Nervous system

• Neurophysiology is the study of how the nervous system works
• • electrical signals are produced when ions move across membranes
  • ions can cross membranes through ion channels
• • Neurons are functional units of the nervous system
  • neurons use electrical signals and chemicals to communicate
• Gates ion channels can be either open or closed

  • 3 types of gated ion channels are
  • ligand gated - respond to chemical stimulus
  • mechanically gated- respond to physical force -light, sound, pressure
  • voltage gated- responds to changes in the membrane potential
  • membrane potential is the charge or voltage difference cross a membrane
• • The member potential of a resting cell= the resting membrane potential is about -70 millivolts
  • the resting membrane potential is mainly due to the sodium Na+ potassium K+ pump
  • Na+K+ pump= pumps about 3 Na+ ions out of cell for ever 2 K+ ions pumped into cell
  • inside of the cell has fewer positive charges (or more negative) than the outside of the membrane
  • the resting membrane potential means NO electrical signal
• • 2 kinds of electrical signals-
  • graded potentials- short distance signals
  • action potentials- long distance signals
  • graded potentials occur when either a ligand gated or mechanically gated ion opens
  • graded potentials decrease with distance
• Action potentials

  • transient reversal of the membrane potential
  • occur along axolemma & sacrolemma
  • action potentials are all or none events caused by diffusion of Na+ & K+ ions
  • threshold (about -55 millivolts) is the membrane potential that must be reached in order for the action potential to begin
• 3 phases of action potentials

  1. depolarization- sodium channels open, membrane potential=positive
  2. repolariztion- K+ channels open, K+ leaves cell= more negative
  3. hyperpolarization- K+ channels close slowly, membrane potential becomes more negative than resting
• The action potential starts at the axon hillock-

  for the action potential to travel along the axolemma it uses either

  • continuous conduction
  • saltatory conduction

  in unmyelinated axons, continuous conduction is slow.

  in myelinated axons , saltatory conduction is fast

  • node or ranvier = action potential occurs - jumps node to node
• How fast action potentials are conducted depend on -—

  • diameter of the axon - increases rate of conduction
  • amount of myelination- increases rate of conduction
• A synapse is a junction Between 2 neurons or between a neuron & an effector cell

  • electrical synapse- gap junctions between neurons (rare)
  • chemical synapse - neuron transmitters communicate between neurons

  pre- synaptic neuron =sends signal

  post-synaptic neuron= receives signal
• Neurotransmitters are chemical messengers released by the nervous system

  neurotransmitters in the CNS=

  • dopamine- motor control = Parkinson’s=poor motor control
  • serotonin- maintain moods, sensory perception= awareness of sensation
• Two main neurotransmitters in the peripheral nervous system -

  • acetylcholine- stimulates skeletal muscle contraction
  • norepinephrine- produced by the autonomic nervous system
• Central nervous system consists of-

  brain - largest part of nervous system

  spinal cord
• The brain has about 100 billion neurons ( almost 99% of the nervous system)

  weighs about 3-3.5 pounds

  It’s main function is integration center & control center
• the brain is divided into 4 main parts

  1. cerebrum
  2. diencephalon- thalamus, hypothalamus, epithalamus
  3. brain stem- mid brain, pons, medulla oblongata
  4. cerebellum

  protection of the brain

  1. skull bones
  2. meninges- CT covers the brain
  3. cerebrospinal fluid CSF- surrounds & cushions brain
  4. blood-brain barrier BBB
• 3 meninges protecting the brain

  1. Dura mater - outer most layer- dense ct
  2. arachnoid mater- middle layer- spider wed
  3. pia mater - inner most layer - soft, directly touches brain
• fluid filled spaces between the meninges-

  subdural space- between dura & arachnoid mater-serous fluid

  subarachnoid space - Between arachnoid & pia mater- CSF
• CSF is made, circulated & removed by brain structures

  CSF is made by the chorid plexus

  CSF enters ventricles after chorid plexus

  • ventricles are spaces inside the brain that circulate CSF
  • there are 4 ventricles =2 lateral ventricles, third ventricle, fourth ventricle
  • fourth ventricle has apertures (holes) that allow CSF to leave ventricles & enter the subarachnoid space
  • CSF is removed/ drained from the brain by the arachnoid villi
  • arachnoid villi are folded regions of the arachnoid mater
  • the CSF is removed from subarachnoid space & returns to the blood
• The blood brain barrier protects the brain from harmful substances

  • BBB consists of endothelial cells (speacialized epithelial cells) that line brain capillaries
  • tight junctions cause the BBB to be very impermeable barrier that keeps harmful substances out of brain
• Substances that cannot or poorly cross the BBB

  • cells
  • proteins
  • waste products (nitrogenous)
  • most antibiotic drugs
  • K+ ions

  substances generally don’t enter brain
• Substances that can cross the BBB

  • glucose
  • some amino acids
  • most ions (not K+)
  • oxygen & carbon dioxide
  • fats
  • alcohol
  • anesthesia
• 4 main parts of the Brian-

  • cerebrum -83% of brain mass
  • diencephalon- about 3%
  • brain stem- about 3%
  • cerebellum- 11%
• Cerebrum

  • consists of 2 cerebral hemispheres
  • convolutions- are at surface, these are folds and creases that increase surface area
  • parts of convolutions- gyrus (ridge), sulcus (groove), fissures (deep groove)
• Some fissures divide the cerebrum or brain-

  • longitudinal fissure - divides the left & right cerebral hemispheres
  • transverse fissure - divides the cerebrum & cerebellum
• Some sulcus divide different lobes of the cerebrum -

  • central sulcus- between frontal & parietal lobes
  • parieto-occipital sulcus- between parietal & occipital lobes
  • lateral sulcus- between temporal lobe & both frontal & parietal lobes
• 5 lobes

  1. frontal
  2. parietal
  3. occipital
  4. temporal
  5. insula 5th lobe behind temporal
• The cerebrum is also divided into regions-

  cerebral cortex- outermost region made of gray matter

  white matter - mainly myelinated axons in inferior or cerebrum

  basal nuclei- gray matter in interior of cerebrum
• Cerebral cortex

  • houses the conscious mind
  • consist of cell bodies of interneurons only
  • wired in a contralateral way- opposite sides
  • some functions exhibit lateralization-
  • speech areas - usually left hemisphere
  • language areas - left
• Different areas of the cerebral cortex have different functions

  • motor areas - control voluntary movement
  • sensory areas- awareness of sensations, perceptions
  • association areas - thinking