical signals toward the cell body

axon

• single, long, joins the cell body at the axon hillock

• initial segment - first part of the axon closest to axon hillock

• transmits electrical impulses away from the cell body

axon terminals

is the most distal portion of a neuron’s axon and is critical for neural communication

anterograde movement

is movement of molecules/organelles outward, from the cell body (also called soma) to the synapse or cell membrane

retrograde movement

is the process where a signal travels backwards from a target source to its original source.

how are neurons classified

structure and function

Structural classification of neurons

@ number of processes extending from the cell body

1. multipolar neurons

2. bipolar neurons

3. unipolar neurons

multipolar neurons

several dendrites and an axon

bipolar neurons

one main dendrite and one axon

unipolar neurons

dendrites and axon fuse together to form a continuous process that emerges from the cell body

functional classification of neurons

@ direction of nerve impulse

1. sensory neurons

2. motor neurons

3. interneurons

Sensory (afferent) neurons

impulses sent into the CNS

• most are unipolar; some bipolar (special senses - retina, olfactory)

motor (efferent) neurons

convey impulses away from the CNS to an effector - “exiting”

• multipolar

interneurons (association) neurons

integrate (process)(interpret) information

• transmit impulses from one neuron to another

• often form a pathway between sensory and motor neurons (ex. reflex)

• mainly found in CNS

• multipolar

Neuroglia (glial cells)

• many

• supportive cells of the CNS and PNS

• gliomas

• 6 types (Schwann cells, Satellite cells, Astrocytes, Oligodendrocytes, Microglia, Ependymal cells

gliomas

brain tumors derived from glia

PNS glial cells

Schwann cells and Satellite cells

Schwann Cells

form myelin sheaths around PNS axons - participate in axon regeneration → more easily accomplished in PNS

Satellite Cells

structural support and regulate neuron internal environment

CNS glial cells

astrocytes, oligodendrocytes, microglia, ependymal cells

Astrocytes

• large, star-shaped, most abundant glial cell

• support neuron: processes wrap around blood capillaries → part of blood-brain-barrier

• help regulate chemical environment

• may play a role in learning and memory by influencing synapses

oligedendrocytes

• small, form myelin sheaths around CNS axons

  • more processes than Schwann cell → myelinates several axons (“white” matter of CNS)

microglia

smallest, spine-like processes; phagocytic (remove debris)

ependymal cells

cuboidal or columnar cells with cilia

• line brain ventricles and central canal of the spinal cord

• produce and circulate CSF

myelin

fatty material (80 lipid and 20% protein) produced by Schwann cells and oligodendrocytes

(think Crisco vegetable)

• white pathways

• myelinated fibers

• insulates axon and increases nerve impulse speed

myelinated fibers

axons covered with myelin (types A and B) -

if axon is myelinated = appears white

white pathways

groups of myelinated fibers

Schwann cells

arranged sequentially along the lengths of both myelinated and unmyelinated axons

Schwann cell organization in myelinated axons

myelin sheath, neurilemma, Nodes of Ranvier

myelin sheath

alternating layers of plasma membrane

• formed by each Schwann cell wrapping itself around a portion of the axon several times

neurilemma

outermost part of Schwann cell containing the cytoplasm and nucleus that were pushed peripherally

Nodes of Ranvier

gaps between adjacent Schwann cells

What is different about CNS myelination?

oligodendrocytes myelinate; no neurilemma

• myelination continues through adolescence

Myelin DIseases

MS (most common in women)

Tay Sachs (genetic - autosomal recessive)

Guillain and Barre Syndrome

Clusters of neuron cell bodies

ganglion and nucleus

• a center = a nucleus or group of nuclei governing a particular function

ganglion

cluster of cell bodies in the PNS

(NOT IN BRAIN OR SPINAL CORD)

Nucleus

cluster of cell bodies in CNS

Bundles of axons

nerve and tract

nerve

bundle of axons located in the PNS

tract

bundle of axons in the CNS

White matter

myelinated axons

Gray matter

neuronal cell bodies + dendrites + unmyelinated axons + neuroglia

Regeneration of nerve fibers and repair of nervous tissue

• more common in PNS, if cell body intact and some neurilemma remains

• if damaged nerve originally led to a skeletal muscle → muscle atrophy BUT muscle regrows after nerve connection is reestablished

NS vs Endocrine (similarities, 3 differences)

• both means of internal communication and regulation

differences -

1. Speed: NS - fast, endocrine - slow and prolonged

2. Target: NS - specific, endocrine - widespread effects (blood goes everywhere)

3. Communication: NS - Neurotransmitters (acetylcholine, dopamine), endocrine - hormones (testosterone)

Resting Membrane Potential (RMP)

electrical charge (voltage) across the plasma membrane in excitable cells when NOT stimulated

= inside of cell is -70 mV

Development of the RMP by

1. Differences in the ion concentration of the ICF vs ECF

2. Differences in the permeabilities of the plasma membrane to different ion types

3. The Na+/K+ pump (active transport) ^ NS needs increases ATP (glucose and O2) → gets things back to way they where (clean up crew)

polarized

plasma membrane has charge difference across it

depolarization

any shift in this potential towards 0 mV

repolarization

plasma membrane returning to the original RMP

WHat do ion channels allow for?

