8: The nervous system ( pre read)

functions of the nervous system

• sensory functions

sensory or afferent nuerones

receptor detects stimulus

afferent neurone relays information to CNS

• integratve

perception and interneurones

• motor

motor or efferent neurones, information to effectos and elicits response

organisation of the nervous system

• CNS : brain and spinal cord

• peripheral nervous system: everything else

PNS subdivided into somatic NS, autonomic NS, and enteric NS

somatic nervous system

sensory neurones from the head, body and limbs

motot neurones to skeletal muscles

voluntary

autonomic nervous system

autonomic neurones from vice versa

motor neurones to smooth and cardiac muscle and glands

involuntary

enteric nervous system

part of the sns

GI tract

what 2 types of cells does nervous tissue contain

neurones

neuroglia

neurones

possess electrical activity, like muscle cells

repsond to stimulus and convert it to action potentials

ap then propogates along membrane

due to the ion movement: Na and K

what are the 3 parts of a neurone

cell body

dendrites

axons

cell body

carries out normal metabolic processes

nucleus surrounded by cytoplasm containing normal organelles in any cell

dendrites

short, taper, branched form input part

axons

long thin

carries nerve impules to other neurones, glands or muscle fibre

axoplasm: substances synthesided in cell body needed in axon, so axonal transport carries it around

synapses at distal end - junction between neurones and other cells

gap is crossed by neurotransmitter

multipolar neurone

several dendrites, one axon

found in brain and spinal cord

motor neurones

bipolar neurone

one dendrite, one axon

found in retina

unipolar neurone

sensory neurones

start as bipolar, axon and dendrite fuse into one which branches after short distance

both have structure and function of axon, but one has dendrites

cell bodies in ganglia

neuroglia

repair function: glial cells divide and replicate in adult NS, in case of injury the gllial cells multiply

no electrical activity

CNS of neuroglia

astrocytes

oligodendrites

microglia

ependymal cells

astrocytes

largest and most numerous of glial cells

star shaped:

protoplasmic- short branched processes in grey matter

fibrous- long unbranched processes in white matter

functions of astrocytes

contains microfilaments, give strength and assist in support of neurones

isolation- processes wrap around bv, form blood brain barrier , islates CNS from harmful substances

in embryo they secrete chemicals which help regulate growth

may help formation of synapses

oligodendrites

smaller, less processes

form and maintain yelin sheath around myelinated cns axons

myelin sinulates axons and increases speed of conduction

microglia

smaller thinner processes

phagocytes- removes debris, microbes and damaged nerve tissue

ependymal cells

cubic or columner

line ventricles

produces and circulates CSF

form blood- CSF barrier

PNS neuroglia

schwann cells

satelite cells

surrounf the pns axons and cell bodies

schwann cells

encircles PNS axons

form myelin sheath

each myelinates a single axon

involved in axon regeneration

satelite cells

flat cells , surround the cell bodies

structural support

regulate exchange of materials

grey and white matter

white- myelinated axons

grey matter- cell bodies, dendrites, unmyelinated axons, neuroglia. not myelinated

both contain bv

how do nerves work

changes in membrane potential

due to movement of ions via ion channels

graded and action potentials both caused by changes in membrane potential

ion channels in neurones

allow ions to move down electrochemical gradient

move from high to low conc, moving towards opposite chage

combination produces electrochemical gradient and creates electrical current

channels also allow movement in and out of cells:

membrane not permable to ions, so has gates which open and close. movement of charged ions causes electrical current to flow

channel types

leakage

voltage gated

ligand gated

mechanically gated

leakage channel type

randomly open and close

more K than Na channels

K moves faster

membrane more permeable to K than Na

voltage gated

open in reponse to a change in membrane potential

involved in action potential generation and propagation

ligand gated

opens and closes in response to chemical stimulus

neurotransmitters, ions , hormones

mechanically gated

opens and closes in reponse to mechanical stimulus

eg vibration ( soung), pressure (touch). stretching

found in ears and skin

resting membrane potential

more negative ions inside the cell: more K and phosphate inside

more positive otside the cell, as more Na and Cl

ions leak both ways, but more K leaks out

membrane is less permeable to Na

negative ions bound to large molecules so not able to follow

ion pumps also move K in and Na out

resting potential maintainsed at around -70mV on outside

graded potentials

ligand or mechanically gated channels open

inside membrane becomes more:

