The Nervous System

Chapter 8

Nervous System Fun Facts

• Human brain ~ 3 pounds (1300-1400 g)

• Elephant brain ~ 6000 g

• Cat ~ 30 g

• Human brain has about 100 billion neurons

• Neurons multiply 250,000/minute in the first months of pregnancy

Body’s Message Centers

Our body has two message centers:

• The Nervous System

• The Endocrine System

Both Systems:

• transmit signals throughout the body

• Differ in the way they transmit information

Overview of the Nervous System

General parts:

• The brain

• The spinal cord

• The nerves and sense organs

General functions:

• controls and coordinates body’s activities

• helps us sense and respond to changes in the environment

Functions Sensory Input: • Sensation of hot and cold via sensory nerves to the

brain • Response to external stimuli

Integration: • Data interpretation by the brain and spinal cord

• Sums up data and sends out impulses

Motor Output: • Signals (nerve impulses) from the brain and spinal cord

to the effectors (muscles, glands, organs) • Responses include muscle contractions, gland secretions, and

changes in organ function

Divisions of the Nervous System

The Central Nervous System (CNS):

• The brain and spinal cord

• Lie in the dorsal body cavity

The Peripheral Nervous System (PNS):

• Consists of cranial and spinal nerves

• Has two subdivisions: – Afferent (sensory)- carry info to

the brain and spinal cord

– Efferent (motor) – carry

Information to effectors

Both systems work together

Nervous Tissue

• Neurons

– nerve cells

– transmit nerve impulses

• Neuroglia:

– Support and nourish neurons

– Outnumber neurons!

Types:

Microglia, Astrocytes, Oligodendrocytes, Ependymal cells, Schwann cells

Structure of a Neuron Cell body – contains the

nucleus and other organelles

Dendrites – receive signals from sensory receptors or other neurons

Axon – conducts nerve signals

– group together in a bundle and form a nerve

– axon bundle is called a tract in the CNS

Structure of a Neuron continued…

• Myelin sheath– fatty (lipid) covering of axons

Function: Insulation and nerve impulse

conduction

Formed by:

• Schwann cells (Neurolemmocytes) in the PNS

• Oligodendrocytes in the CNS

• Nodes of Ranvier (Neurofibril nodes): gaps in the myelin sheath

– Increase the speed of nerve signal conduction

Structural Classification of Neurons

Unipolar neurons

• A single axon enters and leaves the cell body

Bipolar neurons

• One dendrite and one axon

• Only in the retina, ear, and nose

Multipolar neurons

• Many dendrites and one axon

Motor Neurons (efferent) • Found: CNS and PNS

• Function:

– Carry nerves impulses from the CNS to the

muscles , glands and organs (effectors)

– Muscle contraction: walking, shivering

– Organ function modification: change in HR

– Gland secretion: salivation

• Multipolar: many dendrites and a single long axon

• Stimulated by:

interneurons

CNS

Short dendrites

Cell Body

Long Axon to effector

Sensory Neuron (afferent)

Where found: Outside of the brain and spinal cord (PNS)

Function:

bring sensory info from skin, muscles and organs to the spinal cord (CNS)

Unipolar:

Only one exit and entry on the cell body

Sensory

receptors

Sensory receptor

Long dendrites (called

an axon)

Cell Body

Short Axon leading to

spinal cord

Interneurons (Association Neurons)

Found: CNS only

Function:

Convey impulses between

various parts of the CNS

• Integrate messages

• Are involved in thinking, memory, and language processing

• Link other neurons

Multipolar: many dendrites one axon

Sensory Neuron

Short dendrites (of

interneuron)

Cell Body

Short Axon leading

motor neuron

6 Types of Neuroglial or Glial Cells

• Glia = greek word for glue

• Special types of connective tissue cells that help support and protection for nerve cells

• surround neurons and holds them in place

• supply nutrients and oxygen to neurons

• to insulate one neuron from another

• destroy pathogens and remove dead neurons.

