CHAPTER 34ORGANIZATION AND CONTROL OFNEURAL FUNCTION

Essentials of Pathophysiology

PRE LECTURE QUIZ The dorsal horn cell columns contain the afferent (sensory)

neurons and the ventral horn cell columns contain the efferent neurons.

The brain is divided into three regions: the hindbrain, the midbrain, and the forebrain.

The parasympathetic nervous system functions in maintaining vital functions and responding when there is a critical threat to the integrity of the individual—the “fight-or-flight” response.

The blood-brain barrier and the cerebrospinal fluid–brain barrier protect the brain from substances in the blood that would disrupt brain function.

Cerebrospinal fluid helps maintain a constant ionic environment that serves as a medium for diffusion of nutrients, electrolytes, and metabolic end products into the extracellular fluid surrounding central nervous system neurons and glia.

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T

F

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PRE LECTURE QUIZ ______________ are the functioning cells of the

nervous system. The _________________ nervous system contains two

divisions: sympathetic and parasympathetic. Inside the skull and vertebral column, the brain and

spinal cord are loosely suspended and protected by several connective tissue sheaths called the _________________.

Neurons communicate with each other through structures known as ________________, of which there are two types: electrical and chemical.

The main ___________________ for the autonomic nervous system are acetylcholine and the catecholamines, epinephrine and norepinephrine, which control neural function by selectively causing excitation or inhibition of action potentials.

Autonomic

meninges

Neurons

neurotransmitters

synapses

NEURONS

Dendrites receive stimuli

Stimuli pass down axons

Schwann cells contain myelin (“white matter”)

Help increase speed of impulse transmission

SUPPORTING CELLS

Peripheral nervous tissue Schwann cells: wrap a layer of myelin around

axons Satellite cells: separate nervous cells from

supporting tissue Central nervous tissue

Oligodendroglia: myelinating cells Astroglia: regulate ion content in intercellular

fluid Microglia: phagocytes Ependymal cells: line the neural tube cavity

IDENTIFY

Microglial cell Neuron Oligodendritic

cell Ependymal cell Astrocyte

What is the function of each?

E

D

CB

A

QUESTION

Tell whether the following statement is true or false.

All neurons are myelinated.

ANSWER

FalseRationale: The myelin sheath increases

the speed of impulse transmission (the impulse can skip over the myelinated/insulated parts of the neuron), but speed is not important everywhere (like the digestive tract). If every neuron was myelinated, neurons would take up a lot more space, too.

THE BASICS OF CELL FIRING

Stimulus opens Na+ gates

At threshold, more Na+ gates open

Na+ enters cell: depolarization

K+ gates open K+ diffuses out:

repolarization

Threshold potential

Resting membrane potential Stimulus

Action potential

SYNAPTIC TRANSMISSION What is

happening at stages 1–4?

What will result if you block stage 2?

Stage 3? Stage 4?

NEURON SECRETIONS

Neurotransmitters Amino acids Peptides Monoamines

Neuromodulators Attach to receptors and change their

response to neurotransmitters Neurotrophic factors

Neuron survival and to develop connections between neurons

GENERAL ORGANIZATION OF THE NERVOUS SYSTEM

Begins as a hollow tube

First segments of the tube become the brain Forebrain Midbrain Hindbrain

ORGANIZATION OF THE SPINAL CORD

Dorsal Afferent Sensory

Ventral Efferent Motor

SCENARIO

A woman developed polyneuropathy. Her spinal nerves were damaged She lost the ability to tell where her

body was positioned She has to look every time she takes a

step, to tell where she is moving her feet to

Question: What parts of her spinal nerves were

damaged?

CELL COLUMNS OF THE SPINAL CORD

What problems would you expect in someone who suffered ischemia to: Area A Area B Area C

A

B

C

QUESTION

If you place your hand on a hot surface, which ganglion carries the impulse to the spinal cord?

a. Ventralb. Dorsalc. Interneurond. Association neuron

Answer

b. DorsalRationale: Afferent neurons carry

sensory impulses to the spinal cord through the dorsal root ganglion; efferent neurons carry motor responses through the ventral root ganglion to effector cells in the tissue.

