Comprehensive Notes on Biology and Behavior: Neurons and the Nervous System

Neurons

• Neurons: cells that convey sensory information from the body to the brain.
• Two basic functions:
  • Generate electricity (nerve impulses).
  • Release chemicals to communicate with other cells.
• Motor neuron: carries commands to muscles and organs.
• Sensory neuron: carries information from the body and outside world to the brain and spinal cord.
• Interneurons: connect one neuron to another in the same part of the brain and spinal cord.

Energy and the Cell Membrane

• Cell is at rest with an electrical resting potential.
• The cell is stimulated, and electrical charges flow across the cell membrane.
• Reverses the charge of the resting potential, producing an action potential.
• Depolarization: sodium ions go in.
• Restore distributions of ions; the cell is at rest again; potassium ions go out of the cell membrane.
• Polarization: a state where there is a difference in electrical charge between the inside and outside of the cell.
• Voltage: the difference in charge between two points.
• Resting potential: the difference in charge between the inside and outside of a membrane of a neuron at rest.
• Ions: atoms that have lost or gained electrons.

Neuron Impulses

• Three steps
• What moves the ions?
  • Chemically: neurotransmitters.
  • Electrically: change in electrical potential.
• Absolute refractory period: sodium ions are unresponsive to further stimulation.
• Relative refractory period: sodium ion channels could support another action potential, but potassium channels are still open.
• A new action potential can occur if the stimulus is sufficiently strong to overcome the charge.
• Force of diffusion: ions move to the less concentrated side.
• Electrostatic pressure: ions are repelled from similarly charged areas and attracted to oppositely charged areas.
• Sodium-potassium pump: protein molecules that move sodium ions through the cell membrane to the outside.

Ion Channels

• Ion channels: gated pores in the membrane, limiting the flow of ions in and out of the cell.
• Can be chemically gated or electrically gated.

Depolarization

• Action potential: depolarization of the membrane that allows a neuron to communicate.
• The action potential is ungraded: all or none – full strength or none at all.
• The action potential is nondecremental: it travels down without any decrease in size.

Refractory Periods

Rate Law

• The axon encodes stimulus intensity not in the size of the action potential but in how often the neurons fire.

Myelin Sheath

• An insulating layer that wraps around the axon.
• Forces signals to jump ahead to nodes of Ranvier.
• Made up of glial cells.
  • Oligodendrocytes in the central nervous system make up the myelin sheath.
  • Break down dead neurons.
  • Bring nutrients for neurons.

Neurotransmitters and Receptors

• Work like a lock and key; the lock is the receptor, and the key is the neurotransmitter.
• Synapses
• Cells do not make physical contact but communicate across gaps called the synaptic cleft.
• Functional connection.
• End of axon terminals: terminal buttons.
• Vesicles: bubbles containing neurotransmitters.
• Synapse: fluid-filled space between neurons; purely chemical.
• Neurotransmitters: chemical messengers that either excite or inhibit other neurons' firing.
• Excitation: excites.
• Inhibition: inhibits.

Drugs and Receptors

• Agonist: replicates receptor action.
• Antagonist: prevents receptor action.
• Direct: binds at the same site.
• Indirect: binds at a different site.

Pre- and Postsynaptic Potential

• Voltage change at the receptor site: Postsynaptic potential (PSP).
• Positive voltage shift: excitatory PSP.
• Negative voltage shift: inhibitory PSP.

Neural Networks

• Patterns of neural activity.
• Interconnected neurons that fire together in order.
• Synaptic connections.

The Nervous System Organization

• Peripheral nervous system: outside the central nervous system.
  • Somatic: voluntary muscles and receptors.
  • Autonomic: controls automatic, involuntary movements.
    • Sympathetic: fight or flight; raises heart rate and dilates the pupil.
    • Parasympathetic: calms you down after a scare; slows heart rate and constricts the pupil.
• Central nervous system: brain and spinal cord.
  • Afferent: fire signals toward the CNS.
  • Efferent: fire away from the CNS.

The Spinal Cord

• Contains nerve bundles in the spinal cord that carry info to and fro the brain.
• Nerves in each section connect with different parts of the body.
• Dorsal horn: contains afferent endings of sensory nerves.
• Ventral horns: contains cell bodies of efferent nerves.
• Both horns are connected by interneurons.

