Nervous System Overview 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 events in the generation and propagation of an action potential.
• Define the structure and function of a synapse.

Nervous System

• The master controlling and communicating system of the body.
• Functions:
  • Sensory input - stimuli goes to the CNS.
  • Integration - interpretation of sensory input.
  • Motor output - response to stimuli coming from the CNS.

Terminology

• Input (Sensory):
  • Receptors monitor changes.
  • Changes are called stimuli.
  • Information is sent by afferent nerves.
• Integration:
  • Information is processed.
  • A decision is made about what should be done.
• Output (Motor):
  • Effector organs (muscles or glands) are activated.
  • Effected by efferent nerves.

Organization of the Nervous System

• Central nervous system (CNS):
  • Brain and spinal cord.
  • Integration and command center.
• Peripheral nervous system (PNS):
  • Paired spinal and cranial nerves.
  • Carries messages to and from the spinal cord and brain.

Peripheral Nervous System (PNS): Two Functional Divisions

• 1. Sensory (afferent) division:
  • Somatic sensory fibers - carry impulses from 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 - carry impulses from the CNS to skeletal muscles.
• 2. Autonomic Nervous System (ANS):
  • Involuntary - carry impulses from the CNS to smooth muscle, cardiac muscle, and glands.

ANS Division: Two Main Parts

• Sympathetic and Parasympathetic.

Histology of Nerve Tissue

• The two principal cell types of the nervous system are neurons and neuroglia (glial).
  • Neurons: Excitable cells that transmit electrical signals.
  • Neuroglia (glial): Cells that surround and wrap neurons (supporting cells).

Supporting Cells: Neuroglia

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

Neuroglia of the CNS

• Astrocytes:

  • Most abundant, versatile, and highly branched glial cells.
  • Maintain blood-brain barrier.
  • Cling to neurons and their synaptic endings.
  • Wrap around capillaries, regulating their permeability.
  • Provide structural framework for the neuron.
  • Guide migration of young neurons.
  • Control the chemical environment.
  • Repair damaged neural tissue

• Microglia: Small, ovoid cells with spiny processes.

  • Phagocytes that monitor the health of neurons.

• Ependymal cells: Range in shape from squamous to columnar.

  • Line the central cavities of the brain and spinal column.

• Oligodendrocytes: Branched cells with myelin.

  • Myelin: Wraps oligodendrocyte processes around nerve fibers
  • Insulates nerve fibers.

Neuroglia of the PNS

• Schwann cells (neurolemmocytes):
  • Myelin: Wraps itself around nerve fibers, insulating them.
• Satellite cells: Surround neuron cell bodies in ganglia, regulating the environment around the neurons.

Myelin in the Peripheral and Central Nervous Systems

• Myelin sheaths can either be formed by Schwann cells (PNS), or by oligodendrocytes (CNS).

Action Potentials (APs)

• Brief reversal of membrane potential, only generated by muscle cells and neurons.
• Do not decrease in strength over distance.
• Principal means of neural communication.
• An action potential in the axon of a neuron is a nerve impulse.

Action Potential: Resting State

• Na+ and K+ channels are closed.

Action Potential: Depolarization Phase

• Na+ permeability increases, membrane potential reverses. Na+ gates open; K+ gates close.
• Threshold: critical level of depolarization (-55 to -50 mV). Depolarization becomes self-generating at threshold.

Action Potential: Repolarization Phase

• Sodium inactivation gates close; membrane permeability to Na+ declines to resting levels.
• Voltage-sensitive K+ gates open; K+ exits the cell.
• Internal negativity of the resting neuron is restored.

Action Potential: Hyperpolarization

• Potassium gates remain open, causing an excessive efflux of K+.
• This efflux causes hyperpolarization of the membrane (undershoot).
• The neuron is insensitive to stimulus and depolarization during this time.

