Nervous Tissue

Functions of the Nervous System

• Sensory input

– Sensory receptors detect stimuli

• Processing and integration

– The brain interprets stimuli and determines

how to respond

• Motor output

– Effector organs (muscles and glands) are

activated

Organization of

the Nervous

System

• Structural organization

– Central nervous system (CNS)

• Brain

• Spinal cord

– Peripheral nervous system (PNS)

• Cranial nerves

• Spinal nerves

• Ganglia

Organization of the Nervous System

• Functional organization

– Sensory nervous system

• Responsible for sensory input

• Sensory receptors (PNS organs) receive stimuli and nerves

transmit this information to the CNS for interpretation

– Motor nervous system

• Responsible for motor output

• The CNS sends information by way of nerves to effector

organs

Organization of

the Sensory

Nervous

System

• The sensory nervous system consists of somatic

sensory and visceral sensory components

– The somatic sensory component receives stimuli from the skin,

joints, skeletal muscles, and special sense organs

• Voluntary (some control & are aware of the input)

– The visceral sensory component receives stimuli from the

viscera

• Involuntary (no control and generally not aware)

Organization of the

Motor

Nervous System

• The motor nervous system consists of somatic motor

and autonomic motor components

– The somatic motor component sends information to skeletal

muscle

• Voluntary

– The autonomic motor component sends information to cardiac

muscle, smooth muscle, and glands

• Involuntary

Nervous

Tissue

• Two types of cells make up nervous tissue

– Neurons

• The functional (excitable) cells of the nervous system

• Neurons receive and transmit nerve impulses (aka action

potentials)

– Glial cells

• Nonexcitable cells

• Glial cells support neurons

Neurons – Special

Characteristics:

• Larger but less numerous than glial cells

• High metabolic rate

– Neurons require continuous supplies of glucose and oxygen

• Extreme longevity

– Neurons persist for the lifetime of an individual

• Amitotic

– Neurons do not divide to replace themselves

Neuron

Structure • Cell body

– The neuron’s

control center

– Contains the

neuron’s nucleus,

nucleolus, and

other organelles

– Chromatophilic

substances = Nissl

bodies (ribosomes)

– “Interprets” the

input from dendrites

Neuron

Structure • Dendrites

– Multiple short,

branching

processes

projecting off of

the cell body

– Receive nerve

impulses from

other neurons (or

sensory stimuli)

and pass the

signal to the cell

body

Axon

Structure • A single long process projecting off of

the cell body

• Transmits nerve impulses from the

cell body to other neurons (or

effectors)

• Axon structures

– Axon hillock—the connection of

the axon to the cell body

– Axon collateral – side branch of

axon

– Telodendria—branches at the end

of the axon

– Synaptic knobs—expansions at

the ends of the telodendria

Figure 14.3 dendrites

cell body

axon

nucleus

axon hillock

Structural Classification of Neurons

Classified according to the number of processes emanating directly from the cell body of the neuron:

1. Unipolar

2. Bipolar

3. Multipolar

Multipolar—many dendrites and a single axon, most common of all neurons

1. Unipolar—single, short process that branches like a T 2. Bipolar—two processes, one dendrite and one axon

(Note: some neuro-anatomists prefer the term “pseudounipolar”

because of the ‘peripheral process’ of the axon.)

Axon

or

Dendrite:

what is the

difference?

• Dendrites and the Soma can

generate “Graded Potentials”

– Can be either positive or

negative

– Different sized

– Become smaller as they

spread

Axon

or

Dendrite:

what is the

difference?

• Axons & Axon Hillock, generate

“Action Potentials”

– “All or None” = always the

same size & polarity –

regardless of stimulus

– Always the same magnitude

– do NOT become smaller

as they spread.

Correction to text

• The McLaughlin text is imprecise in its use of the

term ‘nerve impulse’ - sometimes using it when

the proper term is graded potential and

sometimes when the precise term is action

potential

• On page page 425 (section 14.3) it states that ‘A

nerve impulse is also known as an action

potential….” but nerve impulse is not a rigorous

scientific term.

• Dendrites & the soma do NOT generate action

potentials.