LAYERS OF THE WHITE MATTER

Archi layer Connects neighboring

segments Contains neurons

reticular activating system

Paleo layer Fibers reach to the brain

stem Neo layer

Pathways for bladder control and fine motor skills

Develop by fifth year of life

BRAIN REGIONS

Cerebrum Thalamus Hypothalamus Cerebral

peduncles Cerebral aqueduct Colliculi Cerebellum Pons Medulla oblongata

FUNCTIONS OF THE HINDBRAIN

Medulla oblongata, cerebellum, and pons Reflex centers for heart and respiration

rates, coughing, swallowing, vomiting, etc. Gives rise to cranial nerves V–XII

controlling viscera, hearing, facial, and mouth/throat functions

Cerebellum allows fine motor coordination

FUNCTIONS OF THE MIDBRAIN

Cerebral peduncles carry nerve fibers from the cerebrum to the hindbrain

Cerebral aqueduct lets cerebrospinal fluid drain from the fourth ventricle inside the cerebrum

Superior colliculi control reflex eye movements

Inferior colliculi control reflex reactions to sound

Gives rise to cranial nerves III and IV, controlling eye movement

FUNCTIONS OF THE FOREBRAIN

Thalamus: “switchboard” or relay station for impulses going to and coming from the cerebrum

Hypothalamus: homeostatic control Cerebrum Gives rise to cranial nerves I and II,

for smell and sight

CEREBRUM

Frontal lobe: motor, anticipation Parietal lobe: somatosensory Temporal lobe: hearing, memory Occipital lobe: vision

Limbic system: emotional

QUESTION

Which part of the brain maintains vital functions like breathing, heart rate, and digestion?

a. Forebrainb. Midbrainc. Hindbraind. Cerebellum

ANSWER

c. HindbrainRationale: Also known as the brain

stem, this is the vasomotor center that controls cardiopulmonary function and digestion.

DURA MATER

Has two layers Inner layer bends

over to form a fold (falx cerebri) that separates the cerebral hemispheres

It forms a second fold (tentorium) that holds the cerebrum up off the cerebellum

DURA MATER (CONT.)

Between the layers of the dura, at the base of each fold, venous blood drains out of the brain in a sinus Bridging veins carry blood from the brain across the inner layer of the dura mater to the sinus The sinus also

collects cerebrospinal fluid

ARACHNOID

Lies just beneath the dura mater

Waterproof Cerebrospinal fluid

(CSF) lies under the arachnoid to cushion the brain

Extensions of the arachnoid (villi) poke through the inner layer of the dura mater into the sinuses, to let CSF drain into the sinuses

PIA MATER

Lies right on the surface of the brain

Holds the cerebral arteries in place

MENINGES AND MENINGEAL SPACES

Epidural space: meningeal arteries Dura mater

Subdural space: bridging veins Arachnoid

Subarachnoid space: cerebral arteries, cerebrospinal fluid

Pia mater

CEREBROSPINAL FLUID

Leaks out of capillaries inside the brain’s hollow ventricles Composition controlled by the blood-brain

barrier Passes out an opening below the

cerebellum Circulates around the brain and spinal cord

in the subarachnoid space Passes through arachnoid villi into blood in

the dural sinuses and is returned to the heart

AUTONOMIC NERVOUS SYSTEM

Sympathetic Catecholamines Epinephrine, norepinephrine,

dopamine Attach to adrenergic receptors

Parasympathetic Acetylcholine Attaches to cholinergic receptors

ADRENERGIC NEUROTRANSMITTERS

Synthesized in the sympathetic system Attach to adrenergic receptors

Alpha-1 receptors: constrict blood vessels Alpha-2 receptors: negative feedback to

stop neurotransmitter release Beta-1 receptors: speed and strengthen

heart Beta-2 receptors: bronchodilation

Neurotransmitter is removed from synapse by reuptake or degraded by enzymes

CHOLINERGIC NEUROTRANSMITTER—ACETYLCHOLINE

Released from parasympathetic system and from motor neurons

Attaches to cholinergic receptors Nicotinic receptors: excite skeletal

muscle cells Muscarinic receptors: slow heart,

stimulate GI tract, vasodilate Neurotransmitter is removed from

synapse by acetylcholinesterase

QUESTION

Tell whether the following statement is true or false.

The sympathetic division of the ANS is also known as fight-or-flight.

ANSWER

TrueRationale: The SNS is characterized by

the release of adrenaline, which results in pupil dilation, bronchodilation, and increased HR, BP, and glucose production—all the things that come in handy when you are running from something!