Unit 5 Part 1 -- Nervous System
A. Central Nervous System (CNS)
ii. Spinal Cord
B. Peripheral Nervous System (PNS)
ii. Autonomic Nervous System (ANS) – Regulates involuntary functions (heart rate, stomach and intestinal contractions, and chemical secretions by glands).
II. Cells of the Nervous System
A. Neuroglia (nerve glue) (Also called Glial Cells)
i. Function to hold neurons in place and protect them.
B. Neurons (nerve cells)
i. Parts of a Neuron
1. Cell body – Main portion --- receives impulses from dendrites and transmits the impulse to an axon to be carried to the next neuron or target organ
2. Dendrite – Branching projections
a. Dendrites receive and conduct nerve impulses to cell body
3. Axon – Tail-like projection
a. Axons conduct impulses away from cell body to another neuron or to a target organ
4. Myelin Sheath
a. White insulating layer around the axon that speeds the nerve impulses along a neuron
5. Nodes of Ranvier
a. Un-insulated gaps in the myelin sheath that can generate electrical activity
6. Schwann’s Cells
a. Cells that wrap around the axon and produce the myelin sheath
7. Axon Terminals
a. Branching ends of the axon which transmit the signal/impulse from one neuron to another OR transmits a signal/impulse to an effector organ such as a muscle or gland.
III. Cells of the Nervous System
a. Sensory neurons (To spinal cord and brain)
b. Interneurons (From sensory neurons to motor neurons
c. Motor neurons (Away from spinal cord and brain)
a. Group of axons bundled together.
1) Called white matter (surrounded by protein myelin).
b. Tissue of cell bodies and dendrites called gray matter.
V. Nerve Impulses
a. Wave of electrical impulses which travel along neuron’s plasma membrane.
b. Stimulus increases permeability of Na+ (cell goes from – to +)
c. Disturbance initiates nerve impulse (reaction)
VI. The Synapse
a. Area where impulses are transmitted from one neuron to another or to a muscle (Pre-synaptic neuron à Post-synaptic neuron)
b. Composed of 3 structures
i. Synaptic knob (contains and releases neurotransmitters)
ii. Neurotransmitter – Chemicals by which neurons communicate.
iii. Synaptic cleft – Receive neurotransmitters and carry out response.
a. The chemical messengers found in the axon terminals that transmit the nervous signals from a neuron to another neuron, a neuron to a muscle fiber, or a neuron to a gland
i. Acetylcholine – Spinal cord and neuromuscular functions.
ii. Catecholamines -- (Sleep, mood, pleasure)
iii. Natural Pain Killers
VIII. Reflex Arcs
a. Neuron pathway – Route of nerve impulse
b. Reflex arc is communication between sensory (afferent) neurons and motor (efferent) neurons.
c. Impulse is conducted in one direction
d. Response to impulse is called reflex.
i. Sometimes, interneurons are involved in a three-neuron reflex (Withdrawal Reflex – Application of a stimulus that causes a withdrawal from stimulus).
e. Autonomic reflex: heart, glands, intestinal muscles (visceral muscle)
f. Somatic reflex: skeletal muscle
g. All reflex arcs have 5 elements: sensory receptor, afferent (sensory) neurons, integration center (CNS-interneurons)), efferent (motor) neurons, effector organ (usually a muscle or gland).