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Fundamentals of the Nervous System and Nervous Tissue

Part 1

Nervous System & Tissue

I. General Functions of the Nervous System II. Organization of the Nervous System III. Nervous Tissue IV. Nerves V. Basic Neuronal Organization VI. Disorders

I. General Functions

• A. Control & Communication Center
• 1. Control through creation & propagation of

electrical impulses (action potentials)

• a. Fast acting & Specific
• 2. Control and communication through

inhibition or excitation at neuron junctions (synapses, neuromuscular junctions . . .)

• a. Determines what happens once action potential

arrives at a junction

• 3. Operates with the endocrine system

I. General Functions

• B. Sensory Function
• 1. Sensory receptors monitor changes inside

and outside the body

• a. The changing condition is the stimulus
• b. The stimulus can ultimately generates an action

potential which is the sensory input

• C. Integrative Function
• 1. Processes and interprets sensory input
• a. Makes decisions – integration
• D. Responsive Function
• 1. Dictates a response by activating effector
• rgans
• a. Response – motor output

II. Organization of the Nervous System

• A. Nervous system divided into two systems:
• 1. Central nervous system (CNS)
• Brain and spinal cord
• Integrating and command center
• 2. Peripheral nervous system (PNS)
• Outside the CNS
• Consists of nerves extending from (and going to)

brain and spinal cord

– Cranial nerves – Spinal nerves

• Peripheral nerves link all regions of the body to the

CNS

II. Organization of the Nervous System

Central Nervous System Brain Spinal Cord Peripheral Nerves Peripheral Nerves Peripheral Nerves Peripheral Nervous System

II. Organization of the Nervous System

• B. Peripheral Nervous System consists of two

major functional aspects

• 1. Sensory (afferent) signals picked up by sensor

receptors

• Carried by nerve fibers of PNS to the CNS
• 2. Motor (efferent) signals are carried away from

the CNS

• Innervate muscles and glands

II. Organization of the Nervous System

• C. Sensory and Motor aspects are divided according

to region they serve

– Somatic (wall of body) – Visceral (guts)

• D. This regional division creates four main

subdivisions:

• 1. Somatic Sensory
• 2. Somatic Motor
• 3. Visceral Sensory
• 4. Visceral Motor

II. Organization of the Nervous System

1. Somatic Senses

a. General somatic senses

• Sensory receptors have a wide dispersion, and include
• Touch, pain, vibration, pressure, and temperature

b. Proprioceptive senses – (sense of body positions)

• Afferent information regarding position and movement of

body in space, due to receptors in tendons, muscles and joints

c. Special somatic senses

• Special due to the presence of a sensory “organ” or a

clustered group of sensory receptors, rather than sensory cells widely dispersed as in the general somatic senses

• Hearing, balance, vision, and smell

II. Organization of the Nervous System

• 2. Somatic motor neurons
• a. General somatic motor – signals contraction of

skeletal muscles

• Under our voluntary control and as such may also

referred to as the “voluntary nervous system”

• b. Branchial motor
• Typical skeletal muscle derived from somitomeres

– Mastication muscles control – Facial expression muscle control – Pharyngeal & laryngeal muscle control – Sternocleidomastoid & Trapezius muscle control

II. Organization of the Nervous System

• 3. Visceral Senses
• a. General visceral senses
• stretch, pain (generally referred to the body wall),

temperature, nausea, and hunger

• Widely felt in digestive and urinary tracts,

reproductive organs

• b. Special visceral senses
• Taste and smell have a visceral afferent component,

involving cranial nerves VII, IX & X (facial, glossopharyngeal & vagus nerve)

II. Organization of the Nervous System

• 4. Visceral Motor Neurons

– Makes up autonomic nervous system

• Further divided into Sympathetic &

Parasympathetic divisions.

– Regulates the contraction of smooth and cardiac muscle & glandular secretion – Controls function of visceral organs – Also called “involuntary nervous system”, as it performs without conscious input.

