Nervous System
The Nervous System
• Parts:
– Brain, spinal cord, nerves, sensory receptors
• Subdivisions:
– Central Nervous System (CNS)
– Peripheral Nervous System (PNS)
• Autonomic – involuntary
– Sympathetic – “fight or flight” responses
– Parasympathetic – “rest & relax” responses
– Enteric- “brain in the gut”
• Somatic - voluntary
• Functions…
– Sensory perceptions, mental activities, stimulating
muscle movements, regulates secretions of glands
Central Nervous System
• Consists of brain and
spinal cord
• The structural &
functional center of
the entire nervous
system which
integrates incoming
pieces of information
& initiates an outgoing
response
Peripheral Nervous System
• All bodily nerves
– Afferent (sensory)
– Efferent (motor)
• All pathways going
toward and away
from the CNS
Nervous System Organization
Cells of Nervous System
• Neurons or nerve cells
– Receive stimuli and
transmit action potentials
(electrical current)
• Organization
– Cell body or soma
– Dendrites: input
– Axons: output
– Myelin or No myelin
• Glia cells ( 5 types)
– Support, clean, provide
nutrition and protect
neurons
Did you know? There are 100 billion
neurons in the brain, but there are
about 10 to 50 times that many glial
cells in the brain
Types of Neurons
• Functional
Classification
– Sensory or afferent:
action potentials
toward CNS (receives
stimuli; could be a
special sense organ)
– Motor or efferent:
action potentials
away from CNS
(attached to a muscle
or gland)
– Interneurons or
association neurons:
within CNS from one
neuron to another
• Structural Classification
–
–
–
Bipolar: a single axon and dendrite arise
at opposite poles of the cell body. Found
only in sensory neurons, such as in the
Retina, Olfactory and auditory systems
Unipolar: a single process or fibre which
divides close to the cell body into two
main branches (axon and dendrite). These
are also sensory neurons
Multipolar: has numerous cell processes
(an axon and many dendrites). These are
Motor neurons
More about the neuron… 1 substance effects
the Speed of the Impulse!
• Myelinated axons
– Myelin protects and
insulates axons from
one another
– Not continuous
• Nodes of Ranvier
• Impulse “jumps” from
node to node
• Fast impulse
• Unmyelinated axons
– Slower impulse
“Saltatory Conduction”
How Neurons Send An Impulse
• Cells produce electrical signals called
action potentials
• Transfer of information from one part
of body to another
• Electrical properties result from ionic
concentration differences across
plasma membrane and permeability
Nerve Impulses – Making the Electricity
Resting Vs. Action Potential
Resting Potential
•Concentration of K+
higher inside than
outside cell
•Na+ higher outside
than inside
Nerve Impulses:
Flow of the Electrical Current
• Propagation of
Action Potential
– Ions (charged particle)
• Na+ (moves inside only)
• K+ (moves outside only)
– A wave of electrical
fluctuation that
travels along the
plasma membrane;
due to changes in
chemical
concentrations by
ions
Action Potentials
• Series of permeability
changes when a local
potential causes
depolarization of
membrane
• Phases
– Depolarization
• More positive
– Repolarization
• More negative
• All-or-none principle
– Neuron will fire or it won’t
The Synapse
• Junction between two
cells
• Electrical message
transferred across
the synapse by
chemicals called
neurotransmitters
How is an electrical impulse ignited?
One Word:Stimulus
• Any change in your
environment.
– Temp, sound, smell
• You may or may not
respond to a specific
stimulus
What receives the Stimulus?
Sensory Receptors
• In order for a stimulus to be detected, it
must be strong enough to elicit an impulse
– It must be at the threshold level- the minimum
stimulus to start an impulse
• The all-or-none response means that either
a neuron will fire or it won’t, there is no
partial impulse
• Sensation- the brain’s interpretation of
what the stimulus is
Classification of Receptors
1. Mechanoreceptors- activated by
mechanical stimuli or deformation of the
receptor
2. Chemoreceptor- changing of the chemical
concentrations around the body
3. Thermoreceptors- detect hot and cold
4. Nociceptors- any stimuli that can cause
tissue damage; sensation of pain
5. Photoreceptors- respond to light
Receptors, Stimulus, & Body Actions
•
•
•
A Stimulus ignites a Receptor
The Receptor receives the message that the Stimulus is sending
The message is sent and a Body Action occurs
•
There are three kinds of actions that our body carries out, namely
voluntary actions, involuntary actions or reflex actions and conditioned
reflex actions. All three actions involve stimuli, an impulse, neurons and
effectors. However they do have their differences.
•
A reflex is a direct connection between stimulus and response, which
does not require conscious thought.
–
Voluntary Reflex is one of your own free will or design; done by choice; not forced or
compelled.
Body Actions
•
Voluntary Actions:
A voluntary action is basically an action in which you initiate by your own
conscious.
–
•
For example, when you see a friend over at the other side of the room, you wish to
attract the attention of that friend of yours, hence you may want to wave your hand
and call out his name. This is done by your brain by sending impulses from it to the
effectors or in this case, your biceps and triceps muscles and also your larynx, via
relaying neurons, the spinal cord, synapes and motor neurons. The impulses upon
reaching their respective effector muscles cause the waving of your hand and you to
shout out your friend's name. This action is under the control of the will thus is known
as a voluntary action.
Involuntary Actions (Reflex Actions):
A voluntary action is under the control of one’s will, involuntary actions as their
name suggest, are total opposites of voluntary actions.