Astrocytes = CNS

– anchor neurons to blood vessels

– Transports and regulates nutrients and wastes

Oligodendrocytes = CNS

Produces the myelin sheath

Microglia= CNS

Small glial cells that phagocytes that engulf invading microbes

Remove debris

Monitor condition of neurons

Ependymal Cells = CNS

Produces , transports and circulates cerebral spinal fluid (CSF)

Schwann Cells = PNS

Flattened cells that wrap around the axons of the PNS neurons

Satellite Cells = PNS

Flattened cells that line the exterior surface of neurons in PNS

Regulate external chemical environment

Nerve Impulse

• An electrochemical charge that travels along a nerve fiber.

• Travels about 200 meters per second

• Rapid deplolarization and repolarization of a small portion of the plasma membrane

Nerve Signal Conduction

Resting potential followed by an action potential

Resting potential • electrical charge across the plasma membrane

(interior of the cell is negative compared to the outside)

• Sodium ions are outside the cell

• Potassium ions are inside the cell

Depolarization

• Neurotransmitters open Sodium (Na+) channels

• Na+ rushes into the cell

• inside of cell is now positive (+)

Repolarization

• Potassium K+ channels open

• K+ flows out of the cell

• Inside becomes negative again

Refractory Period

• Time which the neuron is unable to conduct another action potential

• In mammals, the absolute refractory period is about 1 millisecond and the maximum firing frequency is around 1000 impulses per second

Myelinated Neuron Action Potentials

Unmyelinated Neurons action potentials

Synapses

• The space between a nerve cell and another cell

• To cross a synapse neurotransmitters are released

Transmission across a synapse

• Neurotransmitters stored in vesicles in the axon terminal

• Impulse reaches terminal

• Ca+ channels open

• Vesicles fuse with presynaptic membrane

• Neurotransmitters are released

• Neurotransmitters bind to the receptors on the postsynaptic membrane

Common Neurotransmitters

• Acetylcholine (ACh)

• Norepinephrine (NE)

• Once released must be removed from synapse

– Acetylcholinesterase (AChE) breaks down ACh

CNS: The Brain and Spinal Cord

Gray matter: cell bodies and short nonmyelinated fibers

White matter: myelinated axons

Meninges: Protective Membranes

Dura Mater (Outermost)

Arachnoid Mater

Pia Mater (Innermost)

Dura Mater • Tough, fibrous, connective tissue

• Made of two fused layers

• Separation of layers to form dural venous sinuses

– Collection of venous blood and extra CSF

Arachnoid Mater

• Spider-like connective tissue

• CSF in the subarachnoid space

– CSF-clear fluid, protective cushion

Pia Mater

• Very thin layer

• Follows contours of the brain

CSF

• Made by the ependymal cells lining the ventricles

• Fills all 4 ventricles and the central canal

• Hydrocephalus-CSF buildup in an infant

The Spinal Cord

• Is an extension of the brain

• Exits the cranial cavity via foramen magnum

• Runs through the vertebral column

• Ends at L1

– Cauda equina (horse’s tail)

The Vertebral Column

• Is made of individual vertebrae

• Joined together by intervertebral disks

– Fibrocartilage

– Disk herniation

Anatomy of the Spinal Cord

• Inner gray matter with a central canal

• Outer white matter

Spinal Nerves • Posterior (dorsal) root of a spinal nerve:

– Sensory (afferent) fibers

• Anterior (ventral) root of a spinal nerve:

– Motor (efferent) fibers

• Spinal nerve – joining of posterior & anterior roots

The Human Brain

Has many folds- Gyri

Has many valleys – Sulci

Major Sections of the Human Brain

The cerebrum

The diencephalon:

– Hypothalamus

– Thalamus

– Pineal gland

The cerebellum

The brainstem

Part Location Function

Cerebral Cortex White matter-outside covering of both brain hemispheres

Controls thinking, voluntary movements, language, reasoning and perception

Cerebellum Cauliflower shaped structure located in the lower part of brain near brainstem

