Chapter 37
Human Nervous System The nervous system has three functions 1) Receive sensory input from both external and internal stimuli 2) Perform integration. (Coordinating all of the various inputs of the body to form a single, simple thought or action) 3) Generate motor output instructions for the muscles in the organism The Central Nervous System (CNS) of humans consists of the brain and spinal cord The Peripheral Nervous System (PNS) contains cranial nerves and spinal nerves Cranial nerves connect body to brain; Spinal nerves connect body to spinal cord Both nervous systems work together and are intertwined with each other
Neurons Neurons are the main cells of the nervous system. They contain three distinct sections Dendrites of neurons receive messages and direct them toward the cell body of the neuron (1,000-1,000,000 per neuron cell) Cell bodies contain the majority of cytoplasm, organelles, and the nucleus of the neuron cell The lone axon of each neuron conducts messages from the neuron to adjacent cells (one axon may lead to multiple neurons though) Long axons are covered by myelin sheaths, which insulate the neuron for more rapid transmission of messages
Neurons There are three types of neurons, according to shape and function 1) Motor neurons, which take impulses to muscles and glands 2) Sensory neurons, which take sensory impulses from skin, eyes, tongue, etc, to the CNS 3) Interneurons, which are located within the CNS. These are intermediate nerves which transmit messages from one nerve to the next.
Nerve Impulse: The Resting Potential When the nerve is not conducting an impulse, the relative voltage inside the nerve compared to the outside is -65 mv (millivolts). This is the resting potential of the nerve. During the resting potential, the concentration of Na + ions is higher outside the cell, and the concentration of K + is higher inside the cell. Within the plasma membranes are Na + /K + pumps, active channel proteins that pump three Na + ions out of the cell and two K + ions into the cell The result is an overall lower charge inside the cell.
Nerve Impulse: The Action Potential An action potential is when a neuron transmits a message to other neurons If a cell s personal dendrites are activated by other axons, the charge inside the cell raises. If the internal charge raises enough (to +40mV), then the axon potential begins. The +40mV charge moves along the axon thanks to ion channels in the axon that open and close in succession. Eventually the charge reaches the tip of the axon where the synapse is located. A synapse is where an axon forms a connection with at least one dendrite from another neuron.
Nerve Impulse: The Synapse Each axon swells into a bulb at the synapse and partially surrounds the neighboring cell s dendrite, muscle, etc. When the action potential s electrical impulse reaches the synapse, they open voltage-gated Ca 2+ channels. The sudden rise in Ca 2+ releases a neurotransmitter from the axon bulb The neurotransmitter then crosses the synaptic cleft and reacts with the dendrite of the next neuron. The cycle then repeats in this next neuron.
Neurotransmitters We know of at least 25 neurotransmitters and their functions. Here are a few: Neurotransmitter Acetylcholine Dopamine Epinephrine Histamine Nicotine Norpenephrine Seratonin Function Slows and regulates heartbeat Control of messages from the brain and pituitary gland aka Adrenaline; the Fight or Flight response Triggers response to foreign pathogens Controls volume of release of other neurotransmitters Attention and response control Mood, appetite and sleep
Polarization The overall sequence of activities that occur before, during, and after an action potential can be described in three steps Depolarization Some stimuli in a dendrite are not enough to increase the charge in the neuron beyond the threshold point. If the charge increases and reaches the threshold, the neuron will rapidly depolarize (change charge from to +). Repolarization Once the charge is raised and the Na+ ion channels are closed, only the K+ channels are active. K+ will leave the cell, rapidly lowering the charge Refractory Period The charge drains so low that all channels deactivate while the Na/K pumps return the cell to it s resting state. Until then, no reaction occurs. This is why some actions (repeated muscle use for example) require a pause before re-starting the neuron s action potential
Synaptic Integration Neurons can both excite or inhibit. Excitatory responses trigger an on switch for the nerve, muscle, etc. Inhibitory responses trigger an off switch for the nerve, muscle, etc. Neurons may receive multiple excitatory and inhibitory responses at once from multiple axons. Some even excite or inhibit themselves in a loop to ensure responses increase exponentially or are more once-and-done. Neurons sum up all signals. They may receive a series of excitatory and inhibitory responses all at once, but action potentials will not occur unless the neuron reaches +40mV
