CNS v PNS

• CNS - Central Nervous System

  • Brain and spinal cord enclosed by cranium and vertebral column

• PNS - Peripheral Nervous System

  • All nervous system minue brain and spinal cord - mostly consists of nerves and ganglia

    • Ganglia -  knot-like swelling in a nerve where neuron cell bodies are concentrated

  • contains sensory and motor divisions each with somatic and visceral subdivisions

  • Sensory (afferent) division: carries signals from receptors to CNS

    • Somatic sensory division: carries signals from receptors in the skin, muscles, bones, and joints

    • Visceral sensory division: carries signals from the viscera (heart, lungs, stomach, and urinary bladder)

  • Motor (efferent) division - carries signal from CNS to effectors (glands/muscles that carry out body’s response)

    • Visceral motor division (autonomic nervous system) - carries signal to glands, cardiac muscle, and smooth muscle 

      • Involuntary responses are called visceral reflexes 

      • Sympathetic division - arouse body reaction and speeds up processes

      • Parasympathetic division - slows body reactions down, stimulate digestion and urinary systems

  • Somatic motor division - carries signal to skeletal muscles 

    • Output produces muscular contraction as well as somatic reflexes - involuntary muscle contractions

Neurons

•  Properties

  • Excitability - repsonse to stimuli

  • Conductiity

  • Secretion 

    • When an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell

• Functional classes of neurons

  • Sensory (afferent) neurons - detect stimuli and transmit information about them towards the CNS

  • Interneurons (association neurons) - lies entirely in CNS connecting motor and sensory pathways

    • Receive signals from many neurons and carry out integrative function (make decisions on response)

  • Motor (efferent) neuron - sends out signals to muscles and gland cells (effectors)

• Structure

  • Soma - control center of neuron 

    • single, centrally located nucleus w large nucleolus 

  • Cytoplasm contains golgi, ER, mitochondria, lysosomes, cytoskeleton

    • Cytoskeleton has dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalizes rough ER into dark-staining  Nissl bodies

    • No centrioles, no mitosis

  • Dendrites - branches come off soma

    • Primary site for receiving signals from other neurons 

    • More dendrites, more info 

    • Provide precise pathways for reception and processing of info 

  • Axon 

    • Axon collateral - branches of axon - rapid conduction 

    • Axoplasm

    • Axolemma - plasma membrane of axon

    • One axon per neuron

    • Myelin sheath

  • Distal end of axon has terminal arborization: extensive complex of fine branches

  • Synaptic knob (terminal button)—little swelling that forms a junction (synapse) with the next cell

    • Contains synaptic vesicles full of neurotransmitter

  • Multipolar neuron

    • One axon and multiple dendrites 

    • Most common – most neurons in CNS

• Bipolar neuron

  • One axon and one dendrite

  • Olfactory cells, retina, inner ear

• Unipolar neuron

  • Single process leading away from soma

  • Sensory cells from skin and organs to spinal cord

• Anaxonic neuron

  • Many dendrites but no axon

  • Retina, brain, and adrenal gland

Neuralgia (glial cells)

• Function

  • Protect neurons and help them function

  • Bind neurons together and form framework for nervous tissue 

  • If mature neuron is not in synaptic contact with another neuron, it is covered by glial cells 

    • Prevents neurons from touching each other 

    • Gives precision to conduction pathways

• Types (CNS ONLY)

  • Oligodendrocytes

    • Form myelin sheaths in CNS that speed signal conduction

      • Arm-like processes wrap around nerve fibers

  • Ependymal cells 

    • Line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF)

      • Cuboidal epithelium with cilia on apical surface

  • Microglia

    • Wander through CNS looking for debris and damage

      • Develop from white blood cells (monocytes) and become concentrated in areas of damage

  • Astrocytes

-  Most abundant glial cell in CNS, covering brain surface and most nonsynaptic regions of neurons in the gray matter

-   Diverse functions:

• Form supportive framework 

• Have extensions (perivascular feet) that contact blood capillaries and stimulate them to form a seal called the blood–brain barrier

• Convert glucose to lactate and supply this to neurons

• Secrete nerve growth factors 

• Communicate electrically with neurons 

• Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions

• Astrocytosis or sclerosis—when neuron is damaged, astrocytes form hardened scar tissue and fill in space

• Types (PNS ONLY)

  • Schwann cells

    • Envelope nerve fibers in PNS

    • Wind repeatedly around a nerve fiber

    • Produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS

    • Assist in regeneration of damaged fibers

• Satellite cells

  • Surround the neurosomas in ganglia of the PNS

  • Provide electrical insulation around the soma

  • Regulate the chemical environment of the neurons