🦾Neuroprosthetics Unit 2 – Nervous System Anatomy and Physiology

Key Structures and Functions

• The nervous system consists of the brain, spinal cord, and a complex network of nerves that transmit signals throughout the body
• Neurons are the primary functional units of the nervous system, responsible for receiving, processing, and transmitting information
• Glial cells provide support, protection, and maintenance for neurons, ensuring optimal functioning of the nervous system
• The brain is divided into several regions, each with specific functions (cerebrum, cerebellum, brainstem)
• The spinal cord serves as a conduit for information between the brain and the rest of the body, and also plays a role in reflexes and motor control
• Nerves are bundles of axons that carry signals between the central nervous system and the periphery, enabling communication and coordination
• The blood-brain barrier is a selective barrier that protects the brain from toxins and pathogens while allowing essential nutrients to pass through

Neurons and Synapses

• Neurons are specialized cells that transmit electrical and chemical signals, enabling communication within the nervous system
• The structure of a neuron includes the cell body (soma), dendrites, and axon, each with specific functions in signal transmission
• Dendrites receive signals from other neurons, while the axon conducts signals away from the cell body to other neurons or effector cells
• Synapses are the junctions between neurons where information is transmitted through the release of neurotransmitters
  • Presynaptic neurons release neurotransmitters into the synaptic cleft, which bind to receptors on the postsynaptic neuron
  • The type and amount of neurotransmitter released determines the nature and strength of the signal transmitted
• Neurotransmitters are chemical messengers that enable communication between neurons (glutamate, GABA, dopamine, serotonin)
• Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time, which is crucial for learning, memory, and adaptation

Neuroanatomy Basics

• The cerebrum is the largest part of the brain, responsible for higher cognitive functions, sensory processing, and voluntary movement control
  • It is divided into four lobes: frontal, parietal, temporal, and occipital, each with specific functions
• The cerebellum is located at the back of the brain and plays a crucial role in motor coordination, balance, and fine motor skills
• The brainstem connects the brain to the spinal cord and regulates vital functions such as breathing, heart rate, and sleep-wake cycles
• The limbic system is a group of structures involved in emotion, motivation, and memory formation (amygdala, hippocampus, hypothalamus)
• The thalamus acts as a relay station for sensory and motor information, processing and directing signals to the appropriate brain regions
• The hypothalamus regulates homeostasis, including body temperature, hunger, thirst, and hormonal balance
• The basal ganglia are a group of subcortical structures involved in motor control, learning, and decision-making

Signal Transmission

• Action potentials are the electrical signals that propagate along the axon of a neuron, enabling rapid transmission of information
  • They are generated by the flow of ions across the neuron's membrane, resulting in a brief reversal of the membrane potential
• Saltatory conduction is the rapid propagation of action potentials along myelinated axons, allowing for faster signal transmission
• Graded potentials are localized changes in membrane potential that occur in dendrites and the cell body, enabling the integration of incoming signals
• Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron, triggering changes in the postsynaptic cell's membrane potential
  • Excitatory neurotransmitters (glutamate) increase the likelihood of the postsynaptic neuron firing an action potential
  • Inhibitory neurotransmitters (GABA) decrease the likelihood of the postsynaptic neuron firing an action potential
• Neuromodulators are substances that modify the effects of neurotransmitters, altering the excitability and responsiveness of neurons (dopamine, serotonin, norepinephrine)

Central vs. Peripheral Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord, serving as the primary processing and integration center for nervous system functions
• The peripheral nervous system (PNS) includes all nerves and ganglia outside the brain and spinal cord, connecting the CNS to the rest of the body
• The PNS is divided into the somatic nervous system, which controls voluntary movements and receives sensory input from the external environment
• The autonomic nervous system (ANS) is part of the PNS and regulates involuntary functions such as heart rate, digestion, and respiratory rate
  • The sympathetic division of the ANS is responsible for the "fight or flight" response, preparing the body for action in stressful situations
  • The parasympathetic division of the ANS promotes "rest and digest" functions, conserving energy and maintaining homeostasis
• Sensory neurons in the PNS convey information from sensory receptors to the CNS for processing and interpretation
• Motor neurons in the PNS carry signals from the CNS to muscles and glands, initiating movement and secretion

Sensory and Motor Pathways

• Sensory pathways convey information from sensory receptors to the CNS for processing and interpretation
  • The somatosensory system processes information related to touch, pressure, temperature, and proprioception
  • The visual system processes light stimuli and enables visual perception, with information from the retina relayed to the primary visual cortex
  • The auditory system processes sound waves and enables hearing, with information from the cochlea relayed to the primary auditory cortex
• Motor pathways carry signals from the CNS to muscles and glands, initiating movement and secretion
  • The pyramidal system is responsible for fine motor control and voluntary movements, with upper motor neurons in the cortex projecting to lower motor neurons in the spinal cord
  • The extrapyramidal system is involved in the control of posture, balance, and coordination, with subcortical structures (basal ganglia, cerebellum) modulating motor output
• Reflexes are rapid, involuntary responses to stimuli that bypass the brain, allowing for quick protective actions (withdrawal reflex, knee-jerk reflex)
• Sensorimotor integration is the process by which sensory information is used to guide and refine motor actions, enabling smooth and coordinated movements

Neuroplasticity and Adaptation

• Neuroplasticity refers to the brain's ability to reorganize and adapt in response to experience, learning, or injury
• Synaptic plasticity is a key mechanism underlying neuroplasticity, involving changes in the strength and number of synaptic connections between neurons
  • Long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity, thought to underlie learning and memory formation
  • Long-term depression (LTD) is a persistent weakening of synapses, which may be important for refining neural circuits and eliminating unnecessary connections
• Structural plasticity involves changes in the physical structure of neurons and their connections, such as the growth of new dendrites or the formation of new synapses
• Functional plasticity refers to changes in the activity and responsiveness of neurons and neural circuits, which can occur rapidly in response to changing demands or experiences
• Neurogenesis is the formation of new neurons from neural stem cells, which continues throughout life in specific regions of the brain (hippocampus, olfactory bulb)
• Experience-dependent plasticity is the brain's ability to adapt and reorganize in response to specific experiences or environmental demands, enabling learning and skill acquisition

Relevance to Neuroprosthetics

• Understanding the anatomy and physiology of the nervous system is crucial for the development and implementation of neuroprosthetic devices
• Knowledge of neural signal transmission and processing informs the design of interfaces that can effectively record from or stimulate neural tissue
• Sensory and motor pathways provide targets for neuroprosthetic interventions, such as sensory substitution devices or motor prostheses
  • Cochlear implants bypass damaged hair cells in the inner ear and directly stimulate the auditory nerve, enabling hearing in individuals with profound deafness
  • Brain-computer interfaces (BCIs) record neural activity from the motor cortex and translate it into control signals for external devices, allowing for the restoration of communication or movement in paralyzed individuals
• Neuroplasticity and adaptation are essential considerations in neuroprosthetics, as the brain must learn to interpret and utilize the signals provided by the device
  • Training and rehabilitation protocols can harness the brain's plasticity to optimize the integration of neuroprosthetic devices and enhance their functional outcomes
• The central and peripheral nervous systems offer distinct opportunities for neuroprosthetic interventions, depending on the specific application and target population
• Advancements in our understanding of neuroanatomy and physiology continue to drive the development of more sophisticated and effective neuroprosthetic technologies