Organization of the Nervous System + Cells

What are the three main functions of the nervous system?

Sensory input; integration; and motor output

Sensory input

Detecting stimuli both externally and internally

Integration

Interpretation of sensory input and deciding how to proceed

Motor output

Response to stimuli by activating effector organs

What type of signals does the nervous system use to communicate?

Electrical and chemical signals - fast; efficient; and specific

Central Nervous System (CNS)

Integration/command center made up of the brain and spinal cord

Peripheral Nervous System (PNS)

Everything outside the CNS; carries messages to and from the brain and spinal cord via spinal and cranial nerves

What makes up the PNS?

Spinal nerves and cranial nerves

What are the two functional subdivisions of the PNS?

Sensory (afferent) and motor (efferent)

Afferent

"To" the CNS; sensory

Efferent

"From" the CNS to effector organs; motor

Sensory afferent fibers

Carry impulses from skin; skeletal muscle; and joints to the brain

Visceral afferent fibers

Carry impulses from visceral organs to the brain

Motor (efferent) division

Carries impulses from the CNS to the effector organs

What are the two parts of the PNS motor division?

Somatic nervous system and autonomic nervous system

Somatic nervous system

Conscious (voluntary) control of skeletal muscle

Autonomic nervous system

Involuntary regulation of smooth muscle; cardiac muscle; and glands

What are the two divisions of the autonomic nervous system?

Sympathetic and parasympathetic nervous systems

Neurons

Excitable cells that transmit electrochemical signals and release neurotransmitters; the structural and functional unit of the nervous system; amitotic

What does amitotic mean for neurons?

Mature neurons do not divide

Neuroglia (supporting cells)

Cells that surround and wrap neurons and provide nourishment

Name the supporting cells of the CNS

Astrocytes; ependymal cells; microglia; and oligodendrocytes

Name the supporting cells of the PNS

Satellite cells and Schwann cells

Where are astrocytes found?

CNS

Where are ependymal cells found?

CNS

Where are microglia found?

CNS

Where are oligodendrocytes found?

CNS

Where are satellite cells found?

PNS

Where are Schwann cells found?

PNS

Astrocytes

Most abundant and versatile glial cells; highly branched; found in the CNS

How do astrocytes support neurons metabolically?

They anchor neurons to nutrient supplies; take up glucose from capillaries; and metabolize glucose for neurons to make ATP

Astrocyte role in development and maintenance

Synapse formation; neurogenesis; proper neuronal formation; nurturing; and maintenance (e.g. GDNF)

GDNF

Glial-derived neurotrophic factor; released by astrocytes to support neurons

What are gliotransmitters?

Chemicals released by astrocytes to help regulate neuronal function

Name examples of gliotransmitters

Glutamate (excitatory); ATP (inhibitory); adenosine; and D-serine

Why do astrocytes scavenge extracellular K+?

To buffer K+ released during action potentials and keep the neuronal environment stable

Astrocyte foot processes

Extensions that wrap capillaries and contribute to the blood-brain barrier

Blood-brain barrier (BBB)

Barrier that separates blood-borne substances from neurons and protects the brain

What are the components of the BBB?

Continuous endothelium with tight junctions; thick basal lamina; and astrocyte foot processes

Why are brain capillaries not leaky?

Tight junctions prevent solute movement between endothelial cells

Which substances can cross the BBB by diffusion?

Extremely lipophilic substances; gases (O2 and CO2); and ethanol (EtOH)

Which substances need transporters to cross the BBB?

Ions; polar molecules; and glucose (via GLUT1)

What transporter moves glucose across the BBB?

GLUT1

Where is the BBB absent?

Areas that monitor blood chemical composition such as the vomiting centers and the hypothalamus

Why is the BBB significant?

It protects neurons from harmful substances in the blood

Exam: All are functions of astrocytes EXCEPT?

Defending against microorganisms; releasing inflammatory mediators; and phagocytizing debris (this is microglia)

Ependymal cells

Epithelial cells lining the brain ventricles and the central canal of the spinal cord

What is the function of ependymal cells?

Form a semipermeable barrier between cerebrospinal fluid (CSF) and nervous tissue

Microglia

Immune cells of the CNS ("brain macrophages")

What do microglia do?