• allow specific ions to move across the plasma membrane down their electrochemical gradient from high concentration to low concentration and sometimes towards the opposite charge

Gated ion channels

open and close in response to various signals → change in membrane permeability to the ions moving through them

Leak channels

gates randomly open and close, more K+ leak channels → increase in Potassium (K+) plasma membrane permeability

Ligand (chemically) gated channels

ligand (chemical) binds to R → opening or closing of gate or channel

• chemical signal converted to an electrical signal

Mechanically gated channel

open or close in response to a force (ex. vibration or pressure) that distorts the channel and opens the gate

Voltage gated ion channels

open or close in response to a change in membrane potential (important in AP transmission)

Gated channel can be in one of three states

1. closed but capable of opening

2. open (activated)

3. closed and incapable of opening (inactivated)

Graded potential (local potential)

a small deviation in the RMP @ mechanically gated or ligand-gated channels opening or closing

• occur mainly in dendrites or cells body

1. Graded

2. Decremental

3. Excitatory or Inhibitory

graded potentials have different names @ type of stimulus causing them and where they occur (ex. post synaptic potential, receptor potential, generator potential)

Graded

variable magnitude (amount of membrane voltage change) @ stimulus intensity

Decremental (localized)

signal gets weaker with distance (the local potential may trigger an AP)

Excitatory

depolarization (inside less negative) → increase readiness to fire an AP

Inhibitory

hyperpolarization (inside more neg) → decrease readiness to fire an AP

Action Potentials

propagated changes in transmembrane potential (excited)

• generated on the axon hillock (increased density of voltage-gated ion channels)

• generated if either 1) a strong enough “excitatory local potential” reaches hillock or 2) a combined effect of multiple local potentials reaching the hillock

Unstimulated (resting) neuron (RMP)

most Na+ channels closed (Na+ imperm)

Stimulus → graded potential (what occurs)

most Na+ channels stay closed and NO AP

• if depolarization reaches a critical voltage (threshold)(about -55mV)

Threshold leads to ?

increase in voltage gated Na+ channels opening → depolarization phase (depolarization and polarity reversal)

Membrane depolarization

eventual Na+ gates closing (inactiavted) and K+ gates open → K+ movement from ICF to ECF → membrane potassium drops back (repolarization phase)

After hypterpolarizing phase

increase in K+ (potassium) leaving → membrane potential temporarily becomes more negative than the resting level

AP

spike

• abides and all or none law = maximum firing or not at all and irreversible (cannot be stopped) (gun firing, dominos falling)

Refractory Periods

period of time after an AP begins during which an excitable cell cannot generate another AP

Absolute Refractory Period

from the time the membrane reaches the threshold until repolarization is about 1/3 complete during which no stimulus can trigger another AP (inactivated Na+ channels cannot reopen)

• limits the # of APs produced in a given time period

• shorter in large diameter fibers (more impulses per second)

Relative Refractory Period

brief time following the absolute RP during which a stronger than normal stimulus may initiate another AP (resting Na+ channels)

Propagation of Action Potentials

signal conduction = nerve impulse

• APs must travel from where they arise (trigger zone) to the axon terminal

• each AP triggers by local current flow, a new one identical to the old one, etc, etc. along the axon length (nondecremental)

• only the membrane ahead of the impulse is sensitive to stimulation (membrane is behind refractory) → proper direction of impulse movement

2 types of propagation

continuous conduction and saltatory conduction

continuous conduction

involves step by step depolarization and repolarization of each adjacent membrane segment

• occurs in unmyelinated axons (and muscle fibers)

Saltatory conduction

• occurs in myelinated axons

• myelin is a good insulator that resists current flow and all the NA channels and concentrated at the Nodes of Ranvier (local current that gives rise to propagated AP flowing (jumping) from one N or R to the next

2 consequences of saltatory conduction

faster propagation and more energy efficient (less ion exchange requiring the Na+/K+ pump to correct)

Factors that affect the speed of propagation

1. Myelination - faster (escalade - big white car - myelinated)

2. Axon diameter: larger - faster

3. Temp: higher temp - faster

classification of nerve fibers

Type A, B, C

Type A axon

largest-diameter, myelinated - fastest

Type B axons

medium diameter - myelinated, medium speed

Type C axon

smallest diameter, unmyelinated - slowest

Neural coding

qualitative info, quantitative info (frequency and #)

Qualitive information

WHICH neurons are firing - knowing where pain is

i.e. - shots

Quantitative information about Neurons

stimulus intensity

1. frequency of AP in neuron

2. number of neurons firing

Anesthetic

anesthetic blocks voltage gate → turn it off, don’t perceive pain because it doesn’t go through anywhere

• certain shellfish and other organisms contain neurotoxins, substances that produce their poisonous effects by acting on the nervous system

Synapse

an anatomically specialized junction between 2 neurons, at which the electrical activity in one neuron influences the activity in the 2nd

• synapses change with learning and important in some diseases

presynaptic neuron

a neuron which conducts info towards a synapse (sends message) (talking)

postsynaptic neuron

a neuron that conducts info away from a synapse (receives message) (listening)