Ral OVERVIEW OF THE NS (CHP 8)
The nervous system can be divided into two parts:
1. Central nervous system (CNS): contains the brain and
the spinal cord
a. The integrating center: receives input from the
sensory system of the PNS and determines whether
an output is needed which is executed via the PNS
2. Peripheral nervous system (PNS): contains sensory
(afferent) neurons and efferent neurons
a. Communicates input (to CNS) through afferent
(sensory) neurons
b. Executes output signals through efferent neurons
i. The somatic motor division: controls
skeletal muscles
ii. The autonomic motor division: controls smooth and cardiac muscles, exocrine
glands, some endocrine glands, and types of adipose tissue. Possesses two divisions
1. Sympathetic
2. Parasympathetic
The enteric nervous system is technically a third part. It consists of a third network of neurons that is found
in the walls of the digestive tract.
- Frequently controlled by the autonomic division of the nervous system
- However, also able to function autonomously as its own integrating center
CELLS OF THE NS (CHP 8)
The Cells of the nervous system consist of two types: glial cells (support cells) and neurons (signalling cells).
Neuron Structure:
Neurons are the functional unit of the nervous
system. They possess long processes called
axons and dendrites that extend outwards from the
nerve cell body.
,Neurons can be classified based on the number of
processes they possess.
- Multipolar neurons: neurons possessing many
dendrites and axons
- Pseudounipolar neurons: neurons with a cell
body located off on one side, with a single long
axon
- Bipolar neurons: neurons with a single axon
and single dendrite.
- Anaxonic neurons: neurons possessing no
apparent axon, but numerous dendrites.
Neurons can also be classified based on the information they transmit.
- Sensory neurons carry information about temperature, pressure, light, and other stimuli from
sensory receptors to the CNS.
- PNS sensory neurons are pseudounipolar, with cell bodies close to the CNS and long
processes that extend out to receptors in the limbs and internal organs
- Interneurons are responsible for communication with neurons within the CNS.
- Lie entirely within the CNS and often have complex branching processes.
- Efferent neurons have axons that may divide several times into branches called collaterals and
have enlarged axon terminals (used to stored neurotransmitter)
Axons of both afferent and efferent PNS neurons are bundled together with connective tissue into nerves.
- Nerves carrying only afferent information = sensory nerves
- Nerves carrying only efferent information = motor nerves
- Nerves carrying both efferent and afferent = mixed nerves
Dendrites:
Dendrites receive incoming signals.
- In the PNS, dendrites function to receive incoming information and transfer it to ther integrating center
of the neuron.
- In the CNS, role may be more complex, also sending signals back and forth. Made possible by
dendritic spines.
- Possess dendritic spines, extensions of the dendrites capable of changing structure in response to
signals.
,Axons:
Axons carry outgoing information.
- Most peripheral neurons possess a single axon that originates from the cell body via the axon hillock
- The axon extends and may branch to form collaterals
- Collaterals feed into the bulbous axon terminals
- Axon terminals communicate electrical signal via secretion of neurotransmitter
- Primary function is to transmit outgoing electrical signals.
- Most neurons complete this via NT, but some CNS can do this directly via gap junctions.
Axon terminals lack ribosomes and endoplasmic reticulation.
- All proteins destined for transport must be synthesized in the cell body and then transported to the
terminal (axonal transport)
- Slow transport relies on moving material via axoplasmic or cytoplasmic flow
- Fast transport relies on moving material via microtubules tracks on the axon and requires carrier
proteins kinin and dynamin
- Forward transport = anterograde transport, backward transport = retrograde transport
Synapse
The synapse is the region where an axon terminal of the presynaptic neuron meets the target
postsynaptic cell. The space between the two is called the synaptic cleft.
- Majority of synapses are chemical synapses, in which NT is transmitted.
- Otherwise, electrical synapses, operating using gap junctions.
In the developing embryo, axons of embryonic nerve cells send out tips called growth cones.
- Growth cones use chemical signals from growth factors and other membrane proteins to guide their
way to its target cell.
- Neurotrophic factors, released by neurons and glial cells determine the survival of a neuronal
connection → “use it or lose it”
, Glial Cells:
Glial cells are the support cells of the nervous system: and consist of various types.
1. Myelin forming Glia (CNS): Neurons do not produce ECM to support themselves.
- Glial cells provide structural stability by wrapping around neurons using myelin sheaths.
- Between myelin sheaths, gap left known as nodes of Ranvier
a. Schwann Cells (PNS): supportive cells of the PNS. One schwann cell can serve many axons.
b. Oligodendrocytes (CNS): supportive cells of the CNS. One oligodendrocyte per axon.
2. Non-myelinating glial cells:
a. Satellite Cells (PNS): Non-myelinating Schwann cells.
- Form supportive capsules around the ganglia of the nerve cell bodies
- Ganglion = collection of cell bodies
b. Astrocytes: Highly branched glial cells of the CNS.
- Can provide neurons with substrate for ATP production
- Can absorb and release chemicals at a synapse
- Can maintain homeostasis via uptake of K+ and water
- Can protect BBB
c. Microglia: Specialized immune cells residing permanently in the CNS.
- Can induce oxidative stress leading to neurodegenerative diseases
d. Ependymal Cells: Cells that create a selectively permeable barrier separating fluid
compartments of CNS.