II. Organization of the Nervous System

Sensory (afferent) Division Motor (efferent) Division

Somatic Sensory Visceral sensory

PNS

Somatic Motor Visceral Motor General Somatic Senses Special Somatic Senses General Visceral Senses Special Visceral Senses Sympathetic division of ANS Parasympathetic division of ANS

CNS

Brain Spinal Cord Propri-

• ceptive

Senses

III. Nervous Tissue

• A. Characteristics of nervous tissue

– Cells are densely packed and intertwined – Two main cell types make up nervous tissue

• Neurons

– Excitable

• Neuroglia (supportive cells)

– Nonexcitable – More on these later…

III. Nervous Tissue

• B. General Information on Neurons
• 1. Billions of neurons create the basic functional

structure unit of the nervous system

• 2. Neurons are specialized cells that conduct electrical

impulses (action potentials or nerve impulses) along the plasma membrane

• 3. Longevity – can live and function for a lifetime
• 4. Do not divide – fetal neurons lose their ability to

undergo mitosis; neural stem cells are an exception

• 5. High metabolic rate – require abundant oxygen and

glucose

–

Can account for approx 10% of your metabolic rate . . . When you are thinking

III. Nervous Tissue

C. General Structure of a Neuron consists of (1) a cell body and (2) neuronal processes (dendrites & axons

[axon collaterals, axon terminals & synapses]) 1. Cell body (perikaryon or soma)

• Size varies from 5–140µm
• Contains a normal complement of organelles
• Also contains chromatophilic (Nissl) bodies

– Basically clumps of RER and free ribosomes that “love stain” (namesake) that renew membranes and protein portion of cytosol

• Neurofibrils – bundles of intermediate filaments that form a

network between chromatophilic bodies and provides structural integrity to the soma

Structure of a Typical Large Neuron

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III. Nervous Tissue

• C. General Cell body information cont.
• 2. Location of neuronal cell bodies
• a. Most neuronal cell bodies are located within the

CNS

– Protected by bones of the skull and vertebral column

• b. Some neuronal cell bodies form ganglia

– clusters of cell bodies, axon terminals and dendrites

• utside of the CNS (i.e. part of the PNS)

III. Nervous Tissue

• C. General Structure
• 2. Neuronal Processes – extensions of cell body

membrane forming

• Dendrites
• Axons

• 2. Neuron Processes

a. Dendrites

– Extensively branching from the soma – Transmit electrical signals (graded potentials) toward the cell body

• As graded potentials, they may be affected by other nearby

synaptic events

– Increasing state of excitation (further depolarization of membrane) – Decreasing state of excitation (inhibition – further from a threshold event)

– Contain chromatophilic bodies, but only extend into the basal part of dendrites – Function as receptive sites

• Lots of surface area for this due to the large amount of

branching exhibited by dendrites

• 2. Neuron Processes

b. Axons

1. Neuron has only one

• Exception – anaxionic neuron

2. Impulse generator and conductor

• Generated at axon hillock
• Conducted along the length of the axon

1. Transmits impulses away from the cell body 2. Chromatophilic bodies are absent 3. No protein synthesis in axon 4. Neurofilaments, actin microfilaments, and microtubules

• Provide strength along length of axon
• Aid in the transport of substances to and from the cell body

– Axonal transport or flow – needed as there is no protein synthesis in axon

• 2. Neuron Processes
• b. Axons – cont.
• 5. May have branches along length
• Axon collaterals

– Allow for divergence of action potential – Allows for widespread effect

• Multiple branches at end of axon

– Terminal branches (telodendria) » End in knobs called axon terminals (also called end bulbs or boutons) » Forms a synapse with another nueron, or a neuromuscular junction if a muscle. . .

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Neuron Structure

III. Nervous Tissue

D. Action Potentials (nerve impulses)

– Generated at the axon hillock

• If the membrane reaches threshold potential due to the net

effects of synapse activity on the dendrites & soma

– Conducted along the axon – Releases neurotransmitters at axon terminals – Neurotransmitters – excite or inhibit the post‐ synaptic membrane

• Depending on the action of the neurotransmitter receptor
• n the post‐synaptic membrane

– Neuron receives and sends signals

• Receives on the dendrites and soma
• Sends down the axon, axon collaterals

III. Nervous Tissue

• E. Synapses

– Site at which neurons communicate – Signals pass across synapse in one direction – Presynaptic neuron

• Conducts signal toward a synapse

– Postsynaptic neuron

• Transmits electrical activity away from a synapse

– Only if the effects of the neurotransmitter from the presynpatic neuron are excitatory

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Synapse

Two Neurons Communicating at a Synapse

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III. Nervous Tissue

• E. Synapses cont.

