Nervous System Notes [HAP.6]

Nervous
System

Human Anatomy & Physiology

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I. Two main divisions of the

nervous system.

A. Central Nervous System (CNS) is

composed of the brain and spinal cord.

B. Peripheral Nervous System (PNS) are the

nerves (group of axons traveling together)
that lie outside of the CNS.

II. Functions

A. Sensation (senses) originates with

receptors at the ends of
peripheral nerve cells. Can be …
• Sight
• Hearing
• Smell
• Touch

B. Motor functions begin within the

CNS and carries impulses to
structures that will respond to
these impulses. (Movement)

III. Cells of the

Nervous
System

A. Neuron (nerve cell) is the basic unit of the

nervous system. Anatomy of a neuron includes:

1. Dendrites receive nerve impulses and

transmits them to the cell body.

2. Cell body contains the nucleus and other

organelles so the cell can function. Contains
...

a. Neurofibrils which are fine threadlike

networks that extend into the nerve fiber.

b. Nissle bodies which functions like the ER.

III. Cells of the Nervous System (cont’d)

3. Axon is a single long extension that conducts nerve impulses away from the cell

body. Axons have:
a. Myelin - a sheath that surrounds the axon; insulation that allows the impulse to

travel faster. (gives nerves a white appearance)

b. Neurilemma - outermost sheath that surrounds an axon.
c. Schwann cells - produce the myelin sheath and neurilemma around the

nerves.

d. Nodes of Ranvier - the part of the axon that is exposed.
e. Axon terminal - responsible for transmitting chemical signals to the cells that

the neuron contacts.

Anatomy of a
Neuron

III. Cells of the Nervous System (cont’d)

B. Neuroglial cells are non-conductive cells in the CNS that helps regulate

extracellular environment. Classes of Neuroglial Cells include …

1. Microglial cells are found in the CNS and support neurons and

destroys bacterial cells and debris.

2. Oligodendrocytes are responsible for forming myelin within the CNS

but not the neurilemma.

3. Astrocytes help regulate the concentration of nutrients and forms scar

tissue when there is damage to the CNS. (Are associated with Tau
Proteins)

4. Ependymal cells form membranes that cover specialized brain parts

and inner linings within the brain.

IV. Types of Neurons

A. Neurons by classification

1. Bipolar neurons have two nerve fibers, one from each

end. (found in eyes, nose, and ears)

2. Unipolar neurons has a single nerve fiber that extends

from the cell body and divides into two branches.
a. Contains a ganglia which is the location of the cell

body for the neuron.

b. One end moves from the PNS and the other to

the CNS.

3. Multipolar neurons have many nerve fibers. (found in

the brain and spinal cord)

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B. Neurons Based

on Function

1. Afferent (sensory) neurons transmit information

from tissues of the body to the CNS: from
receptor to CNS.

• Receptor - detects changes within the

environment.

2. Efferent (motor) neurons transmit electrical

signals from the CNS out to muscles or glands:
from CNS to effector.

• Effector - muscle or gland.

Neurons Based on Function (cont’d)

a. Somatic nervous system - the part of the PNS whose nerves go to

the skeletal muscle.

b. Autonomic nervous system - the part of the PNS that controls

internal organs (not controlled consciously). Two divisions of
autonomic:

1. Sympathetic nervous system - the part of the autonomic

associated with "flight or fight" responses.

2. Parasympathetic nervous system - the part of the autonomic

nervous system associated with a relaxed or normal state.

3. Interneurons are found in the spinal cord (CNS) and connect afferent and

efferent neurons.

V. Nerve impulse along an Axon.

Nerve Impulse - the way a nerve transmits information;

due to an increase in the membrane permeability.

A. Resting potential is when a neuron is not conducting

an impulse.
1. The sodium-potassium pump keeps sodium on

the outside and potassium on the inside by active
transport.

• Active transport - movement of ions from an area of lesser

concentration to an area of greater concentration
(requires energy).

2. Inside of the axon is more negative.

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Nerve impulse along an Axon
(cont’d)

B. Action potential is a rapid change in the

charge difference (polarity) along a
nerve fiber. Has two parts:

1. Depolarization - sodium ions move

to the inside of the axon. (inside of
axon becomes more positive)

2. Repolarization - potassium ions

move to the outside of the axon.
(inside of axon becomes more
negative again)

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Nerve impulse along an Axon
(cont’d)

C. Refractory period is the period of time that the sodium-potassium

pump is moving sodium to the outside and potassium to the inside of
the axon.

� An impulse cannot be conducted during this time.

� Inside of axon remains negative on the inside

VI. Nerve impulses from one

neuron to another neuron
(synapse)

• Synapse - junction between two neurons.

