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Week 2
10/5/07
General Principles From an Evolutionary Perspective
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First Multicellular Organisms
o Had no nervous system
o Exhibited behavior by contracting smooth muscle cells to control the flow of
water through its body
 These muscle cells were called Independent Effectors: muscle cells
which responded to stimuli acting directly on the membrane of the cell
Cnidaria: Jellyfish, Hydra, etc.
o Two cellular layers: an outer that faces the environment, an inner that lines the
body cavity
o First animals to possess sensory neurons:
What are main advantages of having sensory neurons?
1) increased sensitivity to stimuli
2) faster response to stimuli
3) individual neurons can connect to groups of muscle cells
4) neurons responding to different types of stimuli can be distributed to
different parts of the body
--Most cnidarians also possess motor neurons . Each sensory neuron connects to a
group of motor neurons and each motor neuron connects to a group of muscle cells.
--Motor neurons connect both “vertically” and “horizontally.”
--Amacrine Processes: horizontal connections that transmit information in either
direction; Violates Cajal’s Functional Polarity theory.
--Neurons are uniformly distributed throughout the body of the hydra: forms the first
nervous system called a nerve net.
--Convergence & Divergence
Flatworms
o Bilaterally symmetric
o Important developments:
 Centralization: neurons cluster in groups called ganglia, which are
connected by bundles of axons called nerve cords.
 Cephalization: neurons and sensory receptors are concentrated at the
rostral end of the body
 Interneurons: first animals to possess interneurons which connect sensory
 Interneurons can modify information as they transmit it by acting
as “switches” or “pacemakers”
Annelids/arthropods
o Body segmentation
o Ventral nerve cord: set of linked pairs of ganglia in each segment
Lancelet
o Notochord: stiffens and protects body
o Dorsal nerve cord
Peripheral Nervous System
-3 Subdivisions:
i) Sensorimotor:
-consists of dorsal root ganglia sensory neurons located in the spinal cord
-these neurons connect through peripheral nerves to sites throughout the body,
where they receive information
-Each spinal nerve connects to a narrow band of skin called a dermatome
ii) Enteric:
-nervous system in the wall of the digestive tract
-controls movements of the gut involved in digestion
iii) Autonomic:
-Sympathetic division: prepares organism for fighting or fleeing by causing pupil
dilation, sweating, increase heart rate, and increased blood pressure
-Parasympathetic division: conteracts sympathetic effects to bring organism
back to resting state
Cerebral Vasculature
-Blood-brain barrier
 Restricts the passage of solutes from the capillaries into the brain
 Without BBB certain hormones would interfere with synaptic communication
 Formed in part by endothelial cells that surround the capillary
 Endothelial cells are in turn surrounded by the basement membrane: outside this
membrane are Astrocytes
 Junctions between endothelial cells act as active gates, allowing only certain molecules to
penetrate
 Therapeutic drugs meant to be taken directly into the brain must bind to receptors on the
surface of endothelial cells in order to be admitted
-Blood reaches brain through two arterial roots:
___________ and ___________
Neuroanatomy
-sulci___________
gyri_______________
-Commissures: bundles of axons that connect areas in the cerebral cortex with the same or
related areas
-Association Pathways: Connect related areas in the same hemisphere
-Nervous system is protected by membraneous coverings:
 Pia Mater: vascular membrane adhering closely to the surface of the CNS
 Arachnoid Mater: web-like layer similar to Pia
 Dura mater: Thick, inelastic covering that is closely apposed to the inner surface of the
skull