Anatomy of the Nervous System Quiz

Nervous System Objectives

• Describe the divisions of the nervous system and their characteristics.
• Identify the structures and functions of a typical neuron.
• Describe the location and function of neuroglia.
• Describe resting membrane potentials.
• Discuss the generation and propagation of action potentials.
• Define the function and structure of a synapse.

Nervous System

• The master controlling and communicating system of the body.
• Functions:
  • Sensory input – stimuli travel to the central nervous system (CNS).
  • Integration – interpretation of sensory input.
  • Motor output – response to stimuli originating in the CNS.

Terminology

• Input (sensory):
  • Receptors monitor changes.
  • Changes are called stimuli (singular: stimulus).
  • Information is sent by afferent nerves.
• Integration:
  • Information is processed.
  • A decision is made about what action should be taken.
• Output (motor):
  • Effector organs (muscles or glands) are activated.
  • Effector organs are activated by efferent nerves.

Organization of the Nervous System

• Central nervous system (CNS):
  • Includes the brain and spinal cord.
  • Integrates and commands the body.
• Peripheral nervous system (PNS):
  • Includes paired spinal and cranial nerves.
  • Transmits messages to and from the CNS.

Peripheral Nervous System (PNS): Two Functional Divisions

1. Sensory (afferent) division:
   • Somatic sensory fibers carry impulses from the skin, skeletal muscles, and joints to the brain.
   • Visceral sensory fibers transmit impulses from visceral organs to the brain.
2. Motor (efferent) division:
   • Carries impulses from the CNS to effector organs (muscles and glands).

Motor Division: Two Main Parts

1. Somatic nervous system (voluntary):
   • Carries impulses from the CNS to skeletal muscles.
2. Autonomic nervous system (ANS) (involuntary):
   • Carries impulses from the CNS to smooth muscle, cardiac muscle, and glands.
   • Includes sympathetic and parasympathetic divisions.

ANS Division: Two Main Parts

• Sympathetic and parasympathetic.

Histology of Nervous Tissue

• Two principle types of nervous system cells:
  • Neurons – excitable cells transmitting electrical signals.
  • Neuroglia (glial) – cells that surround and wrap neurons (supporting cells).

Supporting Cells: Neuroglia

• Neuroglia:
  • Provide a supportive scaffolding for neurons.
  • Segregate and insulate neurons.
  • Guide young neurons to proper connections.
  • Promote health and growth.
  • Six types: 4 in the CNS and 2 in the PNS.

Neuroglia of the CNS

• Astrocytes:
  • Most abundant, versatile, highly branched glial cells.
  • Maintain the blood-brain barrier.
  • Cling to neurons and synapses.
  • Wrap around capillaries, regulating their permeability.
  • Provide structural framework support.
  • Guide migration of young neurons.
  • Control chemical environment.
  • Repair damaged neural tissue.
• Microglia:
  • Small, ovoid cells with spiny processes, functioning as phagocytes monitoring neuron health, removing cell debris and pathogens.
• Ependymal cells:
  • Line the central cavities of the brain and spinal column.
  • Range in shape from squamous to columnar.

Neuroglia of the CNS

• Oligodendrocytes:
  • Branched glial cells.
  • Form myelin sheaths, insulating nerve fibers in the CNS.

Neuroglia of the PNS

• Schwann cells (neurolemmocytes):
  • Myelinate nerve fibers in the PNS.
  • Forms myelin sheaths around nerve fibers.
• Satellite cells:
  • Surround neuron cell bodies within ganglia.
  • Regulate the environment around the neurons.

Myelin in the Peripheral and Central Nervous Systems

• Difference between myelin sheath formation in the CNS vs Peripheral nervous System (PNS)

Application

• Multiple Sclerosis (MS): Myelin sheath destruction. The myelin hardens to a tissue called scleroses, an autoimmune disease.

Neurons

• Structural units of the nervous system.
• Composed of a cell body, axon, and dendrites.
• Long-lived (100+ years), amitotic, high metabolic rate.
• Plasma membrane functions in electrical and cell-to-cell signaling during development.

Nerve Cell Body (Perikaryon or Soma)

• Contains the nucleus and nucleolus.
• Major biosynthetic center for the neuron.
• Focal point for the neuron processes.
• Lacks centrioles, thus amitotic.
• Has well-developed Nissl bodies (rough ER).
• Contains an axon hillock, a cone-shaped area where axons arise.

Processes

• Arm-like extensions from the cell body.
• Two types: axons and dendrites.
• Myelinated axons are called tracts in the CNS and nerves in the PNS.

Dendrites: Structure

• Short, tapering processes.
• Receptive, or input, regions.
• Convey messages toward the cell body.
• Electrical signals are conveyed as graded potentials (not action potentials).

Axons: Structure

• Slender, uniform diameter processes arising from the axon hillock.
• Variable length (very short to 4 feet).
• Usually only one unbranched axon per neuron.
• Axon collaterals (branches of the axon).
• Axonal terminal or synaptic knobs.

Axons: Function

• Generate and transmit action potentials.
• Secrete neurotransmitters from the axonal terminals.
• Movement along axons occurs in two ways (anterograde and retrograde).

Myelin Sheath

• Whitish, fatty (protein-lipoid) segmented sheath around most long axons.
• Functions to protect the axon, electrically insulate fibers from one another, and increase the speed of nerve impulse transmission.

Myelin Sheath and Neurilemma: Formation

• Formed by Schwann cells in the PNS.
• Encloses the axon, with layers that make up the myelin sheath.

Nodes of Ranvier (Neurofibral Nodes)

• Gaps in the myelin sheath between adjacent Schwann cells.
• Sites where axon collaterals can emerge.

Unmyelinated Axons

• Schwann cells partially enclose 15 or more axons.
• Conduct nerve impulses slower than myelinated axons.

