HChapter 9--Bio 163 class notes transmit impulses along nerve fibers to other neurons ... Classification of Neurons ... HChapter_9--Bio_163_class_notes

Chapter 9

Nervous System

Central Nervous System (CNS)

vs. Peripheral Nervous System(PNS)CNS

•Brain

•Spinal cord

PNS

•Peripheral nerves connecting CNS to the body

•Cranial nerves

•Spinal nerves

Neurons transmit impulses along nerve fibers to other neurons

Nerves are bundles of nerve fibers

General Functions•Sensory = gather information and convert into nerve impulses

•Integration = sensory impulses are integrated into perceptions

•Motor functions = conscious or subconscious decisions

carried out via effectors

•Somatic nervous system

•Autonomic nervous system

Organization of the Nervous System

Supporting Cells

Neuroglial cells

CNS PNS

1.Microglial 1.Schwann

2.Oligodendrocytes 2.Satellite

3.Astrocytes

4.Ependyma

Neuron Structure = Nerve Cell

1. Cell Body

• Typical organelles

• Nissl bodies (chromatophilic substance) = Rough ER

• Neurofibrils

2. Processes

• Dendrites

• Axon

Axon and Nerve Impulses

• Axon terminals/synaptic knobs

– Vesicles and neurotransmitters

• Synapse

– Synaptic clefts

• Nerve fiber coverings

– Schwann cells- neurilemma

– Nodes of Ranvier

Cell Body Location

• Gray matter

• White matter

• Ganglia

Classification of NeuronsWays of classification:

1. Structure

2. Function

• Bipolar neurons

• Unipolar neurons

• Multipolar neurons

• Sensory neurons (afferent neurons)

• Interneurons

• Motor neurons (efferent neurons)

Cell Membrane Potential

•Irritability

•Conductivity

A cell membrane is usually polarized, with an excess of

negative charges on the inside of the membrane;

polarization is important to the conduction of nerve

impulses.

Distribution of IonsThe distribution of ions is determined by the membrane channel

proteins that are selective for certain ions.

Potassium ions pass through the membrane readily than do

sodium ions, making potassium ions a major contributor to

membrane polarization.

Resting Potential

1. Due to active transport, the cell membrane maintains a

greater concentration of sodium ions outside and a

greater concentration of potassium ions inside the

membrane.

2. Inside of the membrane has excess negative charges,

while the outside has more positive charges.

3. The separation of charge, potential difference, is called

the resting potential.

Potential Changes1. Stimulation of a membrane locally affect its resting

potential.

2. When the membrane potential becomes less negative, the

membrane is depolarized.

3. If sufficiently strong depolarization occurs, a threshold

potential is achieved as ion channels open.

Action Potential

At threshold potential, membrane permeability to sodium suddenly

changes in the region of stimulation.—Action potential

• As sodium channels open, sodium ions rush in, and the

membrane potential changes and becomes depolarized.

• At the same time, potassium channels open to allow

potassium ions to leave the cell—the membrane is

repolarized and resting potential is reestablished.

• Active transport works to maintain the original concentration

of sodium and potassium ions.

Nerve Impulse

A nerve impulse is conducted as action potential is

reached at the trigger zone and spreads by a local

current flowing down the fiber, and adjacent areas of

the membrane reach action potential. Nerve impulse

is conducted as a series of action potentials occurring

along the length of the axon.

Impulse Conduction1. Unmyelinated fibers

2. Myelinated fibers

***All-or-None Response***

• If a nerve fiber responds at all to a stimulus, it responds

completely by conducting an impulse.

• Greater intensity of stimulation triggers more impulses per

second, not stronger impulses.

SynapseSynapse = junction between two communicating neurons

Synaptic Transmission = Presynaptic neuron transmits impulse

across the synaptic cleft to the postsynaptic neuron

When an impulse reaches the synaptic knobs of an axon, the

synaptic vesicles release neurotransmitter into the synaptic

cleft. The neurotransmitter react with the receptors on the

postsynaptic membrane.

•Excitatory Action

•Inhibitory Action

Neurotransmitters•50 kinds of neurotransmitters

•Synthesized in the synaptic knobs

•Stored in vesicles

•Presence of calcium ions are needed to allow vesicles to

fuse to the membrane and release their contents into the

synaptic cleft.

