KN 230 Exam 3

Study

CNS consists of

brain + spinal cord

PNS consists of

nerves + ganglia

Sensory/Afferent Nervous System

Receives sensory info and transmit to CNS

Somatic sensory system detects stimuli we consciously perceive

Visceral sensory system detects stimuli we do not perceive

Motor/Efferent Nervous System

Initiates motor output and transmit from CNS to effectors

Somatic motor system sends voluntary signals to skeletal muscles

Autonomic motor system (visceral motor) sends involuntary commands to heart, smooth muscle, and glands

Nerve

Bundle of parallel axons in PNS

Epineurium = encloses entire nerve, thick layer of dense irregular

Perineurium = wraps fascicle (bundle of axons in nerve), layer of dense irregular

Endoneurium = wraps each individual axon, delicate areolar connective

Nerves are vascularized, BVs found in epineurium and perineurium

Type of Nerves

Sensory = Sensory neurons send signals to CNS

Motor = Motor neurons send signals from CNS

Mixed nerves contain both sensory and motor; most nerves are mixed

Characteristics of Neurons

Excitability

Conductivity

Secretion

Extreme longevity

Amitotic

Cell body (soma)

• Plasma membrane encloses cytoplasm called perikaryon

• Contains nucleus

• Initiates some graded potentials, receives others from dendrites; conducts these potentials to axon

• Contains chromatophilic substance (Nissl bodies) made of ribosomes (free and bound)

Dendrites

• Short, unmyelinated processes branching off cell body

• Receive input and transfer it to cell body

Axon

• Long process emanating from cell body

• Makes contact with other neurons, muscle cells, or glands

• Attaches to cell body at axon hillock (triangular region of soma)

• Cytoplasm called axoplasm; membrane called axolemma

• Splits into branches called axon collaterals

• Sends in several telodendria (axon terminals)

• Tips of telodendria are synaptic knobs (terminal boutons)

• Synaptic knows house synaptic vesicles containing NT

• Axons function to conduct action potentials and these release NT at synaptic knobs

Cytoskeleton

• Composed of microfilaments, intermediate filaments, and microtubules

• Intermediate filaments = neurofilaments

  • Aggregate to form bundles, neurofibrils

  • Provide tensile strength

Multipolar neurons

Many dendrites, one axons; most common

Bipolar neuron

One dendrite, one axons, limited, found in retina of eye

Unipolar (psuedounipolar) neuron

• Unipolar (pseudounipolar): one process extends from cell body

  • Splits into two processes:

    • 1. Peripheral process splits into several receptive dendrites

    • 2. Central process leads to synaptic knobs in CNS

Anaxonic neurons

Have dendrites but no axons

Sensory neurons (afferent)

• Conduct input from somatic and visceral receptors to CNS

• Most unipolar, some bipolar

Motor neurons (efferent)

• Conduct output from CNS to somatic and visceral effectors

• ALL multipolar

Interneurons (association neurons)

• Receive, process and integrate info from many other neurons

• Communicate between sensory and motor neurons

• Located within CNS, make up 99% of our neurons

• Generally multipolar

Chemical synapses

•  Chemical synapses are more common than electrical

  • Presynaptic neuron’s axon terminal produces signal

  • Postsynaptic neuron receives signal

    • Most commonly with one of its dendrites

  • Synaptic cleft = small fluid-filled gap between the two neurons

  • Events of synaptic communication:

    • NT molecules released from vesicles of synaptic knob into cleft

    • NT diffuses across cleft and binds to postsynaptic receptors

    • Binding of NT to receptor initiates postsynaptic potential (a graded potential)

    • Synaptic delay = time is takes for all these events

Electrical synapse

• Presynaptic and postsynaptic neurons bound together by gap junctions

  • Fast; no synaptic delay in passing electrical signals

Glial cells

• Nonexcitable support cells found in CNS and PNS

• Approximately same number of glial cells as there are neurons

Glial cells in CNS

• CNS: Astrocytes, ependymal, microglia, oligodendrocytes

  • Astrocytes

    • Have processes that end in perivascular feet

    • Most abundant glial cell in CNS

    • Help form blood-brain barrier by wrapping feet around brain capillaries

    • Regulate tissue fluid composition (chemical environment around neurons; can regulate potassium concentration

    • Form structural support for nearby neurons

    • Assist neuronal development

    • Alter synaptic activity (add, eliminate, influence)

    • Occupy the space of dying neurons

  • Ependymal cells

    • Line cavities in brain and spinal cord

    • Part of choroid plexus, which produces cerebrospinal fluid

  • Microglia

    • Small cells that wander CNS and replicate in infection

    • Phagocytic cells of immune system

    • Engulf infectious agents and remove debris

  • Oligodendrocytes

    • Large cells with slender extensions

    • Extensions wrap around axons of neurons forming myelin sheath

Glial cells in PNS

• PNS: Satellite cells, Schwann cells

  • Satellite cells

    • Arranged around neuronal cell bodies in a ganglion

    • Electrically insulate and regulate the exchange of nutrients and wastes

  • Neurolemmocytes (Schwann cells)

    • Elongated, flat cells that ensheath PNS axons with myelin

    • Allows for faster action potential propagation