Neuron Structure and Function Nervous System Nervous system is composed of specialized cells called neurons. Neurons have long “arms” called axons and dendrites. Axons carry impulses away from cell body. Dendrites carry impulses to the cell body Connections between axons and dendrites forms our intelligence. Central Nervous system contains the brain & spinal cord. Cell bodies, axons, & dendrites in the human brain. Peripheral Nervous system (PNS) contains all nerves leading to and from the Central Nervous System Sensory receptors: Nerves that carry impulses towards CNS Detect light, pressure and heat. Motor neurons Nerves that carry impulses from the CNS to muscles or glands. Impulses are referred to as action potentials. Dendrites receive action potentials from other neurons. Axons carry action potentials to other neurons. Dendrites take action potential to cell body, axons away from cell body. Neuron Structure Axons Cell bodies of neurons myelin sheath Serves to insulate axons & dendrites and therefore speed up the action potential. The myelin sheath is made up of Schwann cells. Nodes of Ranvier The gaps between Schwann cells are called the Nodes of Ranvier. The action potential jumps from Node of Ranvier to Node of Ranvier. Resting Potential When no impulse is occurring along neuron. Potential refers to differences in charges inside and outside the membrane. Distribution of charge Positive charges are on the outside of neuron. Negative charges are on the inside of neuron. Sodium / Potassium pump Due to more Na+ on outside & more K+ on the inside. Negative protein ions on inside. The neuron is said to be polarized. This is due to the Na+/K+ pump. Action Potential Voltage gated channels open. Na+ moves in. K+ moves out Proteins stay inside depolarization Charges reverse. Outside turns negative. Inside turns positive. This is how the nerve sends a signal – through electrical impulses. Pair share: do you think it take energy for the cell to reverse the charge after an impulse is sent? Why? Refactory Period Period of time after depolarization when neuron cannot create action potential. Voltage gated channels close. Sodium/potassium pump must create resting potential. Uses ATP to do this. Neuron is now back to resting potential. Neuron Structure The space between neurons is called the synapse. (Synaptic cleft) Synapse neurotransmitter The action potential is transferred across the synapse by a chemical: neurotransmitter. The neurotransmitter is released from vesicles in the axon. The neurotransmitter is released when the action potential reaches the axon terminal.