Chapter 12 and 13 Nervous System

Sensory (afferent) division

Carries signlas from receptors to CNS

Afferent

In

Efferict

Out

Motor (efferent) division

Carries signals from CNS to effectors (glands and muscles that carry out the body’s response)

Somatic motor division

Carries signals to skeletal muscles

Somatic sensory division

Carries signals from receptors in the skin, muscles, bones, and joints

Visceral sensory divison

Carries signals from the viscera (heart, lungs, stomach, and urinary bladder)

Visceral motor division (autonomic nervous system)

Carries signals to glands, cardiac and smooth muscle

-Its involutary responses are visceral reflexes

Enteric nerovus system

Involved with digestive system

Sympathetic division

Tends to arouse body for action

Accelerating heart beat and respiration, while inhibiting digestive and urinary systems

Parasympathetic division

Tends to have calming effect

Slows heart rate and breathing

Stimulates digestive and urinary systems

Brain (Central Nervous System)

Perception and processing of sensory stimuli (somatic/autonomic)

Execution of voluntary motor responses (somatic)

Regulation of homeostatic mechanisms (autonomic)

Nerves (Peripheral Nervous System)

Fibers of sensory and motor neurons (somatic/ autonomic)

Digestive Tract (Enteric Nervous System)

Located in the giestive tract, is responsible for autonomous functions and can operate independently of the brain and spinal cord

Spinal Cord (Central Nervous System)

Initiation of reflexes from ventral horn (somatic) and lateral horn (autonomic) gray matter

Pathways for sensory and motor functions between periphery and brain (somatic/autonomic)

Ganglia (Peripheral Nervous System)

Reception of sensory stimuli by dorsal root and cranial ganlia (somatic/autonomic)

Relay of visceral motor responses by autonomic ganglia (autonomic)

Functions of the nervous system

Sensory Input

Integration

Motor output

Homeostasis

Characteristics of Neurons

Excitable

Longevity

High Metabolic Rate

Sometimes Large

Amitotic

May Regenerate

Sensory (afferent) neurons

Detect stimuli and transmit information about them towards the Central Nervous System

Interneurons (association neurons)

Lie entirely within Central Nervous System connecting motor and sensory!!!

Receive signlas from many neurons and carry out intergrative functions (make decisions on responses)

Motor (efferent) neuron

Send signals out to muscles and gland cells (the effectors)

Soma

Control center of neuron

Also called Neurosoma or cell body

Has a single, centrally located nucleus with large nucleolus

Cytoplasms contains

Mitochondria (power house of the cell)

Mature Neurons have

No centrioles

No mitosis after adolescence

Dendrites

Branches that come off the soma

Primary site for receiving signals from other neurons

The more dendrites the neuron has, the more information it can receive

Structure of Neuron

(Telodendria) smaller end branches of the axon

At the end of the telodendria

Axon terminals which have the synaptic vesicles

Synaptic knob

(terminal button) little swelling that forms a junction (synapse) with the next cell

Contains synaptic vesicles full of neurotransmitter

Axon hillock

Where the axon emerges from the cell body

Axoplasm

Within the axon hillock, the cytoplasm changes to a solution of limited components

Initial segment

Because the axon hillock represents the beginning of the axon, it is called this

Axon (nerve fiber)

Originates from one side of the soma called the axon hillock

Schwann cells

Myelin sheath enclose axon

branches extensively on the distal end

The synaptic knob forms a junction (synapse) with the next cell (nerve, muscle, and gland)

Unipolar

Cells have only one process emerging from the cell

Synaptic knob

(Terminal button) little swelling that forms a junction (synapse) with the next cell

Contains synaptic vesicles full of neurotransmitters

Bipolar

Cells have 2 processes, which extend from each end of the cell body, opposite to each other

Multipolar

Neurons are all of the neurons that are not unipolar or bipolar

They have 1 axon and 2 moredentrites (usually many more)

Myelin Sheath

Process of myelination occurs as the glial cell wraps tighter and tighter around the axon

Inner layes of the glial cell form the insulating myelin sheath while the outer layer is called the neurolemma

Gaps between adjacent glial cells form the nodes of Ranvier

Myelinated fibers

Conduct impulses rapidly

These form the white matter of the nervous tissue

Unmyelinated fibers

Conduct impulse relatively slow compared to myelinated fibers

This type forms the gray matter of the nervous tissue

Nodes of Ranvier

Gaps within the myelin sheath

These aid in impulse transmission

The length of the axon between each gap, which is wrapped in myelin, is reffereed to as an axon segment

Glial cells

Forms the myelin sheath

In the PNS, Schwann cells envelop and then rotate around the axons, forming the sheath, while in the CNS, the myelin sheath is formed by oligodendrocytes

Multiple Sclerosis and Guillain-Barre Syndrome

Neurodegenerative disorders that have loss of myelination

Neuroglia or glial cells

Protect neurons and help them function

Bind neurons together and form framework for nervous tissue

Astrocrytes

Star-shaped glia that make up half of all neural volume

Control the chemical environment around neurons

Degrade and recycle neurotransmitters

Form the blood-brain barrier (BBB)

Microglia

Ovoid-shaped glial cells with highly branched, narrow cell processes

Act as macrophages that engulf microbes and dead neural cells

Generally provide protection for the brain and spinal cord

Oligodendrocytes

There are a few processes that extend from the cell body

Each cell process reaches out and surrounds an axon to insulate it in myelin

One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons

Ependymal Cells

Line the ventricles of the brain and the spinal cord

Ependymal cells assist in producing, monitoring, and circulating CSF

They use their cilia to circulate the CSF from the brain and down into the spinal cord

Schwann cells

Are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others.

