Chapter 12 part 1: Nervous Tissue

Nervous system?

A network of neurons and neuroglia found in two divisions

Central nervous system (CNS)

Peripheral nervous system (PNS)

Central nervous system (CNS)?

Consists of the brain and the spinal cord

CNS processes incoming sensory information

Sends signals that stimulate muscles to contract or glands to → secrete

Peripheral nervous system?

Consists of all the nervous tissue outside of the CNS

Includes nerves, ganglia, enteric plexuses, and sensory receptors

Sensory (afferent) division?

Carries signals from receptors to the → CNS

Somatic sensory division?

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

Visceral sensory division?

Carries signals from the thoracic and abdominal organs (heart, lungs, stomach, and bladder)

Motor (efferent) division?

Sends signals from the CNS to a gland or muscle (effectors) to create a response

Somatic motor division?

Carries signals to skeletal muscles

Voluntary and involuntary muscle contractions

Visceral motor division (autonomic nervous system (ANS)) consists of?

Sensory neurons that take information from autonomic sensory receptors to the → CNS

Motor neurons that take information from CNS to smooth muscle, cardiac muscle, and glands

Involuntary action (Visceral reflexes)

Sympathetic division?

“fight or flight” response, ex. increases heart rate

Parasympathetic division

“rest and digest” activity, ex. can slow heart rate

Neurons (nerve cells)?

Receive information (stimuli) and convert it to an → action potential

Excitability?

All cells respond to stimuli – response to environmental → changes

Conductivity?

Neurons respond to stimuli by producing electrical signals that are sent out to other cells

Secretion?

At the end of the nerve, neurotransmitters are secreted in response to the signal, cross a gap, and stimulate another cell

Sensory (afferent) neuron?

Contain sensory receptors or are neighbors to one

Function to send action potentials into the CNS through cranial or spinal nerves → unipolar

Interneuron?

Association neuron, located within the CNS, between afferent (sensory) and efferent (motor) neurons

Function to integrate incoming sensory information from sensory afferent neurons and cause a motor response by activating motor (efferent) neurons → multipolar

Motor (efferent) neurons?

Function to carry action potentials away from the CNS through cranial or spinal nerves to effectors (muscles and glands) in the PNS → multipolar

What does the Neuron consist of?

Cell body

Dendrites

Axon

Cell body?

Contains a nucleus, cytoplasm, and several organelles

Contains Nissl bodies (RER) – granules in the cytoplasm that help regenerate damaged neurons in the → PNS

Neurofibrils – type of → cytoskeleton

Dendrites?

Emerge from the neurosoma (cell body)

Act as the receiving portion of the neuron

Contain receptor sites for chemical messages

Axon (nerve fiber)?

Is a long cylindrical projection that also emerges from the cell body from the → axon hillock

Propagates (carries) a nerve impulse to another neuron, muscle fiber, or gland

Divide at ends into → axon terminals

Forms a synapse (junction) to the next cell

Multipolar neurons?

Consists of several dendrites and one axon coming off of the cell body

Most common – most neurons in CNS (brain and spinal cord)

Interneurons and motor (efferent) neurons

Bipolar neurons?

Consists of one main dendrite and one axon

Olfactory cells, retina, and inner ear

Unipolar neurons?

Dendrites and a single axon are fused together

Form a continuous structure off a cell body

Sensory (afferent) neurons from skin and organs to spinal cord

Neuroglia (glial cells)?

“glue” that holds the nervous system together

Protect neurons and help them → function

Bind neurons together and form framework for nervous tissue

Oligodendrocytes?

Arm-like processes wrap around nerve axons

Function in forming and maintaining the myelin sheath around axons in the CNS

Myelin sheath speeds up signal conduction

Ependymal cells?

Simple cuboidal epithelium with cilia on apical surface

Line internal cavities of the brain (ventricles) and spinal cord

Secrete and circulate → cerebrospinal fluid (CSF)

Microglia?

Develop from white blood cells (monocytes) and become concentrated in areas of → damage

Wander through CNS looking for debris and damage

Perform → phagocytosis

Astrocytes?

Most abundant glial cell in CNS, covering brain surface and most non-synaptic regions of neurons in the gray matter

How do Astrocytes function?

Form supportive → framework

Have extensions (perivascular feet) that surround blood capillaries and form a seal called the → blood–brain barrier

Monitor neuron activity and regulate blood flow to match metabolic need

Secrete nerve → growth factors

Communicate electrically with neurons

Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions

Schwann cell (neurolemmocyte)?

Envelope (surround) nerve axons in the PNS and wind repeatedly around

Produce the myelin sheath of the PNS, similar to the ones produced by oligodendrocytes in CNS

Assist in regeneration of damaged fibers

Satellite cells?

Flat cells that surround neurosoma (cell body) in the PNS ganglia

Provide electrical insulation around the neurosoma (cell body)

Regulate the chemical environment of the neurons

Myelin sheath?

Insulation around a nerve axon

Formed by oligodendrocytes in CNS and Schwann cells in PNS

Schwann cells start forming myelin during fetal development, in a process called → myelination

Neurilemma?

Thick, outermost coil of myelin sheath in the → PNS

Contains nucleus of the Schwann cell and most of its cytoplasm

Myelin sheath is?

Segmented

Nodes of Ranvier (myelin sheath gap)?

Gaps in the myelin sheath along the axon

Many CNS and PNS fibers are?

Unmyelinated

In PNS, Schwann cells hold 1 to 12 small nerve fibers in surface?

Grooves

Membrane folds how many times around each fiber?

Once

Speed at which a nerve signal travels along surface of nerve fiber depends on two factors?

Diameter of fiber

Larger fibers have more surface area and conduct signals more → rapidly

Presence or absence of → myelin

Myelin speeds up signal conduction

Ganglion?

Clusters of neuronal cell bodies in the → PNS

Including the cranial and spinal nerves

Nucleus?

Clusters of neuronal cell bodies in the → CNS

Nerve?

Bundle of axons in the → PNS

Tract?

Bundle of axons in the → CNS

White matter?

Consist mainly of myelinated axons

Gray matter?

Contains cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia

Regeneration of damaged peripheral nerve fibers can be repaired if?

The cell body is still → intact

At least some neurilemma remains

Fiber distal to the injury cannot survive and?

Degenerates

Macrophages does what?

Clean up tissue debris at point of injury and beyond

Cell body does what?

Swells, ER breaks up, and nucleus moves → off center

Due to loss of nerve growth factors from neuron’s target cell

Axon stump sprouts multiple what?

Growth processes as severed distal end continues to degenerate

Schwann cells, basal lamina, and neurilemma form a?

Regeneration tube

Enables neuron to regrow to original destination and reestablish synaptic contact

Once contact is reestablished with original target what happens next?

The cell body shrinks and returns to its original appearance

Nucleus returns to normal shape

Atrophied muscle fibers → regrow

Schwann cells and endoneurium (connective tissue around myelin sheath) are what?

Are required for repair

The CNS lack both and cannot regenerate at all