Download Nervous System I

Nervous System I
Function
• Input: sensation of matter and energy in
the environment.
• Integration: perception, memory, control
of involuntary body functions, “thinking”
• Output: control of voluntary and
involuntary body functions and activities.
A basic model of NS function
INPUT → PROCESSING → OUTPUT
afferent
sensory
integration
efferent
motor
Divisions of the Nervous System
• Central (CNS)
–Brain & spinal cord
• Peripheral (PNS)
–Cranial and spinal nerves connecting
CNS to all body parts
• Afferent = incoming nerves
• Efferent = outgoing nerves
Expounding on the basic model
1. Sensory receptors release nerve
impulses in response to stimuli
2. Impulse transmitted over peripheral
nerves to CNS
3. Integration of all input
– Sensation
– Thought
– Stored to memory
Integration
Expounding on the basic model
4. “Decision” made
– conscious or subconscious
5. Impulse sent to effectors for action
– muscles
– glands
Motor Divisions
• Within the motor system there are two
divisions
–Somatic: voluntary control
• Skeletal muscles
–Autonomic: involuntary control
• Cardiac muscle
• Smooth muscle
• Glands
Autonomic Divisions
• Parasympathetic
–Relax mode
• Sympathetic
–Stress mode
You’re using both at all times, the
balance varies depending on what
you’re doing and your mental state
Sympathetic
Parasympathetic
Heart rate
↑
↓
Blood pressure
↑
↓
Digestive enzymes
(salivary glands,
stomach, etc.)
Pupil size
↓
↑
↑
↓
Function
Bronchial tubes of lungs
(dilated)
(constricted)
↑
↓
(dilated)
(constricted)
↑
↓
General immune system
function
↓
↑
Epinephrine from adrenal
gland
↑
↓
Blood sugar release from
liver glycogen
CNS Processing
• Messages from peripheral nerves follow
condensing pathways into the spinal cord
• The cord is the junction between the PNS
and CNS
–Ascending nerve tract: sensory to brain
–Descending nerve tract: motor impulse to
effectors
• The brainstem connects the brain and spinal
cord
Spinal Cord
• Each vertebrae corresponds to a spinal
nerve
• Nerve contains a dorsal (sensory) root
and a ventral (motor) root
–Dorsal root ganglion – point of entry
into the spinal cord
Meninges
• The brain and spinal cord are protected
by layers of membranes called meninges
–Dura
• Vascular, innervated
–Arachnoid
• Avascular, innervated
–Pia
• Vascular, innervated
• These layers help create
the “blood-brain barrier”
– Own circulation system
using CSF
• neuron
Neuron Structure
• Dendrites = receivers
–Sensory receptor cells
–Other neurons
• Cell body carries out normal cell processes
–Manufactures neurotransmitters
• Axons= conductors
–Sends impulse away from cell body
–Transports chemicals produced in body to
the synaptic knob
Neuron Variants
Can be classified by structure or by
function
• Sensory = afferent, carry into CNS
• Interneuron – completely within CNS
• Motor= efferent, carry out of CNS
Peripheral Neurons
• Schwann cells wind around the axon
creating a myelin sheath (lipids)
• Gaps between Schwann cells called
nodes of Ranvier
This allows for a faster, more efficient
transmission of impulses
getting on your nerves
• A nerve is a bundle of axons of peripheral
neurons
–organization
• A ganglion is a bundle of cell bodies
–Provide point of consolidation for both
afferent and efferent impulses
CNS Glial Cells
Astrocytes: blood-brain barrier, induce
synapse formation
CNS Glial Cells
Oligodendrocytes: form myelin sheaths,
produce growth factors
CNS Glial Cells
Microglia: structural support, immune
function (phagocytosis)
CNS Glial Cells
Ependyma: line ventricles in brain,
regulate composition of CSF
Glial Cells
• Like neurons, arise from neural stem cells
• Combined, the CNS glial cells form over ½
the volume of your brain
• The only glial cells found in the PNS are
Schwann cells – form myelin
Regeneration
• Cell body damage = cell death
–Cell not replaced unless neural stem
cells stimulated
• Axons can regenerate…slowly
–Often don’t end up in right place
–In CNS regeneration is unlikely because
oligodendrocytes don’t proliferate like
Schwann cells to form sheaths for
guidance
Polarization
• All cell membranes are electrically
charged, or polarized, due to unequal
distribution of ions
–K+ intracellular
–Na+ extracellular
–Na+/K+ pumps maintain the polarization
• This maintains a negative charge inside the
cell and a positive charge outside the cell
Potential
• Since the membrane is charged, it has a
measurable voltage – called membrane or
resting potential
-70mV
• Typically, a stimulus will open an ion
channel in the membrane causing a local
depolarization
–Bigger stimulus, bigger depolarization
Depolarization
• Generally, the depolarization caused by a
single stimulus is not sufficient to cause a
neuron to fire
