Cell organization of the Nervous System

Nervous System (NS) Overview

The Master Controller of the body, responsible for communication

What kinds of signals does the NS use?

Electrical and Chemical Signals

Why does the NS use Electrical and Chemical signalling?

They are fast, efficient, specific

What are the functions of the NS?

Sensory input → Integration → Motor Output

What kinds of Sensory input does the NS process?

both external and internal

What is integration?

The interpretation of sensory input, and making decisions as to how to proceed

Why does integration matter?

It is higher order processing, making decisions about stimuli that differs us from primates

What is Motor Output?

Response to stimuli by activating effector organs

What are the divisions of the NS?

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

What is the CNS responsible for?

Integration/commands

What is in the CNS?

The Brain and the Spinal Cord

What is the PNS?

The NS external to the brain and the spinal cord (outside the CNS)

What is the function of the PNS?

Messages from CNS to smooth muscles, cardiac muscles, or glands

What nerves are involved in the PNS?

Spinal and Cranial Nerves

What is the difference between the Spinal Cord and Spinal Nerves?

Spinal cord carries signals to the brain directly from the spine.

Spinal nerves carry signals from the PNS to the spine.

What are the functional subdivisions of the PNS?

Sensory (afferent) and Motor (efferent) neurons

Sensory Afferent Fibers

Nerves that send signals from the muscles or joints to the brain

Motor Efferent Nerves

Nerves that send signals from the CNS to the effector organs

What are the two main parts of the PNS?

Somatic Nervous System (SNS)

Autonomic Nervous System (ANS)

What does the SNS regulate?

Conscious control of skeletal muscle (voluntary)

What does the ANS regulate?

Smooth muscle, cardiac muscle, and glands (involuntary)

What are the divisions of the ANS?

Sympathetic Nervous System (SyNS)

Parasympathetic Nervous System (PSNS)

Neurons

Excitable cells that transmit electrochemical signals and release transmitters.

• Structural/functional unit of the NS, amitotic

Supporting Cells’ overall purpose

Surround, wrap neurons, and provide nourishment.

• Neuroglia

Supporting Cell Types

• Astrocytes,

• Ependymal cells,

• Microglia,

• Oligodendrocytes,

• Satellite cells,

• 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

Characteristics of Astrocytes

Most abundant, versatile, highly branched

What do Astrocytes do?

• Anchor neurons to their nutrient supplies; and take up glucose from capillaries.

• Metabolize glucose for neurons to make ATP.

• Synapse formation, neurogenesis, proper neuronal formation, nurturing, and maintenance.

• Release gliotransmitters to help regulate neuronal function. Scavenge K+ from the extracellular environment.

Blood brain barrier function

Separates blood-borne substances from neurons

What are the layers of the BBB?

• Continuous endothelium with tight junctions.

• Thick basal laminar membrane.

• Astrocyte foot processes

What is the BBB ineffective against?

• Substances that can diffuse through plasma membranes.

• Extremely lipophilic substances. Gases (O2 and CO2). EtOH.

• Absent in areas that monitor chemical composition of blood (vomiting centers, hypothalamus)

What are substances that require special transporters to move across the BBB?

Ions, Polar molecules, Glucose

All of the following are functions of astrocytes except

Defending against invading microorganisms, releasing inflammatory mediators, and phagocytizing dying cell debris

What are ependymal cells?

Epithelial cells that line the central brain cavities (ventricles) and line the spinal column central canal.

• They form a semipermeable barrier between the cerebrospinal fluid (CSF) and nervous tissue

What are microglia?

Immune cells of the brain, brain macrophages.

What do Microglia do?

• Monitor health of neurons,

• phagocytize microorganisms/pathogens and cellular debris within the CNS.

What are the dangers of excessive or overactive microglia?

They can release free radicals that can lead to oxidative stress and damage to other cells

What are Oligodendrocytes?

Branched cells that wrap multiple axons within the CNS.

• CNS myelin sheath

What do Myelin Sheaths do?

They are phospholipid-rich insulated coverings around axons. They assist in:

• Protection.

• Electrical insulation.

• Increase speed of nerve impulse.

What is the myelin sheath?

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

What are the Neuroglia in the PNS?

Schwann cells

Satellite cells

What are Schwann cells?

They form the PNS myelin sheaths (myelination to form neurilemma)

What do Satellite cells (ganglionic gliocytes) do?

Astrocytes of the PNS.

• Provide nutrient support around PNS ganglia

Neuron anatomy

Structural unit of the NS.

• Long-lived, amitotic, high-metabolic rate.

• Does the electrical signaling between cells for communication.

Perikaryon

Major biosynthetic center; contains the cell nucleus

What makes Neurons different from other cells?

They are amitotic and possess no centrioles but have all other organelles such as the Golgi Apparatus

Parts of the perikaryon

• Mitochondria scattered throughout.

• Neurofibrils.

• Nissl bodies.

• Axon hillock.

Neurofibrils

Bundles of intermediate filaments that maintain neuronal shape

Nissl bodies

Rough ER only found in cell body

Axon hillock

Cone-shaped area from which axons arise

What do Dendrites look like?

Arm-like extensions from the perikaryon

Dendrite function

Receptive areas that transmit electrical impulses to the perikaryon.

• Provide a very high surface area to receive signals from other neurons. Always unmyelinated

Tracts

bundles of neurons in CNS

Nerves

bundles of neurons in the PNS

Axon function

• Transmit nerve impulses.

• Secrete neurotransmitters from the axonal terminal (terminal bouton).

What is the direction of movement in an Axon

Axons are bidirectional, being both anterograde (toward the terminal bouton) and retrograde (away from the terminal bouton)

Collections of nerve cell bodies located within the CNS are called

Nuclei

Collections of nerve cell bodies located within the PNS are called

Ganglia

Neuron Classifications

• Sensory (afferent),

• Interneurons (association neurons),

• Motor (efferent)

Sensory neurons function

transmits impulses toward the CNS (bipolar and unipolar neurons)

Interneurons function

Shuttle signals through CNS pathways (multipolar neurons)

Motor neurons function

carry impulses away from the CNS (multipolar neurons)

Somatic Motor Neurons

Controls skeletal muscles and voluntary movements

Autonomic Motor Neurons

Control involuntary functions performed by smooth and cardiac muscle, and glands

Nodes of Ranvier

Gaps in the myelin sheath (which contain voltage regulated Na+ channels) between adjacent Schwann cells, which occur at regular intervals

Nodes of Ranvier function

Allow for fast nerve conduction via saltatory conduction

Why is nerve regeneration important?

Damage to neurons is significant because mature neurons are amitotic

What happens to a damaged nerve cell if the perikaryon remains intact?

repair is still possible (many times limited to only PNS)

How do Schwann cells contribute to nerve regeneration?

Schwann cells of the regeneration tube secrete neurotrophic factors

How do axons re-establish the proper connection?

Nerve growth factor. Brain-derived neurotrophic factor. Glial cell-time derived neurotrophic factor. Neurotrophin-3

What are the steps of nerve regeneration during axon damage?

1. Distal portion of nerve fiber from the site of injury degenerates and is phagocytized.

2. Proximal end of injured nerve fiber renegerates into tube of Schwann cells.

3. Growth occurs.

4. Former connection re-established