Anatomy Ch 10 Nervous system

nervous system

detects changes, makes decisions, stimulates muscles and glands to respond, and maintains homeostasis

• 2 cell types: neurons (communication), neuroglia (support)

• 2 divisions: central (brain and spinal chord) and peripheral (connection to other body parts)

neurons

nerve cells

excitable cells that transmit electrical signals, react to changes, and communicate

• structural units of the nervous system

• large, highly specialized cells that conduct impulses

• extreme longevity (can live 100 years or more!)

• amitotic (nerve damage is almost always irreversible, other than olfactory nerves sometimes)

• high metabolic rate - requires continuous supply of oxygen and glucose

• cell body (soma) and one or more cell processes that vary and length in size: axons and dendrites

  • classified by structure: multipolar, bipolar, unipolar

peripheral nervous system (pns)

connects CNS to other body parts via the cranial nerves and the spinal nerves

2 divisions:

• sensory (afferent): somatic and visceral sensory fibers that send perceptual information TO the brain to make sense of it

• motor (efferent) division: interprets signals from cns to extremities, somatic ns and autonomic ns

cranial nerves

part of the pns that connects through the holes in the skull to the brain and the rest of the body

• 12 pairs

spinal nerves

part of the pns that reaches to every part of the body via neuron communication, sends and receives signals from and to the cns

• 31 pairs

sensory division

afferent (ascend) division of PNS

• information “ascends” to the brain to make sense of it

• somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to CNS

• visceral sensory fibers: convey impulses from visceral organs to CNS

somatic sensory fibers

convey impulses from skin, skeletal muscles, and joints to CNS

• type of sensory afferent division in the PNS

visceral sensory fibers

convey impulses from vsceral organs to CNS

→ organs and parts inside of body send sensory info to brain

• ex: bladder, stomach, heart, etc

• type of sensory afferent division in the PNS

motor division

efferent (exit) division of the PNS

transmits impulses FROM cns to effector organs, muscles, and glands

• 2 divisions: somatic nervous system and autonomic nervous system

somatic nervous system

sends conscous and reflec impulses to skeletal muscles FROM the cns to the pns to make them contract

interprets sensory information via the 5 senses

autonomic nervous system

controls invountary functions of smooth and cardiac muscle and glands to maintain homeostasis

• ex: lung moevements for breathing, heart movements for beating and providing oxygen

general functions of the nervous system

• sensory input: information gathered by sensory receptors about internal and external changes

• integration: processing and interpretation of sensory input, create sensations, add to memory, produce thoughts

• motor output: activation of effector organs (muscles and glands) produces a response

cell body of a neuron

soma

contains nucleus, cytoplasm, organelles, etc

dendrites

branched receptive (input) surfaces of neuron

may have a lot

convey incoming messages toward cell body as graded potentials (short distance signals)

• can send excitatory or inhibitory signals that trigger the postsynaptic cell to continue or to stop firing

• signal majority rules: more exhitatory allows for a neuron firing or inhibitory doesn’t

axon

conduction region of a neuron

generates nerve impulses

transmits impulses along the membrane to axon terminal which has a secretory region at the end that releases neurotransmitters, triggering muscles to move, reactions, etc

carries on many “conversations” with different neurons at the same time

lacks rough ER and Golgi apparatus: relies on cell body to renew proteins and membranes, efficient transport mechanisms, and will quickly decay if cut or damaged

contains:

• axon hillock

• collaterals

• axon terminal

• synaptic knob

• may or may not be covered in myelin sheaths

multipolar neurons

99% of neuron structures

include many processes

most neuron structures of CNS

bipolar neurons

Structure of neuron with 2 processes

located in the eyes, ears, and nose

unipolar processes

neuron structure with one process and cell bodies in ganglia

often sensory ne

neuron processes in the cns

white matter = proceses

gray matter = cell bodies

neuron processes in the pns

almost all neuron processes (aka nerves) and very few cell bodies

tracts

bundles of neuron processes in CNS

nerves

bundles of neuron processes in PNS

myelination

whiteish, protein-lipid substance

Schwann cells that wrap their membranes around the axon terminal like a vest, one cell = one segment

myelinated fibers: segmented sheaths around most long or large-diameter axons

function: protects and electrically insulates axon, increases speed of nerve impulse transmission

unmyelinated axons

encased by schwann cell cytoplasm but there is no wrapped coating of myelin surrounding the axons

myelin sheath

Concentric layers of schwann cell plasma membrane around axon

• protects and electrically insulates axon terminal, increases speed of nerve impulse transmission

• forms a perinuclear cytoplasm bulge

perinucelar cytoplasm/neurillemma

outer collar of myelin sheath in PNS where the cytoplasm bulges peripherally from the schwann cell containing the nucleus and most of the cytoplasm

nodes of ranvier

myelin sheath gaps between adjcent schwann cells

• where axon collaterals can emerge

multiple sclerosis

destruction of myelin sheaths in CNS by an immune response - attacked by a person’s own antibodies