Axons of the CNS

• Both myelinated and unmyelinated fibers are present.
• Myelin sheaths are formed by oligodendrocytes.
• Nodes of Ranvier are widely spaced.

Regions of the Brain and Spinal Cord

• White matter – dense collections of myelinated fibers.
• Gray matter – mostly soma, dendrites, glial cells, and unmyelinated fibers.

Structural Classification of Neurons

• Three types:
  1. Multipolar – one axon, several dendrites; most abundant in CNS.
  2. Bipolar – one axon, one dendrite; rare, e.g., retinal neurons.
  3. Unipolar (pseudounipolar) – single, short process that branches, peripheral process (more distal branch), mainly found in the PNS.

Functional Classification of Neurons

• Three types:
  1. Sensory (afferent) – transmit impulses toward the CNS.
  2. Motor (efferent) – carry impulses toward the body surface.
  3. Interneurons (association neurons) – any neurons between a sensory and a motor neuron.

Neurophysiology

• Neurons are highly irritable (exhibit responsiveness).
• Action potentials (nerve impulses) are electrical impulses carried along axons.
• Nerve impulses are consistently maintained across distance.

Electrical Current and the Body

• Reflects the flow of ions rather than electrons.
• Potential across membranes exists when the number of ions differs across and there is resistance to ion flow.

Role of Ion Channels

• Nongated (leakage channels): always open.
• Chemically gated channels: open with neurotransmitter binding.
• Voltage-gated channels: open and close in response to membrane potential.
• Mechanically gated channels: open and close in response to physical deformation of receptors.

Electrochemical Gradient

• Ions flow along their chemical gradient (high to low concentration).
• Ions flow along their electrical gradient (toward opposite charge).
• Electrochemical gradient results from the combination of chemical and electrical gradients.

Resting Membrane Potential

• Equal the difference in charge between inside and outside the cell's membrane.
• Polarized = unequal charges across the membrane.
• Ranges from -40 to -90 mV, varying between neuron types. The negative sign indicates that the inside of the neuron is more negatively charged than the outside.

Resting Membrane Potential (V_m)

• The potential difference across the membrane of a resting neuron is roughly -70mV.
• This potential is generated by different ion concentrations (Na+, K+, Cl-, and protein anions).

Resting Membrane Potential (V_m) - Ionic Differences

• Differential permeability of the neurilemma to Na+ and K+.
• Operation of the sodium-potassium pump.

Synapse

• The means by which adjacent neurons communicate in the nervous system.
• Most synapses are between the axon of one neuron and the dendrite or cell body of another neuron (axodendritic or axosomatic).
• Presynaptic neuron: information sender.
• Postsynaptic neuron: information receiver.

Gray and White Matter of the Central Nervous System

• The brain and spinal cord receive impulses, interpret them, then respond with appropriate actions; gray matter is comprised of unsheathed nerve fibers located in the cortex or surface layer; white matter is comprised of insulated nerve fibers located in the internal structure.

Electrical Synapses

• Less common than chemical synapses.
• Correspond to gap junctions found in other cell types.
• Very fast propagation of action potentials.
• Important in CNS for arousal from sleep, mental attention, emotions/memory and ion/water homeostasis.

Chemical Synapses

• Specialized for the release and reception of neurotransmitters.
• Typically composed of presynaptic and postsynaptic neurons.
• Presynaptic neuron contains synaptic vesicles containing neurotransmitters.
• Receptors located typically on dendrites and soma of postsynaptic neuron.

Chemical Synapses - Synaptic Cleft

• Fluid-filled space separating presynaptic and postsynaptic neurons.
• Transmission across the synaptic cleft is a chemical event vs electrical one; unidirectional flow of communication between neurons.

Chemical Synapses - Information Transfer

• Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca²⁺ channels.
• Neurotransmitter is released into the synaptic cleft via exocytosis.

Chemical Synapses - Information Transfer

• Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron.
• Postsynaptic membrane permeability changes leading to excitatory or inhibitory effects.

Termination of Neurotransmitter Effects

• Neurotransmitter bound to a postsynaptic neuron: produces a continuous postsynaptic effect and blocks the reception of additional messages.
• The neurotransmitter must be removed from its receptor.

Postsynaptic Potentials

• Neurotransmitter receptors mediate changes in membrane potential according to the amount of neurotransmitter released, the duration it is bound to receptors, etc.
• Two types of postsynaptic potentials:
• EPSP (excitatory postsynaptic potential)
• IPSP (inhibitory postsynaptic potential)

EPSP's

• Excitatory synapses cause depolarization of the postsynaptic membrane.

IPSP's

• Inhibitory synapses reduce a postsynaptic neuron's ability to generate an action potential by making the inner face of the membrane more negatively charged.

Terminology for quiz

• Neuron=nerve cell
• Neuroglia=supporting cell
• Nerve fiber=long axon
• Nerve=collection of nerve fibers (axons) in PNS
• Tract=collections of nerve fibers (axons) in CNS
• Nucleus=cluster of cell bodies in CNS
• Ganglia=cluster of cell bodies in PNS
• Unilateral=one side
• Ipsilateral =same side
• Contralateral=opposite side
• Input/Output: sensory/afferent, motor/efferent

Examine Yourself

• Includes a variety of questions in relation to the nervous system contents.
• Specific details concerning nerve fibers, central nervous system (CNS), cerebrospinal fluid (CSF) formation, and peripheral nervous system (PNS), components, along with a variety of other nervous system components.

References

• A variety of references are cited in the notes.

Test your knowledge on the central and peripheral nervous systems with this quiz. Explore functions, divisions, and types of nerves and neuroglia that play crucial roles in our body's response and integration. Perfect for students of anatomy and physiology.