🐅Animal Physiology Unit 3 – Nervous System: Structure and Function

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