• scars are left behind, which stop neurons from conducting impulses

• Without input from motor neurons, muscles stop contracting and atrophy

• fatigue, mood problems, blurred vision, weak, numb limbs

• treatments involve drugs that suppress immune activity

neuroglia

glial cells

provide structural support for neurons

in embryo: guide neurons into position, may stimulate specialization

produce growth factors to nourish neurons and remove excess ions and neurotransmitters

aid in formation synapses

4 types: astrocytes, oligodendrocytes, microglia, ependymal cells

astrocytes

most abundant, highly branded glial cells

cling to neurons, synaptic endings, and capillaries (blood-brain barrier)

connect neurons to blood vessels, exchange nutrients and growth factors

• support and brace neurons

• aid metabolism of certain substances

• control chemical environment: regulate ion concentrations, such as K+

• part of blood brain barrier: control what neurons have access to from the vessels and capillaries

• guide migration of young neurons

• respond to nerve impulses and neurotransmitters

• influence neuronal functioning: participate in information processing in the brain

microglial cells

small, ovoid cells with thorny processes that touch and monitor neurons - resident immune cells

migrate toward injured neurons

can transform or “activate” to phagocytize microorganisms and neuronal debris

ependymal cells

range in shape from squamous to columnar

may be ciliated: beat to circulate CSF 2-3 times/day

line central cavities of the brain and spinal column

form permeable barrier between CSF in cavities and tissue fluid bathing CNS cells

oligodendrocytes

branched cells whose processes wrap CNS nerve fibers, forming insulating myelin sheaths for thicker nerve fibers

neuroglia only in PNS

schwann cells and satellite cells (support clusters of neuron cell bodies ganglia)

neuron regeneration in the PNS

regeneration is possible

axon separated from cell body and its myelin sheath will degenerate

schwann cells and neurilemma remain

remaining schwann cells provide guiding sheath for growing axon

if growing axon establishes former connection, function will return, if not, function may be lost

neuron regeneration in CNS

axons here lack neurilemma to act as guiding sheath

oligodendrocytes do not proliferate after injury

regeneration is unlikely

synapse

neuron communication

• site at which a neuron transmits a nerve impulse to another neuron

• presynaptic nerve sends impulse

• postsynaptic neuron receives impulse

• they can be both! depends on whcih synapse, function, location, etc

• sparated by synaptic clef

synaptic transmission

impulse travels down axon of presynaptic neuron to axon terminal

impulse reaches synaptic knob and causes an influx of Ca+2 ions

release of neurotransmitters by synaptic vesicles by exocytosis

NTs exert either excitatory or inhibitory effect on postsynaptic neuron by creating graded potentials (gradient of both excitatory and inhibitory in which the majority wins) to which the enruon responds by generating an action potential (nerve impulse) or not

one-way transfer of information

cell membrane potential

usually electrically charged, polarized, due to unequal distribution of ions on the inside and outside of membranes

• inside of membrane is negatively charged with respect to outside of membrane

• causes resting potential important in synaptic transmission

potassium (K+) ions

major INTRAcellular positive ions (cations)

• 2 enter the neuron and influence the resting potentials of neurons

sodium (Na+) ions

major EXTRAcellular positive ions (cations)

• 3 exit the neuron

sodium-potassium pump

creates distribution of Na+ and K+ across cell membrane of neuron

active transport (requires ATP)

transports 3 Na+ out of the cell for every 2 K+ ions into the cell

ion channels

formed by large membrane proteins to help regulate the passage of specific ions into or out of the cell

selective

2 types: leakage (always open), gated (part of protein changes shape to open/close)

gated ion channels

ion channels in which a part of the protein changes shape to allow for open or close

• 3 types: chemically gated, voltage-gated, mechanically gated

when open: ions diffuse quickly across membrane along electrochemical gradient (ions move towards oppositve charge) and ion flow creates electrical current and voltage changes across membrane

chemically gated channels

gated ion channel that opens in response to binding of the appropriate neurotransmitters

→ facilitate diffusion, passive transport

voltage-gated channels

gated ion channels that open in response to changes in membrane potentials (i.e. inside of membrane changes from positive to negative or vice versa)

→ facilitate diffusion (i.e. for Na+ or K+ for graded potentials), passive transport

electrochemical gradient

ions flow along their chemical gradient and electrical gradient together

• chemical: high concentration to low concentration

• electrical: move toward an area of opposite charge

resting potential

potential difference across membrane of resting cell (considering inside of cell membrane is negative and outside is positive)

• charge rests at approximately -70 mV in neurons: cytoplasmic side of membrane is negatively charged compared to outside