•Enzymes in the cleft and on postsynaptic membrane

decompose the neurotransmitter in order to prevent

continuous stimulation of the postsynaptic neuron

Impulse ProcessingNeuronal Pools

•Facilitation

•Convergence

•Divergence

Nerve Pathways

•Reflex arc

•Reflex behavior

•Knee-jerk reflex

•Withdrawal reflex

Meninges

The spinal cord and

brain are surrounded by

membranes called

meninges that lie

between the bone and

soft tissues.

•Dura mater

•Arachnoid mater

•Subarachnoid space

•Pia mater

Spinal Cord

•31 segments, each gives rise to a pair

of spinal nerves

•Cervical enlargement = nerves leading

to the upper limbs

•Lumbar enlargement = nerves leading

to the lower limbs

•Anterior median fissure

•Posterior median sulcus

•White matter

•Gray matter

•Central canal

Functions of the Spinal Cord

•Transmit impulses to and from the brain

•House spinal reflexes

•Ascending tracts vs. descending tracts

Brain•Largest, most complex portion of the nervous system

•Cerebrum (high mental functions)

•Diencephalon (processes sensory input)

•Cerebellum (coordinates muscular activity)

•Brain stem (coordinates and regulates visceral activities)

Cerebrum•Largest portion of the brain, 2 cerebral hemispheres

•Corpus callosum

•Convolutions, sulci, fissures

•Divided into lobes, named according to the bones they underlie

•Functions of cerebrum

•Higher brain functions

•Integrating information for reasoning

•Memory

•Interpretation of sensory input

•Initiating voluntary muscular movements

•Cerebral cortex

Functions of the Cerebral Cortex•Motor, sensory, and association areas

•Primary motor areas = frontal lobe

•Broca’s area = coordinates muscular activity, speech

•Frontal eye field = voluntary movements of the eyes and

eyelids

•Sensory areas = interpret sensory input, feelings, sensations

•Association areas = analyze and interpret sensory impulses,

reasoning, judgment, emotions, verbalizing ideas, storing

memory

•General interpretive area = junction of the lobes, complex

thought processing

Ventricles and Cerebrospinal

Fluid•Ventricles

•Choroid plexuses

•Cerebrospinal fluid

Diencephalon•Thalamus

•Sorting and directing sensory information

•Hypothalamus

•Maintains homeostasis

•Regulates visceral activities

•Linking endocrine system to this system

•Limbic system

•Controls emotional experience and expression

Brain Stem•Midbrain

•Centers for auditory and visual reflexes

•Pons

•Centers that regulate rate and depth of breathing

•Medulla oblongata

•Control visceral functions

•Cardiac center for heart rate and blood pressure

regulation

•Assists pons with breathing regulation

Cerebellum

•2 hemispheres

•Integrate sensory

information about the

position of body parts

•Coordinates skeletal

muscle activity

•Maintains posture

Peripheral Nervous System= consists of the cranial and spinal nerves that arise from

the CNS and travel to the remainder of the body

= somatic nervous system that oversees voluntary activities

12 pairs of cranial nerves:

Spinal Nerves31 pairs of spinal nerves

Structure of a Nerve

Autonomic Nervous System= maintaining homeostasis of visceral activities without

conscious effort

2 divisions:

•Sympathetic division

•Parasympathetic division

Autonomic nerve fibers:

•Preganglionic fibers that leaves CNS

•Postganglionic fibers that extends to the visceral effectors

Sympathetic Division= operates under

conditions of stress or

emergency

•Fibers arise from the

thoracic and lumbar

regions of the spinal

cord

•Synapse in ganglia

close to the vertebral

column

•Postganglionic axons

lead to the effector

organ

Parasympathetic Division

= operates under

normal conditions

•Fibers arise from

brainstem and

sacral region of the

spinal cord

•Synapse in the

ganglia close to the

effector organ

Autonomic NeurotransmittersPreganglionic fibers release acetylcholine

Postganglionic fibers:

Parsympathetic: acetylcholine

Sympathetic: norepinephrine

Control of Autonomic Activity:

•Reflex centers in the brain and spinal cord

•Limbic system and cerebral cortex