- Provide source of neural stem cells
The nervous system can be divided into two parts:
1. Central nervous system (CNS): contains the brain and
the spinal cord
a. The integrating center: receives input from the
sensory system of the PNS and determines whether
an output is needed which is executed via the PNS
2. Peripheral nervous system (PNS): contains sensory
(afferent) neurons and efferent neurons
a. Communicates input (to CNS) through afferent
(sensory) neurons
b. Executes output signals through efferent neurons
i. The somatic motor division: controls
skeletal muscles
ii. The autonomic motor division: controls smooth and cardiac muscles, exocrine
glands, some endocrine glands, and types of adipose tissue. Possesses two divisions
1. Sympathetic
2. Parasympathetic
The enteric nervous system is technically a third part. It consists of a third network of neurons that is found
in the walls of the digestive tract.
- Frequently controlled by the autonomic division of the nervous system
- However, also able to function autonomously as its own integrating center
CELLS OF THE NS (CHP 8)
The Cells of the nervous system consist of two types: glial cells (support cells) and neurons (signalling cells).
Neuron Structure:
Neurons are the functional unit of the nervous
system. They possess long processes called
axons and dendrites that extend outwards from the
nerve cell body.
,Neurons can be classified based on the number of
processes they possess.
- Multipolar neurons: neurons possessing many
dendrites and axons
- Pseudounipolar neurons: neurons with a cell
body located off on one side, with a single long
axon
- Bipolar neurons: neurons with a single axon
and single dendrite.
- Anaxonic neurons: neurons possessing no
apparent axon, but numerous dendrites.
Neurons can also be classified based on the information they transmit.
- Sensory neurons carry information about temperature, pressure, light, and other stimuli from
sensory receptors to the CNS.
- PNS sensory neurons are pseudounipolar, with cell bodies close to the CNS and long
processes that extend out to receptors in the limbs and internal organs
- Interneurons are responsible for communication with neurons within the CNS.
- Lie entirely within the CNS and often have complex branching processes.
- Efferent neurons have axons that may divide several times into branches called collaterals and
have enlarged axon terminals (used to stored neurotransmitter)
Axons of both afferent and efferent PNS neurons are bundled together with connective tissue into nerves.
- Nerves carrying only afferent information = sensory nerves
- Nerves carrying only efferent information = motor nerves
- Nerves carrying both efferent and afferent = mixed nerves
Dendrites:
Dendrites receive incoming signals.
- In the PNS, dendrites function to receive incoming information and transfer it to ther integrating center
of the neuron.
- In the CNS, role may be more complex, also sending signals back and forth. Made possible by
dendritic spines.
- Possess dendritic spines, extensions of the dendrites capable of changing structure in response to
signals.
,Axons:
Axons carry outgoing information.
- Most peripheral neurons possess a single axon that originates from the cell body via the axon hillock
- The axon extends and may branch to form collaterals
- Collaterals feed into the bulbous axon terminals
- Axon terminals communicate electrical signal via secretion of neurotransmitter
- Primary function is to transmit outgoing electrical signals.
- Most neurons complete this via NT, but some CNS can do this directly via gap junctions.
Axon terminals lack ribosomes and endoplasmic reticulation.
- All proteins destined for transport must be synthesized in the cell body and then transported to the
terminal (axonal transport)
- Slow transport relies on moving material via axoplasmic or cytoplasmic flow
- Fast transport relies on moving material via microtubules tracks on the axon and requires carrier
proteins kinin and dynamin
- Forward transport = anterograde transport, backward transport = retrograde transport
Synapse
The synapse is the region where an axon terminal of the presynaptic neuron meets the target
postsynaptic cell. The space between the two is called the synaptic cleft.
- Majority of synapses are chemical synapses, in which NT is transmitted.
- Otherwise, electrical synapses, operating using gap junctions.
In the developing embryo, axons of embryonic nerve cells send out tips called growth cones.
- Growth cones use chemical signals from growth factors and other membrane proteins to guide their
way to its target cell.
- Neurotrophic factors, released by neurons and glial cells determine the survival of a neuronal
connection → “use it or lose it”
, Glial Cells:
Glial cells are the support cells of the nervous system: and consist of various types.
1. Myelin forming Glia (CNS): Neurons do not produce ECM to support themselves.
- Glial cells provide structural stability by wrapping around neurons using myelin sheaths.
- Between myelin sheaths, gap left known as nodes of Ranvier
a. Schwann Cells (PNS): supportive cells of the PNS. One schwann cell can serve many axons.
b. Oligodendrocytes (CNS): supportive cells of the CNS. One oligodendrocyte per axon.
2. Non-myelinating glial cells:
a. Satellite Cells (PNS): Non-myelinating Schwann cells.
- Form supportive capsules around the ganglia of the nerve cell bodies
- Ganglion = collection of cell bodies
b. Astrocytes: Highly branched glial cells of the CNS.
- Can provide neurons with substrate for ATP production
- Can absorb and release chemicals at a synapse
- Can maintain homeostasis via uptake of K+ and water
- Can protect BBB
c. Microglia: Specialized immune cells residing permanently in the CNS.
- Can induce oxidative stress leading to neurodegenerative diseases
d. Ependymal Cells: Cells that create a selectively permeable barrier separating fluid
compartments of CNS.
- Provide source of neural stem cells