The Brain

• Vital functions and coordinating movement.
• Basic life functions.
  • Hindbrain
    • Medulla: heart functions, breathing, coordination in swallowing and digestion.
    • Pons: relay station for signals between bigger levels of the nervous system and lower levels.
    • Elimination and creation.
    • Synaptic pruning: the excess synapses get eliminated.
    • Long-term potentiation: repeated communication across synapses facilitates future communication.
  • Midbrain
  • Forebrain
    • Amygdala: fight or flight response, remembers events tied to strong emotions, and determines the value of a stimulus.
    • Hippocampus: activates when we form memories.
    • Cingulate gyrus: focuses our attention on things that are unpleasant and connects to muscles and glands in the face and neck.
    • Cerebellum: coordination, balance, and muscle tone, helps with the sequence of actions, and is involved in the memory of motor skills.
    • Fine motor skills
    • Reflect actions and voluntary movements
    • Reticular Formation: regulates awareness and attention (filters out irrelevant stimuli), regulates sleep and wakefulness, and coordinates several brain areas.
    • Highly developed, numerous functions
    • Basal ganglia: a collection of neurons crucial to motor function and muscle memory.
    • Limbic System: Connection between cortex and subcortical structures.
    • Thalamus: relays sensory info through neurons that project to appropriate regions in the cortex and filters sensory info.
    • Hypothalamus: regulates the autonomic nervous system and endocrine systems for hunger, sexual behavior, temperature regulation, and aggression, maintaining homeostasis.
    • Cerebral Cortex Temporal Lobes: auditory projection, visual and auditory association, additional language area, structures important for memory. Inferior temporal cortex: visual identification of objects
    • Aphasia: difficulty processing or producing language
    • Mirror neurons: mirroring other’s actions
    • Making new connections, brain adapting after damage

The Endocrine System

• A series of glands throughout the body that release hormones.
• Hormone: chemical messages in the bloodstream.
• Endocrine glands:
  • Pituitary gland: "director" of other glands; produces growth hormone.
  • Thyroid: regulates metabolic rate.
  • Adrenal: controls salt and carbohydrate metabolism.
  • Pancreas: regulates sugar metabolism.
  • Gonads: produce sex hormones.
• CNS Endocrine Control Centers:
  • Hypothalamus: secretes hormones and controls the pituitary gland.
  • Pineal gland: regulates melatonin secretion and sleep schedules.
  • Pituitary gland: involved in sex, reproduction, circulatory function, and hunger.
• HPA Axis: activated in times of stress, showing how the brain affects the immune system.

Hemispheric Lateralization

• Handedness, footedness, ocular dominance (one eye is more dominant than the other), and aural dominance.
• Right hemisphere:
  • Left visual field is processed in the right hemisphere.
  • Feelings, intuition, humor, relationships, rhythm, physical senses, and motor skills; controls the left side of the body.
• Left hemisphere:
  • The right visual field is processed in the left hemisphere of the brain.
  • Analytical thinking, rules, logic, speech, math, time, language, planning; controls the right side of the body.
• Corpus Callosum: controls the connection between the right and left hemispheres.

Visual Lateralization

Ways the Brain can be Monitored or Tested

• CT or CAT scan: computer-enhanced X-ray of the brain.
• TMS: transcranial magnetic stimulation uses a magnetic field that can penetrate up to 2 cm of the brain to enhance or depress activity in specific parts.
• MRI: uses strong magnetic fields to align molecules, then uses a quick and strong burst of radio waves to disrupt alignment; measures energy when the molecules snap back into alignment.
• fMRI: detects changes in blood oxygenation tied to neural activity; high activity equals high oxygen use and blood flow.
• Need to consider background activity
• DTI: builds a picture of water movement (diffusion) in the brain using an MRI scanner.
• PET: tracks levels of harmless radioactive material that is injected into the bloodstream; higher activity in brain levels equals more blood flow and a higher concentration of radioisotope.
• NIRS: measures changes in blood oxygenation.
• EEG: measures electrical activity via electrodes.
  • Can be inter- or intra-cranial, with electrodes either on or in the skull.
  • Very good time resolution.