Nervous System & Nervous Tissue

Chapter 11

overlapping functions of nervous system (3)

• sensory input

• integration

• motor output

main parts of nervous system (2)

• central

• peripheral

central nervous system (CNS)

brain and spinal cord; interprets sensory input, dictates motor output

peripheral nervous system (PNS)

everything other than CNS; nerves and ganglia; serve as communication lines that link all parts of the body to the CNS

nerves

bundles of axons

ganglia

collections of neuron cell bodies

spinal nerves

carry impulses to and from the spinal cord

cranial nerves

carry impulses to and from the brain

PNS subdivisions (2)

• sensory

• afferent

sensory/afferent division (PNS)

nerve fibres convey impulses to the CNS from sensory receptors throughout the body

somatic sensory fibres

convey impulses from the skin, skeletal muscles, joints

visceral sensor fibres

transmit impulses from visceral organs

motor/efferent division (PNS)

nerve fibres transmuts impulses from the CNS to effectors

parts of motor division (2)

• somatic

• autonomic

somatic nervous system (aka voluntary)

conduct impulses from the CNS to skeletal muscles; somatic motor nerve fibres

autonomic nervous system (ANS) (aka involuntary)

regulate activity of smooth muscle, cardiac muscle, and glands; visceral motor nerve fibres

neuroglia (aka glial cells)

supporting; surround and wrap the more delicate neurons

neurons

excitable (respond to stimuli by changing membrane potential); transmit electrical signals

what type of cells are neurons closely associated with

neuroglia

types of neuroglia (6)

• astrocytes

• microglial cells (microglia)

• ependymal cells

• oligodendrocytes

• satellite cells

• schwann cells

how many types of neuroglia are in the CNS, how many in the PNS

4 in CNS, 2 in PNS

astrocytes

cling to neurons and their synaptic endings, cover nearby capillaries; most abundant and versatile glial cells

microglia

processes touch nearby neurons, monitor they health; migrate toward them when neurons are injured (or other trouble)

ependymal cells

line central cavities of the brain and the spinal cord; form permeable barrier between cerebrospinal fluid and tissue fluid bathing the cells of the CNS

oligodendrocytes

line up along thicker nerve fibres in the CNS; wrap their processes tightly around the fibres, producing an insulating covering (myelin sheath)

satellite cells

surround neuron cell bodies located in the PNS; many of same functions as astrocytes do in the CNS

schwann cells

surround all nerve fibres in the PNS; form myelin sheaths around thick nerve fibres (similar to oligodendrocytes)

neurons (aka nerve cells)

structural units of the nervous system

neurons (aka nerve cells)

conduct messages in the form of action potentials (nerve impulses) from one part of the body to another

neuron characteristics (3)

• extreme longevity

• amitotic (lose ability to divide as time goes on)

• high metabolic rate (require continuous supplies of oxygen and glucose)

parts of neuron (2)

• cell body

• processes

neuron cell body (aka soma)

spherical nucleus (with a conspicuous nucleolus) surrounded by cytoplasm

neuron cell body is which two major centres of the neuron

• biosynthetic centre

• metabolic centre

structures in neuron cell body (4)

• mitochondria

• protein- and membrane-making machinery

• cytoskeletal elements

• pigment inclusions

nuclei

clusters of neuron cell bodies of the CNS

ganglia

clusters of neuron cell bodies on nerves in the PNS

types of neuron processes (2)

• dendrites

• axons

dendrites

main receptive/input regions; provide large surface area for receiving signals from other neurons

how many axons in each neuron

one

axon collaterals

branches on neuron cell body

axon terminals

knob-like distal endings of terminal branches

axon

conducting region of the neuron; generates nerve impulses and transmits them

directions of axonal transport (2)

• anterograde movement (away from cell)

• retrograde movement (toward cell)

myelin sheath

insulating sheath; protects and electrically insulate saxons, increases transmission speed of nerve impulses

can dendrites have myelin sheaths

no - always nonmyelinated

what are myelin sheaths in the PNS formed by

schwann cells

nodes of ranvier

myelin sheath gaps

how are neurons grouped

• structurally, according to number of processes extending from their cell body

• functional, according to the direction in which the nerve impulse travels relative to the CNS

multipolar neurons

three or more processes - one axon, the rest dendrites

bipolar neurons

two processes - one axon, one dendrite

unipolar neurons + what they’re better known as

one short process; emerges from cell body and divides (like a T) into proximal and distal branches