negative- hyperpolarising

positive- depolarising

graded pot can hyper or depolarise membrane

larger the signal larger the potential

generally on cell body or dendrites

action potentials

in 2 phases: depolarisation and repolarisation

uually on axons

if memrane gets to -55mV voltage gated Na channels open and ap occurs

depolarisation

stimulus causes graded potential, depolarises membrane

if graded potential reaches threshold, will get action potential

voltage gated Na open, Na ions rush into cell so membrane becomes more positive

Na begin to close after ap generated

repolarisation

voltage gated K channels begin to open at the same time as Na channels

open more slowly, so become open as Na closing

as Na inflow stops, K outflow begins

resets membrane to -70mV

refractory period

absolute- no stimulus will produce an ap. Na channels inactivated and cant be opened so they reset to resting stage

relative- larger than normal stimulus needed, Na channels reset and K channels still open as are slower

larger axons = larger surface area, more gates so faster

generation of an ap

conduction of nerve impulses

• propogation

from trigger zone to axon terminals

continous conduction- unmyelinated fibres

saltatory conduction- myelinated fibres, leaps from node to node, and fast

• effect of axon diameter

larger diameter fibres faster, with sorter refractory periods

• stimulus intensity encoded by combination of frequ of APs

2 types of synapses

electrical

chemical

electrical synapses

gap junctions

ap cross drectly

cardiac and smooth muscle, CNS

have tubular connections, allow ions to flow from cell to cell

fast and synchronised

chemical synapses

has synaptic cleft

crossed by nueortransmitter

slower

unidirectional

AP opens voltage gated Ca channels

serves as a trigger to move vesicles containing transmitter to membrane and release into synaptic cleft

opens ligand gated Na channels in postsynaptic membrane and GP generated

final mechanism to remove transmitter

neurotransmitters

deoplarising- excitatory : EPSP

hyperpolarising- inhibitory : IPSP

• acetylcholine

EPSP at NMJ, IPSP eg to slow heart rate

• amino acids: glutamate and aspartate, GABA and glycine

many IPSPs occur in brain via GABA

More neurotransmitters

norepinephrine and epinephrine : involved in arousal from sleep, dreas and regulation of mood

dopamine: emotion, pleasure, maintains muscle tone

seratonin: sensory perception, temp regulation, mood, apetite and sleep

nitric oxide: vasodilation,

neurotransmitters (3)

encephalins- potent analgesic, similar to morphine and heroin

endorphins

noth involved in memroy, temp reg, onset of pubertu, depression, sz

modifying the effects of neurotransmitters

used in medicine

• L-dopa: dopamine precurser, boots dopamine production used in parkinsons

• amphetamines promote release of norepinephrine

• botulinum toxin blocks ACh release

• salbutamol- adrenergic agonist used to dilate airways in asthma

• cocaine blocks dopamine reuptake, causes euphoria

neural circuits

diverging

converging

reverberating

parallel

diverging

one presynaptic neurone affects many post syaptic

motor neurones in the brain diverge to si]pinal cord

sensory neurones diverge to differing brain regions

signal amplified

converging

several presynaptic neurones converge to single post synaptic

single motot neurone at NMJ recieves converging input from differing brain regions

more effective stimulation or inhibition of post synaptic cell

reverberating

impulses sent through circuit repeatedly

circuit may be turned off by inhibitory neurone

breathing, co ordinated muscle activit, walking and short term memory

parallel after dishcharge

pre synaptic cll stimulates net, these neurones synapse with single post synaptic cell

last neurone recieves both IPSPS and EPSPS

regeneration and repair

nervous system plastic: changes due to experience

very limited regeneration- PNS will heal if cell body intact and schwann cells alive, CNS cannot heal