Oligodendrocytes

Have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment.

Graded potential

For the nerve to fiber, it has to get stimulated

Strong enough to go down to the axon hillock

Trigger zone

Active Ion Channels

Active, or gated ion, channels open and close on in response to very specific conditions

Ligand-gated ion channels

open and close in the presence of a chemical signal such as a neurotransmitter.

Voltage-gated ion channels

open and close in response to a change in the membrane potential.

Mechanically-gated Ion Channels

Mechanically-gated ion channels open and close in response to distortion in the surface of the plasma membrane.

Leakage Ion Channels

Open and close randomly

Neurophysiology

The concentration of ions in extracellular and intracellular fluids is largely balanced, with a net neutral charge.

However, a slight difference in charge occurs right at the membrane surface, both internally and externally.

It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate nerve impulses as action potentials.

Before these can be described, the resting state of the membrane must be explained

Resting Membrane Potential

The inside of the cell has a relatively negative charge (-70mV) compared to the outside of the cell

Depolarization

It begins to sweep across the cell membrane as a graded potential.

Propagation of Action Potential

If (-55mV) is reached, the graded potential is converted to an action potential and all of the voltage-gated ion channels along the axon open.

A ligand-gated Na+ channel will open when a

Neurotransmitter binds to it and a mechanically gated Na+ channel will open when a physical stimulus affects a sensory receptor

Continuous conduction

Propagation along an unmyelinated axon

Saltatory conduction

Along the length of a myelinated axon

Threshold

Any depolarization that does not change the membrane potential to -55mV or higher will not reach this and thus will not result in an action potential

Depolarization

Once the stimulus is removed, the Na+ ion channels begin to close but the K+ ion channels remain open.

This causes the K+ to flow out of the cell and internal charge begins to move back into a negative direction

Hyperpolarized

Because the K+ channels remain open even once the -70mV is reached, the membrane becomes this (-90mV)

Graded potentials

Can be of 2 sorts, either they are depolarizing or hyper polarizing

Depolarizing graded potentials

Typically the result of Na+ entering the cell (or Ca++ entering)

Hyperpolarizing graded potentials

Can be caused by K+ leaving the cell (or Cl entering the cell)

Making inside the cell MORE NEGATIVE threshold is that much further away

Excitatory postsynaptic potential (EPSP)

Postsynaptic neuron receives a depolarization signal meaning the graded potential is moving in a less negative direction (-70mV toward zero).

Inhibitory postsynaptic potentials (IPSP)

Postsynaptic neuron receives a hyperpolarization signals meaning the graded potential is moving in a more negative direction (-70mV toward -90mV)

Summation

The overal change in the membrane potential

Spatial summation

Related to the activity of multiple inputs to a neuron at the same time but in different locations

Temporal summation

Accumulation of multiple, repeated, and frequent inputs

Synapses

Serve as the site where the first neuron

Chemical synapses

Use neurotransmitters

Electrical synapses

Use gap junctions

Neurotransmitters

Molecules that are released when a signal reaches a synpatic nob that binds to a receptor on another cell and alter that cell’s physiology

Acetylcholine

In a class by itself

Amino acid neurotransmitters

Include glycine, glutamate, asapartate, and y-aminobutyric acid (GABA)

Monoamines

Synthesized froom amino acids by removing the COOH group while retaining the NH2

Cholinergic synapse

Uses acetylcholine (ACh) also can be inhibitory

Noradrenaline

Adrenergic synapse employs the neurotransmitter norepinephrine (NE)

Presynaptice cell stops

The releasing of the neurotransmitter

Endoderm

(Inner germ layer) is responsible for generating the lining tissues of various spaces within the body, such as the mucosae of the digestive and respiratory system

Mesoderm

(Middle germ layer) gives rise to most of the muscle and connective tissues

Ectoderm

(Outer germ layer) develops into the integumentary system (the skin) and the nervous system

Ectoderm is the

Nervous system

Anterior end is the

Brain

Posterior end is the

Spinal cord

Neural crest is the

Structures of the PNS

Neural plate

Portion of the ectoderm differentiates into a specialized region of neuroectoderm to form this

Neural groove

When the neuroectoderm folds inward it makes this

Neural tube

As the two sides of the neural groove converge, they form this which lies beneath the ectoderm

Anterior end of the neural tube

Will develop into the brain, and the posterior end will become the spinal cord. The neural crest (cluster of cells) will develop into the structures of the PNS

Spinal Bifida

Birth defect of the spinal cord caused when the nerual tube fails to close completely, but the rest of development continues

Cerebral Cortex

Thin extensive region of gray matter responsible for higher functions of the nervous system

Longitudinal Fissure

Separates the 2 cerebral hemispheres

Corpus Callosum

White matter deep within the cerebrum for communication between the 2 hemispheres

Gray matter

The seat of neurosomas, dendrites, and synapses

Dull color due to little myelin

Forms surface layer (cortex) over the cerebrum and cerebellum

Forms nuclei deep within the brain