Action Potential: Role of the Sodium-Potassium Pump

• Restores resting electrical conditions of the neuron, but does not restore the resting ionic conditions.

Propagation of an Action Potential:

• Continuous and Saltatory Propagation
• Continuous: Unmyelinated axons, current flows across the entire membrane.
• Saltatory: Myelinated axons, current jumps between Nodes of Ranvier.
• Action potential only generated at the Nodes of Ranvier.

Types of Stimuli

• Threshold Stimulus: Strong enough to bring the membrane potential to a threshold voltage, causing an action potential.
• Subthreshold Stimulus: Weak stimuli that cause depolarization (graded potentials), but not action potentials.

Coding for Stimulus Intensity

• Action potentials are all-or-none; strong stimuli generate action potentials more often than weaker ones.
• The CNS determines stimulus intensity by the frequency of impulse transmission.

Absolute Refractory Period

• Time from Na+ activation gates opening to inactivation gates closing.
• Prevents the neuron from generating another action potential ensuring separate action potentials.
• Enforces one-way transmission of nerve impulses.

Relative Refractory Period

• Interval following the absolute refractory period; Sodium gates are closed, potassium gates are open, and repolarization is occurring.

Synapse

• The means by which adjacent neurons communicate.
• Most synapses occur between the axon of one neuron and the dendrites of another (axodendritic) or between the axon of one neuron and the cell body of another (axosomatic).
• The presynaptic neuron sends information, and the postsynaptic neuron receives it.
• Neurons may have 1000 to 10000 axonal terminals making synapses.

Grey and White Matter

• Grey Matter: Consists of unsheathed nerve fibers, cannot be regenerated if damaged (in cortex or surface layer).
• White Matter: Makes up the internal structures and consists of myelinated nerve fibers. The brain and spinal cord receive impulses, process the information, and respond with the appropriate action.

Electrical Synapses

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

Chemical Synapses

• Specialized for the release and reception of neurotransmitters.
• Composed of presynaptic and postsynaptic neurons.
• Presynaptic Neuron: Contains synaptic vesicles, which release neurotransmitters across the synaptic cleft to the postsynaptic neuron.
• Postsynaptic Neuron: Receptors are located on dendrites and soma.

Chemical Synapses: Other Features

• Synaptic cleft: Fluid-filled space separating the neurons; transmission is a chemical event, unlike the electrical one.
• Synaptic cleft: Information Transfer: Nerve impulses reach the axon terminal of the presynaptic neuron, open Ca2+ channels. Neurotransmitter is released into the synaptic cleft via exocytosis. Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron; postsynaptic membrane permeability changes, causing excitatory or inhibitory effect.

Termination of Neurotransmitter Effects

• Neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect, blocking reception of additional messages. Must be removed from its receptor.

Postsynaptic Potentials

• Neurotransmitter receptors mediate changes in membrane potential according to the amounts of neurotransmitter released and the time the neurotransmitter is bound to receptors.
• Types:
  • EPSPs: Excitatory postsynaptic potentials. Cause depolarization, increasing likelihood of action potential.
  • IPSPs: Inhibitory postsynaptic potentials. Cause hyperpolarization, reducing likelihood of action potential.

Terminology for Quiz

• Neuron: nerve cell
• Neuroglia: supporting cell
• Nerve fiber: long axon
• Nerve: collection of axons in PNS
• Tract: collection of 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
• CNS: Central Nervous System
• PNS: Peripheral Nervous System
• Input/sensory: afferent; to brain
• Output/motor: efferent; from brain

Examine Yourself

• Neurons: inside/outside CNS
• Structures concerned with CSF formation: arachnoid villi, choroid plexus, subdural space, dural venous sinus
• Peripheral nervous system involves: spinal ganglia, spinal cord, brain, tracts
• Lateral ventricle lies in: cerebrum, diencephalon, midbrain, cerebellum

References

• Information was compiled from multiple referenced sources; it is impossible to identify all sources.