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.•