Chapter 4 Notes: The Biological Bases of Behaviour

The Nervous System in Action

  • Historical Perspective:

  • Rene Descartes (1600s) proposed that the human body operates like a machine, leading to the scientific study of neuroscience. His dualism emphasized separation between the mind and body, which fueled debates on consciousness and the nature of human behavior.

Neurons

  • What is a Neuron?

  • Specialized nervous system cells that receive, process, and transmit information through electrical impulses and chemical signals.

  • Neurons vary in shape (multipolar, bipolar, unipolar), size, and function, adapting to their roles as sensory, motor, or interneurons.

Major Structures of a Neuron

  • Dendrites:

  • Branch-like extensions that receive stimulation from sensory receptors or other neurons, facilitating the communication necessary for synaptic transmission.

  • Soma (Cell Body):

  • Contains the nucleus and cytoplasm, integrating incoming signals and determining the output based on total received stimuli before sending action potentials to the axon.

  • Axon:

  • A long, cable-like fiber that conducts neural impulses away from the soma, generating action potentials along its length via the myelin sheath, which insulates segments of the axon and facilitates rapid signal conduction (saltatory conduction).

  • Terminal Buttons:

  • End points of the axon that house synaptic vesicles containing neurotransmitters, initiating communication with nearby neurons or target organs.

Types of Neurons

  • Sensory Neurons (Afferent):

  • Carry signals from sensory receptors (e.g., skin, eyes) to the central nervous system (CNS), allowing for sensory perception and response.

  • Motor Neurons (Efferent):

  • Transmit signals from the CNS to muscles and glands, facilitating voluntary and involuntary actions.

  • Interneurons:

  • Connect sensory and motor neurons within the CNS; essential for reflexes and complex signaling, allowing for integration of multiple signals and processing within neuronal circuits.

The Pain Withdrawal Reflex

  • Process of Responding to Pain:

  1. Pain receptors (nociceptors) are activated due to harmful stimuli in the skin.

  2. Sensory neuron relays this message to an interneuron located in the spinal cord, often leading to a very rapid response (reflex arc).

  3. Interneuron rapidly stimulates a motor neuron to withdraw from the painful stimulus, often before the brain is fully aware of the stimulus.

  4. Afterwards, a message is sent to the brain for awareness of the event, which can lead to future behavioral modifications.

Neuroglial (Glial) Cells

  • Support cells in the CNS:

  • Diverse types of glial cells (e.g., astrocytes, oligodendrocytes, microglia) non-neuronal in nature but essential for maintenance.

  • Functions include holding neurons in place, guiding them during development, disposing of metabolic waste, forming the blood-brain barrier, and insulating axons (myelin sheath) to enhance signal transmission.

Neural Communication

  • Process of Sending Messages:

  • Involves electrochemical signals, where neurons fire based on inputs integrating at dendrites and soma, impacting action potentials.

  • Action Potentials:

  • Brief voltage changes that occur when a neuron is stimulated, involving the flow of ions across the neuronal membrane, allowing for the propagation of electrical impulses along the axon.

Action Potential Phases

  1. Resting Potential:

  • Cell maintains a charge of -70 mV due to ion distribution established by the sodium-potassium pump.

  1. Threshold Potential (-55 mV):

  • When sufficiently excited by combined signals, the neuron reaches threshold and fires an action potential.

  1. Depolarization:

  • Rapid influx of Na+ ions leads to a shift in voltage, causing the internal environment to become more positively charged.

  1. Repolarization:

  • K+ ions exit the neuron, restoring the negative resting potential; this process is critical for returning to a homeostatic state.

  1. Refractory Period:

  • A period during which the neuron cannot fire again; involves absolute (no action potential possible) and relative (a stronger than normal stimulus would be required) phases.

  1. All-or-None Law:

  • An action potential occurs fully or not at all, propagating down the axon; it cannot travel back due to the refractory states of previously depolarized regions.

