NEUR201: Intro to the Nervous System

• NEUR201 introduces neurophysiology.

Lecture Objectives

• To recall the nervous system's role in sensing the environment and controlling behaviour.
• To recall the main nervous system cell types and their major features.
• To describe the major divisions of the nervous system and state their major functions.
• To describe how the body is "mapped" in the brain.
• To recall how information is encoded and conducted around the body and between nerve cells.

Nervous System Function

• The nervous system maintains homeostasis and subconscious body functions.
• It is also responsible for thoughts, feelings, memories, emotions, and sleep.
• The nervous system creates sensations and perceptions from sensory information, pain perceptions.
• It also coordinates voluntary and innate movements and behaviors.
• Lastly, the nervous system controls reproductive functions.
• NEUR201 and NEUR202 explain how the nervous system achieves these functions.

Organization of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord.
• The CNS is composed of neurons and glial cells.
• The peripheral nervous system (PNS), including the enteric nervous system (ENS), consists of peripheral nerves and ganglia.
• The PNS is composed of neurons and glial cells.

Nerve Cells (Neurons)

• Neurons have functional components, including input, summation, conduction, and output zones.
• The input zone consists of dendrites and the cell body, which receive chemical signals.
• The summation zone includes the axon hillock and involves the summation of inputs.
• The conduction zone, mainly the axon, may be long and carries electrical signals between brain areas, to/from the spinal cord, or between sensory receptors and effector cells.
• The output zone consists of axon terminals that contact the input zone of other neurons or effectors and release neurotransmitters (chemical signals).
• There are about 85 billion neurons in the human brain.
• Neurons are classified into four morphological types: multipolar, bipolar, unipolar, and anaxonic.
• Neuron-neuron signaling occurs at synapses.
• There are roughly 100 trillion synapses in the human brain.

Glia

• Glia make up approximately 50 billion cells in the human brain.
• Oligodendrocytes in the CNS form the myelin sheath.
• Astrocytes provide nutrients and maintain the extracellular environment, while offering structural support in the CNS.
• Microglia mount the immune response in the CNS.
• Ependymal cells circulate and produce cerebrospinal fluid.
• Schwann cells in the PNS form the myelin sheath.
• Satellite cells provide nutrients and structural support to neurons in the PNS.

Neurons and Networks

• Neurons receive input from many other neurons on their cell body and dendrites.
• Neurons deliver output to other neurons via axons and nerve terminals.
• Inputs and outputs can be excitatory or inhibitory.
• Information processing in the nervous system occurs through the throughput and balance of excitatory and inhibitory signals across networks of neurons.

Synapses

• Electrical synapses have very fast ion flow from cell to cell and can be bidirectional and involves gap junctions.
• Chemical synapses are slower.
• Chemical synapses rely on chemical neurotransmitters that cross the gap.
• Neurotransmitters are packaged in vesicles.
• Synapse strength in chemical synapses can be modified, and signals are unidirectional.

Brain Organization

• The brain has regions that are organized based on function.
• The frontal lobe contains, the primary motor cortex, and prefrontal cortex.
• There are areas for somatosensory, primary taste, visual association, and visual cortex
• The brain also has dedicated auditory and speech regions.
• Sensory axons from all body locations maintain a strict spatial relationship to one another along their projection pathways.
• Sensory information arrives in the somatosensory cortex with a specific "map" of the body.
• The somatosensory map is distorted, with regions having high receptor density and smaller receptive field sizes occupying a disproportionately large area.

Motor Areas of the Brain

• Key motor areas include the pre-central gyrus, primary motor cortex, and the posterior parietal cortex.

Information Flow

• Sensory information is afferent (in) while motor information is efferent (out).

Autonomic Nervous System

• The autonomic nervous system controls fight, flight, and fright responses.
• It also controls rest and digestive processes.
• The enteric nervous system (ENS) is also part of the autonomic system.

Information in the Brain

• Neural information is conveyed in the form of electrical and chemical signals.
• Signaling within a single nerve cell is electrical.
• The resting membrane potential is the electrical potential across the membrane of an inactive nerve cell.
• Local potentials or action potentials are events that occur when excitable cells are active.
• Signaling between nerve cells, or between nerve and non-nerve cells (e.g., muscles), is mostly chemical.

Electrical Signals

• Action potentials, EPSPs (excitatory postsynaptic potentials), and IPSPs (inhibitory postsynaptic potentials) are crucial for neuronal communication.

Typical MCQ Question

• A typical multiple-choice question might ask about the correct statements regarding axons and myelin.