Monitor neuron health and phagocytize microorganisms and cellular debris in the CNS

What happens when microglia are overactive?

They release free radicals causing oxidative stress

Oligodendrocytes

Branched CNS glial cells that wrap multiple axons and form myelin sheaths around CNS axons

Myelin sheath

Whitish; fatty (lipid-protein); segmented sheath around larger axons

What is myelin made of?

Phospholipid-rich lipid-protein insulating covering

What are the functions of the myelin sheath?

Protection; electrical insulation; and increased speed of nerve impulse transmission

Are myelinated axons faster than unmyelinated axons?

Yes - myelination speeds up nerve conduction

Which cells form myelin in the CNS?

Oligodendrocytes

Which cells form myelin in the PNS?

Schwann cells

Schwann cells (neurolemmocytes)

PNS glial cells that form myelin sheaths around PNS axons

Myelination in the PNS

Wrapping of the Schwann cell membrane around an axon

Neurilemma

The Schwann cell covering that surrounds all PNS axons

Satellite cells (ganglionic gliocytes)

"Astrocytes of the PNS"; provide nutrient support around PNS ganglia

Neuron major function

Electrical signaling and cell-cell communication

Neuron characteristics

Long-lived; amitotic; and high metabolic rate

Perikaryon (cell body/soma)

Major biosynthetic center of the neuron that contains the nucleus

Why do neurons lack centrioles?

Neurons are amitotic so they do not need centrioles; but they have all other organelles

Golgi apparatus in neurons

Well developed and completely encircles the nucleus

Neurofibrils

Bundles of intermediate filaments that maintain neuronal shape

Nissl bodies

Rough ER found only in the cell body (not in axons)

Axon hillock

Cone-shaped area of the perikaryon from which the axon arises

Nuclei vs. ganglia

Nuclei = clusters of neuron cell bodies in the CNS; ganglia = clusters of neuron cell bodies in the PNS

Tracts

Bundles of axons (neuron fibers) in the CNS

Nerves

Bundles of axons (neuron fibers) in the PNS

Exam: Collections of nerve cell bodies in the CNS are called?

Nuclei

Dendrites

Receptive areas that transmit electrical impulses toward the perikaryon

Why do dendrites have so much surface area?

To receive signals from many other neurons

Are dendrites myelinated?

No - dendrites are always unmyelinated

Axons

Transmit nerve impulses away from the cell body and secrete neurotransmitters from the axon terminals

Terminal bouton

Axon terminal where neurotransmitters are secreted

Anterograde transport

Movement toward the terminal bouton

Retrograde transport

Movement away from the terminal bouton (toward the cell body)

Functional classes of neurons

Sensory (afferent); interneurons (association); and motor (efferent)

Sensory (afferent) neurons

Transmit impulses toward the CNS; bipolar and unipolar neurons

Interneurons (association neurons)

Shuttle signals through CNS pathways; multipolar neurons

Motor (efferent) neurons

Carry impulses away from the CNS; multipolar neurons

Which neuron structural types are sensory neurons?

Bipolar and unipolar

Which neuron structural type are interneurons and motor neurons?

Multipolar

Somatic motor neurons

Control skeletal muscles and voluntary movements

Autonomic motor neurons

Control involuntary functions of smooth muscle; cardiac muscle; and glands

Nodes of Ranvier

Gaps in the myelin sheath between adjacent Schwann cells at regular intervals; contain voltage-regulated Na+ channels

What is the function of the nodes of Ranvier?

Allow fast nerve conduction

Saltatory conduction

Impulse "jumps" from node of Ranvier to node along a myelinated axon allowing rapid conduction

Why is neuron damage so serious?

Mature neurons are amitotic and cannot divide to replace themselves

When can a damaged neuron be repaired?

If the perikaryon remains intact (often limited to the PNS)

What do Schwann cells of the regeneration tube secrete?

Neurotrophic factors that guide the axon to re-establish proper connections

Name the neurotrophic factors involved in nerve regeneration

Nerve growth factor (NGF); brain-derived neurotrophic factor (BDNF); glial cell-line derived neurotrophic factor (GDNF); and neurotrophin-3

NGF

Nerve growth factor

BDNF

Brain-derived neurotrophic factor