Nervous System part i
What is the nervous system?
The nervous system
 The master controlling and communicating system of the
body
 Every thought, action, and emotion shows the nervous
system’s activity
How does the nervous system Work?
How does the nervous system Work?
 Electrical signals are how the brain communicates with
the nerves
 Do you think these signals are fast or slow?
Organization of the Nervous
system
Organization
 The nervous system can be broken down into structural and
functional classification
Structural Classification
(What’s it made up of)
 2 subdivisions:
CNS
 PNS

1. CNS
 Central nervous system made of the brain and spinal cord
 Interprets information and responds
2. PNS
 Peripheral nervous system is made up of the nerves outside
the CNS
 Links all parts of the body together through electrical
impulses
Functional Classification
(What does it do)
 Only deals with PNS structures (nerves)
 2 major divisions:
Sensory division (afferent)
 Motor division (efferent)

Sensory division (afferent)
 Convey information TOWARD the CNS from sensory
receptors
Motor division (Efferent)
 Carry impulses AWAY from the CNS to muscles for
movement and glands for hormone secretion
 There are 2 types of motor nerves:
2 types of motor nerves
 Somatic: VOLUNTARY impulses
 Ex) Skeletal muscles
 Autonomic: INVOLUNTARY impulses
 Ex) Cardiac and smooth muscle
 Ex) Glands
 2 types of autonomic nerves
2 types of autonomic responses
Sympathetic: “fight or flight”
2. Parasympathetic “rest and digest”
1.
The 3 steps to Normal activity
 1. Sensory input
 2. Integration
 3. Motor output
1. Sensory input
 The nerves contain sensory receptors to monitor changes
in and out of the body (stimuli) to send to the CNS
2. Integration
 The brain/spinal cord processes the sensory input and
makes a decision about what must be done.
3. Motor output
 A response is then created by activating muscles or glands
(away from CNS)
Reflex Arc
Nervous Tissue
 Complex nervous tissue can be separated into two overall
types of cells:
 Supporting cells
 Neurons
Supporting cells in the CNS
 Called neuroglial cells, or simply called glia
 Each type of glial cell has a unique shape and function
1. Astrocytes
 Star-shaped (hence the name)
 Account for nearly half of neural tissue
 Attached to blood capillaries and neurons
Functions of astrocytes:
 1. Protect neurons from harmful substances in blood (BBB)
 2. Help control chemicals in the brain (Na+, K+, CO2)
2. Microglia
 Look like spiders
 Function to dispose of debris in the brain
3. Ependymal cells
 Thin coat with cilia line cavities of CNS tissue and move
fluid around brain and spinal cord
 Forms a protective cushion around the CNS
4. Oligodendrocytes
 Made up mostly of myelin
 Wrap tightly around nerve fibers to produce myelin sheaths
Supporting cells of the PNS
 2 types:
 1. Schwann cells that help form the myelin sheaths around
nerve fibers
 2. Satellite cells that act as protective cushioning around
cell bodies
Neurons
 AKA nerve cells
 Transmit messages sent as electrical signals (nerve
impulses)
General neuron
Cell Body (soma)
 Contains the nucleus; is the metabolic center of the cell
Dendrites
 Conduct electrical currents TOWARD the cell body, and to
RECEIVE information
Axons
 Generate nerve impulses and send them AWAY from the
cell body
Collateral branch
 A branch of the same axon
 Allow a single neuron to communicate with several other
cells
Schwann cell (PNS only)
 Form the myelin sheath
Myelin Sheath
 Fatty, protective coating around the axon
Nodes of Ranvier
 Gaps or indentations in between Schwann cells
 Occur at regular intervals
Axon terminals
 Branching ends of axons
Synapse
 End of the axon terminal releases the neurotransmitter
Synaptic cleft
 Where an axon terminal interacts with another neuron
General Neuron
Organization of the nervous system
Nervous System
PNS: nerves
CNS:
Brain and
spinal cord
Motor/efferent
Sensory/afferent
Autonomic
Somatic
Sympathetic
Parasympathetic
Classification of Neurons
 2 ways to classify neurons:
Function
 Structure

