Nervous System Basics

Nervous System Overview

Nervous System Overview Central Nervous System (CNS) Composed of brain and spinal cord Peripheral Nervous System (PNS) Composed of nerves outside the CNS Autonomic Nervous System (ANS) Part of the PNS, controls the involuntary organs and muscles

Nervous Tissue Left Photo: Glial cells - red, neurons - green. Right Photo: Glial cells - green, neurons – red, nuclei - blue

Nervous Tissue Nervous tissue is composed of two types of cells Neurons: transmit nerve messages Glial Cells: support cells in the nervous system that help to protect the neurons, as well as produce the myelin sheaths of nerves, secrete cerebrospinal fluid, defend against bacteria, regulate ions About 50% of the weight of the brain is glial cells.

Neurons

Neurons Functional unit of the nervous system Composed of three parts: Humans have about 100 billion neurons in their brain! Composed of three parts: Dendrite: receives information from another cell and transmits to the cell body Cell Body: contains nucleus, mitochondria, and other organelles Axon: conducts messages away from the cell body

Neuron Types

Neuron Types Sensory Neuron: long dendrite, short axon; carry messages from sensory receptors to CNS Motor Neuron: long axon, short dendrite; transmit messages from CNS to muscles/glands Interneuron: only in CNS, connect neuron to neuron

Specialization

Specialization Many axons have a myelin sheath, which acts like insulation, wrapped around them Formed from the plasma membrane of specialized Glial cells called Schwann cells Schwann cells serve as supportive, nutritive, and serve facilities for neurons Gap between Schwann cells is called the node of Ranvier Signals jumping node to node travel hundreds of times faster than along the surface of axons Allows your brain to communicate with your toes in a few thousandths of a second

Synaptic Communication

Synaptic Communication Synapse: junction where neurons communicate Neurons normally do not touch each Synaptic cleft: separation gap Presynaptic neuron: transmitting cell Postsynaptic neuron: receiving cell Electrical activity in the neuron causes the release of chemicals called neurotransmitters into the synaptic cleft This causes electrical activity in another neuron Synaptic vesicles: neurotransmitter releasers

Neurotransmitters Chemicals which relay, amplify, and modulate signals between a neuron and another cell Acetylcholine: triggers muscle contraction Norepinephrine: flight or fight response; increases blood flow to muscles, releases glucose, increases heart rate Dopamine: important in treatment of Parkinson’s Serotonin: relates anger, aggression, body temp, mood, sleep, human sexuality, appetite, and metabolism Endorphin – natural “pain relievers”

CNS: The Brain

CNS: The Brain Cerebrum Cerebellum Hypothalamus Largest part of the human brain, associated with higher brain function such as thought and action Cerebellum “Little brain”, associated with regulation and coordination of movement, posture, and balance. Hypothalamus Produces hormones that control body temp, hunger, moods, hormones, sleep, thirst, and sex drive

Peripheral Nervous System

Peripheral Nervous System Afferent Neurons: Some peripheral neurons collect info and transmit it towards the CNS Efferent Neurons: Others transmit info away from the CNS Nerves are bundled axons and dendrites of neurons outside the CNS

Autonomic Nervous System Has two subdivisions that stimulate or inhibit body systems Sympathetic division – can be activated by physical or emotional stress Parasympathetic division – controls the internal environment during routine conditions

Reflex Arc

Reflex Arc Part of the PNS’ motor division Controls the movement of skeletal muscles Voluntary, but can be operated without conscious control (balance) Reflexes such as knee-jerk Sensory neurons connect sensory receptors to the CNS. The CNS processes the signal, and transmits a message back to an effector organ (an organ that responds to a nerve impulse from the CNS) through a motor neuron.

Resting Potential

Resting Potential Outside the neuron’s plasma membrane has a positive charge, while inside has a negative charge Passage of ions across the cell membrane passes the electrical charge along the cell The voltage potential is -70mV of a cell at rest Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane  Active transport due to the Sodium Potassium Pump

Action Potential

Action Potential A temporary reversal of the electrical potential along the membrane for a few milliseconds Begins at one spot on the membrane, but spreads to adjacent areas of the membrane After, there is a refractory period, which prevents the message from being transmitted backward along the membrane.

Steps in an Action Potential

Step in an Action Potential At rest the outside of the membrane is more positive than the inside Sodium channels open and sodium moves into the cell, causing the membrane potential to become positive. At some point, the potassium channels open allowing potassium ions to flow out of the cell. The sodium channels close and stop the inflow of positive charge. Potassium channels are open, allowing positive charges to outflow and the membrane potential plunges. When the membrane potential reaches its resting state, the potassium channels close. Sodium/potassium pump pumps sodium out of the cell and potassium into the so it is ready for the next action potential.