•
In this case, your spinal cord or your brain takes total control, without your own
conscious, depending on where the stimuli originate. Reflex actions controlled by the
spinal cord, example scratching, are called spinal reflexes while those by the brain,
example blinking, are called cranial reflexes. Blushing, sneezing and salivation are also
reflex actions however, salivation is also known an conditioned reflex action.
Reflexes
•
•
A reflex is a response to a perturbing stimulus that acts to return the body to
homeostasis. This may be subconscious as in the regulation of blood sugar by
the pancreatic hormones, may be somewhat noticeable as in shivering in
response to a drop in body temperature; or may be quite obvious as in stepping
on a nail and immediately withdrawing your foot.
Reflexes require a minimum of two neurons, an sensory neuron (input) and a
motor neuron (output).
The sensory neuron (such as a pain receptor in the skin) detects the stimuli and
sends a signal towards the CNS. This sensory neuron synapses with a motor
neuron which innervates the effector tissue (such as skeletal muscle to pull away
from the painful stimuli). This type of reflex is the "withdrawal" reflex and is
monosynaptic, meaning only one synapse has to be crossed between the sensory
neuron and the motor neuron. It is the simplest reflex arc and the integration
center is the synapse itself. Example: Tendon Jerk
–
Reflexes
•
Polysynaptic reflexes are more complex and more common. They involve
interneurons which are found in the CNS. More complex reflexes may
have their integration center in the spinal cord, in the brainstem, or in
the cerebrum where conscious thoughts are initiated.
In humans: the polysynaptic reflex is the sudden movement to protect life
and limb. An example usually given is walking in a shallow pond and stepping
on a sharp object. The foot immediately raises before you are voluntarily
aware of pending danger. It is more complex than the monosynaptic reflex
because prolonged output from the spinal cord is needed to process “Am I in
danger? or Am I hurt?” The monosynaptic reflex doesn’t need the process
time to initiate a “stretch or jerk”.
Reflex Arc
A predictable response to a stimulus which may or may not be conscious
– A reflex consists of either muscle contraction or glandular secretion
– Neurons involved in reflex
• Afferent neuron- sensory
• Interneuron
• Efferent neuron- motor
Process:
1) Detection of signals from outside environment or detection of deviation change)
from homeostasis from internal environment.
2) Integration of multiple signals from outside and inside to produce appropriate
response.
3) Response to counteract stimulus being detected
•
•
Reflexes Vs. Reactions
Many people consider only the simplest types of responses as "reflexes", those
that are always identical and do not allow conscious actions.
We must not confuse these with "reactions", which are different from reflexes
in that they are voluntary responses to a stimulus from the environment.
–
For example, while the body has various subconscious physiological responses to
mitigate cold, as humans we can simply choose to put on more clothes. This is a
conscious order made by the cerebrum, not an involuntary response to a stimulus. This
is a very complex response involving millions of neurons and some time to process the
voluntary response.
–
In contrast, spinal reflexes occur much faster, not only because they involve fewer
neurons, but also because the electrical signal does not have to travel to the brain and
back. Spinal reflexes only travel to the spinal cord and back which is a much shorter
distance. Because of this and the complexity of conscious reactions, they take more
time to complete than a reflex. On average, humans have a reaction time of 0.25
seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch
stimulus. Did your reaction time fall within the average during the Reaction Time Lab?
–
Reaction times vary from individual to individual. Because of the higher degree of
neural processing, reaction times can be influenced by a variety of factors. Reaction
times can decrease with practice, as often times athletes have faster reaction times
than non-athletes. Sleepiness, emotional distress, or consumption of alcohol can also
impact reaction time.
What about the processing/integration center
OR better known as the BRAIN!
Organization of the Brain
• Hindbrain
– Adjacent to top part of spinal cord
• Midbrain
– Rises above hindbrain
• Forebrain
– Uppermost region of brain
Hindbrain
• Medulla
– Controls vital functions, such as breathing
and heart rate
– Regulates reflexes
• Cerebellum
– Plays important role in motor coordination
• Pons
– Involved in sleep and arousal
Midbrain
• Brain stem
– Includes much of hindbrain (but not cerebellum)
and midbrain
– Determines alertness
– Regulates basic survival functions
• Reticular Formation
– Involved in stereotyped patterns of behavior, such
as walking and sleeping
Forebrain: Limbic System, Thalamus, &
Basal Ganglia
•
Important in both memory and emotion
•
Two principal structures
– Amygdala
• Involved in discrimination of objects necessary for survival
– Hippocampus
• Has special role in storage of memories
•
Thalamus
– Serves as relay station for information
•
Basal Ganglia
– Works with cerebellum and cerebral cortex to control and
coordinate voluntary movements
Forebrain: Hypothalamus
• Monitors . . .
– eating, drinking, sex
– emotion, stress, reward
• Helps direct endocrine system
• Regulator of body’s internal state
• Involved in pleasurable feelings
Forebrain: Cerebral Cortex
•
Occipital lobes
– Responding to visual
stimuli
•
Temporal lobes
– Hearing, language
processing, memory
•
Frontal lobes
– Personality,
intelligence, control of
voluntary muscles
•
Parietal lobes
– Registering spatial
location, attention,
motor control
Somatosensory Cortex
Located at front
of parietal lobes
Processes
information about
body sensations
Motor Cortex
Located just
behind frontal
lobes
Processes
information about
voluntary
movement
Association Cortex
Makes up 75% of
cerebral cortex
Integrates
information
Last but NOT Least…. Spinal Cord
PORTAL for the
nerve pathways
that carry
information from
the arms, legs,
and rest of the
body, and carries
signals from the
brain to the body.