Synaptic Transmission

  • Synapse:

  • The gap where neurotransmitters are exchanged between neurons, facilitating communication; could include electrical synapses via gap junctions for rapid signaling.

  • Process of Synaptic Transmission:

  • Upon reaching the synapse, vesicles in the presynaptic neuron release neurotransmitters into the synaptic cleft upon an action potential.

  • Neurotransmitters bind to receptors on the postsynaptic neuron, resulting in changes in activity (excitatory or inhibitory).

  • Neurotransmitters:

  • Chemicals that transfer signals between neurons; notable examples include Acetylcholine (involved in motor control), Dopamine (involved in motivation and reward), and Serotonin (affecting mood and regulation).

Neurotransmitters and Their Functions

  • Acetylcholine:

  • Important for motor control, cognitive functions, and memory; deficiencies linked to Alzheimer's disease.

  • Monoamines (Dopamine, Serotonin, Norepinephrine):

  • Affect arousal, mood, and motivation; imbalances can contribute to mood disorders.

  • Amino Acids (GABA, Glutamate):

  • Major neurotransmitters involved in inhibitory (GABA) and excitatory (Glutamate) processes in the brain, playing essential roles in maintaining neuronal balance and signaling.

  • Peptides (Endorphins):

  • Modulate neurotransmission related to pain relief and emotional regulation, often released in response to stress or pain stimuli.

The Nervous System Structure

  • Central Nervous System (CNS):

  • Comprises the brain and spinal cord; responsible for processing information and coordinating all body functions.

  • Peripheral Nervous System (PNS):

  • Connects the CNS to the body’s organs and limbs; divides into somatic (voluntary control) and autonomic systems (involuntary control).

  • Somatic Nervous System:

  • Governs voluntary movements by signaling skeletal muscles directly; plays a role in reflex actions.

  • Autonomic Nervous System:

  • Controls involuntary processes; further divided into sympathetic (activating fight or flight response) and parasympathetic (managing rest and digest functions) systems.

Major Brain Structures and Their Functions

  • Brain Stem:

  • Sustains vital life functions like heart rate and breathing; acts as a conduit for communication between the brain and spinal cord.

  • Cerebellum:

  • Coordinates movement, balance, and timing; essential for motor learning and fine-tuning voluntary movements.

  • Limbic System:

  • Integral to emotion regulation, drives, and memory encoding; includes important structures such as the Hypothalamus (homeostasis), Hippocampus (memory), and Amygdala (emotions).

  • Cerebral Cortex:

  • Responsible for higher-order cognitive functions such as thought, planning, sensory processing, and motor control; divided into lobes crucial for specific functions (frontal, parietal, temporal, occipital).

Brain Lobes

  • Frontal Lobe:

  • Associated with executive functions like planning, decision-making, problem-solving, and controlling voluntary movements.

  • Parietal Lobe:

  • Processes sensory information related to touch, temperature, and pain, contributing to the perception of bodily awareness.

  • Temporal Lobe:

  • Involved in auditory processing, language comprehension, and memory; integration of sensory information occurs here.

  • Occipital Lobe:

  • Specialized for visual processing, contributing to the perception and interpretation of visual stimuli from the environment.

Brain Plasticity

  • Definition:

  • The ability of the brain to change and adapt through experience, a cornerstone of learning and memory formation.

  • Neurogenesis:

  • The process through which new neurons are generated from neural stem cells, potentially aiding recovery from brain injury and in learning processes.

  • Hebbian Learning:

  • A principle that emphasizes strengthening of synaptic connections between neurons that are activated simultaneously ("cells that fire together wire together"), fundamental to learning and memory formation.

Mapping the Brain

  • Techniques for studying brain functions include lesion studies (observing behaviors post-injury), EEG (measuring electrical activity), TMS (modulating brain activity), PET scans (visualizing metabolic processes), and MRI (structural imaging).

  • These methods provide various insights into brain activity, its physical structure, and its relation to behavior and function, revealing the complexity of neural interactions.