Transcript Motor Neuron

Structural Classification of the
Nervous System
• Central nervous system (CNS)
– Brain
– Spinal cord
• Peripheral nervous system (PNS)
– Nerves outside the brain and spinal cord
Functions of the Nervous
System
• Sensory input – gathering information
– To monitor changes occurring inside and outside
the body
– Changes = stimuli
• Integration
– To process and interpret sensory input and decide if
action is needed
• Motor output
– A response to integrated stimuli
– The response activates muscles or glands
Nervous Tissue: Neurons
• Neurons = nerve cells
– Cells specialized to transmit messages
– Major regions of neurons
• Cell body – nucleus and metabolic center of
the cell
• Processes – fibers that extend from the cell
body
Neuron Anatomy
• Cell body – contains organelles. Neurons lack
centrioles and are incapable of mitosis
– Nissl substance – specialized rough endoplasmic
reticulum. Attached ribosomes. Function to
synthesize vital protein molecules
– Neurofibrils – intermediate cytoskeleton that
maintains cell shape
• Extensions outside the cell body
– Dendrites – short, branched receptive surfaces
that conduct impulses toward the cell body
– Axons – long, usually singular and conduct
impulses away from the cell body
Axons and Nerve Impulses
• Axons end in axonal terminals
• Axonal terminals contain vesicles with
neurotransmitters
• Axonal terminals are separated from the
next neuron by a gap
– Synaptic cleft – gap between adjacent
neurons
– Synapse – junction between nerves
Nervous Tissue: Support Cells
(Neuroglia)
• 1. Astrocytes
– Abundant, star-shaped cells
– Brace neurons
– Form barrier
between capillaries
and neurons
– Control the chemical
environment of
the brain
Figure 7.3a
• 2.
Microglia
– Spider-like phagocytes
– Dispose of debris
• 3.
Ependymal cells
– Line cavities of the
brain and spinal cord
– Circulate
cerebrospinal
fluid
Nervous Tissue: Support Cells
• 4. Oligodendrocytes
– Produce myelin sheath around nerve fibers in
the central nervous system
Figure 7.3d
Nervous Tissue: Support Cells
• 5. Satellite cells
– Protect neuron cell bodies
• 6. Schwann cells
– Form myelin sheath in the peripheral nervous
system
Functional Classification of
Neurons
• Sensory (afferent) neurons
– Carry impulses from the sensory receptors
• Cutaneous sense organs
• Proprioceptors – detect stretch or tension
• Motor (efferent) neurons
– Carry impulses from the central nervous
system
• Interneurons (association neurons)
– Found in neural pathways in the central
nervous system
– Connect sensory and motor neurons
Structural Classification of
Neurons
• Multipolar neurons – many extensions from
the cell body. Characteristic of Motor
Neurons
Figure 7.8a
Structural Classification of
Neurons
• Bipolar neurons – one axon and one
dendrite. Found only associated with
special senses. (Olfactory, vision)
Figure 7.8b
Structural Classification of
Neurons
• Unipolar neurons – have a short single
process leaving the cell body. Sensory
Neurons
Figure 7.8c
Nerve Impulses
• The surface of the nerve cell membrane is
polarized due to unequal distribution of
ions on either side of the membrane
• When nerve cells are at rest there is a
greater concentration of Na+ ions outside
the membrane and a greater concentration
of K+ ions inside the membrane. (Rest
Potential)
• This arrangement of ions gives the outside
of the membrane a positive charge with
respect to the inside.
• A stimulus causes a change in permeability in a
region of the membrane
• Na+ ions rush into the cell and K+ ions rush out
depolarizing the region of the membrane
• This region of depolarization is an Action
Potential
• An action potential in one region stimulates
adjacent regions to depolarize and the action
potential moves away from the point of stimulus
• This moving action potential is a Nerve
Impulse
• The membrane is repolarized in 1/1000 sec. by
active transport
Continuation of the Nerve Impulse
between Neurons
• Impulses are able to cross the synapse to
another nerve
– Neurotransmitter is released from a nerve’s
axon terminal
– The dendrite of the next neuron has
receptors that are stimulated by the
neurotransmitter
– An action potential is started in the dendrite
Reflexes
• Reflex – automatic, unconscious, and
involuntary responses to stimuli
• Simplest nerve pathways
• Autonomic Reflexes – involve contractions of
smooth muscles
• Somatic Reflexes – involve contractions of
skeletal muscles
• Help maintain homeostasis by controlling: body
temperature, heart rate, breathing rate, blood
pressure and digestive activities
• Patellar Reflex – (knee-jerk reflex)
Employs only two neurons. Helps
maintain posture
• Also swallowing, sneezing, coughing,
vomiting are reflexes
Withdrawl Reflex
Withdrawl Reflex
• 1. Receptor – sets up a nerve impulse when
something painful is touched
• 2. Sensory
the spinal cord
Neuron – carries the impulse to
• 3. Interneuron – conducts the impulse from the
sensory to a motor neuron
• 4. Motor
the effector
Neuron – conducts the impulse to
• 5. Effector – responds to stimulation by muscle
contraction