Neuronal Structure and Function

Neuronal Structure and Function

• Neurons are the fundamental units of the nervous system.
• They consist of a cell body (soma), dendrites, and an axon.
• Axons can extend far from the soma, some reaching a meter in length.
• Dendrites and axons branch extensively near their ends.
• The soma is the primary site for metabolic activity and protein synthesis.
• The nucleus, located in the soma, regulates gene expression in response to neuronal demands.
• Microtubules and microfilaments transport proteins and molecules to the axon terminals.
• Axon hillock is the site where action potentials arise.
• Myelin sheath facilitates rapid action potential propagation.

Neuronal Types and Quantities

• The human brain contains approximately 86-100 billion neurons.
• Neurons are diverse in their shapes and properties, resulting in various functional responses.
• Sensory neurons (afferent) transmit information from sensory receptors to the central nervous system.
• Motor neurons (efferent) transmit instructions from the central nervous system to muscles and glands.
• Interneurons connect afferent and efferent neurons, processing information and determining responses.

Functional Regions of Neurons

• Input region: site of synapses and connections with other neurons.
• Integrative region: summates signals from other neurons and initiates neuronal response (action potential).
• Conductive region: action potential travels away from the soma.
• Output region: site of synapse for communicating with other neurons.

Synapses

• Axon endings form synapses with input regions of other neurons.
• A large number of synapses (estimated 10¹⁵) exists in the human brain.
• Each synapse receives input from a single presynaptic neuron.
• Neurons frequently receive thousands of synapses, allowing for extensive interaction.

Glial Cells

• Glial cells support neuronal function, outnumbering neurons in the central nervous system.
• Astrocytes form the blood-brain barrier, regulating extracellular space and neurotransmitter levels.
• Myelinating glia (oligodendrocytes and Schwann cells) insulate axons for faster signal transmission.
• Microglia are involved in brain inflammatory processes.
• Ependymal cells line fluid-filled spaces in the brain.

Nervous System Anatomy

• The Central Nervous System (CNS) includes the encephalon and spinal cord.
• The encephalon consists of the cerebral hemispheres (cortex), brainstem, cerebellum, and deep structures like the thalamus and hypothalamus.
• The spinal cord is a region for sensory relay and motor commands, connecting the brain to the peripheral body.

Brainstem

• The brainstem processes and relays sensory and motor information from and to the head, coordinating functions like vision, respiration, and postural control.

Spinal Cord

• The spinal cord relays sensory information from the body to the brain and transmits motor signals, connecting the brain to the periphery.
• It controls reflex actions and coordinates basic motor functions.

Cerebellum

• The cerebellum is involved in motor control, posture, and the fluidity of mental processes.

Cerebral Hemispheres

• The cerebral cortex represents the highest level of sensory processing and higher cognitive function.

Basal Ganglia

• The basal ganglia are involved in motor control and cognitive functions.

Peripheral Nervous System

• The peripheral nervous system handles sensory information input from the environment and sends motor commands to the periphery.

Sensory Pathways

• Sensory pathways relay sensory information to higher centers in the brain, such as the thalamus.

Motor Pathways

• Motor Pathways send motor signals from higher brain centers to muscles.

Reflexes

• Reflexes are involuntary motor responses to sensory stimuli.

Autonomic Nervous System

• The autonomic nervous system regulates involuntary functions, including internal organs like the heart, lungs and digestive tract.
• Two subdivisions of the autonomic nervous system are parasympathetic and sympathetic.

Brain Stem and other Controls

• The brainstem regulates many internal organs.
• The medulla regulates the heartbeat/blood vessel diameter.

Memory and Synaptic Plasticity

• Memory involves encoding, storage, and retrieval of information through changes in the connectivity of neurons.
• Short-term memory mechanisms include habituation and sensitization.
• Long-term memory involves changes in synaptic strength and structure.

Neuronal Signaling

• Ions flow across neuronal membranes create electrical signals.
• The ion channels and pumps maintain internal concentrations.
• Voltage differences between inside and outside a neuron produce membrane potential.
• Action potentials are rapid changes in the membrane potential.
• Synaptic transmission converts electrical signals to chemical and back.

Neuronal Synapses

• Synapses allow communication between neurons.
• Neurotransmitters are released, binding to receptors on other neurons.
• These mediate excitation or inhibition.