Neurologic System PDF

The nervous system is involved in nearly everything we do - from how we see, to how we walk and talk. The nervous system is divided into: the central nervous system - the brain and the spinal cord the peripheral nervous system, which is further divided into the - somati...

The nervous system is involved in nearly everything we do - from how we see, to how we walk and talk. The nervous system is divided into: the central nervous system - the brain and the spinal cord the peripheral nervous system, which is further divided into the - somatic and - the autonomic nervous systems. Broadly speaking, the nervous system can be split into: - an afferent and - an efferent division. The afferent division brings sensory information from the outside into the central nervous system, and includes visual receptors, auditory receptors, chemoreceptors, and somatosensory or touch receptors. On the other hand, the efferent division brings motor information from the central nervous system to the periphery, ultimately resulting in contraction of skeletal muscles to trigger movement through the somatic nervous system, as well as contraction of the smooth muscles to trigger activity of the internal organs through the autonomic nervous system. The nervous system is made up of two main types of cells: neurons and glial cells. Neurons are the main cells of the nervous system. They’re composed of 1. a cell body, which contains all the cell’s organelles, and when there’s a group of neuron cell bodies that are next to each other in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies that are located outside of the central nervous system is called a ganglion. Neurons have nerve fibers that extend out from the neuron cell body- these are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons. Where two neurons come together is called a synapse, and that’s where one end of an axon releases neurotransmitters, further relaying the signal to the dendrites or directly to the cell body of the next neuron in the series. To trigger the release of neurotransmitters, neurons use an electrical signal that races down the axon, known as the action potential. To help speed up that electrical signal - the axons are intermittently wrapped by a fatty protective sheath called myelin, which comes from glial cells like - oligodendrocytes in the central nervous system, and - Schwann cells in the peripheral nervous system. Another type of glial cells are called astrocytes, and they’re only present in the central nervous system. - Astrocytes give structural and metabolic support to neurons, as well as act as resident immune cells, and help seal and nourish the blood-brain barrier. The blood-brain barrier consists of tight junctions that connect endothelial cells that line the capillaries in the brain. These tight junctions seal off the space between the endothelial cells, and they’re surrounded by basement membrane as well as astrocytes which further strengthen the barrier. Think of the blood-brain barrier as the brain’s bouncer, a highly selective membrane that turns bacteria and other large, shady-looking molecules that are floating around in the blood away at the door, while letting in nutrients like water, oxygen, glucose, and smaller, fat-soluble molecules. The brain has a few regions - the most obvious is the: cerebrum, which is divided into two cerebral hemispheres. - The right cerebral hemisphere receives afferent fibers and sends efferent fibers to the left side of your body, while the left cerebral hemisphere receives afferent fibers and sends efferent fibers to the right side of the body. If we look at a cross section of the cerebrum, the outermost area is the grey matter or cerebral cortex and is made up of billions of neuron cell bodies, and the innermost area is the white matter and is made up of the axons that come off of all of those neurons. The cerebral cortex is divided into the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe. 1. The frontal lobe controls movement, and executive function, which is our ability to make decisions. 2. The parietal lobe processes sensory information, which lets us locate exactly where we are physically and guides movements in a three-dimensional space. 3. The temporal lobe plays a role in hearing, smell, and memory, as well as visual recognition of faces and languages. - The temporal lobe surrounds and communicates with the hippocampus and helps send information from short-term to long-term memory. 4. Finally, there’s the occipital lobe, which is primarily responsible for vision. *** Within the white matter there are deeper structures that are subcortical - below the cortex - like the A. internal capsule, which is like a highway that allows information to flow through neurons that are going to and from the cerebral cortex. B. There’s also the basal ganglia, which are actually two deep structures: - the pallidum and - the striatum, with the striatum further divided into the caudate nucleus and the putamen. The striatum receives input from the cerebral cortex about a desired movement and then it sends output to the other basal ganglia structures to control smooth movement by inhibiting undesired movements. As an example, when you walk, you have to move one leg at a time - so when one leg steps forward, the other leg gets inhibited by the basal ganglia, so that it’s stationary - and that prevents you from falling!  C. Next, there’s the diencephalon, which is composed of an upper part called the thalamus and a lower part called the hypothalamus. - The thalamus is a collection of nuclei - so millions of nerve cell bodies - that process the sensory information coming in from the body to the cerebral cortex, as well as the motor information going from the cerebral cortex to the body. - The hypothalamus is a small region that does a variety of things like regulate the body temperature, the sleep and wake cycle, and eating and drinking. To help do all of this, the hypothalamus regulates the release of the major endocrine hormones. The hypothalamus sends signals to the pituitary, which is a pea-sized gland, that hangs by a stalk from the base of the brain and has two parts - the anterior and posterior pituitary. *** The pituitary gland produces and secretes hormones when it receives signals the hypothalamus. Together, they form the hypothalamic-pituitary axis. D. Next, there’s the cerebellum, which sits down at the base of the skull. - The cerebellum helps with coordinating movement, precision, and balance. - The cerebellum receives sensory input about body position from the spinal cord and receives motor input from the brain - and integrates them together to help fine-tune motor activity and store it as muscle memory. An example is riding a bicycle, something you typically can do pretty easily, even if you haven’t used a bike in a while. E. And finally there’s