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Ch.48
Nervous System
• I.
Functions
– A. Sensory input
– B. Integration – interpretation of input
– C. Motor output- involves effector cells like the muscles and
endocrine glands
•
II.
Parts of the nervous system
– A. Central (CNS) – Brain and spinal cord
– B. Peripheral (PNS) – Network of nerves that carry input from the body to
the CNS and motor output away from CNS.
– C. Neurons
• 1. Specialized for transmitting chemical and electrical signals
• 2. Large cell body
– i.
Contains most of the cytoplasm and nucleus of cell
– ii.
Usually in CNS or ganglia
• 3. Dendrites – convey signal to cell body (large surface area)
• 4. Axons – conduct impulses away from cell body
– i.
In vertebrate PNS they are wrapped into Schwann cells
which form an insulating myelin sheath
– ii.
Synapse- gap between terminal and target cell
– iii.
Neurotransmitters – chemical that cross the synapse to
relay impulse to next cell.
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5. Classes of neurons
– i.
Sensory – convey info about environment from receptors to
CNS
– ii.
Interneurons –integrate sensory input and motor output
– iii.
Motor –convey impulses from CNS to effector cells
6. Neurons are arranged in groups
– i.
Simple circuit – synapse between sensory and motor (reflex).
– ii.
Convergent – Info from several neurons come together at one
neuron.
– iii.
Divergent – Info from a single neuron spreads to several neurons
– iv.
Reverberating – Circular (memory)
– v.
Nucleus – cluster of nerve cell bodies in the brain
– vi.
Ganglion – Cluster in the PNS
– vii.
Glia – cells which reinforce, insulate and protect neurons
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III.
– A.
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– B.
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Neural signals
Membrane potentials
1. –50 to –100mV in animal cells
2. Outside cell is zero, cytoplasm is negatively charged compared
to outside
3. –70mV = resting neuron
4. Inside cell = K+, outside Na+
5. Amino acids, phosphates (-) inside, Cl- outside
Creation of the membrane potential
1. K+ diffuse out (+ moves outside)
2. Negative molecules are too big to leave
3. Negative charge inside attracts K+ back and Na+ also.
4. This creates a slight + charge inside
5. Na+ is pumped out by Na/K pump
• C. Action potential – only neurons
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1. Due to the presence of gated ion channels
2. Stimuli that open K channels hyperpolarize the neuron (interior -)
3. Stimuli that open Na channels depolarize the cell (interior +)
4. When the cell reaches a threshold an action potential is triggered.
Hyperpolarization makes the cell less likely to reach the threshold
• 5. Four phases of an action potential
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Resting state – no channels open
Large depolarization ( inside +, outside -), Na travels in
Repolarization – Na gates closed, K open
Undershoot – hyperpolarization – refractory period
• 6. Self – propagating
• 7. Electrical synapses – impulses travel from one cell to another (
less common than chemical)
• 8. Chemical synapses – involves release of Ca+ ions which
stimulate the release of neurotransmitters which bind to receptors
and open ion gates
• 9. Types of neurotransmitters
– i.
Acetylcholine – functions between motor neuron and muscle cell
and CNS
– ii. Biogenic amines – derived from amino acids. Epinephrine,
norepinephrine and dopamine from tyrosine and serotonin from
tryptophan (imbalances of dopamine and serotonin are associated with
mental illness)
– iii. Amino acids
– iv. Neuropeptides – endorphins
– v. Gaseous transmitters – NO and CO
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IV.
– A.
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Organization of the nervous system
Correlates to body symmetry
1. Cnidaria have a nerve net ( no central control)
2. Cephalization in bilateral organisms
– i. Flatworms – simple brain
– ii. Annelids and arthropods have ventral nerve cord and brain
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B. Vertebrate nervous system
– 1. CNS- spinal cord and brain covered with meninges (connective
tissue)
– 2. In brain white matter is interior, gray is outer, opposite in spinal cord
– 3. Cerebrospinal fluid – circulates hormones ,nutrients and white blood
cells, also absorbes shock.
– 4. PNS – 12 pairs of cranial nerves and 31 pairs of spinal nerves
• i.
Sensory division – to CNS
• ii.
Motor division – CNS to effector cells
• iii.
Control responses to environment and maintain homeostasis
• iv.
Somatic system - voluntary skeletal muscle
• v.
Autonomic – involuntary (parasympathetic-conserve
energy, sympathetic – increases energy use.