• For this to occur, the threshold stimulus
must be reached
–Defined level at which impulse will be
generated
–Effects of multiple stimuli are summative
allowing the threshold to be reached
Polarization
• All cell membranes are electrically charged,
or polarized, due to unequal distribution of
ions
– K+ intracellular
– Na+ extracellular
– Na+/K+ pumps maintain the polarization
• This maintains a negative charge inside the
cell and a positive charge outside the cell
Potential
• Since the membrane is charged, it has a
measurable voltage – called membrane or
resting potential
-70mV
• Typically, a stimulus will open an ion
channel in the membrane causing a local
depolarization
– Bigger stimulus, bigger depolarization
Depolarization
• Generally, the depolarization caused by a
single stimulus is not sufficient to cause a
neuron to fire
• For this to occur, the threshold stimulus
must be reached
– Defined level at which impulse will be
generated
– Effects of multiple stimuli are
summative allowing the threshold to be
reached
Action Potential
• The input from stimuli is compounded at the
attachment point of the axon – called the trigger zone
• Axons are the only part of a neuron that can generate
an action potential
– Voltage change across the membrane
Action Potential
At rest, Na+ channels are closed. Once
threshold is reached:
1. Na+ channels open
2. Na+ diffuses into the cell
3. Membrane potential becomes positive =
depolarization
4. Na+ channels close as a result but K+
channels open
Action Potential
K+ diffuses out of the cell
Membrane potential returns to negative
K+ channels close
Re-establishment of resting potential until
next stimulus
All of this causes a current to flow a short
distance which stimulates the next section
of the membrane to do the same
5.
6.
7.
8.
Impulse
• All of this continues down the length of the
axon – this is the nerve impulse
– Value does not decrease even with
branching
• There is no partial response – it’s all-or-nothing
– The neuron either fires or it doesn’t
– A stimulus above threshold does not cause
stronger impulses only increased frequency
Refractory Period
• There is a refractory period where an axon will not
respond to another threshold stimulus
– When ion channels are open
– During re-establishment of resting potential
a. Resting potential
-70 mV
b. Depolarization
· Na+ ion channels open
· Na+ rush into cell
· Membrane potential
changes
from –70mV to +35 mV
c. Repolarization
· Na+ gates close & K+ gates
open
· K+ rush out of cell
· High K+ outside cell &
high Na+ inside cell
d. Hyperpolarization
· More K+ moved out than was
necessary
· Neuron cannot be stimulated
e. Refractory period
· Na+/K+ pumps move Na+ out
of
cell and K+ into cell
· Reestablish original
distribution of ions
Benefit of Myelin
• Unmyelinated axons conduct impulse
over entire surface, which is slower
• Myelin causes the impulse to jump down
the axon from node to node – called
saltatory conduction
– Lipids insulate, preventing the outflow
of ions
– The nodes of Ranvier, however, are
covered in Na+ and K+ channels
Saltatory Conduction
When stimulated to threshold:
1. Action potential is generated at trigger
zone
2. Electric current flows through cytoplasm
in axon
3. Current reaches node, stimulating the
membrane to threshold
4. New action potential generated and
process repeats until terminal end of axon
is reached
Synaptic Transmission
• Nerve impulses pass from neuron to
neuron at synapses
– Incoming = presynaptic neuron
– Outgoing = postsynaptic neuron
• The synaptic knob of the presynaptic
neuron forms a synapse with the
dendrites of the postsynaptic neuron
So NOW What?
• When the impulse reaches the synaptic knob it triggers
the opening of calcium channels
• Ca2+ moves into the cell which induces the vesicles to
bind to the membrane and release the
neurotransmitters
Synaptic Potential
• The neurotransmitters diffuse across the
cleft and react with receptor molecules on
the postsynaptic neuron
• This triggers EITHER an opening or a closing
of ion channels
• Since this is triggered chemically rather
than electrically, it’s termed synaptic
potential – which can either excite or
inhibit the neuron
Synaptic Potential
• Multiple inputs are constantly coming in to a
neuron
inhibitory > excitatory ==> no impulse
conducted
inhibitory < excitatory ==> action potential
triggered
IF threshold is reached
Decision-making
• In the CNS, interneurons are organized into
neuronal pools
• Each pool can have an excitatory or inhibitory
effect on another pool or peripheral effectors
– The “decision” stems from summation at
the axon
– If excitatory, the whole process begins
anew – but in the opposite direction – to
send the message to the effectors for
action