Nervoys System

Nervous System

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

Neural Tissue

• 2 cell types

• neurons % neuroglia

Neurons

• React to changes

• Send nerve impulses  

• Communication

• Electrically charged due to unequal distribution of positive and negative ions

Neuroglia

• Surround and support neurons

• 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 of synapses

Central Nervous System

• Brain

• Spinal cord

Peripheral Nervous System

• CONNECTS CNS to other body parts

• Cranial nerves

• Spinal nerves

Sensory Function

• Nervous system receives information

Integrative Function

• Nervous system coordinates sensory information to create sensations, memory, thoughts 

• Nervous system makes decisions on body’s response to sensory information

Motor Function

• Decisions are acted upon 

• Impulses are carried to effectors (muscles or glands)

• Divisions of motor portion of PNS: somatic & autonomic

Somatic Nervous System

•  Transmits voluntary instructions to skeletal muscles

Autonomic Nervous System

• Transmits involuntary instructions from the CNS to smooth muscles, cardiac muscle, and glands

Cell Body (Soma)

• Contains nucleus, cytoplasm, organelles, neurofilaments, chromatophilic substance (Nissl bodies) 

Dendrites

• Branched receptive surfaces; a neuron may have

Axon

• Transmits impulses and releases neurotransmitters to another neuron or effector (another neuron, a muscle cell or a gland cell); a neuron may have only 1 axon

Cell Body Structure

• Neurofilaments

• Chromatophilic substance

Axon Structure

• Axon hillock

• Collaterals

• Axon terminal

• Synaptic knob

Schwann Cells Structure

• Myelin

• Myelin sheath

Schwann Cells

• PNS neuroglia that encase axons in a sheath

• Schwann cells wrap tightly around axon in layers composed of myelin, a lipoprotein mixture

• Coating = Myelin Sheath

• Nodes of Ranvier - gaps in myelin sheath between Schwann cells

• Produce myelin sheath found on some peripheral axons

• Speed up speed of nerve impulse transmission

Multipolar Neurons

• 99% of neurons

• Many processes

• Most neurons of CNS

Bipolar Neurons

• Two processes

• Eyes, ears, nose

Unipolar Neurons

• One process

• Cell bodies are in ganglia

• Sensory

Sensory Neurons

• Afferent neurons

• Carry impulse to CNS

• Most are unipolar

• Some are bipolar

Interneurons

• Association neurons

• Link neurons

• Multipolar

• Located in CNS

Motor Neurons

• Multipolar, efferent

• Carry impulses away from CNS

• Carry impulses to effectors

Astrocytes

• Connect neurons to blood vessels; exchange nutrients and growth factors

• Form scar tissue

• Aid metabolism of certain substances

• Regulate ion concentrations, such as K⁺

• Part of Blood Brain Barrier

Oligodendrocytes

• Myelinate CNS axons; also provide structural support

Microglia

• Phagocytic cell; also provides structural support

Ependyma or Ependymal Cells

• Line central canal of spinal cord & ventricles of brain, cover choroid plexuses

• Help regulate composition of cerebrospinal fluid

• Cuboidal or columnar cells; ciliated

Satellite Cells

• Support clusters of neuron cell bodies (ganglia)

Synaptic Transmission

Transmission of a nerve impulse from one neuron to another

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

• Effects of neurotransmitters vary; some open ion channels and others close ion channels

• Chemically gated ion channels respond to neurotransmitters

Excitatory Postsynaptic Potential (EPSP)

• Membrane change in which neurotransmitter opens Na⁺ channels

• Depolarizes 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

Axon Hillock

• Trigger zone where summation of all inputs occur

• Thick cone-shaped region

Neurotransmitters

• At least 100

• Monoamines, amino acids, peptides

• Produced in Rough ER of cytoplasm

• When impulse reaches synaptic knob of an axon, neurotransmitters are released by exocytosis

Acetylcholine

• Stimulates skeletal muscle contraction

Depolarization

• Neuron is stimulated, the gated channels in the axon hillock open up and allow Na+ ions to flow inside neuron

• Inside = MORE POSITIVE

• Greater stimulus = greater depolarization (potential DECREASES)

Repolarization

• Axon hillock is more positive: the gated channels open and allow K+ ions to pass to the outside

• Inside = MORE NEGATIVE

Selectively Permeable Membrane (SPM)

• Determines the distribution of ions in & outside of cells

• K⁺ ions diffuse out of the cell more rapidly than Na⁺ ions can diffuse in

• Every millisecond, more positive charges leave the cell by diffusion than enter it

  
  

Active Transport

• Neurons have a greater concentration of Na⁺ outside and a greater concentration of K⁺ inside

  
  

Potential Difference

• Difference of electrical charge between the inside and outside of the SPM is called the potential difference

  
  

Resting Potential

• Potential difference in a resting neuron

• -70mV

Threshold Potential

• Depolarization sufficient = potential lvl = -55mV

• Threshold reached = Action Potential