the brainstem, which is located right in front of the cerebellum. - The brainstem is made up of three parts - midbrain, pons, and medulla - and it connects to the spinal cord. The midbrain is the uppermost part and participates in vision, hearing, motor control, the sleep- wake cycle, and consciousness. The pons is the middle part and contains nuclei that control facial expressions and sensation, as well as body equilibrium and posture. The medulla is the lower part and contains nuclei that help regulate blood pressure, breathing, swallowing, coughing, vomiting, and digestion. *** The spinal cord is a long rod of nervous tissue that extends down from the brainstem, to the lumbar region of the vertebral column. - Information travels up the spinal cord through afferent or sensory fibers and - down the spinal cord through efferent or motor fibers. If you take a cross-section of the spinal cord: there’s white matter on the outside and that contains the afferent and efferent fibers, and there’s grey matter on the inside and that contains the nerve cell bodies arranged in three grey columns or horns that look a bit like a butterfly, when you put both sides together. The three pairs of grey horns are divided into anterior or ventral horns, posterior or dorsal horns, and lateral horns. 1. The anterior or ventral horns receive information from the motor cortex of the brain, and then send it to the skeletal muscles to trigger voluntary movement. 2. The posterior or dorsal horns take sensory information from the outside world and send it to the sensory cortex of the brain. Sensory information includes pressure, vibration, fine touch, and proprioception - or the awareness of one’s bodily position in space. 3. Finally, there are the lateral horns, which are mainly involved with the sympathetic division of the autonomic motor system - these help regulate processes like urination, digestion, and heart rate. The spinal cord is responsible for coordinating reflexes, which are fast involuntary responses to a stimulus - like getting banged on the knee with a hammer. The reason that’s possible is that some sensory neurons synapse in the spinal cord, instead of going up to the brain, and shorter distances mean faster signals! Now zooming in, the brain and spinal cord are covered by the meninges, which are three protective layers of the brain. The inner layer of the meninges is the pia mater, the middle layer is the arachnoid mater, and the outer layer is the dura mater. These first two, the pia and arachnoid maters form the subarachnoid space, which houses the cerebrospinal fluid, or CSF. CSF is a clear, watery liquid which is pumped around the spinal cord and brain: - cushioning them from impact and - bathing them in nutrients. Now, the peripheral nervous system consists of nerves which are enclosed bundles of axons, that connect the central nervous system to every other part of the body. There are twelve cranial nerves pairs that exit from the skull and innervate the head and neck. The first two emerge from the cerebrum, these are: - cranial nerve one, called the olfactory nerve, and - cranial nerve two called the optic nerve. - The rest of the cranial nerves emerge from different parts of the brainstem. - Cranial nerve three, called the oculomotor nerve, and cranial nerve four called the trochlear nerve emerge from the midbrain. - Cranial nerve five called the trigeminal nerve, cranial nerve six called the abducens nerve, cranial nerve seven called the facial nerve, and cranial nerve eight called the vestibulocochlear nerve emerge from the pons. And finally, cranial nerve nine called the glossopharyngeal nerve, cranial nerve ten called the vagus nerve, cranial nerve eleven called the accessory nerve, and cranial nerve twelve called the hypoglossal nerve all emerge from the medulla. The cranial nerves are numbered based on their front to back position when viewing the brain, except for cranial nerves eleven and twelve, which are inverted. There are also 31 pairs of spinal nerves which exit various regions of the spinal cord and innervate the rest of the body. There are: 1. eight pairs of cervical nerves 2. twelve pairs of thoracic nerves 3. five pairs of lumbar nerves 4. five pairs of sacral nerves, and 5. one pair of coccygeal nerves. The peripheral nervous system is divided into the: somatic and autonomic nervous systems. The somatic nervous system is made up of - afferent or sensory nerves, which have axons that carry sensory information from the peripheral tissues like the skin, back to the cell bodies in the posterior horns of the spinal cord, as well as - efferent or motor nerves that contain the axons that carry motor information from the cell bodies in the anterior horns of the spinal cord to the neuromuscular junction, which is where these axons come into contact with the skeletal muscle cells. Now the autonomic nervous system - so both sympathetic and parasympathetic - is made up of a relay that includes two neurons: preganglionic neurons, which have their cell bodies in nuclei throughout the spinal cord in the lateral horns, and postganglionic neurons, which have their cell bodies in ganglia out of the spinal cord. - Axons of preganglionic neurons exit the spinal cord to reach the ganglia and synapse with postganglionic neurons. - Then, the axons of postganglionic neurons exit the ganglia to reach the organs and synapse with the target organ cells. Signals for the autonomic nervous system start in some hypothalamic centers. Hypothalamic neurons have really long axons - up to 1.4 meters or 4.5 feet - and they carry signals all the way down to the spinal cord nuclei where they synapse with preganglionic neuron cell bodies. Now the sympathetic and parasympathetic nervous systems have opposite effects on the body. The sympathetic nervous system controls functions like increasing the heart rate and blood pressure, as well as slowing digestion. All of this maximizes blood flow to the muscles and brain, and can help you either run away from a threat or fight it, which is why it’s also called the fight or flight response. The parasympathetic nervous system instead slows the heart rate and stimulates digestion - the effects can be summarized as rest and digest. Summary Quick review, the nervous system includes the central nervous system, so the brain and spinal cord, and the peripheral nervous system, which includes all the nerves that connect the central nervous system to the muscles and organs. The spinal cord is the pathway through which afferent and efferent fibers travel to connect the brain and peripheral nervous system. Finally, the peripheral nervous system can be divided into the somatic nervous system, which controls our skeletal muscles, and the autonomic nervous system, which is further divided into the sympathetic and the parasympathetic systems, which control smooth muscles and glands.