– Elaborate cell junctions – Axodendritic synapses – representative type – Synaptic vesicles on presynaptic side

• Membrane‐bound sacs containing

neurotransmitters

• Mitochondria abundant in axon terminals

– Synaptic cleft separates the plasma membrane

• f the two neurons

Structure of a Synapses

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III. Nervous Tissue

• F. Synapse Types:
• 1. Axodendritic
• Between axon terminals of one neuron and

dendrites of another

• Most common type of synapse
• 2. Axosomatic
• Between axons and neuronal cell bodies
• 3. Axoaxonic, dendrodendritic, and

dendrosomatic

• Uncommon types of synapses

Some Important Types of Synapses

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G. Structural classification of Neurons

1. Multipolar

• possess more than two processes
• Numerous dendrites and one axon

2. Bipolar

• possess two processes
• Rare neurons – found in some special sensory organs

3. Unipolar (aka pseudounipolar)

• possess one short, single process
• Start as bipolar neurons during development

4. Anaxonic

• No identifiable axon off of soma
• Used within the CNS and some sensory organs
• Identified as possibly altering the communication (via gap

junctions & synapses) among sensory neurons in the olfactory bulb.

• III. Nervous Tissue

Neurons Classified by Structure

• H. Functional Classification of Neurons

– According to the direction the nerve impulse travels (towards or away from CNS)

• 1. Sensory (afferent) neurons –

transmit impulses toward the CNS

• Virtually all are unipolar neurons
• Cell bodies in ganglia outside the CNS

– Short, single process divides into

– The central process – runs centrally into the CNS – The peripheral process – extends peripherally to the receptors

• III. Nervous Tissue

H. Functional Classification of Neurons cont.

2. Motor (efferent) neurons

• Carry impulses away from the CNS to effector organs
• Most motor neurons are multipolar
• Cell bodies are within the CNS
• Form junctions with effector cells

3. Interneurons– most are multipolar (some are

anaxonic)

• Lie between motor and sensory neurons
• Confined to the CNS may be

– in the cerebellum (Purkinje cells, stellate cells, granule cells, and basket cells) – located in the cerebral cortex and hippocampus are the pyramidal cells, also called projection neurons

• III. Nervous Tissue

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Anatomy Review: The Nervous System

• H. Functional Classification of Neurons

III. Nervous Tissue

I. Six types of supporting cells (neuroglial cells)

1. Four in the CNS 2. Two in the PNS

–

Provide supportive functions for neurons

–

Cover nonsynaptic regions of the neurons

–

Neuroglia characteristics

• usually only refers to supporting cells in the CNS
• Glial cells have branching processes and a central cell

body

• Outnumber neurons 10 to 1
• Make up half the mass of the brain
• Can divide throughout life . . . cancer impact?

• 1. Supporting Cells in the CNS

Types of neuroglial cells

a. Astrocytes – most abundant glial cell type

• Take up and release ions to control the environment around

neuron

• Form networks around capillaries, creating the blood‐brain‐

barrier

• Recapture and recycle neurotransmitters
• Involved with synapse formation in developing neural tissue
• Produce molecules necessary for neural growth (BDTF) &

promote myelination

• Propagate calcium signals that may be involved in memory
• May modulate synaptic activity

Astrocytes are star-shaped glial cells of the CNS that have long processes. Many of these processes extend to blood vessels where they expand and cover much of the external wall. The expanded endings of the astrocyte processes are known as end-feet. While the blood-brain-barrier is formed by tight junctions between endothelial cells, the end-feet function to induce and maintain the blood-brain barrier. In pathology following stroke the relationship of end-feet to the endothelial cells is altered leading to disruption of the blood-brain barrier and subsequent leakage.

• 1. Supporting Cells in the CNS
• b. Microglia – smallest and least abundant
• Phagocytes –

the macrophages

• f the CNS
• Engulf invading microorganisms and dead neurons
• Derive from blood cells called monocytes

Microglia are a type of glial cell that act as the immune cells of the Central nervous system (CNS). Microglia, the smallest of the glial cells, can act as phagocytes, cleaning up CNS debris. Most serve as representatives of the immune system in the brain and spinal cord, inhabiting the cerebrospinal fluid. Microglia are close cousins of other phagocytic cells including macrophages and dendritic cells. Microglia are derived from myeloid progenitor cells (as are macrophages and dendritic cells) which come from the bone marrow. During embryonic development, however, they migrate to the CNS to differentiate into microglia. Microglia are thought to be highly mobile cells that play numerous important roles in protecting the nervous system. They are also thought to play a role in neurodegenerative disorders such as Alzheimer's disease, dementia, multiple sclerosis and Amyotrophic lateral sclerosis. Microglia are responsible for producing an inflammatory reaction to insults (Streit et al., 2004).