Functional Classification
 Groups neurons based on what direction the electrical
impulse is travelling related to the CNS
 3 general types
1. Afferent Neurons
 AKA sensory neurons
 Carry impulses from the sensory receptors to the CNS
 Sensory neurons keep us informed about what is happening
inside and outside the body
2. Efferent neurons
 AKA motor neurons
 Carry impulses from the CNS to muscles and glands
 The cell bodies of these neurons are always located in the
CNS
3. Interneurons
 Connect the motor and sensory neurons in neural pathways
 Cell bodies are always located in the CNS
Structural classification
 In structural classification, neurons are classified based on
the number and type of projections off the cell body
1. Anaxonic
 Small, have no anatomical clues to distinguish
dendrites from axons
 Found in special sense organs
2. Multipolar
 Most common type
 1 axon
 Many dendrites
 CNS & motor neurons
3. Bipolar
 Most rare type
 2 processes
 1 axon
 1 dendrite
 Sight, smell, & hearing
4. Unipolar
(pseudounipolar)
The dendritic and axonal processes
are continuous, cell body lies off to the
side
Common in sensory neurons
Nerve impulses
 Neurons have 2 major functional properties:
Irritability
 Conductivity

Irritability
 Ability to respond to a stimulus and convert it to a nerve
impulse
 So in order to function, the neuron must first receive a
stimulus, then be able to send the message out
 Let’s then take a look at possible stimuli, then observe how
an impulse is sent out
Stimuli
 Most neurons are stimulated by neurotransmitters (found
in the synaptic cleft)
 However, different types of neurons are stimulated by
different stimuli
 Ex) light excites eye receptors
 Ex) pressure excites cutaneous receptors
Electrical impulses (How it works)
 The following 4 steps are going to explain exactly how an
electrical signal is created in the neurons
 This process is similar to the process in muscles and follows
the same concept
Step 1: Resting state (resting potential)
 At rest, the neuron has a negative charge inside (where K+
ions are) the neuron and a positive charge outside (where
there are more Na+ ions)
Step 2. Generation of action potential
 The nerve is stimulated , which opens up “sodium gates” to
allow Na+ into the neuron
 This “charges” up the neuron, generating electricity (action
potential)
Step 3. Propagation of action potential
 When the neuron is fully charged, the charge is released
 This sends an electrical impulse through the neuron
Step 4: Repolarization
 Once the charge is released, K+ leaves the neuron,
returning the neuron to a negative charge
 Until repolarization is finished, a neuron cannot conduct
another impulse
Action potential video
 https://www.youtube.com/watch?v=OZG8M_ldA1
M
Conductivity
 The ability to transmit an impulse to other neurons,
muscles, or glands
reflexes
 When conductivity occurs across a muscle or gland, it is
called a reflex
 A series of reflexes across many neurons is called a reflex
arc
Autonomic reflexes
 Involuntary reflexes
 Stimulate smooth muscles, heart, and glands
Somatic reflexes
 Voluntary reflexes
 Stimulate skeletal muscles
Nerve fiber classification
 Just like neurons, nerves are classified the same way
Nerve Fiber Classification
 Mixed nerves carry BOTH sensory and motor fibers
 Sensory nerves carry information TOWARD the CNS
 Motor nerves carry information AWAY from the CNS to
muscles and glands etc
Somatic nervous system
 Voluntary impulses
 Effects skeletal muscles
1. Sympathetic division
 Often referred to as the “fight or flight” system
 Signs of activity of this division are:
Pounding heartbeat
 Rapid breathing
 Cold, sweaty skin
 Dilated eye pupils
 Increased adrenaline

How long do these signs last?
 Effects usually last several minutes, until the liver dissolves
released hormones
2. Para-sympathetic division
 Most active when the body is at rest
 Promotes normal digestion, elimination of urine and feces,
and conserving body energy (especially concerning the
cardiovascular system)
Lack of oxygen
 Since the nervous system has the highest metabolic rate in
the body, lack of oxygen for even just a few minutes leads to
death of neurons
Older age
 Neurons cannot reproduce themselves, so we lose neurons
as we age, but cannot replace them
 Loss of neurons and lack of oxygen lead to senility;
symptoms:
 Forgetfulness
 Irritability
 Difficulty concentrating