• aka pseudounipolar neurons

sensory/afferent neurons

transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS

motor/efferent neurons

carry impulses away from the CNS to the effectors (muscles and glands) of the body

interneurons/association neurons

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

types of gated channels (3)

• chemically-gated (aka ligand-gated)

• voltage-gated

• mechanically-gated

electrochemical gradient + components (2)

determines the direction an ion moves (into or out of the cell)

• concentration gradient

• electrical gradient

concentration gradient

ions move from area of higher concentration to an area of lower concentration

electrical gradient

ions move toward an area of opposite electrical change

types of signals produced by changes in membrane potential (2)

• graded potentials

• action potentials

graded potentials

incoming signals operating over short distances with variable (graded) strength

action potentials

long-distance signals of axons that always have the same strength

terms used to describe changes in membrane potential relative to resting membrane potential (2)

• depolarization

• hyperpolarization

depolarization

decrease in membrane potential; inside of membrane becomes less negative (moves closer to zero) than resting potential

hyperpolarization

increase in membrane potential; inside of membrane becomes more negative than resting potential

how do neurons send signals over long distances

generate and propagate (transmit) action potentials

what type of cells can generate action potentials

those with excitable membranes

excitable membranes

neurons and muscle cells

action potential

brief reversal of membrane potential; total amplitude of about 100 mV

• depolarization followed by repolarization (and often hyperpolarization)

nerve impulse

action potential in a neuron

• generated in axons

main steps of action potential generation (4)

• resting state: all voltage-gated Na⁺ and K⁺ channels are closed

• depolarization: voltage-gated Na⁺ channels open

• repolarization: Na⁺ are inactivating, voltage-gated K⁺ channels open

• hyperpolarization: some K⁺ channels remain open, Na⁺ channels reset

presynaptic neuron

neuron conducting impulses toward the synapse; sends info

postsynaptic neuron

neuron transmitting the electrical signal away from the synapse; receives info

axodendritic synapses

between axon endings of one neuron and the dendrites of other neurons

axosomatic synapses

between axon endings of one neuron and the cell body (soma) of another neuron

types of synapses (2)

• electrical

• chemical

electrical synapses

channels connect the cytoplasm of adjacent neurons, allow ions and small molecules to flow directly from one neuron to the next; consist of channel-containing gap junctions

chemical synapses

allow the release and reception of chemical messengers (neurotransmitters)

steps of info transfer across chemical synapses (6)

• action potential arrives at axon terminal

• voltage-gated Ca²⁺ channels open and Ca²⁺ enters the axon terminal

• Ca²⁺ entry causes synaptic vesicles to release neurotransmitter by exocytosis

• neurotransmitter dissuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane

• binding of neurotransmitter opens ion channels, resulting in graded potentials

• neurotransmitter effects are terminated

how long do the effects of neurotransmitters typically last

a few milliseconds

what are the ways neurotransmitters are terminated (3)

• reuptake

• degradation

• diffusion (away from synapse)

how fast do impulses travel

150 m/s

synaptic delay

time required for neurotransmitter to be released, diffuse across the synaptic cleft, and bind to receptors

• 0.3-5.0 ms

classification of neurotransmitters by chemical structure (6)

• acetylcholine

• biogenic amines

• amino acids

• peptides

• purines

• gases and lipids

types of gases and lipids (2)

• gasotransmitters

• endocannabinoids

neurotransmitter receptors (2)

• channel-linked

• G protein-coupled

patterns of neural processing (2)

• serial

• parallel

types of neural circuits (

• diverging

• converging

• reverberating

• parallel after-discharge circuit

diverging circuit

amplifying; one input, many outputs

converging circuit

concentrating; many inputs, one output

reverberating circuit

oscillating; signal travels through a chain of neurons, each feeding back to previous neurons

parallel after-discharge circuit

after-discharge; signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell

nucleus

collection of neuron cell bodies in the CNS

ganglion

collection of neuron cell bodies in the PNS

tract

bundle of axons in the CNS

nerve

bundle of axons in the PNS