• 1. Supporting Cells in the CNS

c. Ependymal cells

• Line the central cavity of the spinal cord and brain
• Bear cilia – help circulate the cerebrospinal fluid

Ependymal cells are the epithelial cells that line the CSF filled ventricles in the brain and the central canal of the spinal cord. The cells are cuboidal/columnar. They are not the cells that produce the CSF. Their apical surfaces are covered in a layer of cilia, which constantly beat to circulate cerebrospinal fluid around the central nervous system.

Section of central canal of medulla spinalis, showing ependymal and neuroglial cells.

• 1. Supporting Cells in the CNS
• d. Oligodendrocytes – have few branches
• Wrap their cell processes around axons in CNS

– Produce myelin sheaths

Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),[1] are a variety of neuroglia. Their main function is the myelination of nerve cells exclusively in the central nervous system of the higher vertebrates, a function performed by Schwann cells in the peripheral nervous

• system. A single oligodendrocyte can extend to up to 50 axons, wrapping around approximately 1 mm of each and

forming the myelin sheath. Oligodendrocytes, as well as other macroglial cells (astrocytes and ependymal cells), are derived from neuroectoderm. The nervous system of mammals depends crucially on this sheath for insulation as it results in decreased ion leakage and lower capacitance of the cell membrane. There is also an overall increase in impulse speed as saltatory propagation of action potentials occurs at the nodes of Ranvier in between Schwann cells (of the PNS) and oligodendrocytes (of the CNS); furthermore miniaturization occurs, whereby impulse speed of myelinated axons increases linearly with the axon diameter, whereas the impulse speed of unmyelinated cells increases only with the square root of the diameter.

Axon

Myelin sheath

Axon

Neurolemmoctye

Axon

• 2. Supporting Cells in the PNS
• a. Satellite cells – surround neuron cell

bodies within ganglia

Satellite cells are small cells that line the exterior surface of PNS neurons and help regulate the external chemical environment. Satellite cell nucleus nucleus Satellite cell membrane Neuron cell membrane

• 2. Supporting Cells in the PNS
• b. Schwann cells (neurolemmocytes) –

surround axons in the PNS

– Form myelin sheaths around axons of the PNS – Aid in neural regeneration of peripheral nerves

Myelin Sheaths in the PNS

• Formed by Schwann cells (neurolemmocytes)
• Develop during fetal period and in the first year of postnatal life
• Schwann cells wrap in concentric layers of the lipoprotein myelin

around the axon

– Cover the axon in a tightly packed coil of membranes

• Neurilemma – material external to myelin layers
• Nodes of Ranvier – gaps along axon (also called neurofibril node)
• Thick axons are myelinated
• Thin axons are unmyelinated

• IV. Nerves
• A. General Nerve Information

– cordlike organs in the PNS – Consists of numerous axons wrapped in connective tissue – Axon is surrounded by Schwann cells

• IV. Nerves
• B. Structure of Nerves
• 1. Endoneurium – layer of delicate connective

tissue surrounding the axon

• 2. Nerve fascicles – groups of axons bound into

bundles

• 3. Perineurium – connective tissue wrapping

surrounding a nerve fascicle

• 4. Epineurium – whole nerve is surrounded by

tough fibrous sheath

radial nerve

Structure of a Nerve

• IV. Nerves
• C. Functional Aspect of Nerves

– Nerves are typically classified as

• Motor
• Sensory
• Mixed
• Along its length a nerve may be all three

i.e. leaving the ventral root of the spinal cord a nerve would be motor, then joins with the incoming afferent nerve to create a mixed nerve, and then at a ramus branches and becomes alone in its motor function.

• V. Basic Neuronal Organization

A. General plan is . . .

receptor Sensory neuron Interneuron (Integration Center) Motor neuron effector Arrows represent flow of information

A reflex pathway (arc) to maintain homeostasis!