Controls movement, balance, posture, and coordination

Hypothalamus * Part of limbic system, internal portion of brain under thalamus

Controls temperature, emotions, hunger, thirst, appetite, digestion, sleep

Thalamus * Part of limbic system, internal portion of brain or center of brain

Controls sensory integration and motor integration

Pituitary Gland Part of limbic system, hangs below rest of the limbic system

Controls your hormones and helps to turn food to energy

Pineal gland Part of limbic system, internal portion of brain

Controls sleep and wake cycle

Amygdala Part of limbic system, internal portion of brain

Controls emotions – regulates when you are happy or mad

Part Location Function

Hippocampus Crescent shaped and found deep in temporal lobe in front of limbic system

Forms and stores memories

Mid-brain Middle of brain behind the frontal lobe

Controls breathing, reflexes, and swallowing reflexes

Pons Superior to medulla oblongata and anterior to cerebellum

Helps control breathing, relays info to the diencephalon and cerebellum

Medulla oblongata

Superior to spinal cord Houses respiratory and cardiovascular control centers – depth and rate of breathing, rate and force of heartbeat and blood pressure, coughing, vomiting, sneezing

Corpus Callosum Connects two cerebral hemispheres,

Sensory and motor tracts that connect cerebral cortex to brain stem and spinal cord

Peripheral Nervous System (PNS)

• All the rest of the nerves in your body

• Two branches

– Somatic NS

• Body Movements

under conscious

control

– Autonomic NS

• Maintains homeostasis

Somatic NS

• 12 pairs of cranial nerves

• 31 pairs of Spinal nerves

Nerve Anatomy: Macro to micro

Nerve – covered by epineurium

Bundles of Axons – covered by

perineurium

Individual axons – covered by

endoneurium

(Sound familiar?

same prefixes as

muscle layers)

Types of Nerves

• Mixed = Both Sensory and Motor

• Sensory = sensory (afferent) towards CNS

• Enters in the dorsal root

• Motor = Motor (efferent) from CNS

• Exits the ventral root

Cranial Nerves

• 2 pairs exit from cerebrum itself

• 10 pairs exit from the brain stem

Spinal Nerves

• 31 pairs of nerves that branch off of the spinal cord

• All 31 pairs are mixed nerves

• Named from where they exit the vertebral column

Spinal Nerve Plexus

• Where nerves converge

• 4 major plexi

Cervical

Brachial

Lumbar

Sacral

Cervical Plexus

• C1 – C4

• Services the muscles and the skin of the neck

Brachial Plexus

• C5 – C8 & T1

• Services the arm and hand

Lumbar Plexus

• L1 – L4

• Services the thigh and leg

Sacral Plexus

• L4 & L5, S1-S3

• Services thigh, lower back & perineal region

Cranial Nerves

Cranial Nerve

Name Function Type of nerve

I Olfactory Sense of smell Sensory

II Optic Sense of sight Sensory

III Oculomotor Movement of eyelid and eyeball

Motor

IV Trochlear Muscles of the eyes Motor

V Trigeminal Muscles for chewing (motor) & pain and touch for face and mouth (sensory)

Mixed (both)

VI Abducens Muscles for eye movement Motor

Cranial Nerves Cranial Nerve

Name Function Type of nerve

VII Facial Sense of taste (sensory) & facial expressions (motor)

Mixed (both)

VIII Vestibulocochlear (auditory)

Sense of Hearing Sensory

IX Glossopharyngeal Sense of taste (sensory), blood pressure, tongue movement (motor)

Mixed (both)

X Vagus Innervates smooth muscle of the gut(motor) & feelings of distension/bloating (sensory)

Mixed (both)

XI Accessory Movement of neck muscles Motor

XII Hypoglossal Movement of the tongue Motor

Memory Tools: Names of cranial nerves

• Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven

• On Old Olympus' Towering Top A Finely Vested German Viewed A Hawk

• On Occasion Our Trusty Truck Acts Funny. Very Good Vehicle Any How

• You have 1 nose, 2 eyes, and 3,4,6 makes my eyes do tricks!

