12 Nervous System

Nervous System

Receive info (sensory input)

Processes info (integration)

Responds (motor output)

Central Nervous (CNS)

Brain and Spinal Cord

cannot regenerate

Peripheral Nervous (PNS)

Nerves and ganglia outside of CNS

axons may regenerate if cell body remains intact and enough neurilemma survives

Sensory

Afferent

carries info to the CNS

Mostly unipolar

Motor

Efferent

carries commands from the CNS to muscles or glands

Mutlipolar

Somatic sensory

Detects things u consciously feel

touch, pain, temperature

Visceral sensory

Detects internal conditions

signals from organs

Somatic motor

Controls skeletal muscles voluntarily

Autonomic motor

Involuntary control of heart, smooth muscle, and glands

Sympathetic and parasymphatic

Sympathetic

prepare body for action or stress

Parasympathetic

supports resting, digestion, and recovery

Neurons

Send signals

Excitability, Conductivity, Secretion, extreme longevity and amitotic

Extreme longevity

neurons can live throughout your lifetime

Amitotic

most neurons cannot divide after fetal development

Cell body (soma)

contains the nucleus and maintains the cell (parts of neuron)

contains perikaryon (cytoplasm)

Dendrites

receive incoming signals (parts of neuron)

unmyelinated

Axon

Carries electrical signals away from cell body (parts of neuron)

Axon terminals (synaptic knobs)

Release neurotransmitters to communicate with the next cell (parts of neuron)

Anterograde

moves materials away from cell body toward axon terminals

Retrograde

moves materials back toward cell body for recycling or breakdown

Multipolar

Many dendrites, 1 axon

most common

Bipolar

1 dendrite, 1 axon

found in retina

Unipolar (Pseudounipolar)

1 process that splits into 2

common sensory neuron structure

2 branches: peripheral process, and central process

Anaxonic

Dendrites but no axon

communicates locally

Interneurons

process info and connect sensory and motor neurons

located in the CNS

99% of neurons

generally multipolar

Nerve

bundle of axons in the PNS

Epineurium

entire nerve

Perineurium

bundle of axons (fascicle)

Endoneurium

individual axon

Mixed nerves

contain both sensory and motor fibers

Synapse

neuron communicates with another neuron or an effector

Electrical synapse

cells connect thru gap junctions; signals pass very quickly

Chemical synapse

A neuron releases neurotransmitters that communicate with the next cell

1. electrical signal reaches axon terminal

2. neurotransmitters are released from vesicles

3. they cross the synaptic cleft

4. they bind to receptors on the next cell

5. the next cell develops a graded potential

Glial Cells

support, protect, and nourish neurons

can divide by mitosis

nonexcitable

Astrocytes

support neurons and help form blood-brain barrier

CNS

Ependymal Cells

Line brain and spinal cord cavities; help produce and circulate cerebrospinal fluid (CSF)

CNS

Microglia

Remove germs, damaged cells, and debris

CNS

Phagocytosis - cell eating

Oligodendrocytes

Form myelin around CNS axons

Satellite Cells

Support neuron cell bodies in ganglia and regulate their environment

PNS

Schwann cells (neruolemmocytes)

Form myelin around PNS axons

Myelin sheath

fatty insulating layer around certain axons

make electrical signals travel faster

Nodes of Ranvier

gaps btwn myelin-covered sections

Pumps

use energy to move substances against their concentration gradients

Leak channels

remain open, allowing ions to diffuse

Chemically gated channels

open when a chemical, such as a neurotransmitter, binds

Voltage-gated channels

Open when membrane’s electrical charge changes

Modality-gated channels

open in response to another type of stimulus, such as pressure

Sodium-potassium pump

-use energy

-moves 3 Na+ out of the neuron

-moves 2 K+ in

-maintains the ion concentration gradients needed for neuron function

Resting Membrane Potential (RMP)

