A&P 2-CH 12 Nervous Tissue Part 1

Nervous system overview

• central nervous system (CNS)

  • brain and spinal cord enclosed in bony coverings

• peripheral nervous system (PNS)

  • nerve: bundle of axons in connective tissue

  • ganglion: swelling of cell bodies in a nerve

Nervous & endocrine systems connection

they maintain internal coordination

nervous system

• three basic steps:

  • sense organs receive info

  • brain and spinal cord determine responses

  • brain and spinal cord issue commands to glands and muscles

endocrine system

chemical messengers (hormones) delivered to the bloodstream.

Functional divisions of PNS

1. CNS

   1. brain and spinal cord

2. PNS

   1. sensory division

      1. visceral sensory division

      2. somatic sensory division

   2. motor division

      1. visceral motor division

         1. sympathetic division (fight or flight)

         2. parasympathetic division (digestion)

      2. somatic motor division

sensory (afferent) divisions

• receptors to CNS

• include the visceral sensory and somatic sensory divisions

motor (efferent) division

• CNS to effectors

  • visceral motor divisions (ANS) effectors: cardiac, smooth muscle, glands

  • somatic motor division effectors: skeletal muscle

excitability (irritability)

property of neurons that gives them the ability to respond to changes in the body and external environment stimuli (changes)

conductivity

property of neurons to produce traveling electrical signals between cells

secretion

property of neurons when electrical signals reach end of nerve fiber, chemical neurotransmitters cross gaps

sensory (afferent) neurons

• detect changes in body and external environment

  • info is transmitted into brain or spinal cord

interneurons (association neurons)

• lie between sensory and motor pathways in CNS

• 90% of our neurons are interneurons

• process, store and retrieve info

motor (efferent) neuron

• send signals out to muscles and gland cells

• organs that carry out responses called effectors

structure of a neuron

• cell body=perikaryon=soma

• vast number of short dendrites

  • receive signals

• single axon

  • nerve fiber arising from axon hillock for rapid conduction

multipolar neurons (neuron classification)

most common (brain and spinal cord)

bipolar neurons (neuron classification)

sensory (ear and nose); retina

unipolar neuron (neuron classification)

• single process leaving soma

• carries a signal to the spinal cord (touch or pain)

  • found in skin

anaxonic neurons (neuron classification)

• no axons: multiple dendrites

• do not produce action potentials

• communicate to local cells through dendrites

axonal transport

• movement or proteins, organelles, and other materials along axon

• anterograde: away from soma and down axon

• retrograde: up the axon and toward the soma

supportive cells: importance

• neurons: ~1 trillion in the nervous system and make up ~50% of nervous system

• neuroglia: outnumber neurons 10:1

  • glial=glue

  • covers neurons when not synapsing, prevents accidental signal transmission

Oligodendrocytes (neuroglia of CNS)

form myelin in brain and spinal cord

ependymal cells (neuroglia of CNS)

line cavities of brain and spinal cord: secrete and circulate cerebrospinal fluid

microglia (neuroglia of CNS)

phagocytize and destroy microorganisms, foreign matter, and dead nervous tissue

astrocytes (neuroglia of CNS)

form blood brain barrier

Schwann cells (neuroglia of PNS)

form neurilemma around all PNS nerve fibers and myelin around most of them

satellite cells (neuroglia of PNS)

surround somas of neurons in the ganglia

myelin

• insulating layer around a nerve fiber

• formed from wrappings of plasma membrane

• all myelination completed by late adolescence

myelin-PNS

• in PNS, hundreds of layers wrap axon

  • the outermost coil is Schwann cell (neurilemma)

myelin-CNS

• in CNS-no neurilemma or endoneurium

• oligodendrocytes myelinate several fibers

  • insulation from ECF: speeds up conduction

nodes or Ranvier

between Schwann cells

myelination

begins during fetal development, but proceeds most rapidly in infancy

unmyelinated nerve fibers

• present in CNS and PNS

• PNS: even “unmyelinated” cells have one Schwann cell wrapped around

  • most nerve fibers go through individual Schwann cells

  • small fibers bundle together-go through single channel

speed of nerve signal

• diameter of fiber and presence of myelin

  • Large fibers have more surface area for signals

• small, unmyelinared fibers= 0.5-2 m/s

• small, myelinated fibers= 3-15 m/s

• large, myelinated fibers= up to 120 m/s

function of nerve signals

• slow signals supply the stomach and dilate pupil

• fast signals supply skeletal muscles and transport sensory signals for vision and balance

nerve regeneration: PNS

1. observe normal nerve fiber

   1. normal features present at NMJ

2. cute nerve fiber (stops protein synthesis)

3. degeneration: distal fibers begin and local Schwann cells follow

   1. soma reacts by swelling and some neurons die (regeneration is not guaranteed)

4. early regeneration: regeneration tube; Schwann cells begin to produce molecules for growth (cell adhesion)

5. late regeneration: guides the growing tube to original damaged cells (synaptic contact reestablished)

6. regenerated fiber: regrowth and connection with original fibers (ex: muscle)