Memory Tools: Types of nerves

• Some Say Marry Money, But My Brother Says Big Brains Matter More

• Small Ships Make Money, But My Brother Says Big Boats Make More

• Some Say Money Matters, But My Brother Says Big Brains Matter More

• S words = sensory nerve

• M words = motor nerve

• B words = both sensory and motor

AUTONOMIC NERVOUS SYSTEM (ANS)

• Two divisions:

–Sympathetic (fight or flight)

• “pushing function”

–Parasympathetic (rest and digest)

• “relaxing function”

General Characteristics of Sympathetic and Parasympathetic

• Both divisions are automatic (involuntary)

• Both innervate all organs

• Both have 2 motor neurons and 1 ganglion

–Ganglion:

–place where cell bodies of the 2nd ANS neurons exits outside the CNS

Some Important functions of the ANS

• Regulation of smooth muscle tone

• Regulation of cardiac muscle contraction

• Control of secretions from glands

Setup of 2 ANS Neurons Neuron 1 (pre-ganglionic neuron):

• Cell body in the CNS (Spinal Cord) and the axon outside of the CNS

• Synapses with neuron 2 in the ganglion

Neuron 2 (post-ganglionic neuron):

• Cell body in the ganglion and the axon continues to the organ

• Dendrites synapse with neuron 1 in the ganglion and the axon terminals synapse with the organ

Neuron Setup in the Parasympathetic Nervous System

Neuron 1

Long pre-ganglionic axon

From spinal cord to ganglion

Neuron 2

Short post-ganglionic axon.

From ganglion to organ

The ganglion lies far from the spinal cord

Neuron Setup in the Sympathetic Nervous System

The ganglion lies close to the

spinal cord Neuron 1 short pre-ganglionic axon From spinal cord to ganglion Neuron 2 long post-ganglionic axon From ganglion to organ

Organ

Organ

Spinal Cord ganglion

Parasympathetic NS – synapse between 2 neurons is

close to the organ (effector)

Neurotransmitter - ACh

Sympathetic NS – synapse between 2 motor neurons

is close to spinal cord

Neurotransmitter = NE

Origin of Sympathetic Nerves

• Thoracolumbar

• Thoracic and lumbar portions of the spinal cord

Origin of Parasympathetic Nerves

• Craniosacral

• The Brainstem:

Cranial nerve (X)- Vagus nerve

• Lower lumbar and sacral portions

of the spinal cord

Functions of the Sympathetic Division

• prepares the body for emergencies – Fight or flight

• increases heart rate • raises blood pressure

(vasoconstriction) • dilates the pupils • dilates the trachea and

bronchi • Converts liver glycogen

into glucose • Diminished perception

of pain

• shunts blood away from the skin and organs (vasoconstriction)

• pushes blood toward the skeletal muscles, brain, and heart

• inhibits peristalsis in the gastrointestinal (GI) tract

• inhibits contraction of the bladder and rectum

Functions of the Parasympathetic Division

• The “housekeeper” division

• “Rest and Digest”

• Manages functions associated with a relaxed state

• Contraction of the pupils

• Promotes digestion of food

• Slows down the heart rate and decreased heart contraction

• Increased blood flow to the visceral organs (GI tract), normal peristalsis

Sympathetic Neurotransmitters

• Norepinephrine (NE)

• Also known as Noradrenaline

• Both a Neurotransmitter and a hormone

– Nerves – secrete neurotransmitters

– Adrenal medulla – secretes the hormone

Parasympathetic Neurotransmitters

• Acetylcholine (ACh)

• Released by cholinergic neurons

• Binds to nicotinic or muscarinic receptors

Reflexes

• Automatic involuntary responses

• Occur quickly

• Protective mechanisms

• Cranial Reflexes:

– Blinking of eyes with sudden clapping near eyes

• Spinal Reflexes:

– Quick movement of hand away from the hot pan

Reflex Arc 1. Sensory transducer

(hitting of patellar tendon)

2. Afferent neuron

3. Interneuron

4. Motor neuron

5. Effector organ

(muscle contraction)