resting neuron is not sending an action potential

approximately -70 mV

inside is more negative than outside

more K+ inside

more NA+, CL-, and Ca2+ are outside

Depolarization

Na+ enter

inside becomes less negative

Repolarization

K+ leaves

Hyperpolarization

K+ continues leaving temporarily

inside becomes more negative

Effectors

Muscles and Glands

Nissl bodies

chromatophilic substance

free and bound ribosomes

Neurofilaments

Intermediate filaments in neurons

Neurofibrils

neurofilaments combine into bundles

Fast Axonal Transport

400 mm/day

uses microtubules

uses motor proteins

requires ATP

can move in both directions

Slow Axonal Transport

0.1-3 mm/day

results from flow of axoplasm

moves only away from cell body

carries enzymes, cytoskeletal components, new axoplasm

Presynaptic Neuron

neuron sending message

Postsynaptic Neuron

cell receiving message

PNS Axon Regeneration

1. axon is severed

2. proximal portion; seals off and swells, distal portion; degenerates

3. neurilemma and endoneurium form a regeneration tube

4. axon begins growing again

5. axon reconnects with its original muscle, gland, or sensory receptor

Channels

tiny protein openings in cell membrane

ions move down their concentration gradients; high > low

Voltage

difference in electrical charge btwn 2 locations

higher = more current

Current

movement of charged particles

opening channels lowers it

Resistance

anything that makes it harder for charged particles to move

higher = less current

Ohm’s Law

Current = Voltage/Resistance

Excitatory Postsynaptic Potential

EPSP

makes neuron more likely to fire action potential

usually causes depolarization

Inhibitory Postsynaptic Potential

IPSP

less likely to fire an action potential

causes hyperpolarization

Spatial Summation

Multiple neurons send messages to different locations on the receiving neuron at approximately the same time

Temporal Summation

1 presynaptic neuron repeatedly sends messages very quickly

Threshold

Mininum electrical lvl required to trigger an action potential

-55mV

neuron needs to become less negative

All or None Law

Action potential either happens completely or doesn’t happen at all

Action Potential

Rapid electrical signal that travels down the axon

Refractory Period

the time after an action potential begins when it is difficult to create another action potential

Absolute Refractory Period

Lasts aprox. 1 milisec

no stimulus can trigger another action potential

sodium channels r open and then become inactivated

1 direction

Relative Refractory Period

Happens immediately after the absolute refractory period

another action potential is possible

but a stronger-than-normal stimulus is required

some K+ channels remain open

Continuous Conduction

Happens in unmyelinated axons

electrical signal travels step by step

Saltatory conduction

Happens in myelinated axons

signal appears to jump btwn Nodes of Ranvier

Faster and uses less ATP

Neurotransmitter Release

1. Action potential reaches synaptic knob

2. Calcium channels open

3. calcium enters

4. Vesicles release neurotransmitters thru exocytosis

5. Neurotransmitters enter the synaptic cleft

Thicker axon and Myelination

Faster signals to action potential

Neuromodulation

change how strongly neurons respond

Facilitation and inhibition

Facilitation

Makes response stronger

inhibition

Makes response weaker

Nitric Oxide

May help strengthen memory

helps dilate blood vessels

Endocannabinoids

Affect neurotransmitter release

influence memory and appetite

Removing Neurotransmitters

Enzymes, reuptake, diffusion

Acetylcholinesterase

break down ACh into Acetate and Choline

Neurotransmitters

Acetylcholine, Biogenic amines, Amino acids, and Neuropeptides

Direct

Opens chemically gated channels

Indirect

Uses G-proteins and second messengers

Receptive Segment

Dendrite and cell body

chemically gated channels

Initial Segment

Axon hillock

Voltage-gated Na+ channels and Voltage-gated K+ channels

Conductive segment

Axon and its branches

Voltage-gated Na+ channels and Voltage-gated K+ channels

Transmissive Segment

Synaptic knobs

Voltage-gated Ca2+ channels and pumps