• V. Basic Neuronal Organization
• B. Types of Reflex Arcs
• 1. Monosynaptic
• The simplest, involve
• nly a sensory

neuron and a motor neuron ‐‐‐ no integration with interneurons

–

• Ex. Stretch reflex

• V. Basic Neuronal Organization
• B. Types of Reflex Arcs
• 2. Polysnaptic
• a. Multiple neurons, may

involve multiple segments of spinal nerves, and higher processing by the brain

–

• Ex. Withdrawal reflex,

crossed extensor reflex …

• V. Basic Neuronal Organization
• C. Neuronal Circuits &

Processing Patterns

• 1. Neuronal Circuits –

typically occur when

• ne neuron is affected

by or controls more than one other neuron

• a. Divergent
• b. Convergent

c. Reverberating (oscillating)

• V. Basic Neuronal Organization
• C. Neuronal Circuits &

Processing Patterns

• A. Processing Patterns
• A. Serial
• B. Parallel

• V. Basic Neuronal Organization

D. Types of Matter

1. Gray

a. Darker in color b. Contains dendrites, cell bodies, and axon terminals c. Site of actual integration

–

• Ex. Cerebral and cerebellar

cortices

2. White

a. Lighter in color due to myelination b. Forms tracts (descending motor & ascending sensory) c. Forms commissural, sensory, motor & associative tracts

– routes of communication between processing areas

• VI. Disorders of the Nervous System
• Demyelination Issues

– Multiple sclerosis – common cause of neural disability

• Varies widely in intensity among those affected
• Cause is not completely understood
• An autoimmune disease

– Immune system attacks the myelin around axons in the CNS

– Heavy metal poisoning –

• messes with development processes
• Interferes with ER form and function
• Causes brain damage . . .

– Guillain‐Barré syndrome

• Possibly due to bacterial or viral infection, leading to…
• autoimmune issues– Ab’s attack the myelin sheath causing inflammation
• f peripheral nerves
• No hereditary issues
• Rabies

– Viral disease of CNS after virus is transported along the axon from neurons at the region of the bite (axonal flow)

Additional Supplemental Information

Pyramidal cells Apical dendrite pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron located in the hippocampus and cerebral

• cortex. These cells have a triangularly shaped

soma, or cell body, a single apical dendrite extending towards the pial surface, multiple basal dendrites, and a single axon. Pyramidal neurons compose approximately 80% of the neurons of the cortex, and release glutamate as their neurotransmitter, making them the major excitatory component of the cortex (see synapse). In the primary motor cortex, layer V pyramidal cells are extremely large. These cells are called Betz cells. Their cell bodies can be as large as 100 micrometers in

• humans. Typical human pyramidal cell

bodies range from 10 to 50 micrometers.

Microcircuitry of the

• cerebellum. Excitatory

synapses are denoted by (+) and inhibitory synapses by (-). MF: Mossy fiber. DCN: Deep cerebellar nuclei. IO: Inferior olive. CF: Climbing fiber. GC: Granule cell. PF: Parallel fiber. PC: Purkinje cell. GgC: Golgi cell. SC: Stellate cell. BC: Basket cell.

These cells are some of the largest neurons in the human brain, with an intricately elaborate dendritic arbor, characterized by a large number of dendritic spines. Purkinje cells are found within the Purkinje layer in the

• cerebellum. Purkinje cells are aligned

like dominos stacked one in front of the other. Their large dendritic arbors form nearly two dimensional layers through which parallel fibers from the deeper-layer granule cells pass. These parallel fibers make relatively weaker excitatory (glutamatergic) synapses to spines in the Purkinje cell dendrite, whereas climbing fibers originating from the inferior olivary nucleus in the medulla provide very powerful excitatory input to the proximal dendrites and cell soma. Parallel fibers pass orthogonally through the Purkinje neuron's dendritic arbor, with up to 200,000 parallel fibers forming a synapse with a single Purkinje cell. Alternatively, each Purkinje cell only receives a synapse from a single climbing fiber. Both basket and stellate cells (found in the cerebellar molecular layer) provide inhibitory (GABAergic) input to the Purkinje cell, with basket cells synapsing on the Purkinje cell body and stellate cells onto the dendrites. Purkinje cells send inhibitory projections to the deep cerebellar nuclei, and constitute the sole output of all motor coordination in the cerebellar cortex.

Neuronal Regeneration

• Neural injuries may cause permanent

dysfunction

• If axons alone are destroyed, cells bodies
• ften survive and the axons may regenerate

– PNS – macrophages invade and destroy axon distal to the injury

• Axon filaments grow peripherally from injured site
• Partial recovery is sometimes possible

– CNS – neuroglia never form bands to guide regrowing axons and may hinder axon growth with growth‐inhibiting chemicals – No effective regeneration after injury to the spinal chord and brain

Regeneration of the Peripheral Nerve Fiber

Figure 12.22

Myelin Sheaths CNS vs. PNS Myelination