Neurons and Glial Cells

Key Learning Objectives

• Neurons and glial cells are the major components of the nervous system.
• Neurons communicate via neural impulses at chemical synapses.
• Acetylcholine, monoamine neurotransmitters, GaBa, and endorphins have different functions in the nervous system.

Nervous Tissue

• The nervous system is composed of living cells.
• Glia and neurons compromise the two main categories of cells.

Neurons

• Neurons are basic links that permit communication within the nervous system.
• Signals from the external world are received and messages from the nervous system are carried to move the body.

Neuron Anatomy

• Dendrites are neuron parts specialized to receive information.
• Information flows from dendrites into the cell body, then away from the soma along the axon.
• The axon is a long, thin fiber that transmits signals from the soma to other neurons, muscles, or glands.
• Axons can be several feet long and may branch off to communicate with multiple cells.
• The myelin sheath is insulating material encasing some axons.
• Myelin sheaths speed up transmission of signals along axons and stabilize axon structure in neural networks.
• Multiple sclerosis results from degeneration of myelin sheaths.
• Not all axons are myelinated; myelinated segments may be interspersed with unmyelinated ones.
• Axons end in terminal buttons, which secrete neurotransmitters, activating neighboring neurons.
• Synapses are junctions where neurons interconnect and information gets transmitted.

Information Transmission

• Information is received at dendrites, passed through the soma and axon, and transmitted through the other cells dendrites at synapses.

Glial Cells

• Glial cells support neurons and are generally smaller.
• The human brain roughly has equal numbers of glial cells and neurons.
• Glial cells provide nourishment to neurons, remove waste products, and insulate axons.
• Myelin sheaths are derived from glial cells.
• Glia are involved in the development of the nervous system in human embryos.
• Glia also send and receive chemical signals, as new research suggests.
• Some glia detect neural impulses, sending signals to other glial cells of which, some can feed signals back to neurons.
• Glia modulate neuronal signaling by dampening or amplifying synaptic activity.
• Glial cells shield synapses from surrounding neuronal activity, enhancing the signal-to-noise ratio in the nervous system.
• Glial cells may play a role in major disorders, including cognitive impairment in schizophrenic disorders, depressive disorders, Alzheimer’s disease, and chronic pain.

Action Potential

• An action potential is a brief shift in a neuron's electrical charge traveling along an axon.
• This "down time" is brief, only 1 or 2 milliseconds.
• Synapses are essential for transmitting nervous impulses from one neuron to another and for enabling rapid, direct communication.

Neurotransmitters and Behavior

• Chemical couriers communicate information between neurons.
• Neurotransmitters are fundamental to behavior and play a role in processes such as muscle movements, moods, and mental health.

Common Neurotransmitters

• Acetylcholine (ACh) is released by motor neurons and regulates attention, arousal, and memory and its dysregulation is associated with Alzheimer's disease.
• Dopamine (Da) controls voluntary movement, memory, reward, motivation, behavior, cognition, attention, sleep, arousal, mood, sleep, and learning, and dysregulation is associated with Parkinsonism, schizophrenic disorders, and addictive disorders.
• Norepinephrine (Ne) contributes to mood and arousal and dysregulation results in depressive disorder.
• Serotonin regulates sleep, wakefulness, eating, and aggression, the associated disorder is depressive disorders, obsessive-compulsive disorders, and eating disorders.
• GABA functions as a widely distributed inhibitory transmitter and regulates anxiety and sleep/arousal.
• Anti-anxiety drugs work at GaBa synapses, disorder= Anxiety.
• Endorphins resemble opiate drugs and play a role in pain relief, stress response, and the regulation of eating behavior.

Peripheral Nervous System

• The peripheral nervous system (PNS) includes nerves outside the brain and spinal cord.
• Nerves are bundles of neuron fibers that are routed together.
• The PNS is subdivided into the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

• The somatic nervous system enables feeling the world and movement.
• It is made up of nerves that connect to voluntary skeletal muscles and sensory receptors.
• Nerves carry information from the skin, muscles, and joint receptors to the central nervous system, and commands from the central nervous system to the muscles.
• Afferent nerve fibers carry information inward to the central nervous system.
• Efferent nerve fibers carry information outward from the central nervous system.

Autonomic Nervous System

• The autonomic nervous system (ANS) is made of nerves connecting to the heart, blood vessels, smooth muscles, and glands.
• The ANS is controlled by the central nervous system.
• The ANS controls automatic, involuntary functions such as heart rate, digestion, and perspiration.
• It is subdivided into the sympathetic and parasympathetic divisions.

Divisions of the Autonomic Nervous System

• The sympathetic division slows digestive processes and drains blood from the periphery.
• Key sympathetic nerves signal the adrenal glands, triggering the release of hormones, readying the body for exertion.
• The parasympathetic division conserves bodily resources while activating processes to save and store energy.

Central Nervous System

• The central nervous system (CNS) lies within the skull and spinal column.
• The CNS consists of the brain and spinal cord.
• The spinal cord connects the brain to the rest of the body through the peripheral nervous system.
• Spinal cord damage can result in paralysis, underscoring the importance of the spinal cord in transmitting signals from the brain to the neurons.

The Brain

• The brain contains billions of interacting cells that integrate information from inside and outside the body.
• The brain coordinates the body's actions, enabling talking, thinking, remembering, planning, creating, and dreaming.

The Hindbrain

• The hindbrain includes the cerebellum, medulla, and pons, located in the lower brainstem.
• The medulla attaches to the spinal cord and controls unconscious essential functions like breathing, muscle tone, and circulation.
• The pons ("bridge") connects the brainstem with the cerebellum, involved with sleep and arousal.
• The cerebellum ("little brain") coordinates movements, motor function, and physical balance.

Midbrain and Forebrain

• The midbrain is the segment of the brainstem between the hindbrain and the forebrain and is concerned with vision and hearing
• The forebrain is the largest and most complex region of the brain.
• The forebrain encompasses the thalamus, hypothalamus, limbic system, and cerebrum.
  • Thalamus: sensory, motor, sleep, wakefulness, consciousness, learning, and memory.
  • Hypothalamus: manage biological needs (blood pressure, mood, temperature etc)
  • Limbic System: Controls emotional responses, interpreting, storing memories, regulating hormones
• The cerebrum is the largest, most complex part of the human brain.
• The cerebrum has areas responsible for complex mental activities, including learning, remembering, thinking, and consciousness.
• The cerebrum is divided into right and left halves, called hemispheres.

Lobes of the Brain

• The corpus callosum connects the two cerebral hemispheres.
• Each cerebral hemisphere is divided into four lobes with specific functions:
  1. Occipital lobe: visual signals are sent and visual processing occur here
  2. Parietal lobe: registers the sense of touch
  3. Temporal lobe: auditory processing occurs here
  4. Frontal lobe: executive functions

Brain Specialization

• Broca's area, in the frontal lobe of the left hemisphere, produces language.
• Wernicke's area, in the temporal lobe of the left hemisphere, comprehends language.
• The left hemisphere is considered dominant and controls high mental processes like reasoning, remembering, planning, and problem-solving.
• The right hemisphere is considered non-dominant but is involved with creativity/arts.

Heredity and Environment

• Nature refers to genetic factors.
• Nurture refers to environmental factors.