Download Lesson 33 - UBC Zoology

Lesson 33
Lesson Outline:
Nervous System Structure and Function
Neuronal Tissue
Supporting Cells
Neurons
Nerves
Functional Classification of Neuronal Tissue
Organization of the Nervous System
Central Nervous System
Peripheral Nervous System
Assemblages of Neurons
Central Nervous System
Spinal Cord
Objectives:
References:
Chapter 16: 387-428
Reading for Next Lesson:
Chapter 16: 387-428
Nervous System Structure and Function
Neuronal Tissue
Made up of two cell types:
Neurons
Supporting Cells
(nerve cells = neurons)
(glial cells = neuroglia)
Supporting Cells
The supporting cells can make up more
than 90% of the cells in the nervous
system of some species. More complex
organisms have more glial cells relative to
neurons.
These cells form a scaffolding or glue,
which holds the tissue together.
They assist the neurons by controlling the
environment around them.
Neurons
Neurons are highly specialized.
They are one of the two types of excitable cells in the body. The other one is muscle.
Neurons conduct messages in the form of nerve impulses from
one part of the body to another.
All neurons have a cell body or soma and numerous thin
extensions.
There are two types of extensions (neuron processes):
Dendrites
Axons
Dendrites
Dendrites are the primary processes that receive information.
They are input regions of the neuron.
Note that cell bodies and axons can also receive information.
Axons
Are processes that can generate and conduct action potentials.
They are processes that transmit information. They are the output
regions of the neuron.
They may be very short or very long depending on where they are
conducting information.
They form synapses at their terminals.
Nerves
Nerves generally consist of bundles of axons that travel together through the body.
Functional Classification of neuronal tissue
Sensory neurons are afferent to CNS.
Motor neurons are efferent to CNS.
Interneurons are interconnections between other neurons.
Organization of the Nervous System
Nervous System
Central Nervous System
Peripheral Nervous System
Sensory (Afferent) Division
Visceral
Motor (Efferent) Division
Somatic
Somatic Nervous System
Sympathetic Division
Autonomic Nervous System
Parasympathetic Division
Central Nervous System
Brain and spinal cord.
These are the primary integrating
and control centre (although
integration also occurs, to some
extent, at every synapse).
Peripheral Nervous System
The peripheral nervous system
consists of all the nerves:
- cranial nerves arise directly
from the brain.
- spinal nerves arise from the spinal cord.
The peripheral nerves are the communication lines
between the CNS and the rest of the body.
There are two functional sub-divisions of the
peripheral nervous system:
- sensory (afferent) nervous system
- motor (efferent) nervous system
Sensory (Afferent) Nervous System
The sensory nervous system conveys impulses to the CNS
from sensory receptors.
It has two sub-divisions:
Somatic sensory neurons convey information from
skin, muscles, joints (i.e. the body).
Visceral sensory neurons convey information from the
viscera.
Motor (Efferent) Nervous System
The motor nervous system conducts information from the CNS
to the effectors (muscles, glands, etc.).
It has two sub-divisions:
The somatic nervous system: conducts information to
skeletal muscles.
The autonomic nervous system: conducts information
to cardiac muscle, smooth muscle, and glands (i.e. viscera,
glands, heart and blood vessels).
The autonomic nervous system has two functional sub-divisions:
- sympathetic nervous system
- parasympathetic nervous system
Autonomic Nervous System
In the sympathetic nervous system the motor neuron
releases norepinephrine (noradrenaline) at the effector site.
The ganglia reside just outside the spinal cord.
In the parasympathetic nervous system the neuron releases
acetylcholine at the effector site.
The ganglia reside close to the effector organ.
Sympathetic Nervous System
Parasympathetic Nervous System
These two usually have opposite effects on the same
visceral organs. If one stimulates, the other inhibits.
Remember, however, that each stimulates some organs!
Assemblages of Neurons
Accumulations of nerve cell bodies = a ganglion
Accumulations of ganglia = brain
Reasons for accumulation = shorter distances for integration
Reasons for an anterior accumulation (brain) = sensory integration at the head end
In vertebrates we see an anterior accumulation of nerve cell bodies into ganglia and an
accumulation of ganglia to form a brain near the source of the most important sites of
sensory input - near the sites that first encounter new environments.
Definitions
Grey Matter - accumulations
of unmyelinated cell bodies and
dendrites.
White Matter - accumulations
of myelinated axons.
Nuclei and Tracts synonymous with grey and white
matter within the central nervous
system.
Central nervous system = brain
and spinal cord.
Peripheral nervous system =
cranial and spinal nerves.
The CNS is an integrating site for
sensory input and motor output.
Central Nervous System
Spinal Cord
Embryonic Origins
During embryonic development, the cells immediately
adjacent to the neurocoel begin to differentiate through the
process of neurulation. These are the actively mitotic stem
cells.
They become surrounded by cells that have been derived
from this layer. Some of these cells become neurons and
some become neuroglia.
The neurons sprout dendrites and an axon and the axons
grow out to form the outer layer of the developing spinal
cord. Because the axons eventually become coated with
myelin, the outer layer appears white and was named the
"white matter". The inner layers, which are composed of nonmyelinated cell bodies, are the "gray matter".
There are dorsal and ventral extensions of the grey matter - the dorsal and ventral horns.
The dorsal horn contains the cell bodies of neurons receiving incoming sensory
information, which they distribute elsewhere in the cord or to the brain. The ventral horns
contain cell bodies of motor neurons that project out from the cord.
The white matter contains the axons of nerve fibres projecting up and down the cord
linking different levels of the spinal cord with each other and to the brain.
Form and Function
The spinal cord has two basic functions. It maintains simple reflexes at the segmental
level - and it transmits information up and down the cord between segmental levels.
Spinal Reflexes (intersegment communication)
Sensory information arriving from the periphery synapses on cell bodies in the dorsal
horns of the spinal cord. These are thus, second order neurons and are referred to as
interneurons or association neurons. For reflex arcs, they carry the information to motor
neuron cell bodies in the ventral horns on the same
side, the opposite side, and at different levels in the
cord. The motor neurons in the ventral horn project to
effectors in the periphery.
Many reflexes occur completely at the segmental
level. One such classical example is the withdrawal
reflex. This reflex excites flexors on the ipsilateral
side and extensors on the contralateral side of the
body leading to withdrawal of the stimulated limb and
extension and transfer of weight to the unstimulated
limb.
The interneurons (association neurons) will also transmit information up the cord to the
brain (ascending information) and can also receive descending information from the brain
(descending information).
Spinal Reflexes (intrasegment communication)
Nerve fibres carrying similar information tend to
travel together in nerve tracts and those carrying
information up the cord are referred to as
ascending tracts and those carrying information
down the cord are referred to as descending tracts
(most tracts are named in terms of where they
originate and where they terminate spinothalamic tract versus tectospinal tract).