• generated by: differences in ions across membranes (Na+/K+ pump), and differential permeability of plasma membrane (related to K+ leak channels

importance of membrane potential

changed in membrane potential in neurons use for communication with other cells: integrate, send, and receive information

produced by membrane permeability to ions

changes produces 2 types of signals: graded potentials (short distance) and action potentials (long-distance)

relative to resting potential: depolarization and hyperpolarization

depolarization

decrease in membrane potential

inside of membrane (cytoplasmic side) becomes less negative (i.e. towards and above 0) rises above -70mV

→ INCREASES probability of producing a nerve impulse = EXCITATORY

hyperpolarization

an increase in membrane potential

the inside of the cell to move away from zero relative to the resting membrane potential

→ DECREASES probability of producing a nerve impluse = INHIBITORY

graded potentials

short-lived, localized changes in membrane potential

happen on dendrites

• magnitude varies depending on stimulus strength

• stronger stimulus → more voltage changes → farther current flows through the axon and to the next neuron

2 types: depolarization/excitatory or hyperpolarization/inhibitory

stimulus opens gated ion channels

current flows but dissipates quickly: only over short distances

SYNAPTIC POTENTIALS

excitatory postsynaptic potentials (EPSP)

membrane change in which neurotransmitter opens Na+ channels,

DEPOLARIZES (less negative on inside of cell) membrane of postsynaptic neuron, as Na+ enters axon

action potential in postsynaptic neuron becomes more likely

inhibitory postsynaptic potential (IPSP)

membrane change in which neurotransmitter opens K+ channels (or Cl- channels)

HYPERPOLARIZES membrane of postsynaptic neuron, as K+ leaves axon

action potential of postsynaptic neuron becomes less likely

summation

EPSPs and IPSPs are added together

→ net excitatory effect leads to greater probability of an action potential does not generate action potentials

occurs at trigger zone: axon hillock

threshold potential

degree of depolarization reaches -55mV → action potential

action potential

main way neurons send signals

• principle mean of long-distance neural comunication- do not decay over distance

• only in muscle cells and axons of neurons

• brief reversal of membrane potential with a change in voltage ~100mV

depolarizing phase of action potentials

depolarizing local currents open voltage-gated Na+ channels causing Na+ to rush into the cell

if neuron reaches threshold of -55 to -50 mV, positive feedback causes opeing of all Na+ channels

repolarizing phase of action potential

Na+ channel gates close

membrane permeability to Na+ declines to resting state

slow voltage-agted K+ channels open → K+ exits the cell restoring internal negativity

hyperpolarization phase of action potential

some K+ channels remain open, allowing excessive K+ efflux, potassium leaves the cytoplasm

→ inside of membrane more negative than resting state → slight dip below resting voltage (-90 mV)

Na+ channels begin to reset (get closer to -70mV

impulse conduction in non-myelinated axons

action potentials are conducted down the length of the axon, each segment must depolarize

salutatory conduction

impulse conduction in myelinated axons

speed increases with myelination: lipids act as an electrical insulator

• ions cross membrane only through nodes of ranvier (gaps) by “jumping” from node to node down the axon

also affected by axon diameter: thick acons transmit faster

synaptic transmission

transmission of nerve impulse from one neuron to another

• impulse travels down axon of presynaptic neuron to axon terminal

• reach synaptic knob → influx of Ca+2 ions

• released neurotransmitters from synaptic vesicles by exocytosis

• cross the synaptic cleft and react with specific receptors in the membrane of postsynaptic neuron

• chemically gated ion channels responds to NTs → synaptic potentials

• excitatory NTs and inhibitory NTs

excitatory neurotransmitters

chemicals released at synaptic transmission that increase permeability to Na+ ions

bring membrane closer to threshold → increase likelihood of generating impulses (action potentials)

inhibitory neurotransmitters

chemicals released at a synaptic transmission that move membrane farther from threshold → decrease likelihood of generating impulses

neurotransmitters

at least 100

monoamines, amino acids, peptides

produced in the rough ER or cytoplasm

• released by exocytosis when impulse reaches synaptic knob of axon

ex disorders associated with NT imbalances: depression, epilepsy, huntington disease, insomnia, mania, parkinson’s disease, schizophrenia, etc

ex drugs that alter NT levels: cocaine, nicotine, antidepressants, SSRIs

vesicle trafficking

process of membrane recycling

synaptic vesicle becomes part of cell membrane as it releases NT

enodcytosis returns membrane to cytoplasm, forms new vesicles

opiate drugs

potent painkillers derived from poppy plant

→ human body produces opiates called endorphins which relieve pain in acupuncture, childbirth, well-being during exercise

very useful for severe pain, also very addictive

withdrawal symptoms are caused by the drug causing the body to stop producing natural endorphins, so when drug is absent, severe pain is felt

subdural space means

between arachnoid