The Nervous System

The Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord.
• The peripheral nervous system (PNS) consists of nerves and ganglia.
• The PNS sends information to the CNS via the sensory system.
• The CNS sends instructions to muscles and glands.

Types of Neurons

• Neurons are the basic units of the nervous system.
• Sensory neurons (afferent) transmit information to the brain.
• Motor neurons (efferent) transmit commands from the brain to muscles and glands.
• Interneurons connect neurons to other neurons.

Neuron Anatomy

• Gray matter consists of cell bodies, dendrites, and unmyelinated axons.
• White matter consists of myelinated axons.
• At the end of the axon are terminal buttons.
• A signal goes down the axon and across the synaptic cleft to the dendrite of another neuron.

Neuron Communication

• Neurons send messages electrochemically.
• Chemicals in the body are electrically charged and called ions.
• Neurons are surrounded by a semi-permeable membrane that allows some ions to pass through but blocks others.
• The most important ions in the nervous system are Sodium (Na+), Potassium (K+), and Chloride (Cl-).

Resting Membrane Potential

• When a neuron is not sending a signal, it is at rest.
• At rest, the inside of the neuron is negatively charged relative to the outside.
• At rest, potassium ions can cross the cell membrane easily, but chloride and sodium have a more difficult time.
• Ion pumps move 3 sodium ions out of the neuron for every 2 potassium ions in.
• The inside of the neuron is 70 mV less than the outside, expressed as -70 mV.

Action Potential

• If the depolarizing current brings the resting potential to -55mV, a neuron will fire.
• If a neuron does not reach the -55mV threshold, it will not fire.
• The neuron either reaches the threshold or fires a full action potential, as per the "all or nothing" principle.

Synapse

• The presynaptic neuron sends a message (action potential).
• The action potential travels to the axon terminal.
• The synapse separates the axon terminal of one neuron from the dendrite of another.
• The synaptic cleft is a microscopic gap between neurons.
• The nerve impulse is carried by neurotransmitters across the synaptic cleft to the postsynaptic neuron.
• Synaptic vesicles inside the terminal button contain neurotransmitters.
• Postsynaptic neurons have receptors for neurotransmitters.
• The action potential causes vesicles to release neurotransmitters into the synaptic cleft.
• The receptor shape must match the neurotransmitter to bind.
• The shape is determined by the chemical structure of the neurotransmitter.
• Neurotransmitters that do not bind break down and may be reabsorbed by the presynaptic neuron.

Effects of Neurotransmitters

• Neurotransmitters can increase (excitatory) or decrease (inhibitory) neural firing.
• Excitatory neurotransmitters "grease the wheels" of neural communication.
• Inhibitory neurotransmitters "apply the brakes".
• Neurotransmitters cause depolarization or hyperpolarization.

Summation

• Summation occurs at the axon hillock.
• Neurotransmitters produce stronger effects through summation.
• Temporal summation involves rapid-firing signals from a single presynaptic neuron to a postsynaptic neuron.
• Spatial summation involves multiple postsynaptic potentials occurring simultaneously at different receptor sites.

The Endocrine System

• Endocrine glands release hormones into the bloodstream.

Subdivisions of the Peripheral Nervous System

• The somatic nervous system consists of peripheral nerve fibers sending sensory information to the CNS and motor fibers projecting to skeletal muscles; it is mostly involved in intentional actions.
• The autonomic nervous system regulates the function of internal organs and controls basic life processes like respiration.
• The sympathetic system is the emergency system (fight or flight).
• The parasympathetic system carries out routine activities that maintain energy resources.