Transcript Motor neuron

Chpt. 34
The Nervous
System
• Nervous System and Endocrine System are responsible
for coordination of activities in the body
• A Nervous System allows an organism to detect and to
respond to stimuli in its internal or external
environment
• The nervous system consists of:
- CNS (central nervous system) – BRAIN & SPINAL
CORD
- PNS (peripheral nervous system) – SPINAL NERVES
Responding to Stimuli
Four processes involved:
• Reception – stimulus detected by neurons and
sense organs
• Transmission – neurons in the PNS carry
messages from receptors to the
CNS and from the CNS to effectors
(e.g. muscles)
• Integration – incoming messages sorted,
processed and a response decided
upon occurs in CNS, especially in
brain.
• Response – carried out by effectors (muscles,
glands) when stimulated by neurons
Neurons
Functional unit of the nervous system is the NEURON
( nerve)
It connects with many other nerves receiving and
transmitting information
A nerve is bundle of neurons that connect one part
of the nervous system with another
3 types of neurons:
- Sensory neuron
- Motor neuron
- Interneuron
Sensory (afferent) Neuron:
Takes messages FROM sense organs TO the CNS
Motor (efferent) Neuron:
Bring messages FROM the CNS TO the effectors of the
body e.g. muscles
Interneuron:
Connect the sensory and motor neurons and are only
found in the CNS
Structure of Neurons
Structure of Neurons
Nerve Endings: connect sensory neurons to
receptor cells or sense organs
Cell Body: contains nucleus and cell organelles.
Control centre, specific neurotransmitter
receptors, synthesis of neurotransmitters.
Note: a group of cell bodies located outside CNS
is called a ganglion
Dendrites: initiates an impulse in a neuron
sending it towards the cell body
Axon: - carries impulses away from the cell body
- many axons combine to form nerve fibre
Axon: - end of each axon forms many axon terminals
each of which ends in a neurotransmitter swelling.
Synaptic Knob (neurotransmitter swelling): secretes a
chemical transmitter (neurotransmitter) that passes an
impulse from one neuron to the next
Myelin Sheath: fat rich membrane which insulates
electrical impulses.
Schwann Cells: - located along length of neuron
- secretes the myelin sheath
Note: MS is caused by degeneration of the myelin
sheath in the CNS
Nodes of Ranvier: gaps in the myelin sheet which help to
speed up the passage of the electrical impulse
Note: An interneuron is a short
neuron found in CNS not enclosed in
myelin sheath!!!!!
Nerve Impulses and their Transmission
Resting Neuron: in its resting state inside of axon is
negatively charged and outside positively charged
Transmission:
• when neuron receives stimulus of sufficient strength
an electrical impulse travels along the dendrite and
axon to the neurotransmitter swellings
• electrical impulse involves movement of ions
• the neuron will only carry messages once there is a
stimulus of sufficient strength at the dendrites. If the
stimulus is not strong enough, no message is
transmitted
Threshold:
• minimum stimulus needed to cause an impulse to be
carried
• once the threshold (minimum intensity of stimulus
needed to generate an impulse) level is reached, a
message is transmitted
All or Nothing Law:
• if threshold is reached an impulse is carried, but if
threshold is not reached no impulse is carried.
• the strength of an impulse is always the same. A
strong stimulus does not cause a stronger impulse but
can cause the receptors to send lots more impulses at
increased occurence.
Movement of Impulse:
• Once threshold reached:
- at site of stimulation the inside of
axon becomes positively charged and the
outside negatively
- this change in charge causes next section of
axon to react in a similar way
- a chain rxn occurs and a movement of positive
charge runs along the inside of the axon.
Note: ATP is required for this chain rxn to occur
- once impulse has moved along, the area it has
passed through returns to its resting state
Refractory Period:
Each section of neuron can only carry one impulse at a time
thus there is a delay of approx 5ms between any two
impulses. This delay is called the REFRACTORY PERIOD.
Speed of Impulse:
• dependent on whether myelin sheath is present or
absent around the neuron
• If myelin absent speed of impulse reduced to approx 2 m/s
• if myelin present impulse can jump from one Ranvier
node to the next – speed approx 120 m/s
• speed also dependent on width of axon/dendrite:
- the larger the diameter the faster impulse travels
Synapse
Synapses are specialised junctions (regions) between a
neuron and it’s target cell. The target cell may be
another neuron, muscle or gland cell
Synaptic Cleft: tiny gap between two neurons at synapse
When the impulse reaches the end synaptic knob, the
electrical impulse cannot be transmitted to the next
neuron as there is a tiny gap called the SYNAPTIC CLEFT
between each neuron
In the synaptic knob are vesicles containing chemicals
called NEUROTRANSMITTERS e.g acetylcholine, dopamine
These chemicals are released when the impulse arrives at
the synapse. They diffuse across the synaptic cleft
causing an impulse to start in the second neuron
Enzymes are released into or are present in the gap which
break down the neurotransmitters. As a result only one
impulse is sent each time a neurotransmitter is released
The broken down neurotransmitters are reabsorbed by
the neuron and used to make more neurotransmitters
The next neuron can be excited or inhibited --- when
neurotransmitters ceases due to over stimulation, the
impulse is inhibited and the effector ceases to be
stimulated
Summary of events at synapse:
electrical
chemical
electrical
impulse
impulse
impulse
SYNAPSE & NEUROTRANSMITTERS
Functions of Synapses
• Transmit impulses from one neuron to another or to
an effector.