A. The axon of the pre-synaptic neuron

depolarizes, and calcium ions diffuse
into the pre-synaptic neuron and the
synaptic cleft. (1)
• Diffusion - movement of molecules

from area of greater concentration
to an area of lesser concentration.

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Nerve impulses from one neuron to another
neuron (synapse)

B. Synaptic vesicles hold neurotransmitters, the chemical messenger that an axon

releases that controls another neuron or effector. (2)
1. Excitatory neurotransmitters increase the permeability of the post-synaptic

membrane to sodium ions.

2. Inhibitory neurotransmitters decrease the permeability of the post-synaptic

membrane to sodium ions.

C. Calcium ions cause the synaptic vesicles to attach to the pre-synaptic

membrane and empty the neurotransmitter into the synaptic cleft. (3)

Nerve impulses from one neuron
to another neuron (synapse)

D. The neurotransmitter substance diffuses across the

synaptic cleft and attaches to receptors found on the
dendrites of the post-synaptic neuron. (4)
• This causes:

1. Ion channels to open on the post-synaptic

membrane.

2. Sodium and potassium ions are able to move

across the membrane of the post-synaptic
neuron (depolarization).

The Peripheral Nervous System

VII. The peripheral nervous system (PNS) carries impulses to and from structures of

the body. Composed of:
A. Cranial nerves are 12 pair of nerves that originate from the brain and

controls sensory and/or motor functions of the head, neck, and face.

B. Spinal nerves are 31 pair of nerves that originate at the spinal cord and

contains:
1. Sensory axons have cell bodies that are found outside of the spinal cord

in an area called the dorsal root ganglion.

2. Motor axons (somatic or autonomic)

Central Nervous System

VIII. Central Nervous System is responsible for sending nerve impulses,

and analyzing sensory data (sight, smell, etc...)

A. The CNS is protected by:

1. Bone

a. Skull protects the brain.

b. Vertebral column protects the spinal cord.

Central Nervous System
(Protection cont’d)

2. Meninges are membranes that surround and

protect the brain and spinal cord. It has 3
layers:

a. Dura mater - outer layer, next to the bone.

b. Arachnoid - middle layer

c. Pia mater - layer next to the nervous tissue

3. Cerebrospinal fluid is a fluid that surrounds

the brain and spinal cord and cushions it.

Central
Nervous
System
(Spinal Cord
Functions)

B. Spinal Cord functions:

1. The center for reflex actions (an

automatic response to a stimulus
without a voluntary response)

2. Communication link between the

brain and the spinal nerves.

Central
Nervous
System
(Spinal Cord)

C. Spinal cord is composed of:

1. White matter which is nerve fibers that is

myelinated; has two tracts:

a. Ascending tracts that takes messages

up the spinal cord to the brain and
carries sensory information.

b. Descending tracts that takes messages

from the brain down the spinal cord
and conducts motor impulses.

2. Gray matter which is nerve fibers that does

not have myelin.

D. Brain has voluntary & involuntary

parts

1. Conscious brain: Cerebrum is responsible for voluntary muscle

movements, personality, interpreting sensory information, and
problem solving.

a. The surface of the cerebrum has many ridges called convolutions,

shallow grooves called sulcus and deep grooves called fissures.

b. Cerebrum is divided into right and left halves called hemispheres

which are connected by the corpus callosum.

What is

meant by this

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Central
Nervous
System
(Brain: Lobes
of The
Cerebrum)

c. Each hemisphere has four

lobes:

1. Frontal - major area

for motor (muscle
movements)

2. Parietal - area for

sensations

3. Temporal - area for

hearing

4. Occipital - area for

vision

Lobes of the Cerebrum

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Central
Nervous
System

(Cerebrum

cont’d)

d. Cerebrum contains

cavities called
ventricles which has
cerebrospinal fluid
which is made by the
chroid plexus found
in the pia mater.

2. Central Nervous System

(Unconscious Brain)

a. The Diencephalon which includes:

1. Thalamus receives sensory

information from various parts of
the nervous system and directs it
to the appropriate region of the
cerebral cortex.

2. Hypothalamus which is

responsible for maintaining
homeostasis.

3. Limbic System is a region of the

conscious and unconscious brain
that is involved with emotions.

b. Cerebellum controls balance, input of

sensory information with appendicular
positions, and ensures that skeletal
muscles work together for a smooth
fluid movement.

The Cerebellum

Cerebellum

Central Nervous

System

(Unconscious Brain)

c. Brain Stem is composed of:

1. Medulla Oblongata controls heartbeat,

breathing, and blood pressure.

2. Pons relays impulses to and from the

medulla oblongata and the cerebrum,
as well as transmits nerve impulses from
the cerebrum to centers within the
cerebellum.

3. Midbrain connects brain stem and spinal

cord with higher parts of the brain and
contains some reflex actions.