Functional

Classification

of Neurons

Functionally, neurons are classified according to the direction that the nerve impulse is traveling relative to the CNS:

1. Sensory (afferent)

2. Motor (efferent)

3. Interneurons

Sensory (afferent)—transmit impulses from sensory receptors to the CNS

Motor (efferent)—transmit impulses from CNS to muscles or glands

Interneurons—facilitate communication between sensory and motor neurons

Structural and Functional class of each of these three neurons

Identify their dendrites and axons

Glial Cells

• Smaller but more numerous than neurons

• Actively mitotic

– the reason they are one of the most common

cells involved in brain cancer

Glial Cells of

the CNS

• Astrocytes – Attach to neurons and to capillaries

– Connect neurons to their nutrient supply

– Form the blood-brain barrier

Glial Cells of

the CNS

• Ependymal cells – Line the internal cavities of the CNS

• The ventricles of the brain and the central canal of

the spinal cord

– Produce cerebrospinal fluid (CSF) and

secrete it into the cavities

Glial Cells of

the CNS

• Microglial cells – Immune cells of the CNS

Glial Cells of

the CNS

• Oligodendrocytes – Wrap around the axons of CNS neurons

– Form the myelin sheath in the CNS

Astrocytes

Oligodendrocytes

Microglial cells

Ependymal cells

Glial Cells of

the PNS

• Satellite cells

– Surround the cell bodies of PNS neurons

– Regulate exchange of nutrients & waste

products with surrounding fluids

Glial Cells of

the PNS

• Neurolemmocytes (aka Schwann cells)

– Wrap around the axons of PNS neurons

– Form the myelin sheath in the PNS

Myelin

Sheath

• Oligodendrocytes (CNS) and

neurolemmocytes (PNS) form

myelin sheath around a

neuron’s axon

– Oligodendrocytes and

neurolemmocytes are full of lipids

– They wrap around the axons like

gauze

• Multiple layers of plasma membrane

lipids

Myelin Sheath

• Gaps exist between

neighboring

oligodendrocytes or

neurolemmocytes

– Termed neurofibril nodes

(aka nodes of Ranvier)

• Nerve impulses “jump”

from node to node

– Myelin sheath significantly

speeds up the

transmission of a nerve

impulse along an axon

Synapse

• A synapse is the junction between the axon of one neuron

and the dendrite of another neuron (or an effector organ)

– Where the nerve impulse is transmitted to the neighboring neuron

(or effector)

Synapse

• A synapse consists

of:

– presynaptic neuron

with its

– synaptic knobs;

– A synaptic cleft (small

space) between it and

the

– postsynaptic neuron

Synapse

• The action potential travels

down the axon of the

presynaptic neuron to its

synaptic knob

• It causes voltage-gated calcium

channels to open, allowing Ca++

to enter

• The Ca++ causes synaptic

vesicles to release the

neurotransmitter (e.g.

acetylcholine – red balls) into

the synaptic cleft

Synapse • Neurotransmitters bind

to receptors (ligand-

gated ion channels) on

the postsynaptic

membrane

• Sodium ions enter the

cell through the channel

• An action potential is

triggered on the

postsynaptic neuron

• Nerve transmission

converts from electrical

to chemical back to

electrical

Figure 14.14b

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Identify:

1. Presynaptic neuron

2. Postsynaptic neuron

3. Synaptic knob

4. Synaptic cleft

5. Voltage-gated Ca++ channels

6. Synaptic vesicles

7. Receptor protein

6. Sodium ions

(do they enter the cell?)

8. Acetylcholine molecules

(do they enter the cell?)

Nerves

• Nerves are bundles of axons running

parallel to each other

– Sensory nerves carry impulses only toward

the CNS

– Motor nerves carry impulses only away from

the CNS

– Mixed nerves carry impulses in both

directions

Nerves

An individual nerve

is an organ

– Nerve axons and

blood vessels run

throughout

– Surrounded by

epineurium

• Dense irregular

connective tissue

Nerves

Within the nerve,

axons are grouped

into bundles called

fascicles

– Fascicles are

separated from

other fascicles by

perineurium

• Dense irregular

connective tissue

Nerves

Individual axons

within fascicles are

surrounded by

endoneurium

– Areolar

connective tissue

– Myelin sheath is

found within the

endoneurium

What is the name of the

CT covering?

What kind of CT is it?

What is the name of this

Structure?

What is the name of the

CT covering?

What kind of CT is it?

What is the name of the

CT covering?

What kind of CT is it?

Identify:

1. Dendrites

2. Soma

3. Axon Hillock

4. Axon

5. Nucleus

6. Nucleolus

7. Chromatophilic substance

8. Axon collateral

9. Neurolemma

10.Neurofibril Node

11.Myelin Sheath

12.Telodendria

13.Synaptic knob