• Control direction of impulse i.e. only allow a one way
flow.
• Only one part of the body reacts to stimulus
• Nerve pathways in the brain can become established
for fast reactions
• Impulse can be blocked by certain chemicals –
anaesthetics, depressants etc.
The Central Nervous System
Consists of:
- the BRAIN
- the SPINAL CORD
The BRAIN:
The BRAIN:
Acts as an interpreting centre to sort and process incoming
impulses and decide on response.
Brain and spinal cord are protected by bone and covered by
3 membranes called MENINGES
Meningitis is an inflammation of the meninges. There are
two causes of meningitis – a virus and a bacterium
Viral: less severe infection
causes headache, fever, neck ache
no specific treatment
Bacterial: dangerous form
causes skin rash, vomiting, convulsions,
coma, death
treated with antibiotics
THE BRAIN
CEREBRUM:
Carries out conscious thought processes and voluntary
actions
CEREBELLUM:
Controls balance and muscle co-ordination
MEDULLA OBLONGATA:
Connects the spinal cord with rest of brain
Controls breathing and heartbeat
HYPOTHALMUS
mainly concerned with homeostasis (maintaining a
constant internal environment) it monitors levels of
sugars & hormones in the blood
THALAMUS:
Acts as a sorting centre, relaying different messages to
relevant parts of the brain
PITUITARY GLAND:
Controls the secretions of most endocrine glands in the
body
Nervous System Disorder
Parkinson’s Disease
Is a neurodegenerative disease that affects voluntary
control of muscles in the body
Description:
•Persistent tremor, especially the limbs
•Later muscles and body become stiff and rigid
•Thought processes affected late in the course of
disease
Cause:
Degeneration of a specific group of brain cells that
secrete dopamine which is responsible for the
stimulation of motor neurons. When dopamine
production is depleted, motor neurons are unable to
control movement & co-ordination
Prevention:
No known preventative measures
Treatment:
No cure available
Drugs that mimic the action of dopamine or that can
be converted to dopamine in the brain are used in
treatment. Dopamine secreting cells can be implanted
into the brain
The SPINAL CORD:
• Composed of nerve tissue and surrounded by vertebrae
(protection)
• Transmits impulses to and from brain and also controls
many reflex actions
• The spinal cord extends down through the vertebral
column (neural canal - lined by meninges) of the back
• Parts of peripheral nerves are attached to the spinal
cord along its length
Parts of the SPINAL CORD:
• GREY MATTER
Contains a lot of cell bodies and dendrites of
interneurons & motor neurons that are not myelinated
• WHITE MATTER (axons only)
This contains a lot of nerve fibres that are myelinated ie.
axons connecting with neurons in the brain
• CENTRAL CANAL (centre of grey matter)
Contains cerebrospinal fluid to supply food and
oxygen to the spinal cord
• DORSAL ROOT:
Contains fibres of the sensory neurons bringing the
nerve impulses INTO the spinal cord
• VENTRAL ROOT:
Contains fibres of motor neurons bringing the nerve
impulses OUT of the spinal cord
•MENINGES:
3 layers of membranes that surround the spinal cord
and the brain with fluid to nourish them & cushion
them from hitting against the backbone & skull
Spinal Cord & Transmission of Impulse
Reflex Action
• Automatic, involuntary, unthinking response to a
stimulus i.e. it is the automatic response to a stimulus
which is not controlled by will
• The function is to protect the body against harm
• A REFLEX ARC is a specific neural pathway involved in
bringing about a reflex action
• Examples of reflex actions:
blinking, breathing, protective actions taken when
falling etc.
Reflex Arc Pathway:
1. Receptors to pressure & pain are stimulated
2. Sensory neurons carry the impulses to the spinal
cord by way of the dorsal root
3. The sensory neuron synapses with many neurons in
the spinal cord of the CNS:
- an interneuron may carry the signal to the brain
to ’advise it’ about the situation.
- another interneuron carries the impulse to a
motor neuron
4. A motor neuron carries the impulse from the spinal
cord to the effector
5. The effector responds to the impulse in a predictable
way by contracting or secreting while at the same
time the impulse reaches the brain allowing us to
feel pain
REFLEX ACTION --- The Reflex Arc
Route Along a Reflex Arc
• Receptor
• Sensory neuron
• Spinal cord
• Interneuron
• Motor neuron
• Effector