The CNS: The Spinal Cord The spinal cord has two functions: reflex actions, and communication between the brain and spinal nerves. The cord contains white matter and gray matter. The gray matter is in the center of the spinal cord and contains sensory neurons, motor neurons, and short interneurons The white matter is filled with myelinated fibers. Myelinated fibers insulate and protect the gray matter and are the locations where neurons cross sides of the cord. The left side of the brain and cord controls the right side of the body, and vice versa. The gray matter are the highways and on/off ramps into city streets; the white matter are the exits from one highway to another
The CNS: The Brain In humans, the cerebrum is the largest portion of the brain. The brain contains one hundred billion neurons, each with an average of 7,000 synaptic connections. This means there are 700 trillion connections in the brain. At birth the number of synaptic connections is approximately 1 quadrillion, a number that slowly drops over time. This is why it s easier for children to learn new skills and languages than adults. The cerebrum receives sensory input and commands voluntary responses from the body. The cerebrum is divided into the left and right halves The only connection between the two halves of the brain (and thus, the only way to coordinate information on both sides) is a series of tracts called the corpus callosum. Surrounding the outer back areas of the cerebrum are the primary somatosensory areas of the brain. These areas are the first (primary) to receive sensory information from skin, eyes, tongue, ears, etc
The CNS: The Brain In the frontal lobe of the brain is the primary motor area and Broca s area, which is responsible for speech. This area of the brain receives signals from the somatosensory areas and organizes them into proper thoughts and responses These association areas are what triggers and searches for memories Once thoughts are organized, they are sent to a second section of the brain: the diencephalon.
The CNS: The Brain The Diencephalon contains the hypothalamus and the thalamus The hypothalamus maintains homeostasis by regulating hunger, sleep, thirst, temperature, and hydration. Links the nervous and endocrine systems The thalamus is the gate keeper. It sends messages to their proper destinations all throughout the brain. Also involved in arousal, emotion, and memory
The CNS: The Brain The cerebellum is the portion of the brain we know the least about. The cerebellum is involved in motor coordination. The ability and skill to play an instrument, hit a baseball, juggle, multitask, etc are all organized here. This is the only known function of the cerebellum, yet it contains 10x as many neurons as the rest of the brain combined. Either coordination requires this much extra neuron capacity, or the cerebellum has multiple other unknown functions.
The CNS: The Brain The brain stem contains the midbrain, pons, and medulla oblongata. The midbrain is involved in spinal reflexes and filtering message transmissions The pons contains the neurons that send messages between the brain s and the spinal cord s tissues The medulla oblongata controls involuntary reflexes such as heartbeats, vomiting, coughing, sneezing, hiccupping, and the chain of swallowing
The Limbic System The limbic system contains the hippocampus and the amygdala, which play a role in learning and memory The hippocampus is like a librarian. It has knowledge of past stimuli and connects these memories to present stimuli. The amygdala attaches and associates feelings and emotions with these memories. Examples Apple Summer Vacation Halloween Funeral
Memory Memory is the ability to recall sensory inputs from the past. Memories take the form of specific patterns of neurons that undergo action potentials under specific stimuli Short-term memory resides in the prefrontal area of the frontal lobe and coordinates instantaneous memory. Skill memory, the ability to perform an action, is also stored here. Long term memory is stored and organized thanks to the hippocampus ability to track action potentials over the course of a life-time. The loss of neuronal connections means the loss of the ability to retain any of these three types of memories.
Reflexes The PNS contains nerves, which are bundles of neurons and axons with similar purposes. Reflexes, or involuntary responses to stimuli, are rapidly transmitted here without wasting time moving through the limbic system. If your hand touches a hot stove, thermoreceptors (temperature) send an impulse through nerves to the spinal cord. The spinal cord, without signaling the brain, recognizes the impulse from these specific receptors and triggers muscle contraction. Nociceptors (pain receptors) then signal the brain (in the midbrain) so that you may analyze the situation Do you need to run? Obtain a medicine? Remember this new experience for future reference?
Autonomic System The autonomic system regulates cardiac and smooth muscle and glands (all of which are involuntary muscles). The sympathetic division of this system activates in stressful conditions It accelerates heartbeat and dilates bronchi It also inhibits digestion, saving the energy for other more necessary actions in times of distress This is why sudden stress triggers loss of appetite in most people The parasympathetic division of this system of nerves activates relaxed-state functions. It promotes pupil contraction, food digestion, and slowed heartbeat.