🐅Animal Physiology Unit 3 – Nervous System: Structure and Function
The nervous system is the body's command center, coordinating sensory input, processing information, and initiating responses. It consists of the central nervous system (brain and spinal cord) and peripheral nervous system (nerves and ganglia), communicating through electrical and chemical signals.
Neurons, the building blocks of the nervous system, transmit signals via synapses using neurotransmitters. The central nervous system processes information, while the peripheral nervous system connects it to the body. Sensory and motor pathways enable perception and action, with neural integration allowing for complex behaviors and learning.
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Nervous System Overview
Coordinates and integrates information from sensory receptors, processes this information, and initiates appropriate responses through effectors (muscles and glands)
Consists of the central nervous system (CNS) which includes the brain and spinal cord, and the peripheral nervous system (PNS) which includes nerves and ganglia outside the CNS
Communicates using electrical and chemical signals called neurotransmitters
Enables animals to respond quickly to stimuli, maintain homeostasis, and exhibit complex behaviors
Plays a crucial role in learning, memory, and decision-making processes
Develops from the ectoderm layer during embryonic development
Exhibits neuroplasticity, the ability to reorganize and form new neural connections throughout life
Neurons: Structure and Types
Specialized cells that transmit electrical and chemical signals throughout the nervous system
Consist of a cell body (soma), dendrites that receive signals from other neurons, and an axon that conducts signals away from the cell body
Contain organelles such as mitochondria for energy production, endoplasmic reticulum for protein synthesis, and Golgi apparatus for packaging neurotransmitters
Classified into three main types: sensory neurons (afferent) that convey information from sensory receptors to the CNS, motor neurons (efferent) that transmit signals from the CNS to effectors, and interneurons that form connections within the CNS
Vary in size and shape depending on their function and location (multipolar, bipolar, and unipolar neurons)
Supported and protected by glial cells, which provide nutrients, insulation (myelin), and maintain homeostasis
Exhibit a resting membrane potential of approximately -70 mV due to the unequal distribution of ions (primarily Na+ and K+) across the cell membrane
Synapses and Neurotransmission
Synapses are specialized junctions between neurons or between a neuron and an effector cell that allow for the transmission of signals
Consist of a presynaptic neuron that releases neurotransmitters and a postsynaptic cell that receives the signal
Neurotransmitters are chemical messengers stored in synaptic vesicles within the presynaptic neuron's axon terminal
Action potential arrival at the presynaptic terminal triggers the release of neurotransmitters into the synaptic cleft, the space between the pre- and postsynaptic cells
Neurotransmitters bind to specific receptors on the postsynaptic cell membrane, causing either excitatory (depolarizing) or inhibitory (hyperpolarizing) effects
Examples of neurotransmitters include acetylcholine (ACh), norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid (GABA)
Neurotransmitters are quickly removed from the synaptic cleft by enzymatic degradation or reuptake to terminate the signal and prevent overstimulation
Central Nervous System (CNS)
Comprised of the brain and spinal cord, which are protected by the skull and vertebral column, respectively
The brain is the main processing center for the nervous system and consists of the cerebrum, cerebellum, and brainstem
Cerebrum is divided into two hemispheres and is responsible for higher cognitive functions, sensory processing, and voluntary motor control
Cerebellum coordinates and fine-tunes motor movements, maintains balance and posture
Brainstem (midbrain, pons, and medulla oblongata) regulates vital functions such as breathing, heart rate, and sleep-wake cycles
Spinal cord serves as a conduit for sensory and motor information between the brain and the rest of the body
Organized into segments with pairs of spinal nerves that innervate specific regions of the body
Contains neural circuits for simple reflexes (knee-jerk reflex) that can occur without input from the brain
Surrounded by three layers of protective membranes called meninges (dura mater, arachnoid mater, and pia mater)
Ventricles within the brain and central canal of the spinal cord contain cerebrospinal fluid (CSF) which provides cushioning, nourishment, and waste removal
Peripheral Nervous System (PNS)
Consists of nerves and ganglia outside the CNS that connect the brain and spinal cord to the rest of the body
Divided into the somatic nervous system, which controls voluntary movements and receives sensory input from the external environment, and the autonomic nervous system, which regulates involuntary functions of internal organs
Somatic nervous system includes 12 pairs of cranial nerves that originate from the brain and 31 pairs of spinal nerves that emerge from the spinal cord
Autonomic nervous system is further subdivided into the sympathetic (fight-or-flight response) and parasympathetic (rest-and-digest functions) divisions
