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Ch.48Nervous 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.
• 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
• III. Neural signals– A. 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
– B. 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– 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– i. Resting state – no channels open– ii. Large depolarization ( inside +, outside -), Na travels in– iii. Repolarization – Na gates closed, K open– iv. 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
• IV. Organization of the nervous system– A. 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
• 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.•