Biology 12 Unit 1:Maintaining Dynamic Equilibrium

Coordination and Regulation in Humans  The Nervous System and Homeostasis  Structure of a neuron  Action Potential  Synaptic Transmission  Structure of the Brain  Organization of the Nervous System Clip 1

Nervous System Central Nervous System Peripheral Nervous System BrainSpinal Cord Somatic Nerves Autonomic Nerves SensoryMotor SympatheticParasympathetic Adapted from Nelson Biology

Structure of a Neuron From Nelson Biology Clip 1

Types of Neurons  Motor Neuron  Connects the central nervous system to a muscle or a gland  Sensory Neuron  Connects a sensory receptor to the central nervous system  Interneuron  Connects two or more neurons

Action Potential  In 1900 Bernstein hypothesized that nerve impulses where electrochemical in nature.  Future experimentation proved this.  Giant Squid Experiment:  Cole and Curtis placed two tiny electrodes – one inside the large axon of a squid and the second across from the first outside the axon.

Cole and Curtis measured the electrical potential across the membrane. The resting membrane potential was found to be about –70mV. Squid Axon Giant Squid Experiment

 When stimulated, the action potential jumped to about +40 mV.  The action potential only lasted for a few milliseconds before the nerve cell returned to the resting potential mV 1234 ms threshold

 Caused by an uneven distribution of positively charged ions across the membrane  Set up and maintained by a Sodium- Potassium pump.  Potassium is pumped into the cell, Sodium out of the cell. Maintaining Resting Potential Clip 1

 These positive ions want to move with their concentration gradient by diffusion.  Potassium diffuses out faster than sodium diffuses in leaving a negative charge inside the cell.  The cell is polarized. At rest there are more ion gates open for potassium.

Clip 1

 When stimulated the ion gates for potassium close and the ion gates for sodium open up.  Positive ions flood into the cell making it positive. This rapid inflow is referred to as depolarization.  After the impulse, the gates return to the resting condition with extra potassium gates open. The flow of potassium ions out of the cell restores the resting potential.  The Na+/K+ pump continues to pump the sodium and potassium across the membrane against the concentration gradient to restore the separation of Na+ and K+. Action Potential

 Repolarization takes about seconds.  Once stimulated, the membrane cannot be polarized until after the refractory period. Repolarization

Summary of Impulse 1. At rest – Na+/K+ pump moving ions – potassium gates open 2. Stimulation – potassium gates close – sodium gates open 3. The flood of sodium into the cytoplasm stimulate adjacent areas 4. Refractory – potassium gates open – sodium gates close 5. At rest – Na+/K+ pump moving ions – potassium gates open

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Reflex Arc Reflexes are autonomic responses to certain stimuli. They are not under conscious control, they are involuntary. The pathway that a nerve impulse takes is called a reflex arc. We need to identify the stimulus, receptor, sensory neuron, motor neuron, effector, and the response.

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Synaptic Transmission  Neurons are not directly connected to each other.  The electrochemical action potential cannot jump the synaptic cleft (or synapse).  Synaptic transmission is entirely chemical in nature.   Not all neurons cause depolarization in the post synaptic membrane. Some neurons are inhibitory.

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Central Nervous System Protection  Meninges: protective membranes  Dura mater: outer layer  Arachnoid: middle layer  Pia mater: inner layer  Cerebrospinal fluid: shock-absorbing fluid that circulates in the inner meninges and the vesicles of the CNS  Skull: the hard bone "helmet"

The Spinal Cord and Vertebrae Dorsal root: sensory information to the CNS Ventral root: motor information to the PNS

White Matter: Myelinated nerve fibres (sensory and motor) Grey Matter: Unmyelinated interneurons

Gross Anatomy  Hindbrain: oldest part of brain.  Cerebellum: limb movements, balance, muscle tone.  Medulla Oblongata: connects PNS and CNS, regulates involuntary muscle movements.  Pons: "bridge" between cerebellum and medulla.  Midbrain: relay center for some eye and ear reflexes  Reticular formation: controls respiration and circulation. Filters sensory stimulus between conscious and unconscious.

 Forebrain:  Cerebrum: conscious thought  Cerebral hemispheres: Left-logical; right-creative  Corpus collosum: connects left and right hemispheres  Cerebral cortex: outer layer of cerebrum, sensory processing area  White matter: myelinated neurons  Gray matter: unmyelinated neurons  Thalamus: directs incoming sensory signals  Hypothalamus: homeostasis

The Brain

Organization of the Nervous System The CNS:  The brain and spinal cord The PNS:  Everything else

The PNS  One division of the PNS is the Autonomic Nervous System.  The ANS controls the unconscious activities of the body (heart rate etc.)  The PNS is further subdivided into the sympathetic (stress responses) and the parasympathetic (relaxation) systems.  Balance between these two systems helps achieve homeostasis.

Sympathetic Nervous System

Parasympathetic Nervous System

Nervous System Central Nervous System Peripheral Nervous System BrainSpinal Cord Somatic Nerves Autonomic Nerves SensoryMotor SympatheticParasympathetic Adapted from Nelson Biology

Compare Endocrine and Nervous System Clip