Sympathetic division increases heart rate, dilates pupils, and diverts blood flow to skeletal muscles during stress or emergency situations
Parasympathetic division promotes digestion, slows heart rate, and conserves energy during restful states
Enteric nervous system, sometimes considered a third division of the ANS, controls the gastrointestinal tract and can function independently of the CNS
Sensory and Motor Pathways
Sensory pathways convey information from sensory receptors to the CNS for processing and interpretation
Receptors transduce stimuli (light, sound, touch) into electrical signals called receptor potentials
Sensory neurons transmit these signals to the spinal cord or brain via ascending pathways (spinothalamic tract for pain and temperature, dorsal column-medial lemniscus pathway for touch and proprioception)
Sensory cortices in the brain process and integrate this information to create perceptions and guide behavior
Motor pathways carry signals from the CNS to effectors (muscles and glands) to initiate responses
Upper motor neurons originate in the motor cortex or brainstem and synapse on lower motor neurons in the spinal cord or cranial nerve nuclei
Lower motor neurons directly innervate effectors and cause them to contract or secrete
Descending pathways (corticospinal tract) enable voluntary control of skeletal muscles
Reflex arcs are simple neural circuits that allow rapid, involuntary responses to stimuli without input from higher brain centers
Example: Withdrawal reflex occurs when a painful stimulus activates sensory neurons that synapse directly on motor neurons in the spinal cord, causing the affected limb to be pulled away
Neural Integration and Processing
Neurons integrate and process information from multiple sources to generate appropriate responses
Temporal summation occurs when a postsynaptic neuron receives multiple subthreshold stimuli in rapid succession, leading to a cumulative depolarization that may trigger an action potential
Spatial summation involves the convergence of signals from multiple presynaptic neurons onto a single postsynaptic neuron, allowing for the integration of information from different sources
Neuronal circuits, such as feedback loops and feedforward pathways, enable complex processing and modulation of signals
Feedback loops can be negative (inhibitory) or positive (excitatory) and help maintain homeostasis or amplify signals
Feedforward pathways allow for the anticipation of future events based on current sensory input
Neuroplasticity, the ability of the nervous system to reorganize and form new connections, underlies learning, memory, and adaptation to changing environments
Long-term potentiation (LTP) and long-term depression (LTD) are forms of synaptic plasticity that strengthen or weaken synaptic connections based on the timing and frequency of neuronal activity
Structural plasticity involves the growth of new neurons (neurogenesis), the formation of new synapses (synaptogenesis), and the pruning of unused connections
Nervous System Disorders and Treatments
Neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's, involve the progressive loss of neurons in specific regions of the brain
Alzheimer's disease is characterized by the accumulation of amyloid plaques and neurofibrillary tangles, leading to memory loss and cognitive decline
Parkinson's disease results from the degeneration of dopaminergic neurons in the substantia nigra, causing tremors, rigidity, and difficulty with movement initiation
Huntington's disease is caused by a genetic mutation that leads to the death of neurons in the striatum, resulting in uncontrolled movements, cognitive impairment, and psychiatric symptoms
Neurological disorders, such as epilepsy, multiple sclerosis, and stroke, disrupt the normal functioning of the nervous system
Epilepsy is characterized by recurrent, unprovoked seizures due to abnormal electrical activity in the brain
Multiple sclerosis is an autoimmune disorder that damages the myelin sheath surrounding axons, leading to impaired signal transmission and a range of neurological symptoms
Stroke occurs when blood flow to a region of the brain is disrupted, causing cell death and loss of function in the affected area
Psychiatric disorders, including depression, anxiety, and schizophrenia, involve imbalances in neurotransmitter systems and altered brain function
Depression is associated with reduced levels of serotonin and norepinephrine, leading to persistent feelings of sadness, loss of interest, and changes in sleep and appetite
Anxiety disorders, such as generalized anxiety disorder and panic disorder, are characterized by excessive worry, fear, and physical symptoms like increased heart rate and sweating
Schizophrenia is a complex disorder involving disturbances in thought, perception, and behavior, believed to result from a combination of genetic and environmental factors
Treatments for nervous system disorders may include medications, surgical interventions, and therapeutic approaches
Medications can target specific neurotransmitter systems (selective serotonin reuptake inhibitors for depression) or alleviate symptoms (levodopa for Parkinson's disease)
Surgical interventions, such as deep brain stimulation (DBS), involve the implantation of electrodes to modulate abnormal brain activity in conditions like Parkinson's disease and dystonia
Therapeutic approaches, including cognitive-behavioral therapy (CBT) and physical therapy, can help individuals manage symptoms, improve function, and enhance quality of life