nerves Flashcards
nervous tissue
Controls and integrates all body activities
3 basic functions :
sensing changes within sensory receptors
interpreting and remembering those changes
Reacting to those changes with effectors - muscular contractions , glandular secretions
function of the nervous systems
DENDRITES
AXONS
NEUROGLIAL CELLS
the sensory function is to sense changes in the internal and external environment through sensory receptors (sensory division - afferent:picks up sensory stimuli)
the integrative function is to analyse the sensory information store some aspects and make decisions regarding appropriate behaviour’s - interneurons do this
the motor function is to respond to stimuli by initiating action (motor division- efferent: sends directions from your brain to muscles and glands)
function of the nervous systems DENDRITES
Conducts impulses towards the cell body typically short highly branched un-myleinated surfaces specialised for contact with other neurons
function of the nervous systems AXONS
conduct impulses away from the body
long thin cylindrical process of cell
begins at axon hillock (trigger zone)
swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters
function of the nervous systems NEUROGLIAL CELLS
vital in supporting the information transferred in the neuron but are no electrically excitable themselves
half the volume of the CNS
smaller cells than neurons
cells can divide - rapid mitosis in tumor formation (gliomas)
4 cells in the CNS : astrocytes ,oligodendrocytes, microglia and ependymal
2 cells types in PNS : schwann and satellite cells
function of CNS Glial cells
astrocytes :
support neuron function and induce tight junctions in the endothelial cells that form the blood capillaries forming the blood brain barrier
(the BBB keeps and protects the brain form drugs and toxic chemicals - there are pumps which remove drugs from the CNS)
microglial cells :
part of the immune system ingestion of foreign particles and dead cells (like macrophages)
macrophages - eat any viral or bacterial contamination in the CNS they also eat dead cells
ependymal cells :
line the brain ventricles and central canal of the spinal cord
keeps the fluid moving- the brain doesnt have a direct blood supply ; the cerebral spinal fluid mediated the oxygen and carbon dioxide exchange
Myelination and axon covering in the PNS
Myelination:
a multilayered lipid and protein covering called a myelin sheath that is produced by schwann cells and oligodendrocytes and surrounds axons of most neurons
Axon covering in the PNS:
All axons are surrounded by a myelin sheath
neurilemma is the cytoplasm and nucleus of schwann cells: they have gaps called Nodes of Raniver
Myelinated fibres appear white :
jelly rolls wrapping made of lipoprotein = myelin
acts as an electrical insulator
spends conduction of nerve impulses
Unmyelinated fibres :
slow diameter fibres
only surrounded by neurilemma but no myelin sheath wrapping
grey and white matter
white matter = Myelinated fibres
grey matter = nerve cell bodies, dendrites , axon terminals, bundles of unmyeilnated axons and nerogila
in the spinal cord = grey matter forms and H shaped inner core surrounded by white matter
in the brain = a thin outer shell of grey matter covers the surface and if found in clusters called nuclei inside the CNS
electrical signals in neurons
neurons are electrically excitable due to voltage difference across their membrane
communicate with 2 types of electrical signals :
action potentials that can travel long distances
graded potential that are local membrane changes only
in living cells a flow of ion occurs through ion channels in the cell membrane
electrical changes due to ion flow is how neurons communicate
ANY DRUGS PRESCRIBED TO ALTER THE BRAINS ACTIVITY ACTUALLY :
Influence neuronal activity (antidepressants)
they alter the electrical signals within neurons changing their electrical activity
Two types of ion channels
Leakage (non gated)
channels are always open:
Nerve cells have more K+ than Na+ in their leakage channels
As a result membrane permeability is higher to the K+ ion
This expaining the resting potential of -70mV in the nerve tissue
Gated channels:
Open and close in response to a stimulus
Results in neuron exicitibility
Ion channels
Gated ion channels respond to voltage change ligands (chemicals) and mechanical pressure:
Voltage gated channels respont to direct change in the membrane potential
Ligand gated channels respond to a spicific chenical stimulus
Mechanically gated ion channels respond to mechnaical vibration or pressure
Main basis of the resting membrane potiential
Na+/K+ ATPase sets up a chemical conc gradient
Potassium diffuses down its conc gradient OUT of the cell
As Na+ permeability is low this sets up an electrical gradient
Net positive flow is +ve charge out of the cell inducing a membrane potential of -70mV
The resting potential is due to :
Outwards K+ gradients
K+ leak channels
Graded potentials
Small deviations form the resting potential of -70mV
Hyperpolerization = membrane has become more negative
Depolerisation = membrane has become more positive
The signs are graded :
The vary in size depending in the stregth of the stimulus and localised signals
How do graded potentials arise ?
Sorce of stimuli :
Mechanical stimulation of membranes with mechanical ion gated channels (pressure)
Chemical stimulation of membranes with ligand gated ion channels (neurotransmitters)
graded /postsynaptic/receptor or generator potential:
Ions flow through the ion channels and change membrane potential locally
Amount of change varies with strength of stimuli
Flow of current (ions)is a local change only
Generation of an action potential
An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarisation) and then restore it to its resting state (repolerisation)
During an action potential voltage gated Na+ and K+ channels open in sequence
According to the all or none principle if a stimulus reaches threshold the action potential is always the same ;
A stronger stimulus will not cause a larger impulse
